Recent Postings from Galactic

HII regions within a compact high velocity cloud. A nearly star-less dwarf galaxy?

Within the SECCO survey we identified a candidate stellar counterpart to the Ultra Compact High Velocity Cloud (UCHVC) HVC274.68+74.70-123, that was suggested by Adams et al. (2013) as a possible mini-halo within the Local Group of galaxies. The spectroscopic follow-up of the brightest sources within the candidate reveals the presence of two HII regions whose radial velocity is compatible with physical association with the UVHVC. The available data does not allow us to give a definite answer on the nature of the newly identified system. A few alternative hypotheses are discussed. However, the most likely possibility is that we have found a new faint dwarf galaxy residing in the Virgo cluster of galaxies, which we name SECCO-1. Independently of its actual distance, SECCO-1 displays a ratio of neutral hydrogen mass to V luminosity of M_{HI}/L_V>= 20, by far the largest among local dwarfs. Hence, it appears as a nearly star-less galaxy and it may be an example of the missing links between normal dwarfs and the dark mini halos that are predicted to exist in large numbers according to the currently accepted cosmological model.

Tailed Radio Galaxies as Probes of Cluster Physics in the Square Kilometre Array Era

In recent years, the use of tailed radio galaxies as environmental probes has gained momentum as a method for galaxy cluster detection, examining the dynamics of individual clusters, measuring the density and velocity flows in the intra-cluster medium, and for probing cluster magnetic fields. To date instrumental limitations in terms of resolution and sensitivity have confined this research to the local (z < 0.7) Universe. The advent of SKA-1 surveys however will allow detection of well over 1 million tailed radio galaxies and their associated galaxy clusters out to redshifts of 2 or more. This is in fact ten times more than the current number of known clusters in the Universe. Such a substantial sample of tailed galaxies will provide an invaluable tool not only for detecting clusters, but also for characterizing their intra-cluster medium, magnetic fields and dynamical state as a function of cosmic time. In this paper we present an analysis of the usability of tailed radio galaxies as tracers of dense environments extrapolated from existing deep radio surveys such the Extended Chandra Deep Field-South.

C-Band All-Sky Survey: A First Look at the Galaxy

We present an analysis of the diffuse emission at 5 GHz in the first quadrant of the Galactic plane using two months of preliminary intensity data taken with the C-Band All Sky Survey (C-BASS) northern instrument at the Owens Valley Radio Observatory, California. Combining C-BASS maps with ancillary data to make temperature-temperature plots we find synchrotron spectral indices of $\beta = -2.65 \pm 0.05$ between 0.408 GHz and 5 GHz and $ \beta = -2.72 \pm 0.09$ between 1.420 GHz and 5 GHz for $-10^{\circ} < |b| < -4^{\circ}$, $20^{\circ} < l < 40^{\circ}$. Through the subtraction of a radio recombination line (RRL) free-free template we determine the synchrotron spectral index in the Galactic plane ($ |b| < 4^{\circ}$) to be $\beta = -2.56 \pm 0.07$ between 0.408 GHz and 5 GHz, with a contribution of $53 \pm 8$ per cent from free-free emission at 5\,GHz. These results are consistent with previous low frequency measurements in the Galactic plane. By including C-BASS data in spectral fits we demonstrate the presence of anomalous microwave emission (AME) associated with the HII complexes W43, W44 and W47 near 30 GHz, at 4.4 sigma, 3.1 sigma and 2.5 sigma respectively. The CORNISH VLA 5 GHz source catalogue rules out the possibility that the excess emission detected around 30\;GHz may be due to ultra-compact HII regions. Diffuse AME was also identified at a 4 sigma level within $30^{\circ} < l < 40^{\circ}$, $-2^{\circ} < b < 2^{\circ}$ between 5 GHz and 22.8 GHz.

Trident: A three-pronged galaxy survey. I. Lyman alpha emitting galaxies at z~2 in GOODS North

Context. Lyman alpha emitting galaxies (LAEs) are used to probe the distant universe and are therefore important for galaxy evolution studies and for providing clues to the nature of the epoch of reionization, but the exact circumstances under which Lyman alpha escapes a galaxy are still not fully understood. Aims. The Trident project is designed to simultaneously examine Lyman alpha, H-alpha and Lyman Continuum emission from galaxies at redshift z~2, thus linking together these three aspects of ionising radiation in galaxies. In this paper, we outline the strategy of this project and examine the properties of LAEs in the GOODS North field. Methods. We performed a narrowband LAE survey in GOODS North using existing and two custom made filters at the Nordic Optical Telescope with MOSCA. We use complementary broad band archival data in the field to make a careful candidate selection and perform optical to near-IR SED fitting. We also estimate far-infrared luminosities by matching our candidates to detections in Spitzer/MIPS 24{\mu}m and Herschel/PACS catalogs. Results. We find a total of 25 LAE candidates, probing mainly the bright end of the LAE luminosity function with L_Ly {\alpha} ~ 1-15e42 erg/s. They display a range of masses of ~0.5-50e9 M_solar, and average ages from a few tens of Myr to 1 Gyr when assuming a constant star formation history. The majority of our candidates also show signs of recent elevated star formation. Three candidates have counterparts in the GOODS-Herschel far-IR catalogue, with luminosities consistent with ultra-luminous infrared galaxies (ULIRGs). Conclusions. The wide range of parameters derived from our SED fitting, as well as part of our sample being detected as ULIRGs, seems to indicate that at these Lyman alpha luminosities, LAEs do not necessarily have to be young dwarfs, and that a lack of dust is not required for Lyman alpha to escape.

The Role of Turbulence and Magnetic Fields in Simulated Filamentary Structure

We use numerical simulations of turbulent cluster-forming regions to study the nature of dense filamentary structures in star formation. Using four hydrodynamic and magnetohydrodynamic simulations chosen to match observations, we identify filaments in the resulting column density maps and analyze their properties. We calculate the radial column density profiles of the filaments every 0.05 Myr and fit the profiles with the modified isothermal and pressure confined isothermal cylinder models, finding reasonable fits for either model. The filaments formed in the simulations have similar radial column density profiles to those observed. Magnetic fields provide additional pressure support to the filaments, making `puffier’ filaments less prone to fragmentation than in the pure hydrodynamic case, which continue to condense at a slower rate. In the higher density simulations, the filaments grow faster through the increased importance of gravity. Not all of the filaments identified in the simulations will evolve to form stars: some expand and disperse. Given these different filament evolutionary paths, the trends in bulk filament width as a function of time, magnetic field strength, or density, are weak, and all cases are reasonably consistent with the finding of a constant filament width in different star-forming regions. In the simulations, the mean FWHM lies between 0.06 and 0.26 pc for all times and initial conditions, with most lying between 0.1 to 0.15 pc; the range in FWHMs are, however, larger than seen in typical Herschel analyses. Finally, the filaments display a wealth of substructure similar to the recent discovery of filament bundles in Taurus.

The Bolocam Galactic Plane Survey. XI. Temperatures and Substructure of Galactic Clumps Based on 350 micron Observations

We present 107 maps of continuum emission at 350 microns from Galactic molecular clumps. Observed sources were mainly selected from the Bolocam Galactic Plane Survey (BGPS) catalog, with 3 additional maps covering star forming regions in the outer Galaxy. The higher resolution of the SHARC-II images (8.5” beam) compared with the 1.1 mm images from BGPS (33” beam) allowed us to identify a large population of smaller substructures within the clumps. A catalog is presented for the 1386 sources extracted from the 350 micron maps. The color temperature distribution of clumps based on the two wavelengths has a median of 13.3 K and mean of 16.3 +- 0.4 K, assuming an opacity law index of 1.7. For the structures with the best determined color temperatures, the mean ratio of gas temperature, determined from NH3 observations, to dust color temperature is 0.88 and the median ratio is 0.76. About half the clumps have more than two substructures and 22 clumps have more than 10. The fraction of the mass in dense substructures seen at 350 microns compared to the mass of their parental clump is ~0.19, and the surface densities of these substructures are, on average, 2.2 times those seen in the clumps identified at 1.1 mm. For a well-characterized sample, 88 structures (31%) exceed a surface density of 0.2 g cm^(-2), and 18 (6%) exceed 1.0 g cm^(-2), thresholds for massive star formation suggested by theorists.

