Jason Tumlinson

THE STAR FORMATION LAW IN ATOMIC AND MOLECULAR GAS

We propose a simple theoretical model for star formation in which the local star formation rate (SFR) in a galaxy is determined by three factors. First, the interplay between the interstellar radiation field and molecular self-shielding determines what fraction of the gas is in molecular form and thus eligible to form stars. Second, internal feedback determines the properties of the molecular clouds that form, which are nearly independent of galaxy properties until the galactic interstellar medium (ISM) pressure becomes comparable to the internal giant molecular cloud (GMC) pressure. Above this limit, galactic ISM pressure determines molecular gas properties. Third, the turbulence driven by feedback processes in GMCs makes star formation slow, allowing a small fraction of the gas to be converted to stars per free-fall time within the molecular clouds. We combine analytic estimates for each of these steps to formulate a single star formation law, and show that the predicted correlation between SFR, metallicity, and surface densities of atomic, molecular, and total gas agree well with observations.

The Large, Oxygen-Rich Halos of Star-Forming Galaxies Are a Major Reservoir of Galactic Metals

Observations with the Hubble Space Telescope show that halos of ionized gas are common around star-forming galaxies. The circumgalactic medium (CGM) is fed by galaxy outflows and accretion of intergalactic gas, but its mass, heavy element enrichment, and relation to galaxy properties are poorly constrained by observations. In a survey of the outskirts of 42 galaxies with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope, we detected ubiquitous, large (150-kiloparsec) halos of ionized oxygen surrounding star-forming galaxies; we found much less ionized oxygen around galaxies with little or no star formation. This ionized CGM contains a substantial mass of heavy elements and gas, perhaps far exceeding the reservoirs of gas in the galaxies themselves. Our data indicate that it is a basic component of nearly all star-forming galaxies that is removed or transformed during the quenching of star formation and the transition to passive evolution.

THE ATOMIC-TO-MOLECULAR TRANSITION IN GALAXIES. II: H I AND H2 COLUMN DENSITIES

Gas in galactic disks is collected by gravitational instabilities into giant atomic-molecular complexes, but only the inner, molecular parts of these structures are able to collapse to form stars. Determining what controls the ratio of atomic-to-molecular hydrogen in complexes is, therefore, a significant problem in star formation and galactic evolution. In this paper, we use the model of H2 formation, dissociation, and shielding developed in the previous paper in this series to make theoretical predictions for atomic-to-molecular ratios as a function of galactic properties. We find that the molecular fraction in a galaxy is determined primarily by its column density and secondarily by its metallicity, and is to a good approximation independent of the strength of the interstellar radiation field. We show that the column of atomic hydrogen required to shield a molecular region against dissociation is ~10 M ☉ pc–2 at solar metallicity. We compare our model to data from recent surveys of the Milky Way and of nearby galaxies, and show that the both the primary dependence of molecular fraction on column density and the secondary dependence on metallicity that we predict are in good agreement with observed galaxy properties.

A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar Molecular Hydrogen in Translucent Clouds

We report the first ensemble results from the Far Ultraviolet Spectroscopic Explorer survey of molecular hydrogen in lines of sight with AV e1 mag. We have developed techniques for fitting computed profiles to the low-J lines of H2, and thus determining column densities for J ¼ 0 and J ¼ 1, which contain e99% of the total H2. From these column densities and ancillary data we have derived the total H2 column densities, hydrogen molecular fractions, and kinetic temperatures for 23 lines of sight. This is the first significant sample of molecular hydrogen column densities of � 10 21 cm � 2 , measured through UV absorption bands. We have also compiled a set of extinction data for these lines of sight, which sample a wide range of environments. We have searched for correlations of our H2-related quantities with previously published column densities of other molecules and extinction parameters. We find strong correlations between H2 and molecules such as CH, CN, and CO, in general agreement with predictions of chemical models. We also find the expected correlations between hydrogen molecular fraction and various density indicators such as kinetic temperature, CN

The COS-Halos Survey: Physical Conditions and Baryonic Mass in the Low-Redshift Circumgalactic Medium

