Recent Postings from Galaxies

Star Clusters in the Magellanic Clouds-1: Parameterisation and Classification of 1072 Clusters in the LMC

We have introduced a semi-automated quantitative method to estimate the age and reddening of 1072 star clusters in the Large Magellanic Cloud (LMC) using the Optical Gravitational Lensing Experiment (OGLE) III survey data. This study brings out 308 newly parameterised clusters. In a first of its kind, the LMC clusters are classified into groups based on richness/mass as very poor, poor, moderate and rich clusters, similar to the classification scheme of open clusters in the Galaxy. A major cluster formation episode is found to happen at 125 +- 25 Myr in the inner LMC. The bar region of the LMC appears prominently in the age range 60 - 250 Myr and is found to have a relatively higher concentration of poor and moderate clusters. The eastern and the western ends of the bar are found to form clusters initially, which later propagates to the central part. We demonstrate that there is a significant difference in the distribution of clusters as a function of mass, using a movie based on the propagation (in space and time) of cluster formation in various groups. The importance of including the low mass clusters in the cluster formation history is demonstrated. The catalog with parameters, classification, and cleaned and isochrone fitted CMDs of 1072 clusters, which are available as online material, can be further used to understand the hierarchical formation of clusters in selected regions of the LMC.

The Chemical Abundance Structure of the Inner Milky Way: A Signature of "Upside-Down" Disk Formation?

We present a model for the [alpha/Fe]-[Fe/H] distribution of stars in the inner Galaxy, R=3-5 kpc, measured as a function of vertical distance |z| from the midplane by Hayden et al. (2015, H15). Motivated by an "upside-down" scenario for thick disk formation, in which the thickness of the star-forming gas layer contracts as the stellar mass of the disk grows, we combine one-zone chemical evolution with a simple prescription in which the scale-height of the stellar distribution drops linearly from z_h=0.8 kpc to z_h=0.2 kpc over a timescale t_c, remaining constant thereafter. We assume a linear-exponential star-formation history, SFR ~ te^{-t/t_sf}. With a star-formation efficiency timescale of 2 Gyr, an outflow mass-loading factor of 1.5, t_sf=3 Gyr, and t_c=2.5 Gyr, the model reproduces the observed locus of inner disk stars in [alpha/Fe]-[Fe/H] and the metallicity distribution functions (MDFs) measured by H15 at |z|=0-0.5 kpc, 0.5-1 kpc, and 1-2 kpc. Substantial changes to model parameters lead to disagreement with the H15 data; for example, models with t_c=1 Gyr or t_sf=1 Gyr fail to match the observed MDF at high-|z| and low-|z|, respectively. The inferred scale-height evolution, with z_h(t) dropping on a timescale t_c ~ t_sf at large lookback times, favors upside-down formation over dynamical heating of an initially thin stellar population as the primary mechanism regulating disk thickness. The failure of our short-t_c models suggests that any model in which thick disk formation is a discrete event will not reproduce the continuous dependence of the MDF on |z| found by H15. Our scenario for the evolution of the inner disk can be tested by future measurements of the |z|-distribution and the age-metallicity distribution at R=3-5 kpc.

Positron excess in the center of the Milky Way from short-lived $\beta^+$ emitting isotopes

Observations of the INTEGRAL satellite revealed the presence of yet unexplained excess in the central region of the Galaxy at the energies around 511 keV. These gamma-rays are produced in the process of positron annihilation, the needed rate is around $10^{42}~\mathrm{s^{-1}}$. In this short paper it is shown that \pos -emitting isotopes that are formed in interactions of subrelativistic cosmic rays (CRs) with light nuclei (CNONe) can account for a considerable fraction -- up to several tens of percent -- of $e^{+}$ production rate in the central region.

Efficiency of gas cooling and accretion at the disc-corona interface

In star-forming galaxies, stellar feedback can have a dual effect on the circumgalactic medium both suppressing and stimulating gas accretion. The trigger of gas accretion can be caused by disc material ejected into the halo in the form of fountain clouds and by its interaction with the surrounding hot corona. Indeed, at the disc-corona interface, the mixing between the cold/metal-rich disc gas (T <~ 10^4 K) and the hot coronal gas (T >~ 10^6 K) can dramatically reduce the cooling time of a portion of the corona and produce its condensation and accretion. We studied the interaction between fountain clouds and corona in different galactic environments through parsec-scale hydrodynamical simulations, including the presence of thermal conduction, a key mechanism that influences gas condensation. Our simulations showed that the coronal gas condensation strongly depends on the galactic environment, in particular it is less efficient for increasing virial temperature/mass of the haloes where galaxies reside and it is fully ineffective for objects with virial masses larger than 10^13 Msun. This result implies that the coronal gas cools down quickly in haloes with low-intermediate virial mass (Mvir <~ 3 x 10^12 Msun) but the ability to cool the corona decreases going from late-type to early-type disc galaxies, potentially leading to the switching off of accretion and the quenching of star formation in massive systems.

Radial Distribution Of ISM Gas-phase Metallicity In CLASH Clusters at z~0.35: A New Outlook On Environmental Impact On Galaxy Evolution

We present the first observation of cluster-scale radial metallicity gradients from star-forming galaxies. We use the DEIMOS spectrograph on the Keck II telescope to observe two CLASH clusters at z~0.35: MACS1115+0129 and RXJ1532+3021. Based on our measured interstellar medium (ISM) properties of star-forming galaxies out to a radius of 2.5 Mpc from the cluster centre, we find that the galaxy metallicity decreases as a function of projected cluster-centric distance (-0.15+/-0.08 dex/Mpc}) in MACS1115+01. On the mass-metallicity relation (MZR), star-forming galaxies in MACS1115+01 are offset to higher metallicity (~0.2 dex) than the local SDSS galaxies at a fixed mass range. In contrast, the MZR of RXJ1532+30 is consistent with the local comparison sample. RXJ1532+30 exhibits a bimodal radial metallicity distribution, with one branch showing a similar negative gradient as MACS1115+01 (-0.14+/-0.05 dex/Mpc) and the other branch showing a positive radial gradient. The positive gradient branch in RXJ1532+30 is likely caused by either interloper galaxies or an in-plane merger, indicating that cluster-scale abundance gradients probe cluster substructures and thus the dynamical state of a cluster. Most strikingly, we discover that neither the radial metallicity gradient nor the offset from the MZR is driven by the stellar mass. We compare our observations with Rhapsody-G cosmological hydrodynamical zoom-in simulations of relaxed galaxy clusters and find that the simulated galaxy cluster also exhibits a negative abundance gradient, albeit with a shallower slope (-0.04+/-0.03 dex/Mpc). Our observations suggest that the negative radial gradient originates from ram-pressure stripping and/or strangulation processes in the cluster environments.

A new quadruple gravitational lens from the Hyper Suprime-Cam Survey: the dilemma of HSC~J115252+004733

We report the serendipitous discovery of a quadruply (quad) lensed source at redshift $z_{\rm s}=3.76$, HSC~J115252+004733, from the Subaru Hyper Suprime-Cam (HSC) Survey. The source is lensed by an early-type galaxy at $z_{\rm l}=0.466$ along with a satellite galaxy. Here, we investigate the nature of the source by studying its size, luminosity and from follow-up spectroscopy, the luminosity and velocity width of the Ly-$\alpha$ emission line. Our analyses suggest that the source is most probably a low-luminosity active galactic nucleus (AGN) or possibly an unusually compact and bright galaxy such as a Lyman-$\alpha$ emitter or a Lyman Break Galaxy. The morphology of the brighter pair of lensed images appears point-like except in the HSC $i$-band which was observed in better seeing conditions (0.5"). The extended feature in the $i$-band image can be explained by the emission from the host galaxy of the AGN, or alternatively, the highly compact lensed galaxy which appears point-like in all bands expect in $i$-band. We also find that the flux ratio of the brighter pair of images show variation in the near-infrared compared to the optical imaging. Phenomena such as differential extinction and intrinsic variability cannot explain this chromatic variation. While microlensing from stars in the foreground galaxy is less likely to be the cause, it cannot be ruled out completely. If the galaxy hosts an AGN, then this represents the highest redshift quadruply imaged AGN known to date. Discovery of this unusually compact and faint source demonstrates the potential of the HSC survey.