Simulating the formation of massive seed black holes in the early Universe. I: An improved chemical model

The direct collapse model for the formation of massive seed black holes in the early Universe attempts to explain the observed number density of supermassive black holes (SMBHs) at $z \sim 6$ by assuming that they grow from seeds with masses M > 10000 solar masses that form by the direct collapse of metal-free gas in atomic cooling halos in which H2 cooling is suppressed by a strong extragalactic radiation field. The viability of this model depends on the strength of the radiation field required to suppress H2 cooling, $J_{\rm crit}$: if this is too large, then too few seeds will form to explain the observed number density of SMBHs. In order to determine $J_{\rm crit}$ reliably, we need to be able to accurately model the formation and destruction of H2 in gas illuminated by an extremely strong radiation field. In this paper, we use a reaction-based reduction technique to analyze the chemistry of H2 in these conditions, allowing us to identify the key chemical reactions that are responsible for determining the value of $J_{\rm crit}$. We construct a reduced network of 26 reactions that allows us to determine $J_{\rm crit}$ accurately, and compare it with previous treatments in the literature. We show that previous studies have often omitted one or more important chemical reactions, and that these omissions introduce an uncertainty of up to a factor of three into previous determinations of $J_{\rm crit}$.

Galaxy Zoo: Evidence for Diverse Star Formation Histories through the Green Valley

Does galaxy evolution proceed through the green valley via multiple pathways or as a single population? Motivated by recent results highlighting radically different evolutionary pathways between early- and late-type galaxies, we present results from a simple Bayesian approach to this problem wherein we model the star formation history (SFH) of a galaxy with two parameters, [t, \tau] and compare the predicted and observed optical and near-ultraviolet colours. We use a novel method to investigate the morphological differences between the most probable SFHs for both disc-like and smooth-like populations of galaxies, by using a sample of 126,316 galaxies (0.01 < z < 0.25) with probabilistic estimates of morphology from Galaxy Zoo. We find a clear difference between the quenching timescales preferred by smooth- and disc-like galaxies, with three possible routes through the green valley dominated by smooth- (rapid timescales, attributed to major mergers), intermediate- (intermediate timescales, attributed to minor mergers and galaxy interactions) and disc-like (slow timescales, attributed to secular evolution) galaxies. We hypothesise that morphological changes occur in systems which have undergone quenching with an exponential timescale \tau < 1.5 Gyr, in order for the evolution of galaxies in the green valley to match the ratio of smooth to disc galaxies observed in the red sequence. These rapid timescales are instrumental in the formation of the red sequence at earlier times; however we find that galaxies currently passing through the green valley typically do so at intermediate timescales.

Space Telescope and Optical Reverberation Mapping Project. I. Ultraviolet Observations of the Seyfert 1 Galaxy NGC 5548 with the Cosmic Origins Spectrograph on Hubble Space Telescope

We describe the first results from a six-month long reverberation-mapping experiment in the ultraviolet based on 170 observations of the Seyfert 1 galaxy NGC 5548 with the Cosmic Origins Spectrograph on the Hubble Space Telescope. Significant correlated variability is found in the continuum and broad emission lines, with amplitudes ranging from ~30% to a factor of two in the emission lines and a factor of three in the continuum. The variations of all the strong emission lines lag behind those of the continuum, with He II 1640 lagging behind the continuum by ~2.5 days and Lyman alpha 1215, C IV 1550, and Si IV 1400 lagging by ~5-6 days. The relationship between the continuum and emission lines is complex. In particular, during the second half of the campaign, all emission-line lags increased by a factor of 1.3-2 and differences appear in the detailed structure of the continuum and emission-line light curves. Velocity-resolved cross-correlation analysis shows coherent structure in lag versus line-of-sight velocity for the emission lines; the high-velocity wings of C IV respond to continuum variations more rapidly than the line core, probably indicating higher velocity BLR clouds at smaller distances from the central engine. The velocity-dependent response of Lyman alpha, however, is more complex and will require further analysis.

The Dark Force: Astrophysical Repulsion from Dark Energy [Cross-Listing]

Dark energy (i.e., a cosmological constant) leads, in the Newtonian approximation, to a repulsive force which grows linearly with distance. We discuss possible astrophysical effects of this "dark" force. For example, the dark force overcomes the gravitational attraction from an object (e.g., dwarf galaxy) of mass $10^7 M_\odot$ at a distance of $~ 23$ kpc. It seems possible that observable velocities of bound satellites (rotation curves) could be significantly affected, and therefore used to measure the dark energy density.

The Dark Force: Astrophysical Repulsion from Dark Energy [Cross-Listing]

Dark energy (i.e., a cosmological constant) leads, in the Newtonian approximation, to a repulsive force which grows linearly with distance. We discuss possible astrophysical effects of this "dark" force. For example, the dark force overcomes the gravitational attraction from an object (e.g., dwarf galaxy) of mass $10^7 M_\odot$ at a distance of $~ 23$ kpc. It seems possible that observable velocities of bound satellites (rotation curves) could be significantly affected, and therefore used to measure the dark energy density.

The Dark Force: Astrophysical Repulsion from Dark Energy

Dark energy (i.e., a cosmological constant) leads, in the Newtonian approximation, to a repulsive force which grows linearly with distance. We discuss possible astrophysical effects of this "dark" force. For example, the dark force overcomes the gravitational attraction from an object (e.g., dwarf galaxy) of mass $10^7 M_\odot$ at a distance of $~ 23$ kpc. It seems possible that observable velocities of bound satellites (rotation curves) could be significantly affected, and therefore used to measure the dark energy density.

Space Telescope and Optical Reverberation Mapping Project. II. Swift and HST Reverberation Mapping of the Accretion Disk of NGC 5548

Recent intensive Swift monitoring of the Seyfert 1 galaxy NGC 5548 yielded 282 usable epochs over 125 days across six UV/optical bands and the X-rays. This is the densest extended AGN UV/optical continuum sampling ever obtained, with a mean sampling rate < 0.5-day. Approximately daily HST UV sampling was also obtained. The UV/optical light curves show strong correlations (r_max = 0.57 – 0.90) and the clearest measurement to date of interband lags. These lags are well-fit by a lambda^4/3 wavelength dependence, with a normalization that indicates an unexpectedly large disk size of ~0.35 +/- 0.05 lt-day at 1367 A, assuming a simple face-on model. The U-band shows a marginally larger lag than expected from the fit and surrounding bands, which could be due to Balmer continuum emission from the broad-line region as suggested by Korista and Goad. The UV/X-ray correlation is weaker (r_max < 0.45) and less consistent over time. This indicates that while Swift is beginning to measure UV/optical lags in agreement with accretion disk theory, the relationship between X-ray and UV variability is less fully understood. Combining this accretion disk size estimate with those estimated from quasar microlensing studies suggests that AGN disk sizes scale approximately linearly with central black hole mass over a wide range of masses.

Which galaxy mass estimator can we trust?

