# Posts Tagged galaxy merger

## Recent Postings from galaxy merger

### Galaxy merger histories and the role of merging in driving star formation at z>1

We use Horizon-AGN, a hydrodynamical cosmological simulation, to explore the role of mergers in the evolution of massive (M > 10^10 MSun) galaxies around the epoch of peak cosmic star formation (1<z<4). The fraction of massive galaxies in major mergers (mass ratio R<4:1) is around 3%, a factor of ~2.5 lower than minor mergers (4:1<R <10:1) at these epochs, with no trend with redshift. At z~1, around a third of massive galaxies have undergone a major merger, while all such systems have undergone either a major or minor merger. While almost all major mergers at z>3 are ‘blue’ (i.e. have significant associated star formation), the proportion of ‘red’ mergers increases rapidly at z<2, with most merging systems at z~1.5 producing remnants that are red in rest-frame UV-optical colours. The star formation enhancement during major mergers is mild (~20-40%) which, together with the low incidence of such events, implies that this process is not a significant driver of early stellar mass growth. Mergers (R < 10:1) host around a quarter of the total star formation budget in this redshift range, with major mergers hosting around two-thirds of this contribution. Notwithstanding their central importance to the standard LCDM paradigm, mergers are minority players in driving star formation at the epochs where the bulk of today’s stellar mass was formed.

### A parsec-resolution simulation of the Antennae galaxies: Formation of star clusters during the merger

We present a hydrodynamical simulation of an Antennae-like galaxy merger at parsec resolution, including a multi-component model for stellar feedback and reaching numerical convergence in the global star formation rate for the first time. We analyse the properties of the dense stellar objects formed during the different stages of the interaction. Each galactic encounter triggers a starburst activity, but the varying physical conditions change the triggering mechanism of each starburst. During the first two pericenter passages, the starburst is spatially extended and forms many star clusters. However, the starburst associated to the third, final passage is more centrally concentrated: stars form almost exclusively in the galactic nucleus and no new star cluster is formed. The maximum mass of stars clusters in this merger is more than 30 times higher than those in a simulation of an isolated Milky Way-like galaxy. Antennae-like mergers are therefore a formation channel of young massive clusters possibly leading to globular clusters. Monitoring the evolution of a few clusters reveals the diversity of formation scenarios including the gathering and merger of gas clumps, the monolithic formation and the hierarchical formation in sub-structures inside a single cloud. Two stellar objects formed in the simulation yield the same properties as ultra-compact dwarf galaxies. They share the same formation scenario than the most massive clusters, but have a larger radius either since birth, or get it after a violent interaction with the galactic center. The diversity of environments across space and time in a galaxy merger can account for the diversity of the stellar objects formed, both in terms of mass and size.

### Should we believe the results of UV-mm galaxy SED modelling? [Replacement]

Galaxy spectral energy distribution (SED) modelling is a powerful tool, but constraining how well it is able to infer the true values for galaxy properties (e.g. the star formation rate, SFR) is difficult because independent determinations are often not available. However, galaxy simulations can provide a means of testing SED modelling techniques. Here, we present a numerical experiment in which we apply the SED modelling code MAGPHYS to ultraviolet (UV)–millimetre (mm) synthetic photometry generated from hydrodynamical simulations of an isolated disc galaxy and a major galaxy merger by performing three-dimensional dust radiative transfer. We compare the properties inferred from the SED modelling with the true values and find that MAGPHYS recovers most physical parameters of the simulated galaxies well. In particular, it recovers consistent parameters irrespective of the viewing angle, with smoothly varying results for neighbouring time steps of the simulation, even though each viewing angle and time step is modelled independently. The notable exception to this rule occurs when we use an SMC-type intrinsic dust extinction curve in the radiative transfer calculations. In this case, the two-component dust model used by MAGPHYS is unable to effectively correct for the attenuation of the simulated galaxies, which leads to potentially significant errors (although we obtain only marginally acceptable fits in this case). Overall, our results give confidence in the ability of SED modelling to infer physical properties of galaxies, albeit with some caveats.

### Should we believe the results of UV-mm galaxy SED modelling?

Galaxy spectral energy distribution (SED) modelling is a powerful tool, but constraining how well it is able to infer the true values for galaxy properties (e.g. the star formation rate, SFR) is difficult because independent determinations are often not available. However, galaxy simulations can provide a means of testing SED modelling techniques. Here, we present a numerical experiment in which we apply the SED modelling code MAGPHYS to ultraviolet (UV)–millimetre (mm) synthetic photometry generated from hydrodynamical simulations of an isolated disc galaxy and a major galaxy merger by performing three-dimensional dust radiative transfer. We compare the properties inferred from the SED modelling with the true values and find that MAGPHYS recovers most physical parameters of the simulated galaxies well. In particular, it recovers consistent parameters irrespective of the viewing angle, with smoothly varying results for neighbouring time steps of the simulation, even though each viewing angle and time step is modelled independently. The notable exception to this rule occurs when we use an SMC-type intrinsic dust extinction curve in the radiative transfer calculations. In this case, the two-component dust model used by MAGPHYS is unable to effectively correct for the attenuation of the simulated galaxies, which leads to potentially significant errors (although we obtain only marginally acceptable fits in this case). Overall, our results give confidence in the ability of SED modelling to infer physical properties of galaxies, albeit with some caveats.

### The Size of Star Clusters Accreted by the Milky Way

We perform N-body simulations of a cluster that forms in a dwarf galaxy and is then accreted by the Milky Way to investigate how a cluster’s structure is affected by a galaxy merger. We find that the cluster’s half mass radius will respond quickly to this change in potential. When the cluster is placed on an orbit in the Milky Way with a stronger tidal field the cluster experiences a sharp decrease in size in response to increased tidal forces. Conversely, when placed on an orbit with a weaker tidal field the cluster expands since tidal forces decrease and stars moving outwards due to internal effects remain bound at further distances than before. In all cases, we find that the cluster’s half mass radius will eventually be indistinguishable from a cluster that has always lived in the Milky Way on that orbit. These adjustments occur within 1-2 half mass relaxation times of the cluster in the dwarf galaxy. We also find this effect to be qualitatively independent of the time that the cluster is taken from the dwarf galaxy. In contrast to the half mass radius, we show the core radius of the cluster is not affected by the potential the cluster lives in. Our work suggests that structural properties of accreted clusters are not distinct from clusters born in the Milky Way. Other cluster properties, such as metallicity and horizontal branch morphology, may be the only way to identify accreted star clusters in the Milky Way.

### Simulating multiple merger pathways to the central kinematics of early-type galaxies

Two-dimensional integral field surveys such as ATLAS^3D are producing rich observational data sets yielding insights into galaxy formation. These new kinematic observations have highlighted the need to understand the evolutionary mechanisms leading to a spectrum of fast-rotators and slow-rotators in early-type galaxies. We address the formation of slow and fast rotators through a series of controlled, comprehensive hydrodynamical simulations sampling idealized galaxy merger scenarios constructed from model spiral galaxies. Idealized and controlled simulations of this sort complement the more ‘realistic’ cosmological simulations by isolating and analyzing the effects of specific parameters, as we do in this paper. We recreate minor and major binary mergers, binary merger trees with multiple progenitors, and multiple sequential mergers. Within each of these categories of formation history, we correlate progenitor gas fraction, mass ratio, orbital pericenter, orbital ellipticity, and spin with remnant kinematic properties. We create kinematic profiles of these 95 simulations comparable to ATLAS^3D data. By constructing remnant profiles of the projected specific angular momentum (lambda_R = <R|V|> / <sqrt(V^2+sigma^2)>, triaxiality, and measuring the incidences of kinematic twists and kinematically decoupled cores, we distinguish between varying formation scenarios. We find that binary mergers nearly always form fast rotators. Slow rotators can be formed from zero initial angular momentum configurations and gas-poor mergers, but are not as round as the ATLAS^3D galaxies. Remnants of binary merger trees are triaxial slow rotators. Sequential mergers form round slow rotators that most resemble the ATLAS^3D rotators.

### Starbursts triggered by inter-galactic tides and interstellar compressive turbulence

Using parsec-resolution simulations of a typical galaxy merger, we study the triggering of starbursts by connecting the (inter-)galactic dynamics to the structure of the interstellar medium. The gravitational encounter between two galaxies enhances tidal compression over large volumes, which increases and modifies the turbulence, in particular its compressive mode with respect to the solenoidal one. This generates an excess of dense gas leading to intense star formation activity. Along the interaction, the compressive turbulence modifies the efficiency of gas-to-star conversion which, in the Schmidt-Kennicutt diagram, drives the galaxies from the sequence of discs to that of starbursts.

