We estimate the star formation efficiency per gravitational free-fall time, epsilon(ff), from observations of nearby galaxies with resolution matched to the typical size of a giant molecular cloud. This quantity, epsilon(ff), is theoretically important but so far has only been measured for Milky Way clouds or inferred indirectly in a few other galaxies. Using new, high-resolution CO imaging from the Physics at High Angular Resolution in nearby Galaxies-Atacama Large Millimeter Array (PHANGS-ALMA) survey, we estimate the gravitational free-fall time at 60-120 pc resolution, and contrast this with the local molecular gas depletion time in order to estimate epsilon(ff). Assuming a constant thickness of the molecular gas layer (H = 100 pc) across the whole sample, the median value of epsilon(ff) in our sample is 0.7%. We find a mild scale dependence, with higher epsilon(ff) measured at coarser resolution. Individual galaxies show different values of epsilon(ff), with the median epsilon(ff) ranging from 0.3% to 2.6%. We find the highest epsilon(ff) in our lowest-mass targets, reflecting both long free-fall times and short depletion times, though we caution that both measurements are subject to biases in low-mass galaxies. We estimate the key systematic uncertainties, and show the dominant uncertainty to be the estimated line-of-sight (LOS) depth through the molecular gas layer and the choice of star formation tracers.

Star formation is a multi-scale process that requires tracing cloud formation and stellar feedback within the local (less than or similar to kpc) and global galaxy environment. We present first results from two large observing programs on the Atacama Large Millimeter/submillimeter Array (ALMA) and the Very Large Telescope/Multi Unit Spectroscopic Explorer (VLT/MUSE), mapping cloud scales (1 '' = 47 pc) in both molecular gas and star-forming tracers across 90 kpc(2) of the central disk of NGC 628 to probe the physics of star formation. Systematic spatial offsets between molecular clouds and H II regions illustrate the time evolution of star-forming regions. Using uniform sampling of both maps on 50-500 pc scales, we infer molecular gas depletion times of 1-3 Gyr, but also find that the increase of scatter in the star formation relation on small scales is consistent with gas and H II regions being only weakly correlated at the cloud (50 pc) scale. This implies a short overlap phase for molecular clouds and H II regions, which we test by directly matching our catalog of 1502 H II regions and 738 GMCs. We uncover only 74 objects in the overlap phase, and we find depletion times > 1 Gyr, significantly longer than previously reported for individual star-forming clouds in the Milky Way. Finally, we find no clear trends that relate variations in the depletion time observed on 500 pc scales to physical drivers (metallicity, molecular and stellar-mass surface density, molecular gas boundedness) on 50 pc scales.

NebulaBayes is a new Bayesian code that implements a general method of comparing observed emission-line fluxes to photoionization model grids. The code enables us to extract robust, spatially resolved measurements of abundances in the extended narrow-line regions (ENLRs) produced by Active Galactic Nuclei (AGN). We observe near-constant ionization parameters but steeply radially declining pressures, which together imply that radiation pressure regulates the ENLR density structure on large scales. Our sample includes four "pure Seyfert" galaxies from the S7 survey that have extensive ENLRs. NGC 2992 shows steep metallicity gradients from the nucleus into the ionization cones. An inverse metallicity gradient is observed in ESO 138-G01, which we attribute to a recent gas inflow or minor merger. A uniformly high metallicity and hard ionizing continuum are inferred across the ENLR of Mrk 573. Our analysis of IC 5063 is likely affected by contamination from shock excitation, which appears to soften the inferred ionizing spectrum. The peak of the ionizing continuum E-peak is determined by the nuclear spectrum and the absorbing column between the nucleus and the ionized nebula. We cannot separate variation in this intrinsic E-peak from the effects of shock or H II region contamination, but E-peak measurements nevertheless give insights into ENLR excitation. We demonstrate the general applicability of NebulaBayes by analyzing a nuclear spectrum from the non-active galaxy NGC 4691 using a H II region grid. The NLR and H II region model grids are provided with NebulaBayes for use by the astronomical community.

