Naveen A. Reddy

The Mass-Metallicity Relation at z≳2*

We use a sample of 87 rest-frame UV-selected star-forming galaxies with mean spectroscopic redshift z = 2.26 ± 0.17 to study the correlation between metallicity and stellar mass at high redshift. Using stellar masses determined from SED fitting to observed 0.3-8 μm photometry, we divide the sample into six bins in stellar mass and construct six composite Hα + [N ] spectra from all of the objects in each bin. We estimate the mean oxygen abundance in each bin from the [N II]/Hα ratio and find a monotonic increase in metallicity with increasing stellar mass, from 12 + log(O/H) < 8.2 for galaxies with M = 2.7 × 109 M☉ to 12 + log(O/H) = 8.6 for galaxies with M = 1.0 × 1011 M☉. We use the empirical relation between SFR density and gas density to estimate the gas fractions of the galaxies, finding an increase in gas fraction with decreasing stellar mass. These gas fractions, combined with the observed metallicities, allow the estimation of the effective yield yeff as a function of stellar mass; in constrast to observations in the local universe, which show a decrease in yeff with decreasing baryonic mass, we find a slight increase. Such a variation of metallicity with gas fraction is best fitted by a model with supersolar yield and an outflow rate ~4 times higher than the SFR. We conclude that the mass-metallicity relation at high redshift is driven by the increase in metallicity as the gas fraction decreases through star formation and is likely modulated by metal loss from strong outflows in galaxies of all masses.

GOODS–Herschel: an infrared main sequence for star-forming galaxies

We present the deepest 100 to 500u2009μm far-infrared observations obtained with the Herschel Space Observatory as part of the GOODS-Herschel key program, and examine the infrared (IR) 3–500u2009μm spectral energy distributions (SEDs) of galaxies at 0 < z < 2.5, supplemented by a local reference sample from IRAS, ISO, Spitzer, and AKARI data. We determine the projected star formation densities of local galaxies from their radio and mid-IR continuum sizes. nWe find that the ratio of total IR luminosity to rest-frame 8u2009μm luminosity, IR8 (≡ L_(IR)^(tot)/L_8), follows a Gaussian distribution centered on IR8 = 4 (σ = 1.6) and defines an IR main sequence for star-forming galaxies independent of redshift and luminosity. Outliers from this main sequence produce a tail skewed toward higher values of IR8. This minority population ( u20093 × 10^(10) L_⊙u2009kpc^(-2)) and a high specific star formation rate (i.e., starbursts). The rest-frame, UV-2700u2009A size of these distant starbursts is typically half that of main sequence galaxies, supporting the correlation between star formation density and starburst activity that is measured for the local sample. nLocally, luminous and ultraluminous IR galaxies, (U)LIRGs (L_(IR)^(tot)≥ 10^(11) L_☉), are systematically in the starburst mode, whereas most distant (U)LIRGs form stars in the “normal” main sequence mode. This confusion between two modes of star formation is the cause of the so-called “mid-IR excess” population of galaxies found at z > 1.5 by previous studies. Main sequence galaxies have strong polycyclic aromatic hydrocarbon (PAH) emission line features, a broad far-IR bump resulting from a combination of dust temperatures (T_(dust)u2009~u200915–50 K), and an effective T_(dust)u2009 ~u200931 K, as derived from the peak wavelength of their infrared SED. Galaxies in the starburst regime instead exhibit weak PAH equivalent widths and a sharper far-IR bump with an effective T_(dust)~u200940 K. Finally, we present evidence that the mid-to-far IR emission of X-ray active galactic nuclei (AGN) is predominantly produced by star formation and that candidate dusty AGNs with a power-law emission in the mid-IR systematically occur in compact, dusty starbursts. After correcting for the effect of starbursts on IR8, we identify new candidates for extremely obscured AGNs.

