We utilize Sloan Digital Sky Survey imaging and spectroscopy of ~53,000 star-forming galaxies at z ~ 0.1 to study the relation between stellar mass and gas-phase metallicity. We derive gas-phase oxygen abundances and stellar masses using new techniques that make use of the latest stellar evolutionary synthesis and photoionization models. We find a tight (?0.1 dex) correlation between stellar mass and metallicity spanning over 3 orders of magnitude in stellar mass and a factor of 10 in metallicity. The relation is relatively steep from 108.5 to 1010.5 M? h, in good accord with known trends between luminosity and metallicity, but flattens above 1010.5 M?. We use indirect estimates of the gas mass based on the H? luminosity to compare our data to predictions from simple closed box chemical evolution models. We show that metal loss is strongly anticorrelated with baryonic mass, with low-mass dwarf galaxies being 5 times more metal depleted than L* galaxies at z ~ 0.1. Evidence for metal depletion is not confined to dwarf galaxies but is found in galaxies with masses as high as 1010 M?. We interpret this as strong evidence of both the ubiquity of galactic winds and their effectiveness in removing metals from galaxy potential wells.

https://iopscience.iop.org/article/10.1086/423264/pdf

The Origin of the Mass-Metallicity Relation: Insights from 53,000 Star-forming Galaxies in the Sloan Digital Sky Survey

We present a comprehensive study of the physical properties of ∼ 10 5 galaxies with measurable star formation in the Sloan Digital Sky Survey (SDSS). By comparing physical information extracted from the emission lines with continuum properties, we build up a picture of the nature of star-forming galaxies at z < 0.2. We develop a method for aperture correction using resolved imaging and show that our method takes out essentially all aperture bias in the star formation rate (SFR) estimates, allowing an accurate estimate of the total SFRs in galaxies. We determine the SFR density to be 1.915 +0.02 −0.01 (random) +0.14

The physical properties of star-forming galaxies in the low-redshift universe

We review progress over the past decade in observations of large-scale star formation, with a focus on the interface between extragalactic and Galactic studies. Methods of measuring gas contents and star-formation rates are discussed, and updated prescriptions for calculating star-formation rates are provided. We review relations between star formation and gas on scales ranging from entire galaxies to individual molecular clouds.

Star Formation in the Milky Way and Nearby Galaxies

We review current understanding of star formation, outlining an overall theoretical framework and the observations that motivate it. A conception of star formation has emerged in which turbulence plays a dual role, both creating overdensities to initiate gravitational contraction or collapse, and countering the effects of gravity in these overdense regions. The key dynamical processes involved in star formation—turbulence, magnetic fields, and self-gravity— are highly nonlinear and multidimensional. Physical arguments are used to identify and explain the features and scalings involved in star formation, and results from numerical simulations are used to quantify these effects. We divide star formation into large-scale and small-scale regimes and review each in turn. Large scales range from galaxies to giant molecular clouds (GMCs) and their substructures. Important problems include how GMCs form and evolve, what determines the star formation rate (SFR), and what determines the initial mass function (IMF). Small scales range from dense cores to the protostellar systems they beget. We discuss formation of both low- and high-mass stars, including ongoing accretion. The development of winds and outflows is increasingly well understood, as are the mechanisms governing angular momentum transport in disks. Although outstanding questions remain, the framework is now in place to build a comprehensive theory of star formation that will be tested by the next generation of telescopes.

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Theory of Star Formation

We present a model for star formation and supernova feedback that describes the multiphase structure of star-forming gas on scales that are typically not resolved in cosmological simulations. Our approach includes radiative heating and cooling, the growth of cold clouds embedded in an ambient hot medium, star formation in these clouds, feedback from supernovae in the form of thermal heating and cloud evaporation, galactic winds and outflows, and metal enrichment. Implemented using smoothed particle hydrodynamics, our scheme is a significantly modified and extended version of the grid-based method of Yepes et al., and enables us to achieve a high dynamic range in simulations of structure formation.



We discuss properties of the feedback model in detail and show that it predicts a self-regulated, quiescent mode of star formation, which, in particular, stabilizes the star-forming gaseous layers of disc galaxies. The parametrization of this mode can be reduced to a single free quantity that determines the overall time-scale for star formation. We fix this parameter numerically to match the observed rates of star formation in local disc galaxies. When normalized in this manner, cosmological simulations employing our model nevertheless overproduce the observed cosmic abundance of stellar material. We are thus motivated to extend our feedback model to include galactic winds associated with star formation.



