We present a new method for inferring the metallicity (Z) and ionization parameter (q) of H II regions and star-forming galaxies using strong nebular emission lines (SELs). We use Bayesian inference ...to derive the joint and marginalized posterior probability density functions for Z and q given a set of observed line fluxes and an input photoionization model. Our approach allows the use of arbitrary sets of SELs and the inclusion of flux upper limits. The method provides a self-consistent way of determining the physical conditions of ionized nebulae that is not tied to the arbitrary choice of a particular SEL diagnostic and uses all the available information. Unlike theoretically calibrated SEL diagnostics, the method is flexible and not tied to a particular photoionization model. We describe our algorithm, validate it against other methods, and present a tool that implements it called IZI. Using a sample of nearby extragalactic H II regions, we assess the performance of commonly used SEL abundance diagnostics. We also use a sample of 22 local H II regions having both direct and recombination line (RL) oxygen abundance measurements in the literature to study discrepancies in the abundance scale between different methods. We find that oxygen abundances derived through Bayesian inference using currently available photoionization models in the literature can be in good (~30%) agreement with RL abundances, although some models perform significantly better than others. We also confirm that abundances measured using the direct method are typically ~0.2 dex lower than both RL and photoionization-model-based abundances.
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 ( ), the star-formation efficiency per free-fall time ( ), and the mass-weighted cloud-scale (40 pc) properties of the molecular gas: surface density, , line width, , and , 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, , lower than adopted in many models and found for local clouds. Furthermore, the efficiency per free-fall time anti-correlates with both and , 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 , tracing the strength of self-gravity, with . The sense of the correlation is that gas with stronger self-gravity (higher b) forms stars at a higher rate (low ). The different regions of the galaxy mostly overlap in as a function of b, so that low b explains the surprisingly high found toward the inner spiral arms found by Meidt et al. (2013).
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* 109.5 M , above which the inter-stellar medium undergoes a sharp increase in its level of chemical enrichment. In the 106-109.5 M range the MZR follows a shallow power law ( ) with slope = 0.14 0.08. Approaching M* 109.5 M the MZR steepens significantly, showing a slope of = 0.37 0.08 in the 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 clumpiness 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.
ABSTRACT We develop a simple analytical model that tracks galactic metallicities governed by star formation and feedback to gain insight from the observed galaxy stellar mass-metallicity relations ...over a large range of stellar masses and redshifts. The model reveals the following implications of star formation and feedback processes in galaxy formation. First, the observed metallicity relations provide a stringent upper limit for the averaged outflow mass-loading factors of local galaxies, which are ∼20 for galaxies and monotonically decrease to ∼1 for galaxies. Second, the inferred upper limit for the outflow mass-loading factor sensitively depends on whether the outflow is metal-enriched with respect to the interstellar medium metallicity. If half of the metals ejected from supernovae leave the galaxy in metal-enriched winds, the outflow mass-loading factor for galaxies at any mass can barely be higher than ∼10, which puts strong constraints on galaxy formation models. Third, the relatively lower stellar-phase to gas-phase metallicity ratio for lower-mass galaxies indicates that low-mass galaxies are still rapidly enriching their metallicities in recent times, while high-mass galaxies are more settled, which seems to show a downsizing effect in the metallicity evolution of galaxies. The analysis presented in the paper demonstrates the importance of accurate measurements of galaxy metallicities and the cold gas fraction of galaxies at different redshifts for constraining star formation and feedback processes, and demonstrates the power of these relations for constraining the physics of galaxy formation.
ABSTRACT
PHANGS-HST is an ultraviolet-optical imaging survey of 38 spiral galaxies within ∼20 Mpc. Combined with the PHANGS-ALMA, PHANGS-MUSE surveys and other multiwavelength data, the data set will ...provide an unprecedented look into the connections between young stars, H ii regions, and cold molecular gas in these nearby star-forming galaxies. Accurate distances are needed to transform measured observables into physical parameters (e.g. brightness to luminosity, angular to physical sizes of molecular clouds, star clusters and associations). PHANGS-HST has obtained parallel ACS imaging of the galaxy haloes in the F606W and F814W bands. Where possible, we use these parallel fields to derive tip of the red giant branch (TRGB) distances to these galaxies. In this paper, we present TRGB distances for 10 PHANGS galaxies from ∼4 to ∼15 Mpc, based on the first year of PHANGS-HST observations. Four of these represent the first published TRGB distance measurements (IC 5332, NGC 2835, NGC 4298, and NGC 4321), and seven of which are the best available distances to these targets. We also provide a compilation of distances for the 118 galaxies in the full PHANGS sample, which have been adopted for the first PHANGS-ALMA public data release.
