Alice E. Shapley

High molecular gas fractions in normal massive star-forming galaxies in the young universe

Stars form from cold molecular interstellar gas. As this is relatively rare in the local Universe, galaxies like the Milky Way form only a few new stars per year. Typical massive galaxies in the distant Universe formed stars an order of magnitude more rapidly. Unless star formation was significantly more efficient, this difference suggests that young galaxies were much more molecular-gas rich. Molecular gas observations in the distant Universe have so far largely been restricted to very luminous, rare objects, including mergers and quasars, and accordingly we do not yet have a clear idea about the gas content of more normal (albeit massive) galaxies. Here we report the results of a survey of molecular gas in samples of typical massive-star-forming galaxies at mean redshifts <z> of about 1.2 and 2.3, when the Universe was respectively 40% and 24% of its current age. Our measurements reveal that distant star forming galaxies were indeed gas rich, and that the star formation efficiency is not strongly dependent on cosmic epoch. The average fraction of cold gas relative to total galaxy baryonic mass at z = 2.3 and z = 1.2 is respectively about 44% and 34%, three to ten times higher than in today’s massive spiral galaxies. The slow decrease between z ≈ 2 and z ≈ 1 probably requires a mechanism of semi-continuous replenishment of fresh gas to the young galaxies.

THE SINS SURVEY: MODELING THE DYNAMICS OF z ∼ 2 GALAXIES AND THE HIGH-z TULLY-FISHER RELATION*

We present the modeling of SINFONI integral field dynamics of 18 star-forming galaxies at z ~ 2 from Hα line emission. The galaxies are selected from the larger sample of the SINS survey, based on the prominence of ordered rotational motions with respect to more complex merger-induced dynamics. The quality of the data allows us to carefully select systems with kinematics dominated by rotation, and to model the gas dynamics across the whole galaxy using suitable exponential disk models. We obtain a good correlation between the dynamical mass and the stellar mass, finding that large gas fractions (M gas ≈ M *) are required to explain the difference between the two quantities. We use the derived stellar mass and maximum rotational velocity V max from the modeling to construct for the first time the stellar mass Tully-Fisher relation at z ~ 2.2. The relation obtained shows a slope similar to what is observed at lower redshift, but we detect an evolution of the zero point. We find that at z ~ 2.2 there is an offset in log(M *) for a given rotational velocity of 0.41 ± 0.11 with respect to the local universe. This result is consistent with the predictions of the latest N-body/hydrodynamical simulations of disk formation and evolution, which invoke gas accretion onto the forming disk in filaments and cooling flows. This scenario is in agreement with other dynamical evidence from SINS, where gas accretion from the halo is required to reproduce the observed properties of a large fraction of the z ~ 2 galaxies. Based on observations obtained at the Very Large Telescope (VLT) of the European Southern Observatory, Paranal, Chile, in the context of guaranteed time programs 073.B-9018, 074.A-9011, 075.A-0466, 076.A-0527, 077.A-0576, 078.A-0600, 078.A-0055, 079.A-0341, 080.A-0330, and 080.A-0635.