880 $\mu$m SMA polarization observations of the quasar 3C 286

For decades the bright radio quasar 3C 286 has been widely recognized as one of the most reliable polarization calibrators at centimeter wavelengths because of its unchanging polarization position angle and high polarization percentage. However, it has become clear in recent years that the polarization position angle of 3C 286 changes with increasing frequency, increasing from ~33$^{\circ}$ at $\lambda \gtrsim 3$ cm to ~38$^{\circ}$ at $\lambda \approx 1$ mm. With the advent of high-sensitivity polarization observations by current and future (sub)millimeter telescopes, knowledge of the position angle of 3C 286 at higher frequencies is critical for calibration. We report the first polarization observations of 3C 286 at submillimeter wavelengths, taken at 880 $\mu$m (340 GHz) with the Submillimeter Array. We find a polarization position angle and percentage of $37.4 \pm 1.5^{\circ}$ and $15.7 \pm 0.8$%, respectively, consistent with previous measurements at 1 mm.

Low-mass disc galaxies and the issue of stability: MOND vs dark matter

We analyse the rotation curves and gravitational stability of a sample of six bulgeless galaxies for which detailed images reveal no evidence for strong bars. We explore two scenarios: Newtonian dark matter models and MOdified Newtonian Dynamics (MOND). By adjusting the stellar mass-to-light ratio, dark matter models can match simultaneously both the rotation curve and bar-stability requirements in these galaxies. To be consistent with stability constraints, in two of these galaxies, the stellar mass-to-light ratio is a factor of ~1.5-2 lower than the values suggested from galaxy colours. In contrast, MOND fits to the rotation curves are poor in three galaxies, perhaps because the gas tracer contains noncircular motions. The bar stability analysis provides a new observational test to MOND. We find that most of the galaxies under study require abnormally-high levels of random stellar motions to be bar stable in MOND. In particular, for the only galaxy in the sample for which the line-of-sight stellar velocity dispersion has been measured (NGC 6503), the observed velocity dispersion is not consistent with MOND predictions because it is far below the required value to guarantee bar stability. Precise measurements of mass-weighted velocity dispersions in (unbarred and bulgeless) spiral galaxies are crucial to test the consistency of MOND.

Spectral analysis of four 'hypervariable' AGN: a micro-needle in the haystack?

We analyze four extreme AGN transients to explore the possibility that they are caused by rare, high-amplitude microlensing events. These previously unknown type-I AGN are located in the redshift range 0.6-1.1 and show changes of > 1.5 magnitudes in the g-band on a timescale of ~years. Multi-epoch optical spectroscopy, from the William Herschel Telescope, shows clear differential variability in the broad line fluxes with respect to the continuum changes and also evolution in the line profiles. In two cases a simple point-source, point-lens microlensing model provides an excellent match to the long-term variability seen in these objects. For both models the parameter constraints are consistent with the microlensing being due to an intervening stellar mass object but as yet there is no confirmation of the presence of an intervening galaxy. The models predict a peak amplification of 10.3/13.5 and an Einstein timescale of 7.5/10.8 years respectively. In one case the data also allow constraints on the size of the CIII] emitting region, with some simplifying assumptions, to to be ~1.0-6.5 light-days and a lower limit on the size of the MgII emitting region to be > 9 light-days (half-light radii). This CIII] radius is perhaps surprisingly small. In the remaining two objects there is spectroscopic evidence for an intervening absorber but the extra structure seen in the lightcurves requires a more complex lensing scenario to adequately explain.

Detailed photometric analysis of young star groups in the galaxy NGC 300

The purpose of this work is to understand the global characteristics of the stellar populations in NGC 300. In particular, we focused our attention on searching young star groups and study their hierarchical organization. The research was conducted using archival point spread function fitting photometry measured from images in multiple bands obtained with the Advanced Camera for Surveys of the Hubble Space Telescope. Using the path linkage criterion, we cataloged young star groups and analyzed them from the observation of individual stars in the galaxy NGC 300. We also built stellar density maps from the bluest stars and applied the SExtractor code to identify overdensities. By plotting isocontours over the density maps and comparing the two methods, we could infer and delineate the hierarchical structure of the blue population in the galaxy. For each region of a detected young star group, we estimated the size and derived the radial surface density profiles for stellar populations of different color. A statistical decontamination of field stars was performed for each region. In this way it was possible to build the color-magnitude diagrams and compare them with theoretical evolutionary models. We also constrained the present-day mass function per group by estimating a value for its slope. The blue population in NGC 300 shows a hierarchical behavior in which the larger and loosely distributed structures split into more compact and denser ones over several density levels. We created a catalog of 1147 young star groups in six fields of the galaxy NGC 300, in which we present their fundamental characteristics. The mean and the mode radius values obtained from the size distribution are both 25 pc, in agreement with the value for the Local Group and nearby galaxies. Additionally, we found an average PDMF slope that is compatible with the Salpeter value.

Precessing Jet in the High-Redshift Blazar J0017+8135

The prominent flat-spectrum radio quasar J0017+8135 (S5 0014+81) at z = 3.366 is one of the most luminous active galactic nuclei (AGN) known. Its milliarcsecond-scale radio jet structure has been studied with very long baseline interferometry (VLBI) since the 1980s. The quasar was selected as one of the original defining objects of the International Celestial Reference Frame, but left out from its current second realization (ICRF2) because of systematic long-term positional variations. Here we analyse archival 8.6- and 2.3-GHz VLBI imaging data collected at nearly 100 different epochs during more than 20 years, to obtain information about the kinematics of jet components. Because of the cosmological time dilation, extensive VLBI monitoring data are essential to reveal changes in the jet structure of high-redshift AGN. In the case of J0017+8135, the data can be described with a simple kinematic model of jet precession with a 12-year periodicity in the observer's frame.

Compression and ablation of the photo-irradiated cloud the Orion Bar

The Orion Bar is the archetypal edge-on molecular cloud surface illuminated by strong ultraviolet radiation from nearby massive stars. Owing to the close distance to Orion (about 1,350 light-year), the effects of stellar feedback on the parental cloud can be studied in detail. Visible-light observations of the Bar(1) show that the transition between the hot ionised gas and the warm neutral atomic gas (the ionisation front) is spatially well separated from the transition from atomic to molecular gas (the dissociation front): about 15 arcseconds or 6,200 astronomical units (one astronomical unit is the Earth-Sun distance). Static equilibrium models(2,3) used to interpret previous far-infrared and radio observations of the neutral gas in the Bar(4,5,6) (typically at 10-20 arcsecond resolution) predict an inhomogeneous cloud structure consisting of dense clumps embedded in a lower density extended gas component. Here we report one-arcsecond-resolution millimetre-wave images that allow us to resolve the molecular cloud surface. In contrast to stationary model predictions(7,8,9), there is no appreciable offset between the peak of the H2 vibrational emission (delineating the H/H2 transition) and the edge of the observed CO and HCO+ emission. This implies that the H/H2 and C+/C/CO transition zones are very close. These observations reveal a fragmented ridge of high-density substructures, photoablative gas flows and instabilities at the molecular cloud surface. The results suggest that the cloud edge has been compressed by a high-pressure wave that currently moves into the molecular cloud. The images demonstrate that dynamical and nonequilibrium effects are important for the cloud evolution.