Using small-scale simulations of individual star-forming disc galaxies, we show that these winds produce either galactic fountains or outflows, depending on the depth of the gravitational potential. In low-mass haloes, winds can greatly suppress the overall efficiency of star formation. When incorporated into cosmological simulations, our combined model for star formation and winds predicts a cosmic star formation density that is consistent with observations, provided that the winds are sufficiently energetic. Moreover, outflows from galaxies in these simulations drive chemical enrichment of the intergalactic medium – in principle, accounting for the presence of metals in the Lyman α forest.

/pdf/cosmological-smoothed-particle-hydrodynamics-simulations-a-377dyc13je.pdf

Cosmological smoothed particle hydrodynamics simulations: a hybrid multiphase model for star formation

We report the first results of a detailed study of the star formation law in a sample of 32 nearby spiral galaxies with well-measured rotation curves, H I and H_2 (as traced by CO) surface density profiles, and new Hα CCD photometry. In this paper we present an atlas of Hα images and radial surface brightness profiles and describe a surface density threshold in the star formation law. Prominent breaks in the Hα surface brightness profiles are identified in nearly all of the actively star-forming disks, confirming previous claims of star formation thresholds based on lower quality data. We measure the ratio of the gas density to the critical density for local gravitational stability at the threshold radii. The outer threshold radii observed in Sab-Sdm galaxies are in general agreement with those expected from the Toomre Q stability criterion, confirming earlier work, but with a significant variation that appears to be weakly correlated with galaxy type. Such a trend could plausibly reflect variations in the relative contribution of the stellar disk to the instability of the gas disk across this range of galaxy types. Among disks with subcritical gas surface densities, and outside the threshold radius in star-forming disks, the number of isolated H II regions increases as the gas surface density approaches the critical density. At the thresholds, the gas surface densities span a wide range, and the atomic/molecular gas fraction is highest in the disks having the lowest total gas surface density. The simple Toomre condition fails to account for the active star formation in the inner disks of low-mass spirals such as NGC 2403 and M33. An alternative stability criterion based on the shear in the disk provides a better description of these disks but is a less accurate indicator of the outer edges of star-forming disks than the Toomre criterion. These results strongly support the view that the formation of gravitationally bound interstellar clouds regulates the onset of widespread star formation, at least in the outer regions of galactic disks.

/pdf/star-formation-thresholds-in-galactic-disks-n6pb28n11w.pdf

Star Formation Thresholds in Galactic Disks

We use high signal-to-noise ratio spectra of 20 H II regions in the giant spiral galaxy M101 to derive electron temperatures for the H II regions and robust metal abundances over radii R = 019-125R0 (6-41 kpc) We compare the consistency of electron temperatures measured from the [O III] ?4363, [N II] ?5755, [S III] ?6312, and [O II] ?7325 auroral lines Temperatures from [O III], [S III], and [N II] are correlated with relative offsets that are consistent with expectations from nebular photoionization models However, the temperatures derived from the [O II] ?7325 line show a large scatter and are nearly uncorrelated with temperatures derived from other ions We tentatively attribute this result to observational and physical effects, which may introduce large random and systematic errors into abundances derived solely from [O II] temperatures Our derived oxygen abundances are well fitted by an exponential distribution over six disk scale lengths, from approximately 13 (O/H)? in the center to 1/15 (O/H)? in the outermost region studied [for solar 12 + log(O/H) = 87] We measure significant radial gradients in N/O and He/H abundance ratios, but relatively constant S/O and Ar/O Our results are in approximate agreement with previously published abundances studies of M101 based on temperature measurements of a few H II regions However, our abundances are systematically lower by 02-05 dex than those derived from the most widely used strong-line empirical abundance indicators, again consistent with previous studies based on smaller H II region samples Independent measurements of the Galactic interstellar oxygen abundance from ultraviolet absorption lines are in good agreement with the Te-based nebular abundances We suspect that most of the disagreement with the strong-line abundances arises from uncertainties in the nebular models that are used to calibrate the empirical scale, and that strong-line abundances derived for H II regions and emission-line galaxies are as much as a factor of 2 higher than the actual oxygen abundances However, other explanations, such as the effects of temperature fluctuations on the auroral line based abundances, cannot be completely ruled out These results point to the need for direct abundance determinations of a larger sample of extragalactic H II regions, especially for objects more metal-rich than solar

https://iopscience.iop.org/article/10.1086/375398/pdf

The Composition Gradient in M101 Revisited. II. Electron Temperatures and Implications for the Nebular Abundance Scale