### The growth of galactic bulges through mergers in LCDM haloes revisited. II. Morphological mix evolution

The mass aggregation and merger histories of present-day distinct haloes selected from the cosmological Millennium Simulations I and II are mapped into stellar mass aggregation and galaxy merger histories of central galaxies by using empirical stellar-to-halo and stellar-to-gas mass relations. The growth of bulges driven by the galaxy mergers/interactions is calculated using analytical recipes. The predicted bulge demographics at redshift z~0 is consistent with observations (Zavala+2012). Here we present the evolution of the morphological mix (traced by the bulge-to-total mass ratio, B/T) as a function of mass up to z=3. This mix remains qualitatively the same up to z~1: B/T<0.1 galaxies dominate at low masses, 0.1<B/T<0.45 at intermediate masses, and B/T>0.45 at large masses. At z>1, the fractions of disc-dominated and bulgeless galaxies increase strongly, and by z~2 the era of pure disc galaxies is reached. Bulge-dominated galaxies acquire such a morphology, and most of their mass, following a downsizing trend. Since our results are consistent with several recent observational studies of the morphological mix at different redshifts, a LCDM-based scenario of merger-driven bulge assembly does not seem to face significant issues. However, if the stellar-to-halo mass relation changes too little with redshift, then some tensions with observations appear.

### The growth of galactic bulges through mergers in LCDM haloes revisited. II. Morphological mix evolution [Replacement]

The mass aggregation and merger histories of present-day distinct haloes selected from the cosmological Millennium Simulations I and II are mapped into stellar mass aggregation and galaxy merger histories of central galaxies by using empirical stellar-to-halo and stellar-to-gas mass relations. The growth of bulges driven by the galaxy mergers/interactions is calculated using dynamical prescriptions. The predicted bulge demographics at redshift z~0 is consistent with observations (Zavala+2012). Here we present the evolution of the morphological mix (traced by the bulge-to-total mass ratio, B/T) as a function of mass up to z=3. This mix remains qualitatively the same up to z~1: B/T<0.1 galaxies dominate at low masses, 0.1<B/T<0.45 at intermediate masses, and B/T>0.45 at large masses. At z>1, the fractions of disc-dominated and bulgeless galaxies increase strongly, and by z~2 the era of pure disc galaxies is reached. Bulge-dominated galaxies acquire such a morphology, and most of their mass, following a downsizing trend. Since our results are consistent with most of the recent observational studies of the morphological mix at different redshifts, a LCDM-based scenario of merger-driven bulge assembly does not seem to face critical issues. However, if the stellar-to-halo mass relation changes too little with redshift, then some tensions with observations appear.

### The use of gravitational lenses in the study of distant galaxy mergers

Gravlenses are efficiently explored for detecting the most distant galaxies (up to z=10 redshifts). As an example of the role played by gravlenses we refer to the observation of the galaxy merger at z=3 (Borys, et al; Berciano Alba, et al). We derived solutions for the Smoluchowski kinetic equation for the mass function of galaxies, which describes mergers in differential approximation (minor mergers). It is shown that the evolution of the slope of luminosity function observed in the Ultra Deep Hubble Field (Bouwence et al) can be described as a result of explosive evolution driven by galaxy mergers.

### Automatic quantitative morphological analysis of interacting galaxies

The large number of galaxies imaged by digital sky surveys reinforces the need for computational methods for analyzing galaxy morphology. While the morphology of most galaxies can be associated with a stage on the Hubble sequence, morphology of galaxy mergers is far more complex due to the combination of two or more galaxies with different morphologies and the interaction between them. Here we propose a computational method based on unsupervised machine learning that can quantitatively analyze morphologies of galaxy mergers and associate galaxies by their morphology. The method works by first generating multiple synthetic galaxy models for each galaxy merger, and then extracting a large set of numerical image content descriptors for each galaxy model. These numbers are weighted using Fisher discriminant scores, and then the similarities between the galaxy mergers are deduced using a variation of Weighted Nearest Neighbor analysis such that the Fisher scores are used as weights. The similarities between the galaxy mergers are visualized using phylogenies to provide a graph that reflects the morphological similarities between the different galaxy mergers, and thus quantitatively profile the morphology of galaxy mergers.

### Galaxy mergers on a moving mesh: a comparison with smoothed-particle hydrodynamics

Galaxy mergers have been investigated for decades using smoothed particle hydrodynamics (SPH), but recent work highlighting inaccuracies inherent in the traditional SPH technique calls into question the reliability of previous studies. We explore this issue by comparing a suite of Gadget-3 SPH simulations of idealised (i.e., non-cosmological) isolated discs and galaxy mergers with otherwise identical calculations performed using the moving-mesh code Arepo. When black hole (BH) accretion and active galactic nucleus (AGN) feedback are not included, the star formation histories (SFHs) obtained from the two codes agree well. When BHs are included, the code- and resolution-dependent variations in the SFHs are more significant, but the agreement is still good, and the stellar mass formed over the course of a simulation is robust to variations in the numerical method. During a merger, the gas morphology and phase structure are initially similar prior to the starburst phase. However, once a hot gaseous halo has formed from shock heating and AGN feedback (when included), the agreement is less good. In particular, during the post-starburst phase, the SPH simulations feature more prominent hot gaseous haloes and spurious clumps, whereas with Arepo, gas clumps and filaments are less apparent and the hot halo gas can cool more efficiently. We discuss the origin of these differences and explain why the SPH technique yields trustworthy results for some applications (such as the idealised isolated disc and galaxy merger simulations presented here) but not others (e.g., gas flows onto galaxies in cosmological hydrodynamical simulations).

### Dual Supermassive Black Hole Candidates in the AGN and Galaxy Evolution Survey

Dual supermassive black holes (SMBHs) with kiloparsec scale separations in merger-remnant galaxies are informative tracers of galaxy evolution, but the avenue for identifying them in large numbers for such studies is not yet clear. One promising approach is to target spectroscopic signatures of systems where both SMBHs are fueled as dual active galactic nuclei (AGNs), or where one SMBH is fueled as an offset AGN. Dual AGNs may produce double-peaked narrow AGN emission lines, while offset AGNs may produce single-peaked narrow AGN emission lines with line-of-sight velocity offsets relative to the host galaxy. We search for such dual and offset systems among 173 Type 2 AGNs at z<0.37 in the AGN and Galaxy Evolution Survey (AGES), and we find two double-peaked AGNs and five offset AGN candidates. When we compare these results to a similar search of the DEEP2 Galaxy Redshift Survey and match the two samples in color, absolute magnitude, and minimum velocity offset, we find that the fraction of AGNs that are dual SMBH candidates increases from z=0.25 to z=0.7 by a factor of ~6 (from 2/70 to 16/91, or 2.9% to 18%). This may be associated with the rise in the galaxy merger fraction over the same cosmic time. As further evidence for a link with galaxy mergers, the AGES offset and dual AGN candidates are tentatively ~3 times more likely than the overall AGN population to reside in a host galaxy that has a companion galaxy (from 16/173 to 2/7, or 9% to 29%). Follow-up observations of the seven offset and dual AGN candidates in AGES will definitively distinguish velocity offsets produced by dual SMBHs from those produced by narrow-line region kinematics, and will help sharpen our observational approach to detecting dual SMBHs.

### Pre-Processing and Post-Processing in Group-Cluster Mergers

Galaxies in clusters are more likely to be of early type and to have lower star formation rates than galaxies in the field. Recent observations and simulations suggest that cluster galaxies may be pre-processed’ by group or filament environments and that galaxies that fall into a cluster as part of a larger group can stay coherent within the cluster for up to one orbital period (post-processing’). We investigate these ideas by means of a cosmological $N$-body simulation and idealized $N$-body plus hydrodynamics simulations of a group-cluster merger. We find that group environments can contribute significantly to galaxy pre-processing by means of enhanced galaxy-galaxy merger rates, removal of galaxies’ hot halo gas by ram pressure stripping, and tidal truncation of their galaxies. Tidal distortion of the group during infall does not contribute to pre-processing. Post-processing is also shown to be effective: galaxy-galaxy collisions are enhanced during a group’s pericentric passage within a cluster, the merger shock enhances the ram pressure on group and cluster galaxies, and an increase in local density during the merger leads to greater galactic tidal truncation.