ABSTRACT
Feedback from massive stars plays a key role in molecular cloud evolution. After the onset of star formation, the young stellar population is exposed by photoionization, winds, supernovae, ...and radiation pressure from massive stars. Recent observations of nearby galaxies have provided the evolutionary timeline between molecular clouds and exposed young stars, but the duration of the embedded phase of massive star formation is still ill-constrained. We measure how long massive stellar populations remain embedded within their natal cloud, by applying a statistical method to six nearby galaxies at $20{-}100~\mbox{${\rm ~pc}$}$ resolution, using CO, Spitzer 24$\rm \, \mu m$, and H α emission as tracers of molecular clouds, embedded star formation, and exposed star formation, respectively. We find that the embedded phase (with CO and 24$\rm \, \mu m$ emission) lasts for 2−7 Myr and constitutes $17{-}47{{\ \rm per\ cent}}$ of the cloud lifetime. During approximately the first half of this phase, the region is invisible in H α, making it heavily obscured. For the second half of this phase, the region also emits in H α and is partially exposed. Once the cloud has been dispersed by feedback, 24$\rm \, \mu m$ emission no longer traces ongoing star formation, but remains detectable for another 2−9 Myr through the emission from ambient CO-dark gas, tracing star formation that recently ended. The short duration of massive star formation suggests that pre-supernova feedback (photoionization and winds) is important in disrupting molecular clouds. The measured time-scales do not show significant correlations with environmental properties (e.g. metallicity). Future JWST observations will enable these measurements routinely across the nearby galaxy population.
We measure the velocity dispersion, , and surface density, , 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 ∼30,000 independent sightlines where CO is detected at good significance. and 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 vir ∝ 2/ , which is ∼1.5-3.0 for most galaxies. We do, however, observe enormous variation in the internal turbulent pressure Pturb ∝ 2, which spans ∼5 dex across our sample. We find , , and Pturb 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 - relation flattens at 10 M pc−2, leading to high for a given and high apparent vir. 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.
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 ∼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 imaging. At a common scale of 140 pc, our massive (log(M M ) = 9.3-10.7), normally star-forming (SFRM yr−1 = 0.3-5.9) galaxies exhibit a significant reservoir of quiescent molecular gas not associated with star formation as traced by H 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 ∼0.5 kpc. As the resolution becomes poorer, the morphological features become indistinct for spatial scales larger than ∼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.
We estimate the star formation efficiency per gravitational free-fall time, , from observations of nearby galaxies with resolution matched to the typical size of a giant molecular cloud. This ...quantity, , 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 . Assuming a constant thickness of the molecular gas layer (H = 100 pc) across the whole sample, the median value of in our sample is 0.7%. We find a mild scale dependence, with higher measured at coarser resolution. Individual galaxies show different values of , with the median ranging from 0.3% to 2.6%. We find the highest 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.
ABSTRACT Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) is an optical fiber-bundle integral-field unit (IFU) spectroscopic survey that is one of three core programs in the ...fourth-generation Sloan Digital Sky Survey (SDSS-IV). With a spectral coverage of 3622-10354 and an average footprint of ∼500 arcsec2 per IFU the scientific data products derived from MaNGA will permit exploration of the internal structure of a statistically large sample of 10,000 low-redshift galaxies in unprecedented detail. Comprising 174 individually pluggable science and calibration IFUs with a near-constant data stream, MaNGA is expected to obtain ∼100 million raw-frame spectra and ∼10 million reduced galaxy spectra over the six-year lifetime of the survey. In this contribution, we describe the MaNGA Data Reduction Pipeline algorithms and centralized metadata framework that produce sky-subtracted spectrophotometrically calibrated spectra and rectified three-dimensional data cubes that combine individual dithered observations. For the 1390 galaxy data cubes released in Summer 2016 as part of SDSS-IV Data Release 13, we demonstrate that the MaNGA data have nearly Poisson-limited sky subtraction shortward of ∼8500 and reach a typical 10 limiting continuum surface brightness = 23.5 AB arcsec−2 in a five-arcsecond-diameter aperture in the g-band. The wavelength calibration of the MaNGA data is accurate to 5 km s−1 rms, with a median spatial resolution of 2.54 arcsec FWHM (1.8 kpc at the median redshift of 0.037) and a median spectral resolution of = 72 km s−1.
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