We address the problem that dynamical masses of high-redshift massive galaxies, derived using virial scaling, often come out lower than stellar masses inferred from population fitting to multi-band photometry. We compare dynamical and stellar masses for various samples spanning ranges of mass, compactness and redshift, including the SDSS. The discrepancy between dynamical and stellar masses occurs both at low and high redshifts, and systematically increases with galaxy compactness. Because it is unlikely that stellar masses show systematic errors with galaxy compactness, the correlation of mass discrepancy with compactness points to errors in the dynamical mass estimates which assume homology with massive, nearby ellipticals. We quantify the deviations from homology and propose specific non-virial scaling of dynamical mass with effective radius and velocity dispersion.

The LyAlpha Line Profiles of Ultraluminous Infrared Galaxies: Fast Winds and Lyman Continuum Leakage

We present new Hubble Space Telescope Cosmic Origins Spectrograph far-ultraviolet (far-UV) spectroscopy and Keck Echellete optical spectroscopy of 11 ultraluminous infrared galaxies (ULIRGs), a rare population of local galaxies experiencing massive gas inflows, extreme starbursts, and prominent outflows. We detect H Lyman alpha emission from 8 ULIRGs and the companion to IRAS09583+4714. In contrast to the P Cygni profiles often seen in galaxy spectra, the H Lyman alpha profiles exhibit prominent, blueshifted emission out to Doppler shifts exceeding -1000 km/s in three HII-dominated and two AGN-dominated ULIRGs. To better understand the role of resonance scattering in shaping the H Lyman alpha line profiles, we directly compare them to non-resonant emission lines in optical spectra. We find that the line wings are already present in the intrinsic nebular spectra, and scattering merely enhances the wings relative to the line core. The H Lyman alpha attenuation (as measured in the COS aperture) ranges from that of the far-UV continuum to over 100 times more. A simple radiative transfer model suggests the H Lyman alpha photons escape through cavities which have low column densities of neutral hydrogen and become optically thin to the Lyman continuum in the most advanced mergers. We show that the properties of the highly blueshifted line wings on the H Lyman alpha and optical emission-line profiles are consistent with emission from clumps of gas condensing out of a fast, hot wind. The luminosity of the H Lyman alpha emission increases non-linearly with the ULIRG bolometric luminosity and represents about 0.1 to 1% of the radiative cooling from the hot winds in the HII-dominated ULIRGs.

A search for H$\alpha$ emission in high-metallicity damped Lyman-$\alpha$ systems at $z \sim 2.4$

We report on a sensitive search for redshifted H$\alpha$ line-emission from three high-metallicity damped Ly$\alpha$ absorbers (DLAs) at $z \approx 2.4$ with the Near-infrared Integral Field Spectrometer (NIFS) on the Gemini-North telescope, assisted by the ALTtitude conjugate Adaptive optics for the InfraRed (ALTAIR) system with a laser guide star. Within the NIFS field-of-view, $\approx 3.22" \times 2.92"$ corresponding to $\approx 25$ kpc $ \times 23$ kpc at $z=2.4$, we detect no statistically significant line-emission at the expected redshifted H$\alpha$ wavelengths. The measured root-mean-square noise fluctuations in $0.4"$ apertures are $1-3\times10^{-18}$ erg s$^{-1}$ cm$^{-2}$. Our analysis of simulated, compact, line-emitting sources yields stringent limits on the star-formation rates (SFRs) of the three DLAs, $< 2.2$~M$_{\odot}$ yr$^{-1}$ ($3\sigma$) for two absorbers, and $< 11$~M$_{\odot}$ yr$^{-1}$ ($3\sigma$) for the third, at all impact parameters within $\approx 12.5$~kpc to the quasar sightline at the DLA redshift. For the third absorber, the SFR limit is $< 4.4$~M$_\odot$ yr$^{-1}$ for locations away from the quasar sightline. These results demonstrate the potential of adaptive optics-assisted, integral field unit searches for galaxies associated with high-$z$ DLAs.

Quantum gravitational anomaly as a dark matter

The general properties of a perfect relativistic fluid resulting from the quantum gravitational anomaly are investigated. It is found that, in the limit of a weak gravitational field, this fluid possesses a polytropic equation of state characterized by two universal constants: the polytropic constant and the natural polytropic index. Based on the astrophysical data, the estimates for the polytropic constant are given. It is shown that this fluid can describe a considerable part of the cold dark matter. The quantum theory of such a fluid is constructed in the framework of the background field method. The Ward identities associated with the entropy and vorticity conservation laws are derived. The leading gradient corrections to the pressure of the perfect fluid are found and the restrictions on their form are obtained. These restrictions guarantee, in particular, the absence of ghosts in the model. The second order nonlinear corrections to the equations of motion of a perfect relativistic fluid are analyzed and the explicit expressions for the transverse and longitudinal perturbations induced by a sufficiently strong sound wave are obtained. A dynamical solution to the problem of time in quantum gravity is proposed.

Quantum gravitational anomaly as a dark matter [Cross-Listing]

The general properties of a perfect relativistic fluid resulting from the quantum gravitational anomaly are investigated. It is found that, in the limit of a weak gravitational field, this fluid possesses a polytropic equation of state characterized by two universal constants: the polytropic constant and the natural polytropic index. Based on the astrophysical data, the estimates for the polytropic constant are given. It is shown that this fluid can describe a considerable part of the cold dark matter. The quantum theory of such a fluid is constructed in the framework of the background field method. The Ward identities associated with the entropy and vorticity conservation laws are derived. The leading gradient corrections to the pressure of the perfect fluid are found and the restrictions on their form are obtained. These restrictions guarantee, in particular, the absence of ghosts in the model. The second order nonlinear corrections to the equations of motion of a perfect relativistic fluid are analyzed and the explicit expressions for the transverse and longitudinal perturbations induced by a sufficiently strong sound wave are obtained. A dynamical solution to the problem of time in quantum gravity is proposed.

Quantum gravitational anomaly as a dark matter [Cross-Listing]

The general properties of a perfect relativistic fluid resulting from the quantum gravitational anomaly are investigated. It is found that, in the limit of a weak gravitational field, this fluid possesses a polytropic equation of state characterized by two universal constants: the polytropic constant and the natural polytropic index. Based on the astrophysical data, the estimates for the polytropic constant are given. It is shown that this fluid can describe a considerable part of the cold dark matter. The quantum theory of such a fluid is constructed in the framework of the background field method. The Ward identities associated with the entropy and vorticity conservation laws are derived. The leading gradient corrections to the pressure of the perfect fluid are found and the restrictions on their form are obtained. These restrictions guarantee, in particular, the absence of ghosts in the model. The second order nonlinear corrections to the equations of motion of a perfect relativistic fluid are analyzed and the explicit expressions for the transverse and longitudinal perturbations induced by a sufficiently strong sound wave are obtained. A dynamical solution to the problem of time in quantum gravity is proposed.

ALMA Multi-line Imaging of the Nearby Starburst Galaxy NGC 253

We present spatially resolved ($\sim$50 pc) imaging of molecular gas species in the central kiloparsec of the nearby starburst galaxy NGC 253, based on observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA). A total of 50 molecular lines are detected over a 13 GHz bandwidth imaged in the 3 mm band. Unambiguous identifications are assigned for 27 lines. Based on the measured high CO/C$^{17}$O isotopic line ratio ($\gtrsim$350), we show that $^{12}$CO(1-0) has moderate optical depths. A comparison of the HCN and HCO$^{+}$ with their $^{13}$C-substituted isotopologues shows that the HCN(1-0) and HCO$^{+}$(1-0) lines have optical depths at least comparable to CO(1-0). H$^{13}$CN/H$^{13}$CO$^{+}$ (and H$^{13}$CN/HN$^{13}$C) line ratios provide tighter constraints on dense gas properties in this starburst. SiO has elevated abundances across the nucleus. HNCO has the most distinctive morphology of all the bright lines, with its global luminosity dominated by the outer parts of the central region. The dramatic variation seen in the HNCO/SiO line ratio suggests that some of the chemical signatures of shocked gas are being erased in the presence of dominating central radiation fields (traced by C$_{2}$H and CN). High density molecular gas tracers (including HCN, HCO$^+$, and CN) are detected at the base of the molecular outflow. We also detect hydrogen $\beta$ recombination lines that, like their $\alpha$ counterparts, show compact, centrally peaked morphologies, distinct from the molecular gas tracers. A number of sulfur based species are mapped (CS, SO, NS, C$_{2}$S, H$_{2}$CS and CH$_{3}$SH) and have morphologies similar to SiO.