### The Inner Kiloparsec of Mrk 273 with Keck Adaptive Optics

There is X-ray, optical, and mid-infrared imaging and spectroscopic evidence that the late-stage ultraluminous infrared galaxy merger Mrk 273 hosts a powerful active galactic nucleus (AGN). However, the exact location of the AGN and the nature of the nuclei have been difficult to determine due to dust obscuration and the limited wavelength coverage of available high-resolution data. Here we present near-infrared integral-field spectra and images of the nuclear region of Mrk 273 taken with OSIRIS and NIRC2 on the Keck II Telescope with laser guide star adaptive optics. We observe three spatially resolved components, and analyze the local molecular and ionized gas emission lines and their kinematics. We confirm the presence of the hard X-ray AGN in the southwest nucleus. In the north nucleus, we find a strongly rotating gas disk whose kinematics indicate a central black hole of mass 1.04 +/- 0.1 x 10^9 Msun. The H2 emission line shows an increase in velocity dispersion along the minor axis in both directions, and an increased flux with negative velocities in the southeast direction; this provides direct evidence for a collimated molecular outflow along the axis of rotation of the disk. The third spatially distinct component appears to the southeast, 640 and 750 pc from the north and southwest nuclei, respectively. This component is faint in continuum emission but shows several strong emission line features, including [Si vi] 1.964 {\mu}m which traces an extended coronal-line region. The geometry of the [Si vi] emission combined with shock models and energy arguments suggest that [Si vi] in the southeast component must be at least partly ionized by the SW AGN or a putative AGN in the northern disk, either through photoionization or through shock-heating from strong AGN- and circumnuclear starburst-driven outflows. This lends support to a scenario in which Mrk 273 may be a dual AGN system.

### The M-sigma relation in simulations of isolated and merging disk galaxies with kinetic or thermal AGN feedback

(Abridged) We investigate two modes of coupling the feedback energy from a central AGN to the neighboring gas in galaxy simulations: kinetic – velocity boost, and thermal – heating. We formulate kinetic feedback models for energy-driven wind (EDW) and momentum-driven wind (MDW), using two free parameters: feedback efficiency epsilon_f, and AGN wind velocity v_w. A novel numerical algorithm is implemented in the SPH code GADGET-3, to prevent the expansion of a hole in the gas distribution around the BH. We perform simulations of isolated evolution and merger of disk galaxies, of Milky-Way mass as well as lower and higher masses. We find that in the isolated galaxy BH kinetic feedback generates intermittent bipolar jet-like gas outflows. We infer that current prescriptions for BH subgrid physics in galaxy simulations can grow the BH to observed values even in an isolated disk galaxy. The BH growth is enhanced in a galaxy merger. Comparing the [M_BH - sigma_star] relation obtained in our simulations with observational data, we conclude that it is possible to find parameter sets for a fit in all the models, except for the case with MDW feedback in a galaxy merger, in which the BH is always too massive. The BH thermal feedback implementation of Springel, Di Matteo & Hernquist (2005) within the multiphase star-formation model is found to have negligible impact on gas properties; and the effect claimed in all previous studies is attributed to gas depletion around the BH by the creation of an artificial hole. The BH mass accretion rate in our simulations exhibit heavy fluctuations. The star formation rate is quenched with feedback by removal of gas. The CGM gas at galactocentric distances (20 – 100)/h kpc are found to give the best metallicity observational diagnostic to distinguish between BH models.

### A scaling relation between merger rate of galaxies and their close pair count

When the galaxy merger rate is measured from the observed close pair count, usually a single merger timescale is assumed. Current calibrations based on simulations have focused mostly on major mergers (stellar mass ratios from 1:1 to 1:4). Using a high-resolution N-body/SPH cosmological simulation, we find an accurate scaling relation between galaxy pair counts and merger rates down to a stellar mass ratio of about 1:30. The relation explicitly accounts for the dependence on redshift (or time), pair separation, and virial masses of the two galaxies in a pair. With this relation, one can easily obtain the mean merger timescale for a close pair of galaxies. The use of virial masses, instead of the stellar mass, is motivated by the fact that the dynamical friction time scale is mainly determined by the dark matter surrounding central and satellite galaxies. This fact can also minimize the error induced by uncertainties in modeling star formation in the simulations. Since the virial mass can be read from the well-established relation between the virial masses and the stellar masses in observation, our scaling relation can be easily applied to observations to obtain the merger rate and merger time scale. For major merger pairs (1:1-1:4) of galaxies above 4*10^10 M_sun/h at z=0.1, it takes about 0.31 Gyr to merge for pairs within a projected distance of 20 kpc/h with stellar mass ratio of 1:1, while the time taken goes up to 1.6 Gyr for mergers with stellar mass ratio of 1:4. Our results indicate that a single timescale is not enough to describe mergers with the stellar mass ratio spanning even a narrow range from 1:1 to 1:4.

### A scaling relation between merger rate of galaxies and their close pair count [Replacement]

When the galaxy merger rate is measured from the observed close pair count, usually a single merger timescale is assumed. Current calibrations based on simulations have focused mostly on major mergers (stellar mass ratios from 1:1 to 1:4). Using a high-resolution N-body/SPH cosmological simulation, we find an accurate scaling relation between galaxy pair counts and merger rates down to a stellar mass ratio of about 1:30. The relation explicitly accounts for the dependence on redshift (or time), pair separation, and virial masses of the two galaxies in a pair. With this relation, one can easily obtain the mean merger timescale for a close pair of galaxies. The use of virial masses, instead of the stellar mass, is motivated by the fact that the dynamical friction time scale is mainly determined by the dark matter surrounding central and satellite galaxies. This fact can also minimize the error induced by uncertainties in modeling star formation in the simulations. Since the virial mass can be read from the well-established relation between the virial masses and the stellar masses in observation, our scaling relation can be easily applied to observations to obtain the merger rate and merger time scale. For major merger pairs (1:1-1:4) of galaxies above 4*10^10 M_sun/h at z=0.1, it takes about 0.31 Gyr to merge for pairs within a projected distance of 20 kpc/h with stellar mass ratio of 1:1, while the time taken goes up to 1.6 Gyr for mergers with stellar mass ratio of 1:4. Our results indicate that a single timescale is not enough to describe mergers with the stellar mass ratio spanning even a narrow range from 1:1 to 1:4.

### Massive black hole pairs in clumpy, self-gravitating circumnuclear disks: stochastic orbital decay

We study the dynamics of massive black hole pairs in clumpy gaseous circumnuclear disks. We track the orbital decay of the light, secondary black hole M_{\bullet2} orbiting around the more massive primary at the center of the disk, using N-body/SPH simulations. We find that the gravitational interaction of M_{\bullet2} with massive clumps M_cl erratically perturbs the otherwise smooth orbital decay. In close encounters with massive clumps, gravitational slingshots can kick the secondary black hole out of the disk plane. The black hole moving on an inclined orbit then experiences the weaker dynamical friction of the stellar background, resulting in a longer orbital decay timescale. Interactions between clumps can also favor orbital decay when the black hole is captured by a massive clump which is segregating toward the center of the disk. The stochastic behavior of the black hole orbit emerges mainly when the ratio M_{\bullet2}/M_cl falls below unity, with decay timescales ranging from ~1 to ~50 Myr. This suggests that describing the cold clumpy phase of the inter-stellar medium in self-consistent simulations of galaxy mergers, albeit so far neglected, is important to predict the black hole dynamics in galaxy merger remnants.

### Active Galactic Nucleus and Extended Starbursts in a Mid-stage Merger VV114

High resolution (0.4 arcsec) Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 0 observations of HCO+(4-3) and HCN(4-3) toward a mid-stage infrared bright merger VV114 have revealed compact nuclear (<200 pc) and extended (3 – 4 kpc) dense gas distribution across the eastern part of the galaxy pair. We find a significant enhancement of HCN(4-3) emission in an unresolved compact and broad (290km/s) component found in the eastern nucleus of VV114, and we suggest dense gas associated with the surrounding material around an Active Galactic Nucleus (AGN), with a mass upper limit of < 4 x 10^8 Msun. The extended dense gas is distributed along a filamentary structure with resolved dense gas concentrations (230pc; 10^6 Msun) separated by a mean projected distance of 600 pc, many of which are generally consistent with the location of star formation traced in Pa alpha emission. Radiative transfer calculations suggest moderately dense (10^5 – 10^6 cm^-3) gas averaged over the entire emission region. These new ALMA observations demonstrate the strength of the dense gas tracers in identifying both the AGN and star formation activity in a galaxy merger, even in the most dust enshrouded environments in the local universe.