We analyze the physical conditions of the cool, photoionized (T ∼ 10 4 K) circumgalactic medium (CGM) using the COS-Halos suite of gas column density measurements for 44 gaseous halos within 160 kpc of L ∼ L ∗ galaxies at z ∼ 0.2. These data are well described by simple photoionization models, with the gas highly ionized (nHii/nH 99%) by the extragalactic ultraviolet background. Scaling by estimates for the virial radius, Rvir, we show that the ionization state (tracked by the dimensionless ionization parameter, U) increases with distance from the host galaxy. The ionization parameters imply a decreasing volume density profile nH = (10 −4.2±0.25 )(R/Rvir) −0.8±0.3 . Our derived gas volume densities are several orders of magnitude lower than predictions from standard two-phase models with a cool medium in pressure equilibrium with a hot, coronal medium expected in virialized halos at this mass scale. Applying the ionization corrections to the Hi column densities, we estimate a lower limit to the cool gas mass M cool CGM > 6.5 × 10

A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar Molecular Hydrogen in the Small and Large Magellanic Clouds

We describe a moderate-resolution Far Ultraviolet Spectroscopic Explorer (FUSE) survey of H2 along 70 sight lines to the Small and Large Magellanic Clouds, using hot stars as background sources. FUSE spectra of 67% of observed Magellanic Cloud sources (52% of LMC and 92% of SMC) exhibit absorption lines from the H2 Lyman and Werner bands between 912 and 1120 A. Our survey is sensitive to N(H2) ≥ 1014 cm-2; the highest column densities are log N(H2) = 19.9 in the LMC and 20.6 in the SMC. We find reduced H2 abundances in the Magellanic Clouds relative to the Milky Way, with average molecular fractions = 0.010 for the SMC and = 0.012 for the LMC, compared with = 0.095 for the Galactic disk over a similar range of reddening. The dominant uncertainty in this measurement results from the systematic differences between 21 cm radio emission and Lyα in pencil beam sight lines as measures of N(H I). These results imply that the diffuse H2 masses of the LMC and SMC are 8 × 106 and 2 × 106 M☉, respectively, 2% and 0.5% of the H I masses derived from 21 cm emission measurements. The LMC and SMC abundance patterns can be reproduced in ensembles of model clouds with a reduced H2 formation rate coefficient, R ~ 3 × 10-18 cm3 s-1, and incident radiation fields ranging from 10-100 times the Galactic mean value. We find that these high-radiation, low formation rate models can also explain the enhanced N(4)/N(2) and N(5)/N(3) rotational excitation ratios in the Clouds. We use H2 column densities in low rotational states (J = 0 and 1) to derive kinetic and/or rotational temperatures of diffuse interstellar gas, and we find that the distribution of rotational temperatures is similar to Galactic gas, with T01 = 82 ± 21 K for clouds with N(H2) ≥ 1016.5 cm-2. There is only a weak correlation between detected H2 and far-infrared fluxes as determined by IRAS, perhaps as a result of differences in the survey techniques. We find that the surface density of H2 probed by our pencil beam sight lines is far lower than that predicted from the surface brightness of dust in IRAS maps. We discuss the implications of this work for theories of star formation in low-metallicity environments.

Zero-Metallicity Stars and the Effects of the First Stars on Reionization

We present stellar structure and atmosphere models of metal-free stars and examine them from a cosmological point of view. Metal-free stars exhibit high effective temperatures and small sizes relative to metal-enriched stars of equal mass. These unique physical characteristics enhance the ionizing photon production by metal-free stars, particularly in the He ii (hnu>/=4 ryd) continuum. The star formation rate of metal-free stars necessary to reionize the hydrogen in the universe by z=5 is consistent with the inferred star formation rate at that epoch. However, the hard stellar spectra are inconsistent with the observations of He ii opacity in the intergalactic medium (IGM) at z approximately 3, indicating that the period of metal-free star formation ended before that epoch. We examine the effects of these stars on the ionization balance of the IGM, the radiative feedback of the first luminous objects, and the extragalactic radiation field. We comment on the prospects for detecting metal-free stellar populations with the lambda1640 and lambda4686 recombination lines of He ii.