Radio Properties of Infrared-selected Galaxies in the IRAS 2 Jy Sample

The radio counterparts to the IRAS Redshift Survey galaxies are identified in the NRAO VLA Sky Survey (NVSS) catalog. Our new catalog of the IR flux-limited (S60 μm ≥ 2 Jy) complete sample of 1809 galaxies lists accurate radio positions, redshifts, and 1.4 GHz radio and IRAS fluxes. This sample is 6 times larger in size and 5 times deeper in redshift coverage (to z ≈ 0.15) than those used in earlier studies of the radio and far-IR (FIR) properties of galaxies in the local volume. The well-known radio-FIR correlation is obeyed by the overwhelming majority (≥98%) of the IR-selected galaxies, and the radio AGNs identified by their excess radio emission constitute only about 1% of the sample, independent of the IR luminosity. These FIR-selected galaxies can account for the entire population of late-type field galaxies in the local volume, and their radio continuum may be used directly to infer the extinction-free star formation rate in most cases. Both the 1.4 GHz radio and 60 μm IR luminosity functions are reasonably well described by linear sums of two Schechter functions, one representing normal, late-type field galaxies and the other representing starbursts and other luminous IR galaxies. The integrated FIR luminosity density for the local volume is (4.8 ± 0.5) × 107 L☉ Mpc-3, less than 10% of which is contributed by the luminous IR galaxies with LFIR ≥ 1011 L☉. The inferred extinction-free star formation density for the local volume is 0.015 ± 0.005 M☉ yr-1 Mpc-3.

A Survey of Star-forming Galaxies in the 1.4 ≲ z ≲ 2.5 Redshift Desert: Overview*

The redshift interval 1.4 z 2.5 has been described by some as the redshift desert because of historical difficulties in spectroscopically identifying galaxies in that range. In fact, galaxies can be found in large numbers with standard broadband color selection techniques coupled with follow-up spectroscopy with UV and blue-sensitive spectrographs. In this paper we present the first results of a large-scale survey of such objects, carried out with the blue channel of the LRIS spectrograph (LRIS-B) on the Keck I Telescope. We introduce two samples of star-forming galaxies, BX galaxies at z = 2.20 ± 0.32 and BM galaxies at z = 1.70 ± 0.34. In seven survey fields we have spectroscopically confirmed 749 of the former and 114 of the latter. Interlopers (defined as objects at z < 1) account for less than 10% of the photometric candidates, and the fraction of faint active galactic nuclei is ~3% in the combined BX/BM sample. Deep near-IR photometry of a subset of the BX sample indicates that, compared with a sample of similarly UV-selected galaxies at z ~ 3, the z ~ 2 galaxies are on average significantly redder in (-Ks), indicating longer star formation histories, increased reddening by dust, or both. Using near-IR Hα spectra of a subset of BX/BM galaxies to define the galaxies systemic redshifts, we show that the galactic-scale winds that are a feature of star-forming galaxies at z ~ 3 are also common at later epochs and have similar bulk outflow speeds of 200-300 km s-1. We illustrate with examples the information that can be deduced on the stellar populations, metallicities, and kinematics of redshift desert galaxies from easily accessible rest-frame far-UV and rest-frame optical spectra. Far from being hostile to observations, the universe at z ~ 2 is uniquely suited to providing information on the astrophysics of star-forming galaxies and the intergalactic medium, and the relationship between the two.