Water around IRAS15398-3359 observed with ALMA

How protostars accrete mass is one of the fundamental problems of star formation. High column densities and complex kinematical structures make direct observations challenging and they only provide a snapshot. Chemical tracers provide an interesting alternative to characterise the infall histories of protostars. Previous observations of H13CO+ towards the low-mass protostar IRAS15398-3359 showed a depression in the abundance. This is a sign of destruction of HCO+ by an enhanced presence of gaseous water in an extended region, possibly related to a recent burst in the accretion. Direct observations of water vapour can determine the exact extent of the emission and confirm the hypothesis that HCO+ is indeed a good tracer of the water snow-line. IRAS15398 was observed using ALMA at 0.5" resolution. Maps of HDO(101-000) and H218O(414-321) were taken simultaneously with observations of the CS(8-7) and N2H+(5-4) lines and continuum at 0.65 and 0.75 mm. The maps were interpreted using dust radiative transfer calculations of the protostellar infalling envelope with an outflow cavity. HDO is clearly detected and extended over the scales of the H13CO+ depression, although it is displaced by ~500 AU in the direction of the outflow. H218O is tentatively detected towards the red-shifted outflow lobe, but otherwise it is absent from the mapped region, which suggests that temperatures are low. Based on the temperature structure obtained from dust radiative transfer models, we conclude that the water was most likely released from the grains in an extended hour-glass configuration during a recent accretion burst. HDO is only detected in the region closest to the protostar, at distances of up to 500 AU. These signatures can only be explained if the luminosity has recently been increased by orders of magnitudes. Additionally, the densities in the outflow cones must be sufficiently low.

The feedback of atomic jets from embedded protostars in NGC 1333

Star-formation feedback onto the parent cloud is conventionally examined through the study of molecular outflows. Little is however known on the effect that atomic ejecta, tracing fast shocks, can have on the global cloud properties. In this study we employ Herschel/PACS [OI] and [CII] spectral line maps of the NGC 1333 star-forming region to assess the relative influence of atomic jets onto the star-formation process. Atomic line maps are compared against molecular outflow tracers and atomic ejecta are associated to individual driving sources. We study the detailed morphology and velocity distribution of [OI] line using channel and line-centroid maps and derive the momentum, energy, and mass flux for all the bipolar jets traced by [OI] line emission. We find that the line-centroid maps can trace velocity structures down to 5 km s$^{-1}$ which is a factor of $\sim$20 beyond the nominal velocity resolution reached by Herschel/PACS. These maps reveal an unprecedented degree of details that assist significantly in the association and characterization of jets and outflows. Comparisons of the dynamical and kinematical properties shows that [OI] momentum accounts for only $\sim$1% of the momentum carried by the large scale CO outflows but the energy released through the jets corresponds to 50 - 100% of the energy released in outflows. The estimated ratios of the jet to the outflow momenta and energies are consistent with the results of two-component, nested jet/outflow simulations, where jets are associated to episodic accretion events. Under this scenario, the energy from atomic jets to the cloud is as important as the energy output from outflows in maintaining turbulence and dissipating the cloud gas.

Metals in the z~3 intergalactic medium: results from an ultra-high signal-to-noise ratio UVES quasar spectrum

In this work, we investigate the abundance and distribution of metals in the intergalactic medium (IGM) at $\langle z \rangle \simeq 2.8$ through the analysis of an ultra-high signal-to-noise ratio UVES spectrum of the quasar HE0940-1050. In the CIV forest, our deep spectrum is sensitive at $3\,\sigma$ to lines with column density down to $\log N_{\rm CIV} \simeq 11.4$ and in 60 percent of the considered redshift range down to $\simeq11.1$. In our sample, all HI lines with $\log N_{\rm HI} \ge 14.8$ show an associated CIV absorption. In the range $14.0 \le \log N_{\rm HI} <14.8$, 43 percent of HI lines has an associated CIV absorption. At $\log N_{\rm HI} < 14.0$, the detection rates drop to $<10$ percent, possibly due to our sensitivity limits and not to an actual variation of the gas abundance properties. In the range $\log N_{\rm HI} \ge 14$, we observe a fraction of HI lines with detected CIV a factor of 2 larger than the fraction of HI lines lying in the circum-galactic medium (CGM) of relatively bright Lyman-break galaxies hosted by dark matter halos with $\langle M\rangle \sim10^{12}$ M$_{\odot}$ (Rudie et al. 2012). The comparison of our results with the output of a grid of photoionization models and of two cosmological simulations implies that the volume filling factor of the IGM gas enriched to a metallicity $\log Z/Z_{\odot} \ge -3$ should be of the order of $\sim 10-13$ percent. In conclusion, our results favour a scenario in which metals are found also outside the CGM of bright star-forming galaxies, possibly due to pollution by lower mass objects and/or to an early enrichment by the first sources.

Dust evolution processes constrained by extinction curves in nearby galaxies

Extinction curves, especially those in the Milky Way (MW), the Large Magellanic Cloud (LMC), and the Small Magellanic Cloud (SMC), have provided us with a clue to the dust properties in the nearby Universe. We examine whether or not these extinction curves can be explained by well known dust evolution processes. We treat the dust production in stellar ejecta, destruction in supernova shocks, dust growth by accretion and coagulation, and dust disruption by shattering. To make a survey of the large parameter space possible, we simplify the treatment of the grain size distribution evolution by adopting the `two-size approximation', in which we divide the grain population into small ($\lesssim 0.03~\mu$m) and large ($\gtrsim 0.03~\mu$m) grains. It is confirmed that the MW extinction curve can be reproduced in reasonable ranges for the time-scale of the above processes with a silicate-graphite mixture. This indicates that the MW extinction curve is a natural consequence of the dust evolution through the above processes. We also find that the same models fail to reproduce the SMC/LMC extinction curves. Nevertheless, this failure can be remedied by giving higher supernova destruction rates for small carbonaceous dust and considering amorphous carbon for carbonaceous dust; these modification fall in fact in line with previous studies. Therefore, we conclude that the current dust evolution scenario composed of the aforementioned processes is successful in explaining the extinction curves. All the extinction curves favor efficient interstellar processing of dust, especially, strong grain growth by accretion and coagulation.

Core-collapse supernova progenitor constraints using the spatial distributions of massive stars in local galaxies

We study the spatial correlations between the H$\alpha$ emission and different types of massive stars in two local galaxies, the Large Magellanic Cloud (LMC) and Messier 33. We compare these to correlations derived for core-collapse supernovae (CCSNe) in the literature to connect CCSNe of different types with the initial masses of their progenitors and to test the validity of progenitor mass estimates which use the pixel statistics method. We obtain samples of evolved massive stars in both galaxies from catalogues with good spatial coverage and/or completeness, and combine them with coordinates of main-sequence stars in the LMC from the SIMBAD database. We calculate the spatial correlation of stars of different classes and spectral types with H$\alpha$ emission. We also investigate the effects of distance, noise and positional errors on the pixel statistics method. A higher correlation with H$\alpha$ emission is found to correspond to a shorter stellar lifespan, and we conclude that the method can be used as an indicator of the ages, and therefore initial masses, of SN progenitors. We find that the spatial distributions of type II-P SNe and red supergiants of appropriate initial mass ($\gtrsim$9 $M_{\odot}$) are consistent with each other. We also find the distributions of type Ic SNe and WN stars with initial masses $\gtrsim$20 $M_{\odot}$ consistent, while supergiants with initial masses around 15 $M_{\odot}$ are a better match for type IIb and II-L SNe. The type Ib distribution corresponds to the same stellar types as type II-P, which suggests an origin in interacting binaries. On the other hand, we find that luminous blue variable stars show a much stronger correlation with H$\alpha$ emission than do type IIn SNe.