We calibrate commonly used star formation rate (SFR) prescriptions using observations in five kiloparsec-sized fields in the nearby galaxy Andromeda (M31) at 10 pc spatial resolution. Our observations at different scales enable us to resolve the star-forming regions and to distinguish them from non-star-forming components. We use extinction-corrected Ha from optical integral field spectroscopy as our reference tracer and have verified its reliability via tests. It is used to calibrate monochromatic and hybrid (H alpha+alpha xIR and far-UV+bxIR) SFR prescriptions, which use far-UV (GALEX), 22 mu m (Wide-field Infrared Survey Explorer), and 24 mu m (MIPS). Additionally, we evaluate other multiwavelength infrared tracers. Our results indicate that the SFR prescriptions do not change (in M31) with spatial scales or with subtraction of the diffuse component. For the calibration factors in the hybrid SFR prescriptions, we find a approximate to 0.2 and b approximate to 22 in M31, which are a factor of 5 higher than in the literature. As the fields in M31 exhibit high attenuation and low dust temperatures, lie at large galactocentric distances, and suffer from high galactic inclination compared to measurements in other galaxies, we propose that the fields probe a dust layer extended along the line of sight that is not directly spatially associated with star-forming regions. This (vertically) extended dust component increases the attenuation and alters the SFR prescriptions in M31 compared to literature measurements. We recommend that SFR prescriptions should be applied with caution at large galactocentric distances and in highly inclined galaxies, due to variations in the relative (vertical) distribution of dust and gas.

We study the shape of the gas-phase mass-metallicity relation (MZR) of a combined sample of present-day dwarf and high-mass star-forming galaxies using IZI, a Bayesian formalism for measuring chemical abundances presented in a previous publication. We observe a characteristic stellar mass scale at M-*similar or equal to 10(9.5) M-circle dot, above which the inter-stellar medium undergoes a sharp increase in its level of chemical enrichment. In the 10(6)-10(9.5)M(circle dot) range the MZR follows a shallow power law (Z proportional to M-*(alpha)) with slope alpha = 0.14 +/- 0.08. Approaching M-*similar or equal to 10(9.5) M-circle dot the MZR steepens significantly, showing a slope of alpha = 0.37 +/- 0.08 in the 10(9.5)-10(10.5)M(circle dot) range, and a flattening toward a constant metallicity at higher stellar masses. This behavior is qualitatively different from results in the literature that show a single power-law MZR toward the low-mass end. We thoroughly explore systematic uncertainties in our measurement, and show that the shape of the MZR is not induced by sample selection, aperture effects, a changing N/O abundance, the adopted methodology to construct the MZR, secondary dependences on star formation activity, or diffuse ionized gas contamination, but rather on differences in the method used to measure abundances. High-resolution hydrodynamical simulations of galaxies can qualitatively reproduce our result, and suggest a transition in the ability of galaxies to retain their metals for stellar masses above this threshold. The MZR characteristic mass scale also coincides with a transition in the scale height and dumpiness of cold gas disks, and a typical gas fraction below which the efficiency of star formation feedback for driving outflows is expected to decrease sharply.

We present a catalog of spectroscopic redshifts for SPT-CLJ0615-5746, the most distant cluster in the Reionization Lensing Cluster Survey. Using Nod & Shuffle multislit observations with LDSS-3 on Magellan, we identify similar to 50 cluster members and derive a cluster redshift of z(c) = 0.972, with a velocity dispersion of sigma = 1244 +/- 162 km s(-1). We calculate a cluster mass using a sigma(200) - M-200 scaling relation of M-200 = (9.6 +/- 3.5) x 10(14) M-circle dot, in agreement with previous, independent mass measurements of this cluster. In addition, we examine the kinematic state of SPT-CLJ0615-5746, taking into consideration prior investigations of this system. With an elongated profile in lensing mass and X-ray emission, a non-Gaussian velocity dispersion that increases with clustercentric radius, and a brightest cluster galaxy not at rest with the bulk of the system, there are multiple cluster properties that, while not individually compelling, combine to paint a picture that SPT-CLJ0615-5746 is currently being assembled.

The scatter (sigma(sSFR)) of the specific star formation rates of galaxies is a measure of the diversity in their star formation histories (SFHs) at a given mass. In this paper, we employ the Evolution and Assembly of GaLaxies and their Environments (EAGLE) simulations to study the dependence of the sigma(sSFR) of galaxies on stellar mass (M-*) through the sigma(sSFR)-M-* relation in z similar to 0-4. We find that the relation evolves with time, with the dispersion depending on both stellar mass and redshift. The models point to an evolving U-shaped form for the sigma(sSFR)-M-* relation, with the scatter being minimal at a characteristic mass M*( )of 10(9.5) M-circle dot and increasing both at lower and higher masses. This implies that the diversity of SFHs increases toward both the low- and high-mass ends. We find that feedback from active galactic nuclei is important for increasing the sigma(sSFR) for high-mass objects. On the other hand, we suggest that feedback from supernovae increases the sigma(sSFR) of galaxies at the low-mass end. We also find that excluding galaxies that have experienced recent mergers does not significantly affect the sigma(sSFR)-M-* relation. Furthermore, we employ the EAGLE simulations in combination with the radiative transfer code SKIRT to evaluate the effect of SFR/stellar mass diagnostics in the sigma(sSFR)-M-* relation, and find that the SFR/M(* )methodologies (e.g., SED fitting, UV+IR, UV+IRX-beta) widely used in the literature to obtain intrinsic properties of galaxies have a large effect on the derived shape and normalization of the sigma(sSFR)-M-* relation.