THE CHARACTERISTIC STAR FORMATION HISTORIES OF GALAXIES AT REDSHIFTS z ∼ 2-7 ∗

A large sample of spectroscopically confirmed star-forming galaxies at redshifts 1.4 ≤ z_(spec) ≤ 3.7, with complementary imaging in the near- and mid-IR from the ground and from the Hubble Space Telescope and Spitzer Space Telescope, is used to infer the average star formation histories (SFHs) of typical galaxies from z ∼ 2 to 7. For a subset of 302 galaxies at 1.5 ≤ z_(spec) < 2.6, we perform a detailed comparison of star formation rates (SFRs) determined from spectral energy distribution (SED) modeling (SFRs[SED]) and those calculated from deep Keck UV and Spitzer/MIPS 24μm imaging (SFRs[IR+UV]). Exponentially declining SFHs yield SFRs[SED] that are 5–10 times lower on average than SFRs[IR+UV], indicating that declining SFHs may not be accurate for typical galaxies at z ≳ 2. The SFRs of z ∼ 2–3 galaxies are directly proportional to their stellar masses (M_*), with unity slope—a result that is confirmed with Spitzer/IRAC stacks of 1179 UV-faint (R > 25.5) galaxies—for M_* ≳ 5 × 10^8M_⊙ and SFRs ≳ 2M_⊙ yr^(−1). We interpret this result in the context of several systematic biases that can affect determinations of the SFR–M_* relation. The average specific SFRs at z ∼ 2–3 are remarkably similar within a factor of two to those measured at z ≳ 4, implying that the average SFH is one where SFRs increase with time. A consequence of these rising SFHs is that (1) a substantial fraction of UV-bright z ∼ 2–3 galaxies had faint sub-L* progenitors at z ≳ 4; and (2) gas masses must increase with time from z = 2 to 7, over which time the net cold gas accretion rate—as inferred from the specific SFR and the Kennicutt–Schmidt relation—is ∼2–3 times larger than the SFR. However, if we evolve to higher redshift the SFHs and masses of the halos that are expected to host L* galaxies at z ∼ 2, then we find that ≾10% of the baryons accreted onto typical halos at z ≳ 4 actually contribute to star formation at those epochs. These results highlight the relative inefficiency of star formation even at early cosmic times when galaxies were first assembling.

PHYSICAL CONDITIONS IN A YOUNG, UNREDDENED, LOW-METALLICITY GALAXY AT HIGH REDSHIFT*

Increasingly large samples of galaxies are now being discovered at redshifts z ~ 5-6 and higher. Many of these objects are inferred to be young, low in mass, and relatively unreddened, but detailed analysis of their high quality spectra will not be possible until the advent of future facilities. In this paper, we shed light on the physical conditions in a plausibly similar low-mass galaxy by presenting the analysis of the rest-frame optical and UV spectra of Q2343-BX418, an L* galaxy at z = 2.3 with a very low mass-to-light ratio and unusual properties: BX418 is young (<100 Myr), low mass (M_* ~ 109 M_☉), low in metallicity (Z ~ 1/6 Z_☉), and unreddened (E(B – V) ≃ 0.02, UV continuum slope β = –2.1). We infer a metallicity 12 + log(O/H) = 7.9 ± 0.2 from the rest-frame optical emission lines. We also determine the metallicity via the direct, electron temperature method, using the ratio O III] λλ1661, 1666/[O III] λ5007 to determine the electron temperature and finding 12 + log(O/H) = 7.8 ± 0.1. These measurements place BX418 among the most metal-poor galaxies observed in emission at high redshift. The rest-frame UV spectrum, which represents ~12 hr of integration with the Keck telescope, contains strong emission from Lyα (with rest-frame equivalent width 54 A), He II λ1640 (both stellar and nebular), C III] λλ1907, 1909 and O III] λλ1661, 1666. The C IV/C III] ratio indicates that the source of ionization is unlikely to be an active galactic nucleus. Analysis of the He II, O III], and C III] line strengths indicates a very high ionization parameter log U ~ –1, while Lyα and the interstellar absorption lines indicate that outflowing gas is highly ionized over a wide range of velocities. It remains to be determined how many of BX418s unique spectral features are due to its global properties, such as low metallicity and dust extinction, and how many are indicative of a short-lived phase in the early evolution of an otherwise normal star-forming galaxy.