Recent far-UV (FUV) and near-UV (NUV) observations of the nearby galaxy NGC4625 made by the Galaxy Evolution Explorer (GALEX) show the presence of an extended UV disk reaching 4 times the optical radius of the galaxy. The UV-to-optical colors suggest that the bulk of the stars in the disk of NGC4625 are currently being formed, providing a unique opportunity to study today the physics of star formation under conditions similar to those when the normal disks of spiral galaxies like the Milky Way first formed. In the case of NGC4625, the star formation in the extended disk is likely to be triggered by interaction with NGC4618 and possibly also with the newly discovered galaxy NGC4625A. The positions of the FUV complexes in the extended disk coincide with peaks in the H I distribution. The masses of these complexes are in the range 10^3-10^4 Msun with their Halpha emission (when present) being dominated by ionization from single stars.

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Discovery of an extended ultraviolet disk in the nearby galaxy NGC4625

We present a spectroscopic study of six damped Lya absorption (DLA) systems at z 10^{11} h^{-1} M_sun appear to be common at intermediate redshifts. (3) Using an ensemble of six galaxy-DLA pairs, we derive an abundance profile that is characterized by a radial gradient of -0.041 +/- 0.012 dex per kiloparsec (or equivalently a scale length of 10.6 h^{-1} kpc) from galactic center to 30 h^{-1} kpc radius. (4) Adopting known N(HI) profiles of nearby galaxies and the best-fit radial gradient, we further derive an N(HI)-weighted mean metallicity _weighted = -0.50 +/- 0.07 for the DLA population over 100 random lines of sight, consistent with _weighted = -0.64 (-0.86, +0.40) observed for z~1 DLA systems from Prochaska et al. Our analysis demonstrates that the low metal content of DLA systems does not rule out the possibility that the DLA population trace the field galaxy population.

Abundance Profiles and Kinematics of Damped Lyman-alpha Absorbing Galaxies at z < 0.65

We present optical spectroscopy of a sample of metal-rich extragalactic HII regions in the spiral galaxies M83, NGC 3351 and NGC 6384, including a number of circumnuclear HII regions (hot spots). Different age estimators (equivalent width of the Hbeta emission line W(Hb), Balmer line absorption profiles, UV spectra) provide consistently young ages (4-6 Myr) for the hot spots. We have detected the W-R bump feature at 4650 A in five of the objects, and in fewer cases possibly the WC features at 5696 A and 5808 A. Six additional objects showing W-R features are drawn from our previous work on extragalactic HII regions. From the measured luminosity of the W-R blue bump we estimate a small number of WN stars (from 1-2 to about 30). By assuming instantaneous bursts of star formation the ages derived from W(Hb) (3-6 Myr) and the strength of the W-R bump are in agreement with the predictions of evolutionary stellar population models, provided we account for stochastic effects on the IMF. From a comparison with published photoionization models based on synthetic cluster spectral energy distributions we find some evidence for an overestimation of the number of He ionizing photons in the model fluxes. In general, however, the massive star diagnostics are in agreement with the predictions of recent evolutionary models calculated with a Salpeter IMF and a high mass limit. We find no compelling evidence for a depletion of massive stars (M > 40-50 solar masses) in the IMF of metal-rich clusters, contrary to our previous conclusions based on older evolutionary models.

Robert C. Kennicutt

Papers

Star Formation Thresholds in Galactic Disks

The Composition Gradient in M101 Revisited. II. Electron Temperatures and Implications for the Nebular Abundance Scale

Discovery of an extended ultraviolet disk in the nearby galaxy NGC4625

Abundance Profiles and Kinematics of Damped Lyman-alpha Absorbing Galaxies at z < 0.65

Optical spectroscopy of metal-rich HII regions and circumnuclear hot spots in M83 and NGC 3351