### The most luminous quasars do not live in the most massive dark matter haloes at any redshift

Quasars represent the brightest Active Galactic Nuclei (AGN) in the Universe and are thought to indicate the location of prodigiously growing Black Holes (BHs), with luminosities as high as 10^48 erg/sec. It is often expected though that such an extremely energetic process will take place in the most massive bound structures in the dark matter (DM) distribution. We show that in contrast to this expectation, in a galaxy formation model which includes AGN feedback, quasars are predicted to live in average DM halo environments with typical masses of a few times 10^12 Msun. This fundamental prediction arises from the fact that quasar activity (i.e., BH accretion with luminosity greater than 10^46 erg/sec) is inhibited in DM haloes where AGN feedback operates. The galaxy hosts of quasars in our simulations are identified with over massive (in gas and stars) spheroidal galaxies, in which BH accretion is triggered via a galaxy merger or secular processes. We further show that the z=0 descendants of high redshift (z~6) QSOs span a wide range of morphologies, galaxy and halo masses. The z~6 BHs typically grow only by a modest factor by the present day. Remarkably, high redshift QSOs never inhabit the largest DM haloes at that time and their descendants are very seldom found in the most massive haloes at z=0. We also show that observationally it is very likely to find an enhancement in the abundance of galaxies around quasars at z~5. However, these enhancements are considerably weaker compared to the overdensities expected at the extreme peaks of the DM distribution. Thus, it is very unlikely that a quasar detected in the $z\gtrsim5$ Universe pinpoints the location of the progenitors of superclusters in the local Universe.

### Constrained simulations of the Antennae Galaxies: Comparison with Herschel-PACS observations

We present a set of hydro-dynamical numerical simulations of the Antennae galaxies in order to understand the origin of the central overlap starburst. Our dynamical model provides a good match to the observed nuclear and overlap star formation, especially when using a range of rather inefficient stellar feedback efficiencies (0.01 < q_EoS < 0.1). In this case a simple conversion of local star formation to molecular hydrogen surface density motivated by observations accounts well for the observed distribution of CO. Using radiative transfer post-processing we model synthetic far-infrared spectral energy distributions (SEDs) and two-dimensional emission maps for direct comparison with Herschel-PACS observations. For a gas-to-dust ratio of 62:1 and the best matching range of stellar feedback efficiencies the synthetic far-infrared SEDs of the central star forming region peak at values of ~65 – 81 Jy at 99 – 116 um, similar to a three-component modified black body fit to infrared observations. Also the spatial distribution of the far-infrared emission at 70 um, 100 um, and 160 um compares well with the observations: >50% (> 35%) of the emission in each band is concentrated in the overlap region while only < 30% (< 15%) is distributed to the combined emission from the two galactic nuclei in the simulations (observations). As a proof of principle we show that parameter variations in the feedback model result in unambiguous changes both in the global and in the spatially resolved observable far-infrared properties of Antennae galaxy models. Our results strengthen the importance of direct, spatially resolved comparative studies of matched galaxy merger simulations as a valuable tool to constrain the fundamental star formation and feedback physics.

### Constrained simulations of the Antennae Galaxies: Comparison with Herschel-PACS observations [Replacement]

We present a set of hydro-dynamical numerical simulations of the Antennae galaxies in order to understand the origin of the central overlap starburst. Our dynamical model provides a good match to the observed nuclear and overlap star formation, especially when using a range of rather inefficient stellar feedback efficiencies (0.01 < q_EoS < 0.1). In this case a simple conversion of local star formation to molecular hydrogen surface density motivated by observations accounts well for the observed distribution of CO. Using radiative transfer post-processing we model synthetic far-infrared spectral energy distributions (SEDs) and two-dimensional emission maps for direct comparison with Herschel-PACS observations. For a gas-to-dust ratio of 62:1 and the best matching range of stellar feedback efficiencies the synthetic far-infrared SEDs of the central star forming region peak at values of ~65 – 81 Jy at 99 – 116 um, similar to a three-component modified black body fit to infrared observations. Also the spatial distribution of the far-infrared emission at 70 um, 100 um, and 160 um compares well with the observations: >50% (> 35%) of the emission in each band is concentrated in the overlap region while only < 30% (< 15%) is distributed to the combined emission from the two galactic nuclei in the simulations (observations). As a proof of principle we show that parameter variations in the feedback model result in unambiguous changes both in the global and in the spatially resolved observable far-infrared properties of Antennae galaxy models. Our results strengthen the importance of direct, spatially resolved comparative studies of matched galaxy merger simulations as a valuable tool to constrain the fundamental star formation and feedback physics.

### Star Formation and Metallicity Gradients in Semi-analytic Models of Disk Galaxy Formation

We updated our radially-resolved SAMs of galaxy formation to track the radial distribution of stars, metals, atomic and molecular gas in galactic disks. The models are run on both MS and MS II using the recipes outlined in Fu et al. (2010) and Guo et al. (2011), with 3 main changes: (1) We adopt a simple star formation law where \Sigma_SFR \propto \Sigma_H2. (2) We inject the heavy elements produced by SNe directly into the halo, instead of first mixing them with the disk cold gas. (3) We include radial gas inflows in disks using a model of the form v_inflow=alpha r. The average \Sigma_H2 profiles in L_* galaxies strongly constrains the inflow velocities, favoring models where v_inflow ~ 7 km/s at r=10 kpc. The radial inflow model has little influence on the gas and stellar metallicity gradients in the outer disks. Gas metallicity gradients are affected much more strongly by the fraction of metals that are directly injected into the halo gas, rather than mixed with the interstellar cold gas. Metals ejected out of the galaxy at early epochs result in late infall of pre-enriched gas and flatter present-day gas metallicity gradients. A prescription in which 80% of the metals produced by stars are injected into the halo gas provides the best fit to the relatively flat observed metallicity gradients of galaxies with stellar masses greater than 10^10 M_sun. Such a prescription also results in a good fit to the relation between gas metallicity and sSFR in the outer parts of disks. We examine the correlation between gas metallicity gradient and some global galaxy properties, finding that it is most strongly correlated with the B/T ratio of the galaxy. This is because gas is consumed when the bulge forms during the galaxy merger, and the gas metallicity gradient is then set by newly-accreted gas. These model predictions appear to be in good agreement with observations from Moran et al. (2012).

### The Spatial Extent and Distribution of Star Formation in 3D-HST Mergers at z~1.5

We present an analysis of the spatial distribution of star formation in a sample of 60 visually identified galaxy merger candidates at z>1. Our sample, drawn from the 3D-HST survey, is flux-limited and was selected to have high star formation rates based on fits of their broad-band, low spatial resolution spectral energy distributions. It includes plausible pre-merger (close pairs) and post-merger (single objects with tidal features) systems, with total stellar masses and star formation rates derived from multi-wavelength photometry. Here we use near-infrared slitless spectra from 3D-HST which produce Halpha or [OIII] emission line maps as proxies for star-formation maps. This provides a first comprehensive high-resolution, empirical picture of where star formation occurred in galaxy mergers at the epoch of peak cosmic star formation rate. We find that detectable star formation can occur in one or both galaxy centres, or in tidal tails. The most common case (58%) is that star formation is largely concentrated in a single, compact region, coincident with the centre of (one of) the merger components. No correlations between star formation morphology and redshift, total stellar mass, or star formation rate are found. A restricted set of hydrodynamical merger simulations between similarly massive and gas-rich objects implies that star formation should be detectable in both merger components, when the gas fractions of the individual components are the same. This suggests that z~1.5 mergers typically occur between galaxies whose gas fractions, masses, and/or star formation rates are distinctly different from one another.

### Fast and Furious: Shock Heated Gas as the Origin of Spatially Resolved Hard X-ray Emission in the Central 5 kpc of the Galaxy Merger NGC 6240

We have obtained a deep, sub-arcsecond resolution X-ray image of the nuclear region of the luminous galaxy merger NGC 6240 with Chandra, which resolves the X-ray emission from the pair of active nuclei and the diffuse hot gas in great detail. We detect extended hard X-ray emission from kT~6 keV (~70 million K) hot gas over a spatial scale of 5 kpc, indicating the presence of fast shocks with velocity of ~2200 km/s. For the first time we obtain the spatial distribution of this highly ionized gas emitting FeXXV, which shows a remarkable correspondence to the large scale morphology of H_2(1-0) S(1) line emission and H\alpha filaments. Propagation of fast shocks originated in the starburst driven wind into the ambient dense gas can account for this morphological correspondence. With an observed L(0.5-8 keV)=5.3E+41 erg/s, the diffuse hard X-ray emission is 100 times more luminous than that observed in the classic starburst galaxy M82. Assuming a filling factor of 1% for the 70 MK temperature gas, we estimate its total mass (M_{hot}=1.8E+8 Msun) and thermal energy (E_{th}=6.5E+57 ergs). The total iron mass in the highly ionized plasma is M_{Fe}=4.6E+5 Msun. Both the energetics and the iron mass in the hot gas are consistent with the expected injection from the supernovae explosion during the starburst that is commensurate with its high star formation rate. No evidence for fluorescent Fe I emission is found in the CO filament connecting the two nuclei.