On the formation of a quasi-stationary twisted disc after a tidal disruption event

We investigate misaligned accretion discs formed after tidal disruption events that occur when a star encounters a supermassive black hole. We employ the linear theory of warped accretion discs to find the shape of a disc for which the stream arising from the disrupted star provides a source of angular momentum that is misaligned with that of the black hole. For quasi-steady configurations we find that when the warp diffusion or propagation time is large compared to the local mass accretion time and/or the natural disc alignment radius is small, misalignment is favoured. These results have been verified using SPH simulations. We also simulated 1D model discs including gas and radiation pressure. As accretion rates initially exceed the Eddington limit the disc is initially advection dominated. Assuming the $\alpha$ model for the disc, where it can be thermally unstable it subsequently undergoes cyclic transitions between high and low states. During these transitions the aspect ratio varies from $\sim 1$ to $\sim 10^{-3}$ which is reflected in changes in the degree of disc misalignment at the stream impact location. For maximal black hole rotation and sufficiently large values of viscosity parameter $\alpha > \sim 0.01-0.1$ the ratio of the disc inclination to that of the initial stellar orbit is estimated to be $0.1-0.2$ in the advection dominated state, while reaching of order unity in the low state. Misalignment descreases with decrease of $\alpha$, but increases as the black hole rotation parameter decreases. Thus, it is always significant when the latter is small.

On the formation of a quasi-stationary twisted disc after a tidal disruption event [Cross-Listing]

We investigate misaligned accretion discs formed after tidal disruption events that occur when a star encounters a supermassive black hole. We employ the linear theory of warped accretion discs to find the shape of a disc for which the stream arising from the disrupted star provides a source of angular momentum that is misaligned with that of the black hole. For quasi-steady configurations we find that when the warp diffusion or propagation time is large compared to the local mass accretion time and/or the natural disc alignment radius is small, misalignment is favoured. These results have been verified using SPH simulations. We also simulated 1D model discs including gas and radiation pressure. As accretion rates initially exceed the Eddington limit the disc is initially advection dominated. Assuming the $\alpha$ model for the disc, where it can be thermally unstable it subsequently undergoes cyclic transitions between high and low states. During these transitions the aspect ratio varies from $\sim 1$ to $\sim 10^{-3}$ which is reflected in changes in the degree of disc misalignment at the stream impact location. For maximal black hole rotation and sufficiently large values of viscosity parameter $\alpha > \sim 0.01-0.1$ the ratio of the disc inclination to that of the initial stellar orbit is estimated to be $0.1-0.2$ in the advection dominated state, while reaching of order unity in the low state. Misalignment descreases with decrease of $\alpha$, but increases as the black hole rotation parameter decreases. Thus, it is always significant when the latter is small.

Outer Disk Star Formation in HI selected Galaxies

The HI in galaxies often extends past their conventionally defined optical extent. I report results from our team which has been probing low intensity star formation in outer disks using imaging in H-alpha and ultraviolet. Using a sample of hundreds of HI selected galaxies, we confirm that outer disk HII regions and extended UV disks are common. Hence outer disks are not dormant but are dimly forming stars. Although the ultraviolet light in galaxies is more centrally concentrated than the HI, the UV/HI ratio (the Star Formation Efficiency) is nearly constant, with a slight dependency on surface brightness. This result is well accounted for in a model where disks maintain a constant stability parameter Q. This model also accounts for how the ISM and star formation are distributed in the bright parts of galaxies, and how HI appears to trace the distribution of dark matter in galaxy outskirts.

The MOSDEF survey: AGN multi-wavelength identification, selection biases and host galaxy properties

We present results from the MOSFIRE Deep Evolution Field (MOSDEF) survey on the identification, selection biases and host galaxy properties of 55 X-ray, IR and optically-selected active galactic nuclei (AGN) at $1.4 < z < 3.8$. We obtain rest-frame optical spectra of galaxies and AGN and use the BPT diagram to identify optical AGN. We examine the uniqueness and overlap of the AGN identified at different wavelengths. There is a strong bias against identifying AGN at any wavelength in low mass galaxies, and an additional bias against identifying IR AGN in the most massive galaxies. AGN host galaxies span a wide range of star formation rate (SFR), similar to inactive galaxies once stellar mass selection effects are accounted for. However, we generally identify IR AGN in less dusty galaxies with relatively higher SFR and optical AGN in dusty galaxies with relatively lower SFR. X-ray AGN selection does not display a bias with host galaxy SFR. These results are consistent with those from larger studies at lower redshifts. Once selection biases are accounted for, we find AGN in galaxies with similar physical properties as inactive galaxies, with no evidence for AGN activity in particular types of galaxies which is consistent with AGN being fueled scholastically in all types of host galaxies. We do not detect a significant correlation between SFR and AGN luminosity for individual AGN host galaxies, which may indicate the timescale difference between the growth of galaxies and their supermassive black holes.

A new insight into the consistency of smoothed particle hydrodynamics [Cross-Listing]

In this paper the problem of consistency of smoothed particle hydrodynamics (SPH) is solved. A novel error analysis is developed in $n$-dimensional space using the Poisson summation formula, which enables the treatment of the kernel and particle approximation errors in combined fashion. New consistency integral relations are derived for the particle approximation which correspond to the cosine Fourier transform of the classically known consistency conditions for the kernel approximation. The functional dependence of the error bounds on the SPH interpolation parameters, namely the smoothing length $h$ and the number of particles within the kernel support ${\cal{N}}$ is demonstrated explicitly from which consistency conditions are seen to follow naturally. As ${\cal{N}}\to\infty$, the particle approximation converges to the kernel approximation independently of $h$ provided that the particle mass scales with $h$ as $m\propto h^{\beta}$, with $\beta >n$. This implies that as $h\to 0$, the joint limit $m\to 0$, ${\cal{N}}\to\infty$, and $N\to\infty$ is necessary for complete convergence to the continuum, where $N$ is the total number of particles. The analysis also reveals the presence of a dominant error term of the form $(\ln {\cal{N}})^{n}/{\cal{N}}$, which tends asymptotically to $1/{\cal{N}}$ when ${\cal{N}}\gg 1$, as it has long been conjectured based on the similarity between the SPH and the quasi-Monte Carlo estimates.

Optimal fitting of gaussian-apodized or under-resolved emission lines in Fourier Transform spectra providing new insights on the velocity structure of NGC 6720

An analysis of the kinematics of NGC 6720 is performed on the commissioning data obtained with SITELLE, the Canada-France-Hawaii Telescope's new imaging Fourier transform spectrometer. In order to measure carefully the small broadening effect of a shell expansion on an unresolved emission line, we have determined a computationally robust implementation of the convolution of a Gaussian with a sinc instrumental line shape which avoids arithmetic overflows. This model can be used to measure line broadening of typically a few km/s even at low spectral resolution (R less than 5000). We have also designed the corresponding set of Gaussian apodizing functions that are now used by ORBS, the SITELLE's reduction pipeline. We have implemented this model in ORCS, a fitting engine for SITELLE's data, and used it to derive the [SII] density map of the central part of the nebula. The study of the broadening of the [NII] lines shows that the Main Ring and the Central Lobe are two different shells with different expansion velocities. We have also derived deep and spatially resolved velocity maps of the Halo in [NII] and Halpha and found that the brightest bubbles are originating from two bipolar structures with a velocity difference of more than 35 km/s lying at the poles of a possibly unique Halo shell expanding at a velocity of more than 15 km/s.

Odds for an enlightened rather than barren future

We are at a stage in our evolution where we do not yet know if we will ever communicate with intelligent beings that have evolved on other planets, yet we are intelligent and curious enough to wonder about this. We find ourselves wondering about this at the very beginning of a long era in which stellar luminosity warms many planets, and by our best models, continues to provide equally good opportunities for intelligent life to evolve. By simple Bayesian reasoning, if, as we believe, intelligent life forms have the same propensity to evolve later on other planets as we had to evolve on ours, it follows that they will likely not pass through a similar wondering stage in their evolution. This suggests that the future holds some kind of interstellar communication that will serve to inform newly evolved intelligent life forms that they are not alone before they become curious.