THE GASEOUS ENVIRONMENT OF HIGH-z GALAXIES: PRECISION MEASUREMENTS OF NEUTRAL HYDROGEN IN THE CIRCUMGALACTIC MEDIUM OF z ∼ 2-3 GALAXIES IN THE KECK BARYONIC STRUCTURE SURVEY*

We present results from the Keck Baryonic Structure Survey (KBSS), a unique spectroscopic survey of the distant universe designed to explore the details of the connection between galaxies and intergalactic baryons within the same survey volumes, focusing particularly on scales from ∼ 50 kpc to a few Mpc. The KBSS is optimized for the redshift range z ∼ 2-3, combining S/N ∼ 100 Keck/HIRES spectra of 15 of the brightest QSOs in the sky at z ≃ 2.5-2.9 with very densely sampled galaxy redshift surveys within a few arcmin of each QSO sightline. In this paper, we present quantitative results on the distribution, column density, kinematics, and absorber line widths of neutral hydrogen (H_I) surrounding a subset of 886 KBSS star-forming galaxies with 2.0 ≾ z ≾ 2.8 and with projected distances ≤ 3 physical Mpc from a QSO sightline. Using Voigt profile decompositions of the full Lyα forest region of all 15 QSO spectra, we compiled a catalog of ∼6000 individual absorbers in the redshift range of interest, with 12 ≤ log(N_(HI)) ≤ 21. These are used to measure H I absorption statistics near the redshifts of foreground galaxies as a function of projected galactocentric distance from the QSO sightline and for randomly chosen locations in the intergalacticmedium (IGM) within the survey volume. We find that NHI and the multiplicity of velocity-associated H I components increase rapidly with decreasing galactocentric impact parameter and as the systemic redshift of the galaxy is approached. The strongest H_I absorbers within ≃ 100 physical kpc of galaxies have N_(HI) ∼ 3 orders of magnitude higher than those near random locations in the IGM. The circumgalactic zone of most significantly enhanced H_I absorption is found within transverse distances of ≾ 300 kpc and within ±300 km s^(-1) of galaxy systemic redshifts. Taking this region as the defining bounds of the circumgalactic medium (CGM), nearly half of absorbers with log(N_(HI)) > 15.5 are found within the CGM of galaxies meeting our photometric selection criteria, while their CGM occupy only 1.5% of the cosmic volume. The spatial covering fraction, multiplicity of absorption components, and characteristic NHI remain significantly elevated to transverse distances of ∼2 physical Mpc from galaxies in our sample. Absorbers with N_(HI) > 10^(14.5) cm^(-2) are tightly correlated with the positions of galaxies, while absorbers with lower N_(HI) are correlated with galaxy positions only on ≳ Mpc scales. Redshift anisotropies on these larger scales indicate coherent infall toward galaxy locations, while on scales of ∼ 100 physical kpc peculiar velocities of Δv ≃ ±260 km s^(-1) with respect to the galaxies are indicated. The median Doppler widths of individual absorbers within 1-3 r_(vir) of galaxies are larger by ≃ 50% than randomly chosen absorbers of the same NHI, suggesting higher gas temperatures and/or increased turbulence likely caused by some combination of accretion shocks and galactic winds around galaxies with M_(halo) ≃ 10^(12) M_⊙ at z ∼ 2-3.

CONSTRAINTS ON THE ASSEMBLY AND DYNAMICS OF GALAXIES. I. DETAILED REST-FRAME OPTICAL MORPHOLOGIES ON KILOPARSEC SCALE OF z ∼ 2 STAR-FORMING GALAXIES*

We present deep and high-resolution Hubble Space Telescope NIC2 F160W imaging at 1.6 μm of six z ~ 2 star-forming galaxies with existing near-infrared integral field spectroscopy from SINFONI at the Very Large Telescope. The unique combination of rest-frame optical imaging and nebular emission-line maps provides simultaneous insight into morphologies and dynamical properties. The overall rest-frame optical emission of the galaxies is characterized by shallow profiles in general (Sersic index n < 1), with median effective radii of R_e ~ 5 kpc. The morphologies are significantly clumpy and irregular, which we quantify through a non-parametric morphological approach, estimating the Gini (G), multiplicity (Ψ), and M_(20) coefficients. The estimated strength of the rest-frame optical emission lines in the F160W bandpass indicates that the observed structure is not dominated by the morphology of line-emitting gas, and must reflect the underlying stellar mass distribution of the galaxies. The sizes and structural parameters in the rest-frame optical continuum and Hα emission reveal no significant differences, suggesting similar global distributions of the ongoing star formation and more evolved stellar population. While no strong correlations are observed between stellar population parameters and morphology within the NIC2/SINFONI sample itself, a consideration of the sample in the context of a broader range of z ~ 2 galaxy types (K-selected quiescent, active galactic nucleus, and star forming; 24 μm selected dusty, infrared-luminous) indicates that these galaxies probe the high specific star formation rate and low stellar mass surface density part of the massive z ~ 2 galaxy population, with correspondingly large effective radii, low Sersic indices, low G, and high Ψ and M_(20). The combined NIC2 and SINFONI data set yields insights of unprecedented detail into the nature of mass accretion at high redshift.