### Supermassive Black Hole Binary Evolution in Axisymmetric Galaxies: the final parsec problem is not a problem

During a galaxy merger, the supermassive black hole (SMBH) in each galaxy is thought to sink to the center of the potential and form a supermassive black hole binary; this binary can eject stars via 3-body scattering, bringing the SMBHs ever closer. In a static spherical galaxy model, the binary stalls at a separation of about a parsec after ejecting all the stars in its loss cone — this is the well-known final parsec problem. Earlier work has shown that the centrophilic orbits in triaxial galaxy models are key in refilling the loss cone at a high enough rate to prevent the black holes from stalling. However, the evolution of binary SMBHs has never been explored in axisymmetric galaxies, so it is not clear if the final parsec problem persists in these systems. Here we use a suite of direct N-body simulations to follow SMBH binary evolution in galaxy models with a range of ellipticity. For the first time, we show that mere axisymmetry can solve the final parsec problem; we find the the SMBH evolution is independent of N for an axis ratio of c/a=0.8, and that the SMBH binary separation reaches the gravitational radiation regime for c/a=0.75.

### The Exceptional Soft X-ray Halo of the Galaxy Merger NGC 6240

We report on a recent ~150-ks long Chandra observation of the ultraluminous infrared galaxy merger NGC 6240, which allows a detailed investigation of the diffuse galactic halo. Extended soft X-ray emission is detected at the 3-sigma confidence level over a diamond-shaped region with projected physical size of ~110×80 kpc, and a single-component thermal model provides a reasonably good fit to the observed X-ray spectrum. The hot gas has a temperature of ~7.5 million K, an estimated density of 2.5×10^{-3} cm^{-3}, and a total mass of ~10^10 M_sun, resulting in an intrinsic 0.4-2.5 keV luminosity of 4×10^41 erg s^{-1}. The average temperature of 0.65 keV is quite high to be obviously related to either the binding energy in the dark-matter gravitational potential of the system or the energy dissipation and shocks following the galactic collision, yet the spatially-resolved spectral analysis reveals limited variations across the halo. The relative abundance of the main alpha-elements with respect to iron is several times the solar value, and nearly constant as well, implying a uniform enrichment by type II supernovae out to the largest scales. Taken as a whole, the observational evidence is not compatible with a superwind originated by a recent, nuclear starburst, but rather hints at widespread, enhanced star formation proceeding at steady rate over the entire dynamical timescale (~200 Myr). The preferred scenario is that of a starburst-processed gas component gently expanding into, and mixing with, a pre-existing halo medium of lower metallicity (Z ~ 0.1 solar) and temperature (kT ~ 0.25 keV). This picture cannot be probed more extensively with the present data, and the ultimate fate of the diffuse, hot gas remains uncertain. Under some favorable conditions, at least a fraction of it might be retained after the merger completion, and evolve into the hot halo of a young elliptical galaxy.

### Gravitational waves and stalled satellites from massive galaxy mergers at z <= 1 [Replacement]

We present a model for merger-driven evolution of the mass function for massive galaxies and their central supermassive black holes at late times. We discuss the current observational evidence in favor of merger-driven massive galaxy evolution during this epoch, and demonstrate that the observed evolution of the mass function can be reproduced by evolving an initial mass function under the assumption of negligible star formation. We calculate the stochastic gravitational wave signal from the resulting black-hole binary mergers in the low redshift universe (z <= 1) implied by this model, and find that this population has a signal-to-noise ratio as much as ~5x larger than previous estimates for pulsar timing arrays, with an expectation value for the characteristic strain h_c (f=1 yr^{-1}) = 4.1 x 10^{-15} that may already be in tension with observational constraints, and a {2-sigma, 3-sigma} lower limit within this model of h_c (f=1 yr^{-1}) = {1.1 x 10^{-15}, 6.8 x 10^{-16}}. The strength of this signal is sufficient to make it detectable with high probability under conservative assumptions within the next several years, if the principle assumption of merger-driven galaxy evolution since z = 1 holds true. For cases where a galaxy merger fails to lead to a black hole merger, we estimate the probability for a given number of satellite unmerged black holes to remain within a massive host galaxy, and interpret the result in light of ULX observations. In particular, we find that the brightest cluster galaxies should have 1-2 such sources with luminosities above 10^{39} erg/s, which is consistent with the statistics of observed ULXs.

### Gravitational waves and stalled satellites from massive galaxy mergers at z < 1

We present a model for merger-driven evolution of the mass function for massive galaxies and their central supermassive black holes at late times. We discuss the current observational evidence in favor of merger-driven massive galaxy evolution during this epoch, and demonstrate that the observed evolution of the mass function can be reproduced by evolving an initial mass function under the assumption of negligible star formation. We calculate the stochastic gravitational wave signal from the resulting black-hole binary mergers in the low redshift universe (z < 1) implied by this model, and find that this population has a signal-to-noise ratio as much as ~5x larger than previous estimates for pulsar timing arrays, with an expectation value for the characteristic strain h_c(f =1 yr^{-1})=5.8 x 10^{-15} that is already in tension with observational constraints, and a 2-sigma lower limit within this model of h_c(f =1 yr^{-1})=2.0 x 10^{-15}. The strength of this signal may therefore be detectable with the data already collected using the current generation of pulsar timing arrays, and could be detected with high statistical significance under conservative assumptions within the next few years, if the principle assumption of merger-driven galaxy evolution since z=1 holds true. For cases where a galaxy merger fails to lead to a black hole merger, we estimate the probability for a given number of satellite unmerged black holes to remain within a massive host galaxy, and interpret the result in light of ULX observations. In particular, we find that the brightest cluster galaxies should have 1-2 such sources with luminosities above 10^{39} erg/s, which is consistent with the statistics of observed ULXs.

### Systematic investigation of the expected gravitational wave signal from supermassive black hole binaries in the pulsar timing band

In this letter we carry out the first systematic investigation of the expected gravitational wave (GW) background generated by supermassive black hole (SMBH) binaries in the nHz frequency band accessible to pulsar timing arrays (PTAs). We take from the literature several estimates of the redshift dependent galaxy mass function and of the fraction of close galaxy pairs to derive a wide range of galaxy merger rates. We then exploit empirical black hole-host relations to populate merging galaxies with SMBHs. The result of our procedure is a collection of a large number of phenomenological SMBH binary merger rates consistent with current observational constraints on the galaxy assembly at z<1.5. For each merger rate we compute the associated GW signal, eventually producing a large set of estimates of the nHz GW background that we use to infer confidence intervals of its expected amplitude. When considering the most recent SMBH-host relations, accounting for ultra-massive black holes in brightest cluster galaxies, we find that the nominal $1\sigma$ interval of the expected GW signal is only a factor of 3-to-10 below current PTA limits, implying a non negligible chance of detection in the next few years.

### The imminent detection of gravitational waves from massive black-hole binaries with pulsar timing arrays

Recent observations of massive galaxies indicate that they double in mass and quintuple in size between redshift z = 1 and the present, despite undergoing very little star formation, suggesting that galaxy mergers drive the evolution. Since these galaxies will contain supermassive black holes, this suggests a larger black hole merger rate, and therefore a larger gravitational-wave signal, than previously expected. We calculate the merger-driven evolution of the mass function, and find that merger rates are 10 to 30 times higher and gravitational waves are 3 to 5 times stronger than previously estimated, so that the gravitational-wave signal may already be detectable with existing data from pulsar timing arrays. We also provide an explanation for the disagreement with past estimates that were based on dark matter halo simulations.