Globular Clusters, Ultracompact Dwarfs, and Dwarf Galaxies in Abell 2744 at the Redshift of 0.308

We report a photometric study of globular clusters (GCs), ultracompact dwarfs (UCDs), and dwarf galaxies in the giant merging galaxy cluster Abell 2744 at z = 0.308. Color-magnitude diagrams of the point sources derived from deep F814W (restframe r') and F105W (restframe I) images of Abell 2744 in the Hubble Space Telescope Frontier Field show a rich population of point sources whose colors are similar to those of typical GCs. These sources are as bright as -14.9 < M_r' < -11.4 (26.0 < F814W < 29.5) mag, being mostly UCDs and bright GCs in Abell 2744. The luminosity function (LF) of these sources shows a break at M_r' ~ -12.9 (F814W ~ 28.0) mag, indicating a boundary between UCDs and bright GCs. The numbers of GCs and UCDs are estimated to be N_GC = 385,000+-24,000, and 147 +- 26, respectively. The clustercentric radial number density profiles of the UCDs and bright GCs show similar slopes, but these profiles are much steeper than that of the dwarf galaxies and the mass density profile based on gravitational lensing analysis. We derive an LF of the red sequence galaxies for -22.9 < M_r' < -13.9 mag. The faint end of this LF is fit well by a flat power law with a = -1.14 +- 0.08, showing no faint upturn. These results support the galaxy-origin scenario for bright UCDs: they are the nuclei of dwarf galaxies that were stripped when they pass close to the center of massive galaxies or a galaxy cluster, while some of the faint UCDs are the bright end of the GCs.

Low-Redshift Lyman Limit Systems as Diagnostics of Cosmological Inflows and Outflows

We use cosmological hydrodynamic simulations with stellar feedback from the FIRE project to study the physical nature of Lyman limit systems (LLSs) at z<1. At these low redshifts, LLSs are closely associated with dense gas structures surrounding galaxies, such as galactic winds, dwarf satellites, and cool inflows from the intergalactic medium. Our analysis is based on 14 zoom-in simulations covering the halo mass range M_h~10^9-10^13 Msun at z=0, which we convolve with the dark matter halo mass function to produce cosmological statistics. We find that the majority of cosmologically-selected LLSs are associated with halos in the mass range 10^10 < M_h < 10^12 Msun. The incidence and HI column density distribution of simulated absorbers with columns 10^16.2 < N_HI < 2x10^20 cm^-2 are consistent with observations. High-velocity outflows (with radial velocity exceeding the halo circular velocity by a factor >~2) tend to have higher metallicities ([X/H] ~ -0.5) while very low metallicity ([X/H] < -2) LLSs are typically associated with gas infalling from the intergalactic medium. However, most LLSs occupy an intermediate region in metallicity-radial velocity space, for which there is no clear trend between metallicity and radial kinematics. Metal-enriched inflows arise in the FIRE simulations as a result of galactic winds that fall back onto galaxies at low redshift. The overall simulated LLS metallicity distribution has a mean (standard deviation) [X/H] = -0.9 (0.4) and does not show significant evidence for bimodality, in contrast to recent observational studies but consistent with LLSs arising from halos with a broad range of masses and metallicities.

Tidal disruption events from supermassive black hole binaries

We investigate the pre-disruption gravitational dynamics and post-disruption hydrodynamics of the tidal disruption of stars by supermassive black hole (SMBH) binaries. We focus on binaries with relatively low mass primaries ($10^6M_{\odot}$), moderate mass ratios, and separations with reasonably long gravitational wave inspiral times (tens of Myr). First, we generate a large ensemble (between 1 and 10 million) of restricted three-body integrations to quantify the statistical properties of tidal disruptions by circular SMBH binaries of initially-unbound stars. Compared to the reference case of a disruption by a single SMBH, the binary potential induces significant variance into the specific energy and angular momentum of the star at the point of disruption. Second, we use Newtonian numerical hydrodynamics to study the detailed evolution of the fallback debris from 120 disruptions randomly selected from the three-body ensemble (excluding only the most deeply penetrating encounters). We find that the overall morphology of the debris is greatly altered by the presence of the second black hole, and the accretion rate histories display a wide range of behaviors, including order of magnitude dips and excesses relative to control simulations that include only one black hole. Complex evolution persists, in some cases, for many orbital periods of the binary. We find evidence for power in the accretion curves on timescales related to the binary orbital period, though there is no exact periodicity. We discuss our results in the context of future wide-field surveys, and comment on the prospects of identifying and characterizing the subset of events occurring in nuclei with binary SMBHs.

Dynamical evidence for a strong tidal interaction between the Milky Way and its satellite, Leo V

We present a chemodynamical analysis of the Leo~V dwarf galaxy, based on Keck II DEIMOS spectra of 8 member stars. We find a systemic velocity for the system of $\langle v_r\rangle = 170.9^{+ 2.1}_{-1.9}$kms$^{-1}$, and barely resolve a velocity dispersion for the system, with $\sigma_{vr} = 2.3^{+3.2}_{-1.6}$kms$^{-1}$, consistent with previous studies of Leo~V. The poorly resolved dispersion means we are unable to concretely determine whether Leo~V is dark matter dominated. We find an average metallicity for the dwarf of [Fe/H]$ = -2.48\pm0.21$, and measure a significant spread in the iron abundance of its member stars, with $-3.1\le$[Fe/H]$\le-1.9$ dex, which cleanly identifies Leo~V as a dwarf galaxy that has been able to self-enrich its stellar population through extended star formation. Owing to the tentative photometric evidence for tidal substructure around Leo~V, we also investigate whether there is any dynamical evidence for tidal stripping or shocking of the system. We measure a significant velocity gradient across the system, of $\frac{{\rm d}v}{{\rm d}\chi} = -4.1^{+2.8}_{-2.6}$kms$^{-1}$ per arcmin (or $\frac{{\rm d}v}{{\rm d}\chi} = -71.9^{+50.8}_{-45.6}$kms$^{-1}$~kpc$^{-1}$), which points almost directly toward the Galactic centre. We argue that Leo~V is likely a dwarf on the brink of dissolution, having just barely survived a past encounter with the centre of the Milky Way.

Stream-subhalo interactions in the Aquarius simulations

We perform the first self-consistent measurement of the rate of interactions between stellar tidal streams created by disrupting satellites and dark subhalos in a cosmological simulation of a Milky-Way-mass galaxy. Using a retagged version of the Aquarius A dark-matter-only simulation, we selected 18 streams of tagged star particles that appear thin at the present day and followed them from the point their progenitors accrete onto the main halo, recording in each snapshot the characteristics of all dark-matter subhalos passing within several distance thresholds of any tagged star particle in each stream. We considered distance thresholds corresponding to constant impact parameters (1, 2, and 5 kpc), as well as those proportional to the region of influence of each subhalo (one and two times its half-mass radius $r_{1/2}$). We then measured the age and present-day, phase-unwrapped length of each stream in order to compute the interaction rate in different mass bins and for different thresholds, and compared these to analytic predictions from the literature. We measure a median rate of $1.5^{+3.0}_{-1.1}\ (9.1^{+17.5}_{-7.1},\ 61.8^{+211}_{-40.6})$ interactions within 1 (2, 5) kpc of the stream per 10 kpc of stream length per 10 Gyr. Resolution effects (both time and particle number) affect these estimated rates by lowering them.

A VLT/FORS2 spectroscopic survey of individual stars in a transforming dwarf galaxy

Understanding the properties of dwarf galaxies is important not only to put them in their proper cosmological context, but also to understand the formation and evolution of the most common type of galaxies. Dwarf galaxies are divided into two main classes, dwarf irregulars (dIrrs) and dwarf spheroidals (dSphs), which differ from each other mainly because the former are gas-rich objects currently forming stars, while the latter are gas-deficient with no on-going star formation. Transition types (dT) are thought to represent dIs in the process of losing their gas, and can therefore shed light into the possible process of dwarf irregulars (dIrrs) becoming gas-deficient, passively evolving galaxies. Here we present preliminary results from our wide-area VLT/FORS2 MXU spectroscopic survey of the Phoenix dT, from which we obtained line-of-sight velocities and metallicities from the nIR Ca II triplet lines for a large sample of individual Red Giant Branch stars.