Average extinction curves and relative abundances for quasi-stellar object absorption-line systems at 1 ≤zabs < 2

We have studied a sample of 809 Mg II absorption systems with 1.0 ≤ z abs ≤ 1.86 in the spectra of Sloan Digital Sky Survey quasi-stellar objects (QSOs), with the aim of understanding the nature and abundance of the dust and the chemical abundances in the intervening absorbers. Normalized, composite spectra were derived, for abundance measurements, for the full sample and several subsamples, chosen on the basis of the line strengths and other absorber and QSO properties. Average extinction curves were obtained for the subsamples by comparing their geometric mean spectra with those of matching samples of QSOs without absorbers in their spectra. There is clear evidence for the presence of dust in the intervening absorbers. The 2175-A feature is not present in the extinction curves, for any of the subsamples. The extinction curves are similar to the Small Magellanic Cloud (SMC) extinction curve with a rising ultraviolet (UV) extinction below 2200 A. The absorber rest-frame colour excess, E(B - V), derived from the extinction curves, depends on the absorber properties and ranges from <0.001 to 0.085 for various subsamples. The column densities of Mg II, Al II, Si II, Ca II, Ti II, Cr II, Mn II, Fe II, Co II, Ni II and Zn II do not show such a correspondingly large variation. The overall depletions in the high E(B - V) samples are consistent with those found for individual damped Lyman a systems, the depletion pattern being similar to halo clouds in the Galaxy. Assuming an SMC gas-to-dust ratio, we find a trend of increasing abundance with decreasing extinction; systems with N H1 ∼ 10 20 cm -2 show solar abundance of Zn. The large velocity spread of strong Mg II systems seems to be mimicked by weak lines of other elements. The ionization of the absorbers, in general appears to be low: the ratio of the column densities of Al III to Al II is always less than 1/2. QSOs with absorbers are, in general, at least three times as likely to have highly reddened spectra as compared to QSOs without any absorption systems in their spectra.

A Budget and Accounting of Metals at z ~ 0: Results from the COS-Halos Survey

We present a budget and accounting of metals in and around star-forming galaxies at z ∼ 0. We combine empirically derived star formation histories with updated supernova and AGB yields and rates to estimate the total mass of metals produced by galaxies with present-day stellar mass of 10 9.3 – 10 11.6 M⊙. On the accounting side of the ledger, we show that a surprisingly constant 20–25% mass fraction of produced metals remain in galaxies’ stars, interstellar gas and interstellar dust, with little dependence of this fraction on the galaxy stellar mass (omitting those metals immediately locked up in remnants). Thus, the bulk of metals are outside of galaxies, produced in the progenitors of today’s L ∗ galaxies. The COS-Halos survey is uniquely able to measure the mass of metals in the circumgalactic medium (to impact parameters of < 150kpc) of low-redshift ∼ L ∗ galaxies. Using these data, we map the distribution of CGM metals as traced by both the highly ionized O VI ion and a suite of low-ionization species; combined with constraints on circumgalactic dust and hotter X-ray emitting gas out to similar impact parameters, we show that ∼ 40% of metals produced by M⋆ ∼ 10 10 M⊙ galaxies can be easily accounted for out to 150kpc. With the current data, we cannot rule out a constant mass of metals within this fixed physical radius. This census provides a crucial boundary condition for the eventual fate of metals in galaxy evolution models.

Chemical Evolution in Hierarchical Models of Cosmic Structure. I. Constraints on the Early Stellar Initial Mass Function

I present a new Galactic chemical evolution model motivated by and grounded in the hierarchical theory of galaxy formation, as expressed by a halo merger history of the Galaxy. This model accurately reproduces the metallicity distribution function (MDF) for Population II stars residing today in the Galactic halo. Model MDFs are calculated for a fiducial Galaxy formation scenario and a range of assumptions about the astrophysics of star formation and chemical enrichment at early times. The observed MDF and the apparent absence of true Population III stars from the halo strongly imply that there is some critical metallicity, Zcr 10-4 Z?, below which low-mass star formation is inhibited and perhaps impossible. The observed constraints from the halo MDF, relative metal abundances from extremely metal-poor Galactic halo stars, and the ionizing photon budget needed to reionize the IGM together imply a stellar IMF below Zcr that is peaked in the range of massive stars that experience core-collapse supernovae, with mean mass M = 8-42 M?. This mass range is similar to the masses predicted by models of primordial star formation that account for formation feedback. A set of five plausible IMF cases is presented, ranging from broadly peaked with mean mass ~15 M? to narrowly peaked at mean mass ~70 M?. These IMF cases cannot be distinguished formally by the available constraints, but the models with lower characteristic mass produce overall better fits to the available data. The model also implies that metal-poor halo stars below [Fe/H] -3 had only 1-10 metal-free stars as their supernova precursors, such that the relative abundances in these halo stars exhibit IMF-weighted averages over the intrinsic yields of the first supernovae. This paper is the first part of a long-term project to connect the high-redshift in situ indicators of early star formation with the low-z, old remnants of the first stars.