### Massive black hole seeds born via direct gas collapse in galaxy mergers: their properties, statistics and environment

We study the statistics and cosmic evolution of massive black hole seeds formed during major mergers of gas-rich late-type galaxies. Generalizing the results of the hydro-simulations from Mayer et al. 2010, we envision a scenario in which a supermassive star can form at the center of galaxies that just experienced a major merger owing to a multi-scale powerful gas inflow, provided that such galaxies live in haloes with masses above 10^{11} Msun, are gas-rich and disc-dominated, and do not already host a massive black hole. We assume that the ultimate collapse of the supermassive star leads to the rapid formation of a black hole of 10^5 Msun following a quasi-star stage. Using a model for galaxy formation applied to the outputs of the Millennium Simulation, we show that the conditions required for this massive black hole formation route to take place in the concordance LambdaCDM model are actually common at high redshift, and can be realized even at low redshift. Most major mergers above z~4 in haloes with mass > 10^{11} Msun can lead to the formation of a massive seed and, at z~2, the fraction of favourable mergers decreases to about half. Interestingly, we find that even in the local universe a fraction (~20%) of major mergers in massive haloes still satisfy the conditions for our massive black hole formation route. Those late events take place in galaxies with a markedly low clustering amplitude, that have lived in isolation for most of their life, and that are experiencing a major merger for the first time. We predict that massive black hole seeds from galaxy mergers can dominate the massive end of the mass function at high (z>4) and intermediate (z~2) redshifts relative to lighter seeds formed at higher redshift, for example, by the collapse of Pop III stars. Finally, a fraction of these massive seeds could lie, soon after formation, above the MBH-MBulge relation.

### NGC6240: extended CO structures and their association with shocked gas

We present deep CO observations of NGC6240 performed with the IRAM Plateau de Bure Interferometer (PdBI). NGC6240 is the prototypical example of a major galaxy merger in progress, caught at an early stage, with an extended, strongly-disturbed butterfly-like morphology and the presence of a heavily obscured active nucleus in the core of each progenitor galaxy. The CO line shows a skewed profile with very broad and asymmetric wings detected out to velocities of -600 km/s and +800 km/s with respect to the systemic velocity. The PdBI maps reveal the existence of two prominent structures of blueshifted CO emission. One extends eastward, i.e. approximately perpendicular to the line connecting the galactic nuclei, over scales of ~7 kpc and shows velocities up to -400 km/s. The other extends southwestward out to ~7 kpc from the nuclear region, and has a velocity of -100 km/s with respect to the systemic one. Interestingly, redshifted emission with velocities 400 to 800 km/s is detected around the two nuclei, extending in the east-west direction, and partly overlapping with the eastern blue-shifted structure, although tracing a more compact region of size ~1.7 kpc. The overlap between the southwestern CO blob and the dust lanes seen in HST images, which are interpreted as tidal tails, indicates that the molecular gas is deeply affected by galaxy interactions. The eastern blueshifted CO emission is co-spatial with an Halpha filament that is associated with strong H2 and soft X-ray emission. The analysis of Chandra X-ray data provides strong evidence for shocked gas at the position of the Halpha emission. Its association with outflowing molecular gas supports a scenario where the molecular gas is compressed into a shock wave that propagates eastward from the nuclei. If this is an outflow, the AGN are likely the driving force.

### Spectroscopic evidence of distinct stellar populations in the counter-rotating stellar disks of NGC 3593 and NGC 4550

We present the results of integral-field spectroscopic observations of the two disk galaxies NGC 3593 and NGC 4550 obtained with VIMOS/VLT. Both galaxies are known to host 2 counter-rotating stellar disks, with the ionized gas co-rotating with one of them. We measured in each galaxy the ionized gas kinematics and metallicity, and the surface brightness, kinematics, mass surface density, and the stellar populations of the 2 stellar components to constrain the formation scenario of these peculiar galaxies. We applied a novel spectroscopic decomposition technique to both galaxies, to separate the relative contribution of the 2 counter-rotating stellar and one ionized-gas components to the observed spectrum. We measured the kinematics and the line strengths of the Lick indices of the 2 counter-rotating stellar components. We modeled the data of each stellar component with single stellar population models that account for the alpha/Fe overabundance. In both galaxies we successfully separated the main from the secondary stellar component that is less massive and rotates in the same direction of the ionized-gas component. The 2 stellar components have exponential surface-brightness profiles. In both galaxies, the two counter-rotating stellar components have different stellar populations: the secondary stellar disk is younger, more metal poor, and more alpha-enhanced than the main galaxy stellar disk. Our findings rule out an internal origin of the secondary stellar component and favor a scenario where it formed from gas accreted on retrograde orbits from the environment fueling an in situ outside-in rapid star formation. The event occurred ~ 2 Gyr ago in NGC 3593, and ~ 7 Gyr ago in NGC 4550. The binary galaxy merger scenario cannot be ruled out, and a larger sample is required to statistically determine which is the most efficient mechanism to build counter-rotating stellar disks (abridged).

### Numerical hydrodynamic simulations based on semi-analytic galaxy merger trees: method and Milky-Way like galaxies

We present a new approach to study galaxy evolution in a cosmological context. We combine cosmological merger trees and semi-analytic models of galaxy formation to provide the initial conditions for multi-merger hydrodynamic simulations. In this way we exploit the advantages of merger simulations (high resolution and inclusion of the gas physics) and semi-analytic models (cosmological background and low computational cost), and integrate them to create a novel tool. This approach allows us to study the evolution of various galaxy properties, including the treatment of the hot gaseous halo from which gas cools and accretes onto the central disc, which has been neglected in many previous studies. This method shows several advantages over other methods. As only the particles in the regions of interest are included, the run time is much shorter than in traditional cosmological simulations, leading to greater computational efficiency. Using cosmological simulations, we show that multiple mergers are expected to be more common than sequences of isolated mergers, and therefore studies of galaxy mergers should take this into account. In this pilot study, we present our method and illustrate the results of simulating ten Milky Way-like galaxies since z=1. We find good agreement with observations for the total stellar masses, star formation rates, cold gas fractions and disc scale length parameters. We expect that this novel numerical approach will be very useful for pursuing a number of questions pertaining to the transformation of galaxy internal structure through cosmic time.

### Red Galaxies from Hot Halos in Cosmological Hydro Simulations

I highlight three results from cosmological hydrodynamic simulations that yield a realistic red sequence of galaxies: 1) Major galaxy mergers are not responsible for shutting off star-formation and forming the red sequence. Starvation in hot halos is. 2) Massive galaxies grow substantially (about a factor of 2 in mass) after being quenched, primarily via minor (1:5) mergers. 3) Hot halo quenching naturally explains why galaxies are red when they either (a) are massive or (b) live in dense environments.

### Major Galaxy Mergers Only Trigger the Most Luminous AGN

Using multiwavelength surveys of active galactic nuclei across a wide range of bolometric luminosities (10^{43}<L_{bol}(erg/s<5×10^{46}) and redshifts (0<z<3), we find a strong, redshift-independent correlation between the AGN luminosity and the fraction of host galaxies undergoing a major merger. That is, only the most luminous AGN phases are connected to major mergers, while less luminous AGN appear to be driven by secular processes. Combining this trend with AGN luminosity functions to assess the overall cosmic growth of black holes, we find that ~50% by mass is associated with major mergers, while only 10% of AGN by number, the most luminous, are connected to these violent events. Our results suggest that to reach the highest AGN luminosities -where the most massive black holes accreted the bulk of their mass – a major merger appears to be required. The luminosity dependence of the fraction of AGN triggered by major mergers can successfully explain why the observed scatter in the M-\sigma relation for elliptical galaxies is significantly lower than in spirals. The lack of a significant redshift dependence of the L_{bol}-f_{merger} relation suggests that downsizing, i.e., the general decline in AGN and star formation activity with decreasing redshift, is driven by a decline in the frequency of major mergers combined with a decrease in the availability of gas at lower redshifts.

### Eccentric black hole mergers and zoom-whirl behavior from elliptic inspirals to hyperbolic encounters [Cross-Listing]

We perform a parameter study of non-spinning, equal and unequal mass black hole binaries on generic, eccentric orbits in numerical relativity. The linear momentum considered ranges from that of a circular orbit to ten times that value. We discuss the different manifestations of zoom-whirl behavior in the hyperbolic and the elliptic regime. The hyperbolic data set applies to dynamical capture scenarios (e.g. in globular clusters). Evolutions in the elliptic regime correspond to possible end states of supermassive black hole binaries. We spot zoom-whirl behavior for eccentricities as low as $e\sim0.5$, i.e. within the expected range of eccentricities in massive black hole binaries from galaxy mergers and binaries near galactic centers. The resulting gravitational waveforms reveal a rich structure, which will effectively break degeneracies in parameter space improving parameter estimation.