The Shape of Dark Matter Haloes IV. The Structure of Stellar Discs in Edge-on Galaxies

We present optical and near-infrared archival observations of eight edge-on galaxies. These observations are used to model the stellar content of each galaxy using the FitSKIRT software package. Using FitSKIRT, we can self-consistently model a galaxy in each band simultaneously while treating for dust. This allows us to accurately measure both the scale length and scale height of the stellar disc, plus the shape parameters of the bulge. By combining this data with the previously reported integrated magnitudes of each galaxy, we can infer their true luminosities. We have successfully modelled seven out of the eight galaxies in our sample. We find that stellar discs can be modelled correctly, but have not been able to model the stellar bulge reliably. Our sample consists for the most part of slow rotating galaxies, and we find that the average dust layer is much thicker than what is reported for faster rotating galaxies.

G11.92-0.61 MM1: A Keplerian disc around a massive young proto-O star

The formation process of massive stars is not well understood, and advancement in our understanding benefits from high resolution observations and modelling of the gas and dust surrounding individual high-mass (proto)stars. Here we report sub-arcsecond (<1550 au) resolution observations of the young massive star G11.92-0.61 MM1 with the SMA and VLA. Our 1.3 mm SMA observations reveal consistent velocity gradients in compact molecular line emission from species such as CH$_3$CN, CH$_3$OH, OCS, HNCO, H$_2$CO, DCN and CH$_3$CH$_2$CN, oriented perpendicular to the previously reported bipolar molecular outflow from MM1. Modelling of the compact gas kinematics suggests a structure undergoing rotation around the peak of the dust continuum emission. The rotational profile can be well fit by a model of a Keplerian disc, including infall, surrounding an enclosed mass of 30-60M$_{\odot}$, of which 2-3M$_{\odot}$ is attributed to the disc. From modelling the CH$_3$CN emission, we determine that two temperature components, of 150 K and 230 K, are required to adequately reproduce the spectra. Our 0.9 and 3.0cm VLA continuum data exhibit an excess above the level expected from dust emission; the full centimetre-submillimetre wavelength spectral energy distribution of MM1 is well reproduced by a model including dust emission, an unresolved hypercompact H{\i}{\i} region, and a compact ionised jet. In combination, our results suggest that MM1 is an example of a massive proto-O star forming via disc accretion, in a similar way to that of lower mass stars.

The Shape of Dark Matter Haloes III. Kinematics and Structure of the HI disc

We present a new strategy for fitting the structure and kinematics of the HI in edge-on galaxies using a fit to the terminal-velocity channel maps of a HI data cube. The strategy can deal with self-absorbing HI gas and the presence of warps. The method is first tested on a series of models. We demonstrate that fitting optically thin models to real galaxies will lead to an overestimation of the thickness and velocity dispersion, and to a serious underestimation of the HI face-on column densities. We subsequently fit both self-absorption and optically thin models to the HI data of six edge-on galaxies. In three of these we have also measured the velocity dispersion. On average 27 \pm 6 % of the total HI mass of edge-on galaxies is hidden by self-absorption. This implies that the HI mass, thickness and velocity dispersion of galaxies is typically underestimated in the literature.

The Shape of Dark Matter Haloes II. The Galactus HI Modelling & Fitting Tool

We present a new HI modelling tool called \textsc{Galactus}. The program has been designed to perform automated fits of disc-galaxy models to observations. It includes a treatment for the self-absorption of the gas. The software has been released into the public domain. We describe the design philosophy and inner workings of the program. After this, we model the face-on galaxy NGC2403, using both self-absorption and optically thin models, showing that self-absorption occurs even in face-on galaxies. It is shown that the maximum surface brightness plateaus seen in Paper I of this series are indeed signs of self-absorption. The apparent HI mass of an edge-on galaxy can be drastically lower compared to that same galaxy seen face-on. The Tully-Fisher relation is found to be relatively free from self-absorption issues.

Where does the gas fueling star formation in BCGs originate?

We investigate the relationship between X-ray cooling and star formation in brightest cluster galaxies (BCGs). We present an X-ray spectral analysis of the inner regions, 10-40 kpc, of six nearby cool core clusters (z<0.35) observed with Chandra ACIS. This sample is selected on the basis of the high star formation rate (SFR) observed in the BCGs. We restrict our search for cooling gas to regions that are roughly cospatial with the starburst. We fit single- and multi-temperature mkcflow models to constrain the amount of isobarically cooling intracluster medium (ICM). We find that in all clusters, below a threshold temperature ranging between 0.9 and 3 keV, only upper limits can be obtained. In four out of six objects, the upper limits are significantly below the SFR and in two, namely A1835 and A1068, they are less than a tenth of the SFR. Our results suggests that a number of mechanisms conspire to hide the cooling signature in our spectra. In a few systems the lack of a cooling signature may be attributed to a relatively long delay time between the X-ray cooling and the star burst. However, for A1835 and A1068, where the X-ray cooling time is shorter than the timescale of the starburst, a possible explanation is that the region where gas cools out of the X-ray phase extends to very large radii, likely beyond the core of these systems.

Self-gravitating disc candidates around massive young stars

There have been several recent detections of candidate Keplerian discs around massive young protostars. Given the relatively large disc-to-star mass ratios in these systems, and their young ages, it is worth investigating their propensity to becoming self-gravitating. To this end, we compute self-consistent, semi-analytic models of putative self-gravitating discs for five candidate disc systems. Our aim is not to fit exactly the observations, but to demonstrate that the expected dust continuum emission from marginally unstable self-gravitating discs can be quite weak, due to high optical depth at the midplane even at millimetre wavelengths. In the best cases, the models produce "observable" disc masses within a factor of <1.5 of those observed, with midplane dust temperatures comparable to measured temperatures from molecular line emission. We find in two cases that a self-gravitating disc model compares well with observations. If these discs are self-gravitating, they satisfy the conditions for disc fragmentation in their outer regions. These systems may hence have as-yet-unresolved low mass stellar companions, and are thus promising targets for future high angular resolution observations.

Galaxy And Mass Assembly (GAMA): the Stellar Mass Budget of Galaxy Spheroids and Disks

We build on a recent photometric decomposition analysis of 7506 Galaxy and Mass Assembly (GAMA) survey galaxies to derive stellar mass function fits to individual spheroid and disk component populations down to a lower mass limit of log(M_*/M_sun)= 8. We find that the spheroid/disk mass distributions for individual galaxy morphological types are well described by single Schechter function forms. We derive estimates of the total stellar mass densities in spheroids (rho_spheroid = 1.24+/-0.49 * 10^8 M_sun Mpc^-3 h_0.7) and disks (rho_disk = 1.20+/-0.45 * 10^8 M_sun Mpc^-3 h_0.7), which translates to approximately 50% of the local stellar mass density in spheroids and 48% in disks. The remaining stellar mass is found in the dwarf "little blue spheroid" class, which is not obviously similar in structure to either classical spheroid or disk populations. We also examine the variation of component mass ratios across galaxy mass and group halo mass regimes, finding the transition from spheroid to disk mass dominance occurs near galaxy stellar mass ~10^11 M_sun and group halo mass ~10^12.5 M_sun/h. We further quantify the variation in spheroid-to-total mass ratio with group halo mass for central and satellite populations as well as the radial variation of this ratio within groups.