AN HST/WFC3-IR MORPHOLOGICAL SURVEY OF GALAXIES AT z = 1.5-3.6. I. SURVEY DESCRIPTION AND MORPHOLOGICAL PROPERTIES OF STAR-FORMING GALAXIES*

We present the results of a 42-orbit Hubble Space Telescope Wide-Field Camera 3 (HST/WFC3) survey of the rest-frame optical morphologies of star-forming galaxies with spectroscopic redshifts in the range z = 1.5-3.6. The survey consists of 42 orbits of F160W imaging covering ~65 arcmin2 distributed widely across the sky and reaching a depth of 27.9 AB for a 5σ detection within a 0.2 arcsec radius aperture. Focusing on an optically selected sample of 306 star-forming galaxies with stellar masses in the range M_* = 10^9-10^(11) M_☉, we find that typical circularized effective half-light radii range from ~0.7 to 3.0 kpc and describe a stellar mass-radius relation as early as z ~ 3. While these galaxies are best described by an exponential surface brightness profile (Sersic index n ~ 1), their distribution of axis ratios is strongly inconsistent with a population of inclined exponential disks and is better reproduced by triaxial stellar systems with minor/major and intermediate/major axis ratios ~0.3 and 0.7, respectively. While rest-UV and rest-optical morphologies are generally similar for a subset of galaxies with HST/Advanced Camera for Surveys imaging data, differences are more pronounced at higher masses M_* > 3 × 10^(10) M_☉. Finally, we discuss galaxy morphology in the context of efforts to constrain the merger fraction, finding that morphologically identified mergers/non-mergers generally have insignificant differences in terms of physical observables such as stellar mass and star formation rate, although merger-like galaxies selected according to some criteria have statistically smaller effective radii and correspondingly larger Σ_(SFR).

A Refined Estimate of the Ionizing Emissivity from Galaxies at z ≃ 3: Spectroscopic Follow-up in the SSA22a Field

We investigate the contribution of star-forming galaxies to the ionizing background at z ~ 3, building on previous work based on narrowband (NB3640) imaging in the SSA22a field. We use new Keck/LRIS spectra of Lyman break galaxies (LBGs) and narrowband-selected Lyα emitters (LAEs) to measure redshifts for 16 LBGs and 87 LAEs at z > 3.055, such that our NB3640 imaging probes the Lyman-continuum (LyC) region. When we include the existing set of spectroscopically confirmed LBGs, our total sample with z > 3.055 consists of 41 LBGs and 91 LAEs, of which 9 LBGs and 20 LAEs are detected in our NB3640 image. With our combined imaging and spectroscopic data sets, we critically investigate the origin of NB3640 emission for detected LBGs and LAEs. We remove from our samples three LBGs and three LAEs with spectroscopic evidence of contamination of their NB3640 flux by foreground galaxies and statistically model the effects of additional, unidentified foreground contaminants. The resulting contamination and LyC-detection rates, respectively, are 62% ± 13% and 8% ± 3% for our LBG sample, and 47% ± 10% and 12% ± 2% for our LAE sample. The corresponding ratios of non-ionizing UV to LyC flux density, corrected for intergalactic medium (IGM) attenuation, are 18.0^(+34.8)_(–7.4) for LBGs and 3.7^(+2.5)_(–1.1) for LAEs. We use these ratios to estimate the total contribution of star-forming galaxies to the ionizing background and the hydrogen photoionization rate in the IGM, finding values larger than, but consistent with, those measured in the Lyα forest. Finally, the measured UV to LyC flux-density ratios imply model-dependent LyC escape fractions of f^(LyC)_(esc) ~ 5%-7% for our LBG sample and f^(LyC)_(esc) ~ 10%-30% for our fainter LAE sample.