### PKS0347+05: a radio-loud/radio-quiet double AGN system triggered in a major galaxy merger

We present optical, infrared and radio observations of the powerful FRII radio source PKS0347+05 (z=0.3390), and demonstrate that it is a rare example of a radio-loud/radio-quiet double AGN system, comprising a weak line radio galaxy (WLRG) separated by 25 kpc (in projection) from a Seyfert 1 nucleus at the same redshift. Our deep Gemini optical images show a highly disturbed morphology, with a warped dust lane crossing through the halo and nuclear regions of the radio galaxy host, tidal tails, and a bridge connecting the radio galaxy to the Seyfert 1 nucleus. Spectral synthesis modelling of our Gemini optical spectrum of the radio galaxy shows evidence for a reddened young stellar population of age <100 Myr. Further evidence for recent star formation activity in this source is provided by the detection of strong PAH features in mid-IR Spitzer/IRS spectra. Together, these observations support a model in which both AGN have been triggered simultaneously in a major galaxy merger. However, despite the presence of a powerful FRII radio source, and the apparently plentiful supply of fuel provided by the merger, the nucleus of the radio galaxy shows only weak, low ionization emission line activity. We speculate that the fuel supply to nuclear regions of the radio galaxy has recently switched off (within the last ~10^6 yr), but the information about the resulting decrease in nuclear AGN activity has yet to reach the extended lobes and hotspots of the FRII radio source. Based on this scenario, we estimate that powerful, intermediate redshift FRII radio sources have lifetimes of t\sim 5×10^6 yr. Overall, our observations emphasise that the fuelling of AGN activity in major galaxy mergers is likely to be highly intermittent.

### Submillimetre galaxies in a hierarchical universe: number counts, redshift distribution, and implications for the IMF [Replacement]

High-redshift submillimetre galaxies (SMGs) are some of the most rapidly star-forming galaxies in the Universe. Historically, galaxy formation models have had difficulty explaining the observed number counts of SMGs. We combine a semi-empirical model with 3-D hydrodynamical simulations and 3-D dust radiative transfer to predict the number counts of unlensed SMGs. Because the stellar mass functions, gas and dust masses, and sizes of our galaxies are constrained to match observations, we can isolate uncertainties related to the dynamical evolution of galaxy mergers and the dust radiative transfer. The number counts and redshift distributions predicted by our model agree well with observations. Isolated disc galaxies dominate the faint (S_1.1 < ~1 mJy, or S_850 < ~2 mJy) population. The brighter sources are a mix of merger-induced starbursts and galaxy-pair SMGs; the latter subpopulation accounts for ~30 – 50 per cent of all SMGs at all S_1.1 > ~0.5 mJy (S_850 > ~1 mJy). The mean redshifts are ~3.0 – 3.5, depending on the flux cut, and the brightest sources tend to be at higher redshifts. Because the galaxy-pair SMGs will be resolved into multiple fainter sources by ALMA, the bright ALMA counts should be as much as 2 times less than those observed using single-dish telescopes. The agreement between our model, which uses a Kroupa IMF, and observations suggests that the IMF in high-redshifts starbursts need not be top-heavy; if the IMF were top-heavy, our model would over-predict the number counts. We conclude that the difficulty some models have reproducing the observed SMG counts is likely indicative of more general problems — such as an under-prediction of the abundance of massive galaxies or a star formation rate–stellar mass relation normalisation lower than that observed — rather than a problem specific to the SMG population.

### Submillimetre galaxies in a hierarchical universe: number counts, redshift distribution, and implications for the IMF

High-redshift submillimetre galaxies (SMGs) are some of the most rapidly star-forming galaxies in the Universe. Historically, galaxy formation models have had difficulty explaining the observed number counts of SMGs. We combine a semi-empirical model with 3-D hydrodynamical simulations and 3-D dust radiative transfer to predict the number counts of unlensed SMGs. Because the stellar mass functions, gas and dust masses, and sizes of our galaxies are constrained to match observations, we can isolate uncertainties related to the dynamical evolution of galaxy mergers and the dust radiative transfer. The number counts and redshift distributions predicted by our model agree well with observations. Isolated disc galaxies dominate the faint (S_1.1 < ~1 mJy, or S_850 < ~2 mJy) population. The brighter sources are a mix of merger-induced starbursts and galaxy-pair SMGs; the latter subpopulation accounts for ~30 – 50 per cent of all SMGs at all S_1.1 > ~0.5 mJy (S_850 > ~1 mJy). The mean redshifts are ~3.0 – 3.5, depending on the flux cut, and the brightest sources tend to be at higher redshifts. Because the galaxy-pair SMGs will be resolved into multiple fainter sources by ALMA, the bright ALMA counts should be as much as 2 times less than those observed using single-dish telescopes. The agreement between our model, which uses a Kroupa IMF, and observations suggests that the IMF in high-redshifts starbursts need not be top-heavy; if the IMF were top-heavy, our model would over-predict the number counts.

### Evolution along the sequence of S0 Hubble types induced by dry minor mergers. I - Global bulge-to-disk structural relations

Recent studies have argued that galaxy mergers are not important drivers for the evolution of S0′s, on the basis that mergers cannot preserve the coupling between the bulge and disk scale-lengths observed in these galaxies and the lack of correlation of their ratio with the S0 Hubble type. We investigate whether the remnants resulting from collision-less N-body simulations of intermediate and minor mergers onto S0 galaxies evolve fulfilling global structural relations observed between the bulges and disks of these galaxies. Different initial bulge-to-disk ratios of the primary S0 have been considered, as well as different satellite densities, mass ratios, and orbits of the encounter. We have analysed the final morphology of the remnants in images simulating the typical observing conditions of S0 surveys. We derive bulge+disk decompositions of the final remnants to compare their global bulge-to-disk structure with observations. We show that all remnants present undisturbed S0 morphologies according to the prescriptions of specialized surveys. The dry intermediate and minor mergers induce noticeable bulge growth (S0c –> S0b and S0b –> S0a), but affect negligibly to the bulge and disk scale-lengths. Therefore, if a coupling between these two components exists prior to the merger, the encounter does not break this coupling. This fact provides a simple explanation for the lack of correlation between the ratio of bulge and disk scale-lengths and the S0 Hubble type reported by observations. These models prove that dry intermediate and minor mergers can induce global structural evolution within the sequence of S0 Hubble types compatible with observations, meaning that these processes should not be discarded from the evolutionary scenarios of S0′s just on the basis of the strong coupling observed between the bulge and disk scale-lengths in these galaxies (abridged).

### Evolution along the sequence of S0 Hubble types induced by dry minor mergers. I - Global bulge-to-disk structural relations [Replacement]

Recent studies have argued that galaxy mergers are not important drivers for the evolution of S0′s, on the basis that mergers cannot preserve the coupling between the bulge and disk scale-lengths observed in these galaxies and the lack of correlation of their ratio with the S0 Hubble type. We investigate whether the remnants resulting from collision-less N-body simulations of intermediate and minor mergers onto S0 galaxies evolve fulfilling global structural relations observed between the bulges and disks of these galaxies. Different initial bulge-to-disk ratios of the primary S0 have been considered, as well as different satellite densities, mass ratios, and orbits of the encounter. We have analysed the final morphology of the remnants in images simulating the typical observing conditions of S0 surveys. We derive bulge+disk decompositions of the final remnants to compare their global bulge-to-disk structure with observations. We show that all remnants present undisturbed S0 morphologies according to the prescriptions of specialized surveys. The dry intermediate and minor mergers induce noticeable bulge growth (S0c –> S0b and S0b –> S0a), but affect negligibly to the bulge and disk scale-lengths. Therefore, if a coupling between these two components exists prior to the merger, the encounter does not break this coupling. This fact provides a simple explanation for the lack of correlation between the ratio of bulge and disk scale-lengths and the S0 Hubble type reported by observations. These models prove that dry intermediate and minor mergers can induce global structural evolution within the sequence of S0 Hubble types compatible with observations, meaning that these processes should not be discarded from the evolutionary scenarios of S0′s just on the basis of the strong coupling observed between the bulge and disk scale-lengths in these galaxies (abridged).