A case study for a tidal interaction between dwarf galaxies in UGC 6741

We present a case study of the tidal interaction between low mass, star-forming, galaxies initially found exploring the Sloan Digital Sky Survey (SDSS) images and further analyzed with SDSS spectroscopy and UV GALEX photometry. With a luminosity of M$_{r}$ = $-$17.7 mag and exhibiting a prominent tidal filament, UGC 6741 appears as a scale down version of massive gas–rich interacting systems and mergers.The stellar disk of the smaller companion, UGC 6741_B, which is three times less massive, has likely been already destroyed. Both galaxies, which are connected by a 15 kpc long stellar bridge, have a similar oxygen abundance of 12+log(O/H)$\sim$8.3. Several knots of star-forming regions are identified along the bridge, some with masses exceeding $\sim$10$^{7}$ M$_{\sun}$. The most compact of them, which are unresolved, may evolve into globular clusters or Ultra Compact Dwarf galaxies (UCDs). This would be the first time progenitors of such objects are detected in mergers involving dwarf galaxies. UGC 6741 has currently the color and star formation properties of Blue Compact Dwarf galaxies (BCDs). However the analysis of its surface photometry suggests that the galaxy lies within the scaling relations defined by early-type dwarf galaxies (dEs). Thus UGC 6741 appears as a promising system to study the possible transformation of BCDs into dEs, through possibly a merger episode. The frequency of such dwarf-dwarf mergers should now be explored.

Constraining globular cluster formation through studies of young massive clusters - V. ALMA observations of clusters in the Antennae

Some formation scenarios that have been put forward to explain multiple populations within Globular Clusters (GCs) require that the young massive cluster have large reservoirs of cold gas within them, which is necessary to form future generations of stars. In this paper we use deep observations taken with Atacama Large Millimeter/sub-millimeter Array (ALMA) to assess the amount of molecular gas within 3 young (50-200 Myr) massive (~10^6 Msun) clusters in the Antennae galaxies. No significant CO(3–2) emission was found associated with any of the three clusters. We place upper limits for the molecular gas within these clusters of ~1×10^5 Msun (or <9 % of the current stellar mass). We briefly review different scenarios that propose multiple episodes of star formation and discuss some of their assumptions and implications. Our results are in tension with the predictions of GC formation scenarios that expect large reservoirs of cool gas within young massive clusters at these ages.

Towards simulating star formation in turbulent high-z galaxies with mechanical supernova feedback

Feedback from supernovae is essential to understanding the self-regulation of star formation in galaxies. However, the efficacy of the process in a cosmological context remains unclear due to excessive radiative losses during the shock propagation. To better understand the impact of SN explosions on the evolution of galaxies, we perform a suite of high-resolution (12 pc), zoom-in cosmological simulations of a Milky Way-like galaxy at z=3 with adaptive mesh refinement. We find that SN explosions can efficiently regulate star formation, leading to the stellar mass and metallicity consistent with the observed mass-metallicity relation and stellar mass-halo mass relation at z~3. This is achieved by making three important changes to the classical feedback scheme: i) the different phases of SN blast waves are modelled directly by injecting radial momentum expected at each stage, ii) the realistic time delay of SNe, commencing at as early as 3 Myr, is required to disperse very dense gas before a runaway collapse sets in at the galaxy centre via mergers of gas clumps, and iii) a non-uniform density distribution of the ISM is taken into account below the computational grid scale for the cell in which SN explodes. The last condition is motivated by the fact that our simulations still do not resolve the detailed structure of a turbulent ISM in which the fast outflows can propagate along low-density channels. The simulated galaxy with the SN feedback model shows strong outflows, which carry approximately ten times larger mass than star formation rate, as well as smoothly rising circular velocity. Other feedback models that do not meet the three conditions form too many stars, producing a peaked rotation curve. Our results suggest that understanding the structure of the turbulent ISM may be crucial to assess the role of SN and other feedback processes in galaxy formation theory. [abridged]

The effect of dark matter resolution on the collapse of baryons in high redshift numerical simulations

We examine the impact of dark matter particle resolution on the formation of a baryonic core in high resolution adaptive mesh refinement simulations. We test the effect that both particle smoothing and particle splitting have on the hydrodynamic properties of a collapsing halo at high redshift (z > 20). Furthermore, we vary the background field intensity, with energy below the Lyman limit (< 13.6 eV), as may be relevant for the case of metal-free star formation and super-massive black hole seed formation. We find that using particle splitting methods greatly increases our particle resolution without introducing any numerical noise and allows us to achieve converged results over a wide range of external background fields. Furthermore, we find that for lower values of the background field a lower dark matter particle mass is required. We use the characteristic Jeans length of the gas to define the core of a collapsing halo, $\rm{R_{core} \lesssim 1\ pc}$ for T $\lesssim 8000$ K, and number density, $\rm{n \sim 1 \times 10^6\ cm^{-3}}$. We find that in order to produce converged results which are not affected by dark matter particles requires that the relationship ${M_{\rm{core}} / M_{\rm{DM}}} > 100.0$ be satisfied, where ${M_{\rm{core}}}$ is the enclosed baryon mass within the core and $M_{\rm{DM}}$ is the minimum dark matter particle mass. This ratio should provide a very useful starting point for conducting convergence tests before any production run simulations. We find that dark matter particle smoothing is a useful adjunct to already highly resolved simulations.

Galaxy formation in the Planck cosmology III: star-formation histories and post-processing magnitude reconstruction

We adapt the L-Galaxies semi-analytic model to follow the star-formation histories (SFH) of galaxies — by which we mean a record of the formation time and metallicities of the stars that are present in each galaxy at a given time. We use these to construct stellar spectra in post-processing, which offers large efficiency savings and allows user-defined spectral bands and dust models to be applied to data stored in the Millennium data repository. We contrast model SFHs from the Millennium Simulation with observed ones from the VESPA algorithm as applied to the SDSS-7 catalogue. The overall agreement is good, with both simulated and SDSS galaxies showing a steeper SFH with increased stellar mass. The SFHs of blue and red galaxies, however, show poor agreement between data and simulations, which may indicate that the termination of star formation is too abrupt in the models. The mean star-formation rate (SFR) of model galaxies is well-defined and is accurately modelled by a double power law at all redshifts: SFR proportional to 1/(x^{-1.39}+x^{1.33}), where x=(T-t)/3.0 Gyr, t is the age of the stars and T is the loopback time to the onset of galaxy formation; above a redshift of unity, this is well approximated by a gamma function: SFR proportional to x^{1.5}e^{-x}, where x=(T-t)/2.0 Gyr. Individual galaxies, however, show a wide dispersion about this mean. When split by mass, the SFR peaks earlier for high-mass galaxies than for lower-mass ones, and we interpret this downsizing as a mass-dependence in the evolution of the quenched fraction: the SFHs of star-forming galaxies show only a weak mass dependence.

