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.

The distribution of metals within a galaxy traces the baryon cycle and the buildup of galactic disks, but the detailed gas phase metallicity distribution remains poorly sampled. We have determined the gas phase oxygen abundances for 7138 H II regions across the disks of eight nearby galaxies using Very Large Telescope/Multi Unit Spectroscopic Explorer (MUSE) optical integral field spectroscopy as part of the PHANGS-MUSE survey. After removing the first-order radial gradients present in each galaxy, we look at the statistics of the metallicity offset (Delta O/H) and explore azimuthal variations. Across each galaxy, we find low (sigma = 0.03-0.05 dex) scatter at any given radius, indicative of efficient mixing. We compare physical parameters for those H II regions that are 1 sigma outliers toward both enhanced and reduced abundances. Regions with enhanced abundances have high ionization parameter, higher H alpha luminosity, lower H alpha velocity dispersion, younger star clusters, and associated molecular gas clouds showing higher molecular gas densities. This indicates recent star formation has locally enriched the material. Regions with reduced abundances show increased H alpha velocity dispersions, suggestive of mixing introducing more pristine material. We observe subtle azimuthal variations in half of the sample, but cannot always cleanly associate this with the spiral pattern. Regions with enhanced and reduced abundances are found distributed throughout the disk, and in half of our galaxies we can identify subsections of spiral arms with clearly associated metallicity gradients. This suggests spiral arms play a role in organizing and mixing the interstellar medium.

We report one of the first extragalactic observations of electron temperature variations across a spiral arm. Using Multi Unit Spectroscopic Explorer mosaic observations of the nearby galaxy NGC 1672, we measure the [N ii]?5755 auroral line in a sample of 80 H ii regions in the eastern spiral arm of NGC 1672. We discover systematic temperature variations as a function of distance perpendicular to the spiral arm. The electron temperature is lowest on the spiral arm itself and highest on the downstream side. Photoionization models of different metallicity, pressure, and age of the ionizing source are explored to understand what properties of the interstellar medium drive the observed temperature variations. An azimuthally varying metallicity appears to be the most likely cause of the temperature variations. The electron temperature measurements solidify recent discoveries of azimuthal variations of oxygen abundance based on strong lines, and rule out the possibility that the abundance variations are artifacts of the strong-line calibrations.

The processes regulating star formation in galaxies are thought to act across a hierarchy of spatial scales. To connect extragalactic star formation relations from global and kiloparsec-scale measurements to recent cloud-scale resolution studies, we have developed a simple, robust method that quantifies the scale dependence of the relative spatial distributions of molecular gas and recent star formation. In this paper, we apply this method to eight galaxies with similar to 1 ''. resolution molecular gas imaging from the Physics at High Angular resolution in Nearby GalaxieS-ALMA (PHANGS-ALMA) survey and PdBI Arcsecond Whirlpool Survey (PAWS) that have matched resolution, high-quality narrowband H alpha imaging. At a common scale of 140 pc, our massive (log(M-*[M-circle dot]) = 9.3-10.7), normally star-forming (SFR[M(circle dot)yr(-1)] = 0.3-5.9) galaxies exhibit a significant reservoir of quiescent molecular gas not associated with star formation as traced by H alpha emission. Galactic structures act as backbones for both molecular gas and H II region distributions. As we degrade the spatial resolution, the quiescent molecular gas disappears, with the most rapid changes occurring for resolutions up to similar to 0.5 kpc. As the resolution becomes poorer, the morphological features become indistinct for spatial scales larger than similar to 1 kpc. The method is a promising tool to search for relationships between the quiescent or star-forming molecular reservoir and galaxy properties, but requires a larger sample size to identify robust correlations between the star-forming molecular gas fraction and global galaxy parameters.