The Relationship between Stellar Populations and Lyα Emission in Lyman Break Galaxies

We present the results of a photometric and spectroscopic survey of 321 Lyman break galaxies (LBGs) at z ~ 3 to investigate systematically the relationship between Lyα emission and stellar populations. Lyα equivalent widths (W_(Lyα)) were calculated from rest-frame UV spectroscopy and optical/near-infrared/Spitzer photometry was used in population synthesis modeling to derive the key properties of age, dust extinction, star formation rate (SFR), and stellar mass. We directly compare the stellar populations of LBGs with and without strong Lyα emission, where we designate the former group (W_(Lyα) ≥ 20 A) as Lyα emitters (LAEs) and the latter group (W_(Lyα) < 20 A) as non-LAEs. This controlled method of comparing objects from the same UV luminosity distribution represents an improvement over previous studies in which the stellar populations of LBGs and narrowband-selected LAEs were contrasted, where the latter were often intrinsically fainter in broadband filters by an order of magnitude simply due to different selection criteria. Using a variety of statistical tests, we find that Lyα equivalent width and age, SFR, and dust extinction, respectively, are significantly correlated in the sense that objects with strong Lyα emission also tend to be older, lower in SFR, and less dusty than objects with weak Lyα emission, or the line in absorption. We accordingly conclude that, within the LBG sample, objects with strong Lyα emission represent a later stage of galaxy evolution in which supernovae-induced outflows have reduced the dust covering fraction. We also examined the hypothesis that the attenuation of Lyα photons is lower than that of the continuum, as proposed by some, but found no evidence to support this picture.

THE MOSDEF SURVEY: MEASUREMENTS OF BALMER DECREMENTS AND THE DUST ATTENUATION CURVE AT REDSHIFTS z ∼ 1.4–2.6*

We present results on the dust attenuation curve of z~2 galaxies using early observations from the MOSFIRE Deep Evolution Field (MOSDEF) survey. Our sample consists of 224 star-forming galaxies with nebular spectroscopic redshifts in the range z= 1.36-2.59 and high S/N measurements of, or upper limits on, the H-alpha and H-beta emission lines obtained with Keck/MOSFIRE. We construct composite SEDs of galaxies in bins of specific SFR and Balmer optical depth in order to directly constrain the dust attenuation curve from the UV through near-IR for typical star-forming galaxies at high redshift. Our results imply an attenuation curve that is very similar to the SMC extinction curve at wavelengths redward of 2500 Angstroms. At shorter wavelengths, the shape of the curve is identical to that of the Calzetti relation, but with a lower normalization (R_V). Hence, the new attenuation curve results in SFRs that are ~20% lower, and log stellar masses that are 0.16 dex lower, than those obtained with the Calzetti attenuation curve. Moreover, we find that the difference in the reddening---and the total attenuation---of the ionized gas and stellar continuum correlates strongly with SFR, such that for dust-corrected SFRs larger than 20 Msun/yr assuming a Chabrier IMF, the nebular emission lines suffer an increasing degree of obscuration relative to the continuum. A simple model that can account for these trends is one in which the UV through optical stellar continuum is dominated by a population of less reddened stars, while the nebular line and bolometric luminosities become increasingly dominated by dustier stellar populations for galaxies with large SFRs, as a result of the increased dust enrichment that accompanies such galaxies. Consequently, UV- and SED-based SFRs may underestimate the total SFR at even modest levels of ~20 Msun/yr. [Abridged]