Spatially extended and high-velocity dispersion molecular component in spiral galaxies: single-dish vs. interferometric observations

Recent studies of the molecular medium in nearby galaxies have provided mounting evidence that the molecular gas can exist in two phases: one that is clumpy and organized as molecular clouds and another one that is more diffuse. This last component has a higher velocity dispersion than the clumpy one. In order to investigate these two molecular components further, we compare the fluxes and line widths of CO in NGC 4736 and NGC 5055, two nearby spiral galaxies for which high-quality interferometric as well as single-dish data sets are available. Our analysis leads to two main results: 1) Employing three different methods, we determine the flux recovery of the interferometer as compared to the single-dish to be within a range of 35-74% for NGC4736 and 81-92% for NGC5055, and 2) when focusing on high (SNR>5) lines of sight, the single-dish line widths are larger by ~(40+-20)% than the ones derived from interferometric data; which is in agreement with stacking all lines of sight. These results point to a molecular gas component that is distributed over spatial scales larger than 30"(~1kpc), and is therefore filtered out by the interferometer. The available observations do not allow us to distinguish between a truly diffuse gas morphology and a uniform distribution of small clouds that are separated by less than the synthesized beam size (~3" or ~100pc), as they would both be invisible for the interferometer. This high velocity dispersion component has a dispersion similar to what is found in the atomic medium, as traced through observations of the HI line.

Stacking of SKA data: comparing uv-plane and image-plane stacking

Stacking as a tool for studying objects that are not individually detected is becoming popular even for radio interferometric data, and will be widely used in the SKA era. Stacking is typically done using imaged data rather than directly using the visibilities (the uv-data). We have investigated and developed a novel algorithm to do stacking using the uv-data. We have performed exten- sive simulations comparing to image-stacking, and summarize the results of these simulations. Furthermore, we disuss the implications in light of the vast data volume produced by the SKA. Having access to the uv-stacked data provides a great advantage, as it allows the possibility to properly analyse the result with respect to calibration artifacts as well as source properties such as size. For SKA the main challenge lies in archiving the uv-data. For purposes of robust stacking analysis, it would be strongly desirable to either keep the calibrated uv-data at least in an aver- age form, or implement a stacking queue where stacking positions could be provided prior to the observations and the uv-stacking is done almost in real time.

The Structure of Nuclear Star Clusters in Nearby Late-type Spiral Galaxies from Hubble Space Telescope Wide Field Camera 3 Imaging [Replacement]

We obtained Hubble Space Telescope/Wide Field Camera 3 imaging of a sample of ten of the nearest and brightest nuclear clusters residing in late-type spiral galaxies, in seven bands that span the near-ultraviolet to the near-infrared. Structural properties of the clusters were measured by fitting two-dimensional surface brightness profiles to the images using GALFIT. The clusters exhibit a wide range of structural properties. For six of the ten clusters in our sample, we find changes in the effective radius with wavelength, suggesting radially varying stellar populations. In four of the objects, the effective radius increases with wavelength, indicating the presence of a younger population which is more concentrated than the bulk of the stars in the cluster. However, we find a general decrease in effective radius with wavelength in two of the objects in our sample, which may indicate extended, circumnuclear star formation. We also find a general trend of increasing roundness of the clusters at longer wavelengths, as well as a correlation between the axis ratios of the NCs and their host galaxies. These observations indicate that blue disks aligned with the host galaxy plane are a common feature of nuclear clusters in late-type galaxies, but are difficult to detect in galaxies that are close to face-on. In color-color diagrams spanning the near-UV through the near-IR, most of the clusters lie far from single-burst evolutionary tracks, showing evidence for multi-age populations. Most of the clusters have integrated colors consistent with a mix of an old population (> 1 Gyr) and a young population (~100-300 Myr). The wide wavelength coverage of our data provides a sensitivity to populations with a mix of ages that would not be possible to achieve with imaging in optical bands only.

The Structure of Nuclear Star Clusters in Nearby Late-type Spiral Galaxies from Hubble Space Telescope Wide Field Camera 3 Imaging

We obtained Hubble Space Telescope/Wide Field Camera 3 imaging of a sample of ten of the nearest and brightest nuclear clusters residing in late-type spiral galaxies, in seven bands that span the near-ultraviolet to the near-infrared. Structural properties of the clusters were measured by fitting two-dimensional surface brightness profiles to the images using GALFIT. The clusters exhibit a wide range of structural properties. For six of the ten clusters in our sample, we find changes in the effective radius with wavelength, suggesting radially varying stellar populations. In four of the objects, the effective radius increases with wavelength, indicating the presence of a younger population which is more concentrated than the bulk of the stars in the cluster. However, we find a general decrease in effective radius with wavelength in two of the objects in our sample, which may indicate extended, circumnuclear star formation. We also find a general trend of increasing roundness of the clusters at longer wavelengths, as well as a correlation between the axis ratios of the NCs and their host galaxies. These observations indicate that blue disks aligned with the host galaxy plane are a common feature of nuclear clusters in late-type galaxies, but are difficult to detect in galaxies that are close to face-on. In color-color diagrams spanning the near-UV through the near-IR, most of the clusters lie far from single-burst evolutionary tracks, showing evidence for multi-age populations. Most of the clusters have integrated colors consistent with a mix of an old population (> 1 Gyr) and a young population (~100-300 Myr). The wide wavelength coverage of our data provides a sensitivity to populations with a mix of ages that would not be possible to achieve with imaging in optical bands only.

Effective power-law dependence of Lyapunov exponents on the central mass in galaxies

Using both numerical and analytical approaches, we demonstrate the existence of an effective power-law relation $L\propto m^p$ between the mean Lyapunov exponent $L$ of stellar orbits chaotically scattered by a supermassive black hole in the center of a galaxy and the mass parameter $m$, i.e. ratio of the mass of the black hole over the mass of the galaxy. The exponent $p$ is found numerically to obtain values in the range $p \approx 0.3$–$0.5$. We propose a theoretical interpretation of these exponents, based on estimates of local `stretching numbers’, i.e. local Lyapunov exponents at successive transits of the orbits through the black hole’s sphere of influence. We thus predict $p=2/3-q$ with $q\approx 0.1$–$0.2$. Our basic model refers to elliptical galaxy models with a central core. However, we find numerically that an effective power law scaling of $L$ with $m$ holds also in models with central cusp, beyond a mass scale up to which chaos is dominated by the influence of the cusp itself. We finally show numerically that an analogous law exists also in disc galaxies with rotating bars. In the latter case, chaotic scattering by the black hole affects mainly populations of thick tube-like orbits surrounding some low-order branches of the $x_1$ family of periodic orbits, as well as its bifurcations at low-order resonances, mainly the Inner Lindbland resonance and the 4/1 resonance. Implications of the correlations between $L$ and $m$ to determining the rate of secular evolution of galaxies are discussed.

Spectro-photometric distances to stars: a general-purpose Bayesian approach

We have developed a procedure that estimates distances to stars using measured spectroscopic and photometric quantities. It employs a Bayesian approach to build the probability distribution function over stellar evolutionary models given the data, delivering estimates of expected distance for each star individually. Our method provides several alternative distance estimates for each star in the output, along with their associated uncertainties. The code was first tested on simulations, successfully recovering input distances to mock stars with errors that scale with the uncertainties in the adopted spectro-photometric parameters, as expected. The code was then validated by comparing our distance estimates to parallax measurements from the Hipparcos mission for nearby stars (< 60 pc), to asteroseismic distances of CoRoT red giant stars, and to known distances of well-studied open and globular clusters. The photometric data of these reference samples cover both the optical and near infra-red wavelengths. The spectroscopic parameters are also based on spectra taken at various wavelengths, with varying spectral coverage and resolution: the Radial Velocity Experiment, the Sloan Digital Sky Survey programs SEGUE and APOGEE, and the ESO HARPS instrument. For Hipparcos and CoRoT samples, the typical random distance scatter is 20% or less, both for the nearby and farther data. There is a trend towards underestimating the distances by < 10%. The comparison to star clusters from SEGUE and APOGEE has led to systematic differences < 5% for most cluster stars although with significant scatter. Finally, we tested our distances against those previously determined for a high quality sample of giant stars from the RAVE survey, again finding a reasonable agreement, with only a small systematic trend. Efforts are underway to provide our code to the community by running it on a public server.