The Distant Outer Gas Arm Between l=35d and l=45d

The Galactic plane has been mapped from l=34.75d to 45.25d and b=-5.25d to 5.25d in the CO (J=1-0) emission with the 13.7 m telescope of the Purple Mountain Observatory. The unbiased survey covers a large area of 100 square degrees sampled every 30" with a velocity resolution of ~0.2km/s. In this paper, we present the result of an unbiased CO survey of this longitude and latitude range in the velocity range from -60km/s to -10km/s. Over 500 molecular clouds (MCs) are picked out from the 12CO emission, and 131 of these MCs are associated with 13CO emission.The distant MCs, which lie beyond the solar circle and are mostly concentrated in the Galactic plane, trace the large-scale molecular gas structure over 10 degrees of Galactic azimuth. We suggest that the CO emission of the segment is from the Outer Arm. The physical mid-plane traced by the Outer Arm seems to be slightly displaced from the IAU-defined plane on a large scale, which could be explained by the warped plane at large Galactocentric distances of >~10 kpc and the apparent tilted mid-plane to the projected IAU-defined plane caused by the Sun's z-height above the disk for distances near and within the Solar circle. If the inner plane of our Galaxy is flat, we can derive an upper limit of the Sun's offset of ~17.1 pc above the physical mid-plane of the Milky Way. We also discuss the correlations between the physical parameters of the distant MCs, which is quite consistent with the result of other studies of this parameter.

Point mass Cosmological Black Holes

Real black holes in the universe are located in the expanding accelerating background which are called the cosmological black holes. Hence, it is necessary to model these black holes in the cosmological background where the dark energy is the dominant energy. In this paper, we argue that most of the dynamical cosmological black holes can be modeled by point mass cosmological black holes. Considering the de Sitter background for the accelerating universe, we present the point mass cosmological background in the cosmological de Sitter space time. Our work also includes the point mass black holes which have charge and angular momentum. We study the mass, horizons, redshift structure and geodesics properties for these black holes.

Point mass Cosmological Black Holes [Cross-Listing]

Real black holes in the universe are located in the expanding accelerating background which are called the cosmological black holes. Hence, it is necessary to model these black holes in the cosmological background where the dark energy is the dominant energy. In this paper, we argue that most of the dynamical cosmological black holes can be modeled by point mass cosmological black holes. Considering the de Sitter background for the accelerating universe, we present the point mass cosmological background in the cosmological de Sitter space time. Our work also includes the point mass black holes which have charge and angular momentum. We study the mass, horizons, redshift structure and geodesics properties for these black holes.

The Redshifted Hydrogen Balmer and Metastable He I Absorption Line System in Mini-FeLoBAL Quasar SDSS J112526.12+002901.3: A Parsec Scale Accretion Inflow?

The accretion of interstellar medium onto the central super massive black holes is widely accepted as the source of the gigantic energy released by the active galactic nuclei. But few pieces of observational evidence have been confirmed directly demonstrating the existence of the inflows. The absorption line system in the spectra of quasar SDSS J112526.12+002901.3 presents an interesting example, in which the rarely detected hydrogen Balmer and metastable He I absorption lines are found redshifted to the quasar's rest frame along with the low-ionization metal absorption lines Mg II, Fe II, etc. The repeated SDSS spectroscopic observations suggest a transverse velocity smaller than the radial velocity. The motion of the absorbing medium is thus dominated by infall. The He I* lines present a powerful probe to the strength of ionizing flux, while the Balmer lines imply a dense environment. With the help of photoionization simulations, we find the absorbing medium is exposed to the radiation with ionization parameter $U\approx 10^{-1.8}$, and the density is $n(\mathrm{H})\approx 10^9\ \mathrm{cm}^{-3}$. Thus the absorbing medium is located $\sim 4\ \mathrm{pc}$ away from the central engine. According to the similarity in the distance and physical conditions between the absorbing medium and the torus, we strongly propose the absorption line system as a candidate for the accretion inflow which originates from the inner surface of the torus.

Studying Relation Between Star Formation and Molecular Clumps on Subparsec Scales in 30 Doradus

We present $\mathrm{^{12}CO}$ and $\mathrm{^{13}CO}$ molecular gas data observed by ALMA, massive early stage young stellar objects identified by applying color-magnitude cuts to \textit{Spitzer} and \textit{Herschel} photometry, and low-mass late stage young stellar objects identified via H$\mathrm{\alpha}$ excess. Using dendrograms, we derive properties for the molecular cloud structures. This is the first time a dendrogram analysis has been applied to extragalactic clouds. The majority of clumps have a virial parameter equal to unity or less. The size-linewidth relations of $\mathrm{^{12}CO}$ and $\mathrm{^{13}CO}$ show the clumps in this study have a larger linewidth for a given size (by factor of 3.8 and 2.5, respectively) in comparison to several, but not all, previous studies. The larger linewidths in 30 Doradus compared to typical Milky Way quiescent clumps are probably due to the highly energetic environmental conditions of 30 Doradus. The slope of the size-linewidth relations of $\mathrm{^{12}CO}$, 0.65 $\pm$ 0.04, and $\mathrm{^{13}CO}$, 0.97 $\pm$ 0.12, are on the higher end but consistent within 3$\mathrm{\sigma}$ of previous studies. Massive star formation occurs in clumps with high masses ($> 1.83 \times 10^{2}\;\mathrm{M_{\odot}}$), high linewidths (v $> 1.18\;\mathrm{km/s}$), and high mass densities ($> 6.67 \times 10^{2}\;\mathrm{M_{\odot}\;pc^{-2}}$). The majority of embedded, massive young stellar objects are associated with a clump. However the majority of more evolved, low-mass young stellar objects are not associated with a clump.

Hierarchical black hole triples in young star clusters: impact of Kozai-Lidov resonance on mergers

Mergers of compact object binaries are one of the most powerful sources of gravitational waves (GWs) in the frequency range of second-generation ground-based gravitational wave detectors (Advanced LIGO and Virgo). Dynamical simulations of young dense star clusters (SCs) indicate that ~27 per cent of all double compact object binaries are members of hierarchical triple systems (HTs). In this paper, we consider 570 HTs composed of three compact objects (black holes or neutron stars) that formed dynamically in N-body simulations of young dense SCs. We simulate them for a Hubble time with a new code based on the Mikkola's algorithmic regularization scheme, including the 2.5 post-Newtonian term. We find that ~88 per cent of the simulated systems develop Kozai-Lidov (KL) oscillations. KL resonance triggers the merger of the inner binary in three systems (corresponding to 0.5 per cent of the simulated HTs), by increasing the eccentricity of the inner binary. Accounting for KL oscillations leads to an increase of the total expected merger rate by ~50 per cent. All binaries that merge because of KL oscillations were formed by dynamical exchanges (i.e. none is a primordial binary) and have chirp mass >20 Msun. This result might be crucial to interpret the formation channel of the first recently detected GW events.

Coevolution of metallicity and star formation in galaxies to z=3.7: II. A theoretical model

Recent work suggests that galaxy evolution, and the build-up of stellar mass (M*) over cosmic time, is characterized by changes with redshift of star formation rate (SFR) and oxygen abundance (O/H). In a companion paper, we have compiled a large dataset to study Metallicity Evolution and Galaxy Assembly (MEGA), consisting of roughly 1000 galaxies to z=3.7 with a common O/H calibration. Here we interpret the MEGA scaling relations of M*, SFR, and O/H with an updated version of the model presented by Dayal et al. (2013). This model successfully reproduces the observed O/H ratio of 80,000 galaxies selected from the Sloan Digital Sky Survey to within 0.05-0.06 dex. By extending the model to the higher redshift MEGA sample, we find that although the specific mass loading of outflows does not change measurably during the evolution, the accretion rate and gas content of galaxies increase significantly with redshift. These two effects can explain, either separately or possibly in tandem, the observed lower metal abundance of high-z galaxies.