### Disentangling the circumnuclear environs of Centaurus A: Gaseous Spiral Arms in a Giant Elliptical Galaxy

We report the existence of spiral arms in the recently formed gaseous and dusty disk of the closest giant elliptical, NGC 5128 (Centaurus A), using high resolution 12CO(2-1) observations of the central 3 arcmin (3 kpc) obtained with the Submillimeter Array (SMA). This provides evidence that spiral-like features can develop within ellipticals if enough cold gas exists. We elucidate the distribution and kinematics of the molecular gas in this region with a resolution of 4.4 x 1.9 (80 pc x 40 pc). The spiral arms extend from the circumnuclear gas at a radius of 200 pc to at least 1 kiloparsec. The general properties of the arms are similar to those in spiral galaxies: they are trailing, the width is \sim 500 \pm 200 pc, and the pitch angle is 20 degrees. From independent estimates of the time when the HI-rich galaxy merger occurred, we infer that the formation of spiral arms happened on a time scale of less than \sim10^8 yr. The formation of spiral arms increases the gas density and thus the star formation efficiency in the early stages of the formation of a disk.

### Post-merger Signatures of Red-sequence Galaxies in Rich Abell Clusters at $z\lesssim 0.1$

We have investigated the post-merger signatures of red-sequence galaxies in rich Abell clusters at $z \lesssim$ 0.1: A119, A2670, A3330 and A389. Deep images in u’, g’, r’ and medium-resolution galaxy spectra were taken using MOSAIC 2 CCD and Hydra MOS mounted on a Blanco 4-m telescope at CTIO. Post-merger features are identified by visual inspection based on asymmetric disturbed features, faint structures, discontinuous halo structures, rings and dust lanes. We found that ~ 25% of bright (M_r < -20) cluster red-sequence galaxies show post-merger signatures in four clusters consistently. Most (~ 71%) of the featured galaxies were found to be bulge-dominated, and for the subsample of bulge-dominated red-sequence galaxies, the post-merger fraction rises to ~ 38%. We also found that roughly 4% of bulge-dominated red-sequence galaxies interact (on-going merger). A total of 42% (38% post-merger, 4% on-going merger) of galaxies show merger-related features. Compared to a field galaxy study with a similar limiting magnitude (van Dokkum 2005), our cluster study presents a similar post-merger fraction but a markedly lower on-going merger fraction. The merger fraction derived is surprisingly high for the high density of our clusters, where the fast internal motions of galaxies are thought to play a negative role in galaxy mergers. The fraction of post-merger and on-going merger galaxies can be explained as follows. Most of the post-merger galaxies may have carried over their merger features from their previous halo environment, whereas interacting galaxies interact in the current cluster in situ. According to our semi-analytic calculation, massive cluster haloes may very well have experienced tens of halo mergers over the last 4-5 Gyr; post-merger features last that long, allowing these features to be detected in our clusters today. (Abridged)

### Galaxy Zoo: Bulgeless Galaxies With Growing Black Holes [Replacement]

The growth of supermassive black holes appears to be driven by galaxy mergers, violent merger-free processes and/or secular’ processes. In order to quantify the effects of secular evolution on black hole growth, we study a sample of active galactic nuclei (AGN) in galaxies with a calm formation history free of significant mergers, a population that heretofore has been difficult to locate. Here we present an initial sample of 13 AGN in massive (M_* >~ 1e10 M_sun) bulgeless galaxies — which lack the classical bulges believed inevitably to result from mergers — selected from the Sloan Digital Sky Survey using visual classifications from Galaxy Zoo. Parametric morphological fitting confirms the host galaxies lack classical bulges; any contributions from pseudobulges are very small (typically < 5%). We compute black hole masses for the two broad-line objects in the sample (4.2e6 and 1.2e7 M_sun) and place lower limits on black hole masses for the remaining sample (typically M_BH >~ 1e6 M_sun), showing that significant black hole growth must be possible in the absence of mergers or violent disk instabilities. The black hole masses are systematically higher than expected from established bulge-black hole relations. However, if the mean Eddington ratio of the systems with measured black hole masses (L/L_Edd = 0.065) is typical, 10 of 13 sources are consistent with the correlation between black hole mass and total stellar mass. That pure disk galaxies and their central black holes may be consistent with a relation derived from elliptical and bulge-dominated galaxies with very different formation histories implies the details of stellar galaxy evolution and dynamics may not be fundamental to the co-evolution of galaxies and black holes.

### Galaxy Zoo: Bulgeless Galaxies With Growing Black Holes

The growth of supermassive black holes appears to be driven by both galaxy mergers and secular’ processes that occur in their absence. In order to quantify the effects of secular evolution on black hole growth, we require a sample of active galactic nuclei (AGN) in galaxies that have formed without significant mergers, a population that heretofore has been difficult to locate. Here we present an initial sample of 13 AGN in massive (M_* \gtrsim 1e10 M_sun) bulgeless galaxies — which lack the classical bulges believed inevitably to result from mergers — selected from the Sloan Digital Sky Survey using visual classifications from Galaxy Zoo. Parametric morphological fitting confirms the host galaxies lack classical bulges; any contributions from pseudobulges are very small (typically < 5%). This is the largest such sample yet assembled. We compute black hole masses for the two broad-line objects in the sample (4.2e6 and 1.2e7 M_sun) and place lower limits on black hole masses for the remaining sample (typically M_BH \gtrsim 1e6 M_sun), showing that significant black hole growth must be possible in the absence of mergers. The black hole masses are systematically higher than expected from established bulge-black hole relations. However, if the mean Eddington ratio of the systems with measured black hole masses (L/L_Edd \approx 0.065) is typical, 10 of 13 sources are consistent with the correlation between black hole mass and total stellar mass. That pure disk galaxies and their central black holes may be consistent with a relation derived from elliptical and bulge-dominated galaxies with very different formation histories implies the details of stellar galaxy evolution and dynamics may not be fundamental to the co-evolution of galaxies and black holes.

### Quadruple-peaked spectral line profiles as a tool to constrain gravitational potential of shell galaxies

Stellar shells observed in many giant elliptical and lenticular as well as a few spiral and dwarf galaxies, presumably result from galaxy mergers. Line-of-sight velocity distributions of the shells could, in principle, if measured with a sufficiently high S/N, constitute one of methods to constrain the gravitational potential of the host galaxy. Merrifield & Kuijken (1998) predicted a double-peaked line profile for stationary shells resulting from a nearly radial minor merger. In this paper, we aim at extending their analysis to a more realistic case of expanding shells, inherent to the merging process, in the same type of merger and the same orbital geometry. We use analytical approach as well as test particle simulations to predict the line-of-sight velocity profile across the shell structure. Simulated line profiles are convolved with spectral PSFs to estimate the peak detectability. The resulting line-of-sight velocity distributions are more complex than previously predicted due to non-zero phase velocity of the shells. In principle, each of the Merrifield & Kuijken (1998) peaks splits into two, giving a quadruple-peaked line profile, which allows more precise determination of the potential of the host galaxy and, moreover, contains additional information. We find simple analytical expressions that connect the positions of the four peaks of the line profile and the mass distribution of the galaxy, namely the circular velocity at the given shell radius and the propagation velocity of the shell. The analytical expressions were applied to a test-particle simulation of a radial minor merger and the potential of the simulated host galaxy was successfully recovered. The shell kinematics can thus become an independent tool to determine the content and distribution of the dark matter in shell galaxies, up to ~100 kpc from the center of the host galaxy.

### Quadruple-peaked spectral line profiles as a tool to constrain gravitational potential of shell galaxies [Replacement]

Stellar shells observed in many giant elliptical and lenticular as well as a few spiral and dwarf galaxies, presumably result from galaxy mergers. Line-of-sight velocity distributions of the shells could, in principle, if measured with a sufficiently high S/N, constitute one of methods to constrain the gravitational potential of the host galaxy. Merrifield & Kuijken (1998) predicted a double-peaked line profile for stationary shells resulting from a nearly radial minor merger. In this paper, we aim at extending their analysis to a more realistic case of expanding shells, inherent to the merging process, whereas we assume the same type of merger and the same orbital geometry. We use analytical approach as well as test particle simulations to predict the line-of-sight velocity profile across the shell structure. Simulated line profiles are convolved with spectral PSFs to estimate the peak detectability. The resulting line-of-sight velocity distributions are more complex than previously predicted due to non-zero phase velocity of the shells. In principle, each of the Merrifield & Kuijken (1998) peaks splits into two, giving a quadruple-peaked line profile, which allows more precise determination of the potential of the host galaxy and, moreover, contains additional information. We find simple analytical expressions that connect the positions of the four peaks of the line profile and the mass distribution of the galaxy, namely the circular velocity at the given shell radius and the propagation velocity of the shell. The analytical expressions were applied to a test-particle simulation of a radial minor merger and the potential of the simulated host galaxy was successfully recovered. The shell kinematics can thus become an independent tool to determine the content and distribution of the dark matter in shell galaxies, up to ~100 kpc from the center of the host galaxy.