Bar pattern speeds in CALIFA galaxies: I. Fast bars across the Hubble sequence

The bar pattern speed ($\Omega_{\rm b}$) is defined as the rotational frequency of the bar, and it determines the bar dynamics. Several methods have been proposed for measuring $\Omega_{\rm b}$. The non-parametric method proposed by Tremaine \& Weinberg (1984; TW) and based on stellar kinematics is the most accurate. This method has been applied so far to 17 galaxies, most of them SB0 and SBa types. We have applied the TW method to a new sample of 15 strong and bright barred galaxies, spanning a wide range of morphological types from SB0 to SBbc. Combining our analysis with previous studies, we investigate 32 barred galaxies with their pattern speed measured by the TW method. The resulting total sample of barred galaxies allows us to study the dependence of $\Omega_{\rm b}$ on galaxy properties, such as the Hubble type. We measured $\Omega_{\rm b}$ using the TW method on the stellar velocity maps provided by the integral-field spectroscopy data from the CALIFA survey. Integral-field data solve the problems that long-slit data present when applying the TW method, resulting in the determination of more accurate $\Omega_{\rm b}$. In addition, we have also derived the ratio $\cal{R}$ of the corotation radius to the bar length of the galaxies. According to this parameter, bars can be classified as fast ($\cal{R}$ $< 1.4$) and slow ($\cal{R}$>1.4). For all the galaxies, $\cal{R}$ is compatible within the errors with fast bars. We cannot rule out (at 95$\%$ level) the fast bar solution for any galaxy. We have not observed any significant trend between $\cal{R}$ and the galaxy morphological type. Our results indicate that independent of the Hubble type, bars have been formed and then evolve as fast rotators. This observational result will constrain the scenarios of formation and evolution of bars proposed by numerical simulations.

Dust-regulated galaxy formation and evolution:A new chemodynamical model with live dust particles

Interstellar dust plays decisive roles in the conversion of neutral to molecular hydrogen (H_2), the thermodynamical evolution of interstellar medium (ISM), and the modification of spectral energy distributions (SEDs) of galaxies. These important roles of dust have not been self-consistently included in previous numerical simulations of galaxy formation and evolution. We have therefore developed a new model by which one can investigate whether and how galaxy formation and evolution can be influenced by dust-related physical processes such as photo-electric heating, H_2 formation on dust, and stellar radiation pressure on dust in detail. A novel point of the model is that different dust species in a galaxy are represented by `live dust’ particles (i.e., not test particles). Therefore, dust particles in a galaxy not only interact gravitationally with all four components of the galaxy (i.e., dark matter, stars, gas, and dust) but also are grown and destroyed through physical processes of ISM. First we describe a way to include dust-related physical processes in Nbody+hydrodynamical simulations of galaxy evolution in detail. Then we show some preliminary results of dust-regulated galaxy evolution. The preliminary results suggest that the evolution of dust distributions driven by radiation pressure of stars is very important for the evolution of star formation rates, chemical abundances, H_2 fractions, and gas distributions in galaxies.

Theoretical Derivation And Statistical Result of $\Sigma$-$D$ Relation for Shell-Type Galactic Supernova Remnant

Theoretical derivation of the relation between radio surface brightness~($\Sigma$) and diameter~($D$) for shell-type galactic supernova remnant (SNR) at the adiabatic phase and radiative phase is investigated respectively in our paper. We find that a transition point exists in 30~pc between these two $\Sigma$-$D$ relations, which can be consistent with the statistical results made by other authors. In addition, we also obtain the statistical result of $\Sigma$-$D$ relation on 57 shell-type galactic remnants, which suggests that the best fit line of the $\Sigma$-$D$ relation should be slightly flatter than those proposed by some other authors before. An extra interesting result is that our theoretical derivation also implicates that a new state may exist between the adiabatic phase and radiative phase.

Detection of a methanol megamaser in a major-merger galaxy

We have detected emission from both the 4_{-1}-3_{0} E (36.2~GHz) class I and 7_{-2}-8_{-1} E (37.7~GHz) class II methanol transitions towards the centre of the closest ultra-luminous infrared galaxy Arp 220. The emission in both the methanol transitions show narrow spectral features and have luminosities approximately 8 orders of magnitude stronger than that observed from typical class I methanol masers observed in Galactic star formation regions. The emission is also orders of magnitude stronger than the expected intensity of thermal emission from these transitions and based on these findings we suggest that the emission from the two transitions are masers. These observations provides the first detection of a methanol megamaser in the 36.2 and 37.7 GHz transitions and represents only the second detection of a methanol megamaser, following the recent report of an 84 GHz methanol megamaser in NGC1068. We find the methanol megamasers are significantly offset from the nuclear region and arise towards regions where there is Ha emission, suggesting that it is associated with starburst activity. The high degree of correlation between the spatial distribution of the 36.2 GHz methanol and X-ray plume emission suggests that the production of strong extragalactic class I methanol masers is related to galactic outflow driven shocks and perhaps cosmic rays. In contrast to OH and H2O megamasers which originate close to the nucleus, methanol megamasers provide a new probe of feedback (e.g. outflows) processes on larger-scales and of star formation beyond the circumnuclear starburst regions of active galaxies.

The VMC Survey - XIV. First results on the look-back time star-formation rate tomography of the Small Magellanic Cloud

We analyse deep images from the VISTA survey of the Magellanic Clouds in the YJKs filters, covering 14 sqrdeg (10 tiles), split into 120 subregions, and comprising the main body and Wing of the Small Magellanic Cloud (SMC). We apply a colour–magnitude diagram reconstruction method that returns their best-fitting star formation rate SFR(t), age-metallicity relation (AMR), distance and mean reddening, together with 68% confidence intervals. The distance data can be approximated by a plane tilted in the East-West direction with a mean inclination of 39 deg, although deviations of up to 3 kpc suggest a distorted and warped disk. After assigning to every observed star a probability of belonging to a given age-metallicity interval, we build high-resolution population maps. These dramatically reveal the flocculent nature of the young star-forming regions and the nearly smooth features traced by older stellar generations. They document the formation of the SMC Wing at ages <0.2 Gyr and the peak of star formation in the SMC Bar at 40 Myr. We clearly detect periods of enhanced star formation at 1.5 Gyr and 5 Gyr. The former is possibly related to a new feature found in the AMR, which suggests ingestion of metal-poor gas at ages slightly larger than 1 Gyr. The latter constitutes a major period of stellar mass formation. We confirm that the SFR(t) was moderately low at even older ages.

NANOGrav Constraints on Gravitational Wave Bursts with Memory [Cross-Listing]

Among efforts to detect gravitational radiation, pulsar timing arrays are uniquely poised to detect "memory" signatures, permanent perturbations in spacetime from highly energetic astrophysical events such as mergers of supermassive black hole binaries. The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) observes dozens of the most stable millisecond pulsars using the Arecibo and Green Bank radio telescopes in an effort to study, among other things, gravitational wave memory. We herein present the results of a search for gravitational wave bursts with memory (BWMs) using the first five years of NANOGrav observations. We develop original methods for dramatically speeding up searches for BWM signals. In the directions of the sky where our sensitivity to BWMs is best, we would detect mergers of binaries with reduced masses of $10^9$ $M_\odot$ out to distances of 30 Mpc; such massive mergers in the Virgo cluster would be marginally detectable. We find no evidence for BWMs. However, with our non-detection, we set upper limits on the rate at which BWMs of various amplitudes could have occurred during the time spanned by our data–e.g., BWMs with amplitudes greater than $10^{-13}$ must occur at a rate less than 1.5 yr$^{-1}$.

