The spatial distribution of dust in galaxies affects the global attenuation, and hence inferred properties, of galaxies. We trace the spatial distribution of dust in five approximately kiloparsec fields of M31 by comparing optical attenuation with the total dust mass distribution. We measure the attenuation from the Balmer decrement using Integral Field Spectroscopy and the dust mass from Herschel far-IR observations. Our results show that M31's dust attenuation closely follows a foreground screen model, contrary to what was previously found in other nearby galaxies. By smoothing the M31 data, we find that spatial resolution is not the cause for this difference. Based on the emission-line ratios and two simple models, we conclude that previous models of dust/gas geometry need to include a weakly or non-attenuated diffuse ionized gas (DIG) component. Due to the variation of dust and DIG scale heights with galactic radius, we conclude that different locations in galaxies will have different vertical distributions of gas and dust and therefore different measured attenuation. The difference between our result in M31 with that found in other nearby galaxies can be explained by our fields in M31 lying at larger galactic radii than the previous studies that focused on the centers of galaxies.

We compare the structure of molecular gas at 40 pc resolution to the ability of gas to form stars across the disk of the spiral galaxy M51. We break the PAWS survey into 370 pc and 1.1 kpc resolution elements, and within each we estimate the molecular gas depletion time (tau(mol)(Dep)), the star-formation efficiency per free-fall time (epsilon(ff)), and the mass-weighted cloud-scale (40 pc) properties of the molecular gas: surface density, Sigma, line width, sigma, and b equivalent to Sigma/sigma(2) proportional to alpha(-1)(vir), a parameter that traces the boundedness of the gas. We show that the cloud-scale surface density appears to be a reasonable proxy for mean volume density. Applying this, we find a typical star-formation efficiency per free-fall time, epsilon(ff)() similar to 0.3%-0.36%, lower than adopted in many models and found for local clouds. Furthermore, the efficiency per free-fall time anti-correlates with both Sigma and sigma, in some tension with turbulent star-formation models. The best predictor of the rate of star formation per unit gas mass in our analysis is b equivalent to Sigma/sigma(2), tracing the strength of self-gravity, with tau(mol)(Dep) proportional to b(-0.9). The sense of the correlation is that gas with stronger self-gravity (higher b) forms stars at a higher rate (low tau(mol)(Dep)). The different regions of the galaxy mostly overlap in tau(mol)(Dep) as a function of b, so that low b explains the surprisingly high tau(mol)(Dep) found toward the inner spiral arms found by Meidt et al. (2013).

We present a spectroscopic survey of high-redshift, luminous galaxies over four square degrees on the sky, aiming to build a large and homogeneous sample of Ly alpha emitters (LAEs) at z approximate to 5.7 and 6.5, and Lyman-break galaxies (LBGs) at 5.5 < z < 6.8. The fields that we choose to observe are well studied, such as by the Subaru XMM-Newton Deep Survey and COSMOS. They have deep optical imaging data in a series of broad and narrow bands, allowing for the efficient selection of galaxy candidates. Spectroscopic observations are being carried out using the multi-object spectrograph M2FS on the Magellan Clay telescope. M2FS is efficient enough to identify high-redshift galaxies, owing to its 256 optical fibers deployed over a circular field of view 30' in diameter. We have observed similar to 2.5 square degrees. When the program is completed, we expect to identify more than 400 bright LAEs at z approximate to 5.7 and 6.5, and a substantial number of LBGs at z >= 6. This unique sample will be used to study a variety of galaxy properties and to search for large protoclusters. Furthermore, the statistical properties of these galaxies will be used to probe cosmic reionization. We describe the motivation, program design, target selection, and M2FS observations. We also outline our science goals, and present a sample of the brightest LAEs at z approximate to 5.7 and 6.5. This sample contains 32 LAEs with Ly alpha luminosities higher than 10(43) erg s(-1). A few of them reach >= 3 x 10(43) erg s(-1), comparable to the two most luminous LAEs known at z >= 6, "CR7" and "COLA1." These LAEs provide ideal targets to study extreme galaxies in the distant universe.