We present optical integral field unit observations of the Taffy system (UGC 12914/15), named for the radio emission that stretches between the two galaxies. Given that these gas-rich galaxies are believed to have recently collided head-on, the pair exhibits a surprisingly normal total (sub-LJRG) IR luminosity (L-FIR similar to 4.5 x 10(10) L-circle dot). Previous observations have demonstrated that a large quantity of molecular and neutral gas has been drawn out of the galaxies into a massive multiphase bridge. We present, for the first time, spatially resolved spectroscopy of the ionized gas in the system. The results show that the ionized gas is highly disturbed kinematically, with gas spread in two main filaments between the two galaxies. The line profiles exhibit widespread double components in both the bridge and parts of the disks of the galaxies. We investigate the spatial distribution of the excitation properties of the ionized gas using emission-line diagnostic diagrams and conclude that a large quantity (up to 40%) of the emission from the entire system is consistent with gas heated in similar to 200 km s(-1) shocks. While the shocked gas is mainly associated with the bridge, there is a significant amount of shocked gas associated with both galaxies. Confirming other multiwavelength indicators, the results suggest that the effects of shocks and turbulence can continue to be felt in a high-speed galaxy collision long after the collision has occurred. The persistence of shocks in the Taffy system may explain the relatively low current star formation rates in the system as a whole.

Galaxy clusters trace the largest structures of the Universe and provide ideal laboratories for studying galaxy evolution and cosmology(1,2). Clusters with extended X-ray emission have been discovered at redshifts of up to z approximate to 2.5 (refs (3-7)). Meanwhile, there has been growing interest in hunting for protoclusters, the progenitors of clusters, at higher redshiftss(8-)(14). It is, however, very challenging to find the largest protoclusters at early times, when they start to assemble. Here, we report a giant protocluster of galaxies at z approximate to 5.7, when the Universe was only one billion years old. This protocluster occupies a volume of about 35(3) cubic comoving megaparsecs. It is embedded in an even larger overdense region with at least 41 spectroscopically confirmed, luminous Ly alpha-emitting galaxies (Ly alpha emitters, or LAEs), including several previously reported LAEs(9). Its LAE density is 6.6 times the average density at z approximate to 5.7. It is the only one of its kind in an LAE survey in 4 deg(2) on the sky. Such a large structure is also rarely seen in current cosmological simulations. This protocluster will collapse into a galaxy cluster with a mass of (3.6 +/- 0.9) x10(15) solar masses, comparable to those of the most massive clusters or protoclusters known so far.

We examine the diagnostic power of rest-frame ultraviolet (UV) nebular emission lines, and compare them to more commonly used rest-frame optical emission lines, using the test case of a single star-forming knot of the bright lensed galaxy RCSGA 032727-132609 at redshift z similar to 1.7. This galaxy has complete coverage of all the major rest-frame UV and optical emission lines from Magellan/MagE and Keck/NIRSPEC. Using the full suite of diagnostic lines, we infer the physical properties: nebular electron temperature (T-e), electron density (n(e)), oxygen abundance (log (O/H), ionization parameter [log (q), and interstellar medium (ISM) pressure (log (P/k)]. We examine the effectiveness of the different UV, optical, and joint UV-optical spectra in constraining the physical conditions. Using UV lines alone we can reliably estimate log (q), but the same is difficult for log (O/H). UV lines yield a higher (similar to 1.5 dex) log (P/k) than the optical lines, as the former probes a further inner nebular region than the latter. For this comparison, we extend the existing Bayesian inference code IZI, adding to it the capability to infer ISM pressure simultaneously with metallicity and ionization parameter. This work anticipates future rest-frame UV spectral data sets from the James Webb Space Telescope (JWST) at high redshift and from the Extremely Large Telescope (ELT) at moderate redshift.