NANOGrav Constraints on Gravitational Wave Bursts with Memory

Among efforts to detect gravitational radiation, pulsar timing arrays are uniquely poised to detect "memory" signatures, permanent perturbations in spacetime from highly energetic astrophysical events such as mergers of supermassive black hole binaries. The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) observes dozens of the most stable millisecond pulsars using the Arecibo and Green Bank radio telescopes in an effort to study, among other things, gravitational wave memory. We herein present the results of a search for gravitational wave bursts with memory (BWMs) using the first five years of NANOGrav observations. We develop original methods for dramatically speeding up searches for BWM signals. In the directions of the sky where our sensitivity to BWMs is best, we would detect mergers of binaries with reduced masses of $10^9$ $M_\odot$ out to distances of 30 Mpc; such massive mergers in the Virgo cluster would be marginally detectable. We find no evidence for BWMs. However, with our non-detection, we set upper limits on the rate at which BWMs of various amplitudes could have occurred during the time spanned by our data–e.g., BWMs with amplitudes greater than $10^{-13}$ must occur at a rate less than 1.5 yr$^{-1}$.

Passive galaxies as tracers of cluster environments at z~2

Even 10 billion years ago, the cores of the first galaxy clusters are often found to host a characteristic population of massive galaxies with already suppressed star formation. Here we search for distant cluster candidates at z~2 using massive passive galaxies as tracers. With a sample of ~40 spectroscopically confirmed passive galaxies at 1.3<z<2.1, we tune photometric redshifts of several thousands passive sources in the full 2 sq.deg. COSMOS field. This allows us to map their density in redshift slices, probing the large scale structure in the COSMOS field as traced by passive sources. We report here on the three strongest passive galaxy overdensities that we identify in the redshift range 1.5<z<2.5. While the actual nature of these concentrations is still to be confirmed, we discuss their identification procedure, and the arguments supporting them as candidate galaxy clusters (likely mid-10^13 M_sun range). Although this search approach is likely biased towards more evolved structures, it has the potential to select still rare, cluster-like environments close to their epoch of first appearance, enabling new investigations of the evolution of galaxies in the context of structure growth.

Observations of the Icy Universe

Freeze-out of the gas phase elements onto cold grains in dense interstellar and circumstellar media builds up ice mantles consisting of molecules that are mostly formed in situ (H2O, NH3, CO2, CO, CH3OH, and more). This review summarizes the detected infrared spectroscopic ice features and compares the abundances across Galactic, extragalactic, and solar system environments. A tremendous amount of information is contained in the ice band profiles. Laboratory experiments play a critical role in the analysis of the observations. Strong evidence is found for distinct ice formation stages, separated by CO freeze out at high densities. The ice bands have proven to be excellent probes of the thermal history of their environment. The evidence for the long-held idea that processing of ices by energetic photons and cosmic rays produces complex molecules is weak. Recent state of the art observations show promise for much progress in this area with planned infrared facilities.

An Emerging Wolf-Rayet Massive Star Cluster in NGC 4449

We present a panchromatic investigation of the partially-embedded, emerging massive cluster Source 26 (= S26) in NGC 4449 with optical spectra obtained at Apache Point Observatory and archival Hubble, Spitzer, and Herschel Space Telescope images. First identified as a radio continuum source with a thermal component due to ionized material, the massive cluster S26 also exhibits optical Wolf-Rayet (WR) emission lines that reveal a large evolved massive star population. We find that S26 is host to $\sim$240 massive stars, of which $\sim$18 are Wolf-Rayet stars; the relative populations are roughly consistent with other observed massive star forming clusters and galaxies. We construct SEDs over two spatial scales (roughly 100 pc and 300 pc) that clearly exhibit warm dust and polycyclic aromatic hydrocarbon (PAH) emission. The best fit dust and grain models reveal that both the intensity of the exciting radiation and PAH grain destruction increase toward the cluster center. Given that the timescale of evacuation is important for the future dynamical evolution of the cluster, it is important to determine whether O- and WR stars can evacuate the material gradually before supernova do so on a much faster timescale. With a minimum age of $\approx$ 3 Myr, it is clear that S26 has not yet fully evacuated its natal material, which indicates that unevolved O-type stars alone do not provide sufficient feedback to remove the gas and dust. We hypothesize that the feedback of WR stars in this cluster may be necessary for clearing the material from the gravitational potential of the cluster. We find S26 is similar to Emission Line Clusters observed in the Antennae Galaxies and may be considered a younger analog to 30 Doradus in the LMC.

Overturning the Case for Gravitational Powering in the Prototypical Cooling Lyman-alpha Nebula

The Nilsson et al. (2006) Lyman-alpha nebula has often been cited as the most plausible example of a Lyman-alpha nebula powered by gravitational cooling. In this paper, we bring together new data from the Hubble Space Telescope and the Herschel Space Observatory as well as comparisons to recent theoretical simulations in order to revisit the questions of the local environment and most likely power source for the Lyman-alpha nebula. In contrast to previous results, we find that this Lyman-alpha nebula is associated with 6 nearby galaxies and an obscured AGN that is offset by $\sim$4"$\approx$30 kpc from the Lyman-alpha peak. The local region is overdense relative to the field, by a factor of $\sim$10, and at low surface brightness levels the Lyman-alpha emission appears to encircle the position of the obscured AGN, highly suggestive of a physical association. At the same time, we confirm that there is no compact continuum source located within $\sim$2-3"$\approx$15-23 kpc of the Lyman-alpha peak. Since the latest cold accretion simulations predict that the brightest Lyman-alpha emission will be coincident with a central growing galaxy, we conclude that this is actually a strong argument against, rather than for, the idea that the nebula is gravitationally-powered. While we may be seeing gas within cosmic filaments, this gas is primarily being lit up, not by gravitational energy, but due to illumination from a nearby buried AGN.

Uncertainties in The Interstellar Extinction Curve and the Cepheid Distance to M101

I revisit the Cepheid-distance determination to the nearby spiral galaxy M101 (Pinwheel Galaxy) of Shappee & Stanek (2011), in light of several recent investigations questioning the shape of the interstellar extinction curve at $\lambda \approx 8,000$ \AA (i.e. I-band). I find that the relatively steep extinction ratio $A_{I}/E(V-I)=1.1450$ (Fitzpatrick & Massa 2007) is slightly favoured relative to $A_{I}/E(V-I)=1.2899$ (Fitzpatrick 1999) and significantly favoured relative the historically canonical value of $A_{I}/E(V-I)=1.4695$ (Cardelli et al. 1989). The steeper extinction curves, with lower values of $A_{I}/E(V-I)$, yield fits with reduced scatter, metallicity-dependences to the dereddened Cepheid luminosities that are closer to values inferred in the local group, and that are less sensitive to the choice of reddening cut imposed in the sample selection. The increase in distance modulus to M101 when using the preferred extinction curve is ${\Delta}{\mu} \sim 0.06$ mag, resulting in an estimate of the distance modulus to M101 relative to the LMC of $ {\Delta}\mu_{\rm{LMC}} \approx 10.72 \pm 0.03$ (stat). The best-fit metallicity-dependence is $dM_{I}/d\rm{[O/H]} \approx (-0.38 \pm 0.14$ (stat)) mag dex$^{-1}$.