We investigate the evolution of the galaxy star formation rate function (SFRF) and cosmic star formation rate density (CSFRD) of z similar to 0-8 galaxies in the Evolution and Assembly of GaLaxies and their Environments (EAGLE) simulations. In addition, we present a compilation of ultraviolet, infrared and H alpha SFRFs and compare these with the predictions from the EAGLE suite of cosmological hydrodynamic simulations. We find that the constraints implied by different indicators are inconsistent with each other for the highest star-forming objects at z < 2, a problem that is possibly related to selection biases and the uncertainties of dust attenuation effects. EAGLE's feedback parameters were calibrated to reproduce realistic galaxy sizes and stellar masses at z = 0.1. In this work we test if and why those choices yield realistic star formation rates (SFRs) for z similar to 0-8 as well. We demonstrate that supernovae feedback plays a major role at setting the abundance of galaxies at all star-forming regimes, especially at high redshifts. On the contrary, active galactic nuclei (AGN) feedback becomes more prominent at lower redshifts and is a major mechanism that affects only the highest star-forming systems. Furthermore, we find that galaxies with SFR similar to 1-10M(circle dot) yr(-1) dominate the CSFRD at redshifts z <= 5, while rare high star-forming galaxies (SFR similar to 10-100M(circle dot) yr(-1)) contribute significantly only briefly around the peak era (z similar to 2) and then are quenched by AGN feedback. In the absence of this prescription objects with SFR similar to 10-100M(circle dot) yr(-1) would dominate the CSFRD, while the cosmic budget of star formation would be extremely high. Finally, we demonstrate that the majority of the cosmic star formation occurs in relatively rare high-mass haloes (M-Halo similar to 10(11-13)M(circle dot)) even at the earliest epochs.

Modern extragalactic molecular gas surveys now reach the scales of star-forming giant molecular clouds (GMCs; 20-50 pc). Systematic variations in GMC properties with galaxy environment imply that clouds are not universally self-gravitating objects, decoupled from their surroundings. Here we re-examine the coupling of clouds to their environment and develop a model for 3D gas motions generated by forces arising with the galaxy gravitational potential defined by the background disk of stars and dark matter. We show that these motions can resemble or even exceed the motions needed to support gas against its own self-gravity throughout typical galactic disks. The importance of the galactic potential in spiral arms and galactic centers suggests that the response to self-gravity does not always dominate the motions of gas at GMC scales, with implications for observed gas kinematics, virial equilibrium, and cloud morphology. We describe how a uniform treatment of gas motions in the plane and in the vertical direction synthesizes the two main mechanisms proposed to regulate star formation: vertical pressure equilibrium and shear/Coriolis forces as parameterized by Toomre Q approximate to 1. As the modeled motions are coherent and continually driven by the external potential, they represent support for the gas that is distinct from that conventionally attributed to turbulence, which decays rapidly and thus requires maintenance, e.g., via feedback from star formation. Thus, our model suggests that the galaxy itself can impose an important limit on star formation, as we explore in a second paper in this series.

We present a model for the origin of the extended law of star formation in which the surface density of star formation (Sigma(SFR)) depends not only on the local surface density of the gas (Sigma(g)) but also on the stellar surface density (Sigma(*)), the velocity dispersion of the stars and on the scaling laws of turbulence in the gas. We compare our model with the spiral, face-on galaxy NGC 628 and show that the dependence of the star formation rate on the entire set of physical quantities for both gas and stars can help explain both the observed general trends in the Sigma(g) - Sigma(SFR) and Sigma(*) - Sigma(SFR) relations, but also, and equally important, the scatter in these relations at any value of Sigma(g) and Sigma(*). Our results point out to the crucial role played by existing stars along with the gaseous component in setting the conditions for large scale gravitational instabilities and star formation in galactic discs.

We measure the velocity dispersion, sigma, and surface density, Sigma, of the molecular gas in nearby galaxies from CO spectral line cubes with spatial resolution 45-120. pc, matched to the size of individual giant molecular clouds. Combining 11 galaxies from the PHANGS-ALMA survey with four targets from the literature, we characterize similar to 30,000 independent sightlines where CO is detected at good significance. Sigma and sigma show a strong positive correlation, with the best-fit power-law slope close to the expected value for resolved, self-gravitating clouds. This indicates only a weak variation in the virial parameter alpha(vir) proportional to sigma(2)/Sigma, which is similar to 1.5-3.0 for most galaxies. We do, however, observe enormous variation in the internal turbulent pressure P-turb proportional to Sigma sigma(2), which spans similar to 5 dex across our sample. We find Sigma, sigma , and P-turb to be systematically larger in more massive galaxies. The same quantities appear enhanced in the central kiloparsec of strongly barred galaxies relative to their disks. Based on sensitive maps of M31 and M33, the slope of the sigma-Sigma relation flattens at Sigma less than or similar to 10M(circle dot) pc(-2), leading to high s for a given S and high apparent avir. This echoes results found in the Milky Way and likely originates from a combination of lower beam-filling factors and a stronger influence of local environment on the dynamical state of molecular gas in the low-density regime.

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.