Coevolution of metallicity and star formation in galaxies to z=3.7: I. A fundamental plane

With the aim of understanding the coevolution of star formation rate (SFR), stellar mass (M*), and oxygen abundance (O/H) in galaxies up to redshift z=3.7, we have compiled the largest available dataset for studying Metallicity Evolution and Galaxy Assembly (MEGA); it comprises roughly 1000 galaxies with a common O/H calibration and spans almost two orders of magnitude in metallicity, a factor of 10^6 in SFR, and a factor of 10^5 in stellar mass. From a Principal Component Analysis, we find that the 3-dimensional parameter space reduces to a Fundamental Plane of Metallicity (FPZ) given by 12+log(O/H) = -0.14 log (SFR) + 0.37 log (M*) + 4.82. The mean O/H FPZ residuals are small (0.16 dex) and consistent with trends found in smaller galaxy samples with more limited ranges in M*, SFR, and O/H. Importantly, the FPZ is found to be redshift-invariant within the uncertainties. In a companion paper, these results are interpreted with an updated version of the model presented by Dayal et al. (2013).

The survival of gas clouds in the Circumgalactic Medium of Milky-Way-like galaxies

Observational evidence shows that low-redshift galaxies are surrounded by extended haloes of multiphase gas, the so-called 'circumgalactic medium' (CGM). To study the survival of relatively cool gas (T < 10^5 K) in the CGM, we performed a set of hydrodynamical simulations of cool (T = 10^4 K) neutral gas clouds travelling through a hot (T = 2x10^6 K) and low-density (n = 10^-4 cm^-3) coronal medium, typical of Milky Way-like galaxies at large galactocentric distances (~ 50-150 kpc). We explored the effects of different values of relative velocity and radius of the clouds. Our simulations include radiative cooling, photoionization heating and thermal conduction. The main result is that large clouds (radii larger than 250 pc) may survive for very long time (at least 250 Myr): their mass decreases during their trajectory but at very low rates. We found that thermal conduction plays a significant role: its effect is to prevent formation of Kelvin-Helmholtz instabilities at the cloud-corona interface, keeping the cloud compact and therefore more difficult to destroy. The distribution of column densities in our simulations are compatible with those observed for low-temperature ions (e.g. SiII and SiIII) and for high-ions (OVI) once we take into account that OVI covers much more extended regions than the cool gas and, therefore, it is more likely to be detected along a generic line of sight.

Observational Evidence of Dynamic Star Formation Rate in Milky Way Giant Molecular Clouds

Star formation on galactic scales is known to be a slow process, but whether it is slow on smaller scales is uncertain. We cross-correlate 5469 giant molecular clouds (GMCs) from a new all-sky catalog with 256 star forming complexes (SFCs) to build a sample of 191 SFC-GMC complexes---collections of multiple clouds each matched to 191 SFCs. The total mass in stars harbored by these clouds is inferred from WMAP free-free fluxes. We measure the GMC mass, the virial parameter, the star formation efficiency $\epsilon$ and the star formation rate per free-fall time $\epsilon_{\rm ff}$. Both $\epsilon$ and $\epsilon_{\rm ff}$ range over 3--4 orders of magnitude. We find that 68.3% of the clouds fall within $\sigma_{\log\epsilon}=0.79\pm0.22\,{\rm dex}$ and $\sigma_{\log\epsilon_{\rm ff}}=0.91\pm0.22\,{\rm dex}$ about the median. Compared to these observed scatters, a simple model with a time independent $\epsilon_{\rm ff}$ that depends on the host GMC properties predicts $\sigma_{\log\epsilon_{\rm ff}}=0.24$. Allowing for a time-variable $\epsilon_{\rm ff}$, we can recover the large dispersion in the rate of star formation. This strongly suggests that star formation in the Milky Way is a dynamic process on GMC scales. We also show that the surface star formation rate profile of the Milky Way correlates well with the molecular gas surface density profile.

Bulge-forming galaxies with an extended rotating disk at z~2

We present 0".2-resolution Atacama Large Millimeter/submillimeter Array observations at 870 um for 25 Halpha-seleced star-forming galaxies (SFGs) around the main-sequence at z=2.2-2.5. We detect significant 870 um continuum emission in 16 (64%) of these SFGs. The high-resolution maps reveal that the dust emission is mostly radiated from a single region close to the galaxy center. Exploiting the visibility data taken over a wide $uv$ distance range, we measure the half-light radii of the rest-frame far-infrared emission for the best sample of 12 SFGs. We find nine galaxies to be associated with extremely compact dust emission with R_{1/2,870um}<1.5 kpc, which is more than a factor of 2 smaller than their rest-optical sizes, R_{1/2,1.6um}=3.2 kpc, and is comparable with optical sizes of massive quiescent galaxies at similar redshifts. As they have an exponential disk with Sersic index of n=1.2 in the rest-optical, they are likely to be in the transition phase from extended disks to compact spheroids. Given their high star formation rate surface densities within the central 1 kpc of Sigma SFR1kpc=40 Msol/yr/kpc^2, the intense circumnuclear starbursts can rapidly build up a central bulge with Sigma M*1kpc>1e10 Msol/kpc^2 in several hundred Myr, i.e. by z~2. Moreover, ionized gas kinematics reveal that they are rotation-supported with an angular momentum as large as that of typical SFGs at z=1-3. Our results suggest bulges are commonly formed in extended rotating disks by internal processes, not involving major mergers.

Anisotropies in the HI gas distribution toward 3C196

The local Galactic HI gas was found to contain cold neutral medium (CNM) filaments that are aligned with polarized dust emission. These filaments appear to be dominated by the magnetic field and in this case turbulence is expected to show distinct anisotropies. We use the Galactic Effelsberg--Bonn HI Survey (EBHIS) to derive 2D turbulence spectra for the HI distribution in direction to 3C196 and two more comparison fields. Prior to Fourier transform we apply a rotational symmetric 50% Tukey window to apodize the data. We derive average as well as position angle dependent power spectra. Anisotropies in the power distribution are defined as the ratio of the spectral power in orthogonal directions. We find strong anisotropies. For a narrow range in position angle, in direction perpendicular to the filaments and the magnetic field, the spectral power is on average more than an order of magnitude larger than parallel. In the most extreme case the anisotropy reaches locally a factor of 130. Anisotropies increase on average with spatial frequency as predicted by Goldreich and Sridhar, at the same time the Kolmogorov spectral index remains almost unchanged. The strongest anisotropies are observable for a narrow range in velocity and decay with a power law index close to --8/3, almost identical to the average isotropic spectral index of $-2.9 < \gamma < -2.6$. HI filaments, associated with linear polarization structures in LOFAR observations in direction to 3C196, show turbulence spectra with marked anisotropies. Decaying anisotropies appear to indicate that we witness an ongoing shock passing the HI and affecting the observed Faraday depth.

Anisotropies in the HI gas distribution toward 3C196 [Replacement]

The local Galactic HI gas was found to contain cold neutral medium (CNM) filaments that are aligned with polarized dust emission. These filaments appear to be dominated by the magnetic field and in this case turbulence is expected to show distinct anisotropies. We use the Galactic Effelsberg--Bonn HI Survey (EBHIS) to derive 2D turbulence spectra for the HI distribution in direction to 3C196 and two more comparison fields. Prior to Fourier transform we apply a rotational symmetric 50% Tukey window to apodize the data. We derive average as well as position angle dependent power spectra. Anisotropies in the power distribution are defined as the ratio of the spectral power in orthogonal directions. We find strong anisotropies. For a narrow range in position angle, in direction perpendicular to the filaments and the magnetic field, the spectral power is on average more than an order of magnitude larger than parallel. In the most extreme case the anisotropy reaches locally a factor of 130. Anisotropies increase on average with spatial frequency as predicted by Goldreich and Sridhar, at the same time the Kolmogorov spectral index remains almost unchanged. The strongest anisotropies are observable for a narrow range in velocity and decay with a power law index close to --8/3, almost identical to the average isotropic spectral index of $-2.9 < \gamma < -2.6$. HI filaments, associated with linear polarization structures in LOFAR observations in direction to 3C196, show turbulence spectra with marked anisotropies. Decaying anisotropies appear to indicate that we witness an ongoing shock passing the HI and affecting the observed Faraday depth.

 

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