The Spatial Structure of Young Stellar Clusters. II. Total Young Stellar Populations

We investigate the intrinsic stellar populations (estimated total numbers of OB and pre-main-sequence stars down to 0.1 Mo) that are present in 17 massive star-forming regions (MSFRs) surveyed by the MYStIX project. The study is based on the catalog of >31,000 MYStIX Probable Complex Members with both disk-bearing and disk-free populations, compensating for extinction, nebulosity, and crowding effects. Correction for observational sensitivities is made using the X-ray Luminosity Function (XLF) and the near-infrared Initial Mass Function (IMF)–a correction that is often not made by infrared surveys of young stars. The resulting maps of the projected structure of the young stellar populations, in units of intrinsic stellar surface density, allow direct comparison between different regions. Several regions have multiple dense clumps, similar in size and density to the Orion Nebula Cluster. The highest projected density of ~34,000 stars/pc^2 is found in the core of the RCW38 cluster. Histograms of surface density show different ranges of values in different regions, supporting the conclusion of Bressert et al. (2010, B10) that no universal surface-density threshold can distinguish between clustered and distributed star-formation. However, a large component of the young stellar population of MSFRs resides in dense environments of 200-10,000 stars/pc^2 (including within the nearby Orion molecular clouds), and we find that there is no evidence for the B10 conclusion that such dense regions form an extreme "tail" of the distribution. Tables of intrinsic populations for these regions are used in our companion study of young cluster properties and evolution.

Kiloparsec-scale jets in three radio-loud narrow-line Seyfert 1 galaxies

We have discovered kiloparsec-scale extended radio emission in three narrow-line Seyfert 1 galaxies (NLS1s) in sub-arcsecond resolution 9 GHz images from the Karl G. Jansky Very Large Array (VLA). We find all sources show two-sided, mildly core-dominated jet structures with diffuse lobes dominated by termination hotspots. These span 20-70 kpc with morphologies reminiscent of FR II radio galaxies, while the extended radio luminosities are intermediate between FR I and FR II sources. In two cases the structure is linear, while a $45^{\circ}$ bend is apparent in the third. Very Long Baseline Array images at 7.6 GHz reveal parsec-scale jet structures, in two cases with extended structure aligned with the inner regions of the kiloparsec-scale jets. Based on this alignment, the ratio of the radio core luminosity to the optical luminosity, the jet/counter-jet intensity and extension length ratios, and moderate core brightness temperatures ($\lesssim10^{10}$ K), we conclude these jets are mildly relativistic ($\beta\lesssim0.3$, $\delta\sim1$-$1.5$) and aligned at moderately small angles to the line of sight (10-15$^{\circ}$). The derived kinematic ages of $\sim10^6$-$10^7$ y are much younger than radio galaxies but comparable to other NLS1s. Our results increase the number of radio-loud NLS1s with known kiloparsec-scale extensions from seven to ten and suggest that such extended emission may be common, at least among the brightest of these sources.

The rms-flux relation in accreting white dwarfs: another nova-like variable and the first dwarf nova

We report on the detection of the linear rms-flux relation in two accreting white dwarf binary systems: V1504 Cyg and KIC 8751494. The rms-flux relation relates the absolute root-mean-square (rms) variability of the light curve to its mean flux. The light curves analysed were obtained with the Kepler satellite at a 58.8 s cadence. The rms-flux relation was previously detected in only one other cataclysmic variable, MV Lyr. This result reenforces the ubiquity of the linear rms-flux relation as a characteristic property of accretion-induced variability, since it has been observed in several black hole binaries, neutron star binaries and active galactic nuclei. Moreover, its detection in V1504 Cyg is the first time the rms-flux relation has been detected in a dwarf nova-type CV during quiescence. This result, together with previous studies, hence points towards a common physical origin of accretion-induced variability, independent of the size, mass, or type of the central accreting compact object.

On the uniqueness of kinematical signatures of intermediate-mass black holes in globular clusters

Finding an intermediate-mass black hole (IMBH) in a globular cluster (GC), or proving its absence, is a crucial ingredient in our understanding of galaxy formation and evolution. The challenge is to identify a unique signature of an IMBH that cannot be accounted for by other processes. Observational claims of IMBH detection are often based on analyses of the kinematics of stars, such as a rise in the velocity dispersion profile towards the centre. In this contribution we discuss the degeneracy between this IMBH signal and pressure anisotropy in the GC. We show that that by considering anisotropic models it is possible to partially explain the innermost shape of the projected velocity dispersion profile, even though models that do not account for an IMBH do not exhibit a cusp in the centre.

Probabilities for Solar Siblings

We have shown previously (Bobylev et al 2011) that some of the stars in the Solar neighborhood today may have originated in the same star cluster as the Sun, and could thus be called Solar Siblings. In this work we investigate the sensitivity of this result to Galactic models and to parameters of these models, and also extend the sample of orbits. There are a number of good candidates for the Sibling category, but due to the long period of orbit evolution since the break-up of the birth cluster of the Sun, one can only attach probabilities of membership. We find that up to 10% (but more likely around 1 %) of the members of the Sun’s birth cluster could be still found within 100 pc from the Sun today.

Circularization of Tidally Disrupted Stars around Spinning Supermassive Black Holes

We study the circularization of tidally disrupted stars on bound orbits around spinning supermassive black holes by performing three-dimensional smoothed particle hydrodynamic simulations with Post-Newtonian corrections. Our simulations reveal that debris circularization depends sensitively on the efficiency of radiative cooling. There are two stages in debris circularization if radiative cooling is inefficient: first, the stellar debris streams self-intersect due to relativistic apsidal precession; shocks at the intersection points thermalize orbital energy and the debris forms a geometrically thick, ring-like structure around the black hole. The ring rapidly spreads via viscous diffusion, leading to the formation of a geometrically thick accretion disk. In contrast, if radiative cooling is efficient, the stellar debris circularizes due to self-intersection shocks and forms a geometrically thin ring-like structure. In this case, the dissipated energy can be emitted during debris circularization as a precursor to the subsequent tidal disruption flare. The possible radiated energy is up to ~2*10^{52} erg for a 1 Msun star orbiting a 10^6 Msun black hole. We also find that a retrograde (prograde) black hole spin causes the shock-induced circularization timescale to be shorter (longer) than that of a non-spinning black hole in both cooling cases. The circularization timescale is remarkably long in the radiatively efficient cooling case, and is also sensitive to black hole spin. Specifically, Lense-Thirring torques cause dynamically important nodal precession, which significantly delays debris circularization. On the other hand, nodal precession is too slow to produce observable signatures in the radiatively inefficient case. We also discuss the relationship between our simulations and the parabolic TDEs that are characteristic of most stellar tidal disruptions.

Oxygen Abundance Measurements of SHIELD Galaxies

We have derived oxygen abundances for 8 galaxies from the Survey of HI in Extremely Low-mass Dwarfs (SHIELD). The SHIELD survey is an ongoing study of very low-mass galaxies, with M$_{\rm HI}$ between 10$^{6.5}$ and 10$^{7.5}$ M$_{\odot}$, that were detected by the Arecibo Legacy Fast ALFA (ALFALFA) survey. H$\alpha$ images from the WIYN 3.5m telescope show that these 8 SHIELD galaxies each possess one or two active star-forming regions which were targeted with long-slit spectral observations using the Mayall 4m telescope at KPNO. We obtained a direct measurement of the electron temperature by detection of the weak [O III] $\lambda$4363 line in 2 of the HII regions. Oxygen abundances for the other HII regions were estimated using a strong-line method. When the SHIELD galaxies are plotted on a B-band luminosity-metallicity diagram they appear to suggest a slightly shallower slope to the relationship than normally seen. However, that offset is systematically reduced when the near-infrared luminosity is used instead. This indicates a different mass-to-light ratio for the galaxies in this sample and we suggest this may be indicative of differing star-formation histories in the lowest luminosity and surface brightness dwarf irregulars.

 

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