The Stellar, Gas, and Dynamical Masses of Star-forming Galaxies at z ~ 2

We present analysis of the near-infrared spectra of 114 rest-frame UV-selected star-forming galaxies at z ~ 2. By combining the Hα spectra with photometric measurements from observed 0.3-8 μm, we assess the relationships among kinematics, dynamical masses, inferred gas fractions, and stellar masses and ages. The Hα line widths give a mean dynamical mass M_(dyn) = (6.9 ± 0.6) × 10^(10) M_⊙ within a typical radius of ~6 kpc, after excluding AGNs. The average dynamical mass is ~2 times larger than the average stellar mass, and the two agree to within a factor of several for most objects. However, ~15% of the sample has M_(dyn)≫ M. These objects are best fit by young stellar populations and tend to have high Hα equivalent widths, W_(Hα) ≳ 200 A, suggesting that they are young starbursts with large gas masses. Rest-frame optical luminosity and velocity dispersion are correlated with 4 σ significance. Using the local empirical correlation between star formation rate per unit area and gas surface density, we estimate the mass of the gas associated with star formation and find a mean gas fraction of ~50% and a strong decrease in gas fraction with increasing stellar mass. The masses of gas and stars combined are considerably better correlated with the dynamical masses than are the stellar masses alone, and agree to within a factor of 3 for 85% of the sample. The combination of kinematic measurements, estimates of gas masses, and stellar population properties suggest that the factor of ~500 range in stellar mass across the sample cannot be fully explained by intrinsic differences in the total masses of the galaxies, which vary by a factor of ~40; the remaining variation is due to the evolution of the stellar population and the conversion of gas into stars.

Hα Observations of a Large Sample of Galaxies at z ~ 2: Implications for Star Formation in High-Redshift Galaxies*

Using Hα spectra of 114 rest-frame UV-selected galaxies at z ~ 2, we compare inferred star formation rates (SFRs) with those determined from the UV continuum luminosity. After correcting for extinction using standard techniques based on the UV continuum slope, we find excellent agreement between the indicators, with = 31 M_☉ yr^(-1) and = 29 M_☉ yr^(-1). The agreement between the indicators suggests that the UV luminosity is attenuated by a typical factor of ~4.5 (ranging from no attenuation to a factor of ~100 for the most obscured object in the sample), in good agreement with estimates of obscuration from X-ray, radio, and mid-IR data. The Hα luminosity is attenuated by a factor of ~1.7 on average, and the maximum Hα attenuation is a factor of ~5. In agreement with X-ray and mid-IR studies, we find that the SFR increases with increasing stellar mass and at brighter K magnitudes to ~ 60 M_☉ yr^(-1) for galaxies with K_s 10^(11) M_☉) have had higher SFRs in the past.

Multiwavelength Constraints on the Cosmic Star Formation History from Spectroscopy: The Rest-Frame Ultraviolet, Hα, and Infrared Luminosity Functions at Redshifts 1.9 ≲ z ≲ 3.4*

We use a sample of rest-frame UV-selected and spectroscopically observed galaxies at redshifts 1.9 ≤ z < 3.4, combined with ground-based spectroscopic Hα and Spitzer MIPS 24 μm data, to derive the most robust measurements of the rest-frame UV, Hα, and infrared (IR) luminosity functions (LFs) at these redshifts. Our sample is by far the largest of its kind, with over 2000 spectroscopic redshifts in the range 1.9 ≤ z < 3.4 and ~15,000 photometric candidates in 29 independent fields covering a total area of almost a square degree. Our method for computing the LFs takes into account a number of systematic effects, including photometric scatter, Lyα line perturbations to the observed optical colors of galaxies, and contaminants. Taking into account the latter, we find no evidence for an excess of UV-bright galaxies over what was inferred in early z ~ 3 LBG studies. The UV LF appears to undergo little evolution between z ~ 4 and z ~ 2. Corrected for extinction, the UV luminosity density (LD) at z ~ 2 is at least as large as the value at z ~ 3 and a factor of ~9 larger than the value at z ~ 6, primarily reflecting an increase in the number density of bright galaxies between z ~ 6 and z ~ 2. Our analysis yields the first constraints anchored by extensive spectroscopy on the infrared and bolometric LFs for faint and moderately luminous (L_(bol) ≾ 10^(12) L⊙) galaxies. Adding the IR to the emergent UV luminosity, incorporating independent measurements of the LD from ULIRGs, and assuming realistic dust attenuation values for UV-faint galaxies, indicates that galaxies with L_(bol) < 10^(12) L⊙ account for ≈80% of the bolometric LD and SFRD at z ~ 2–3. This suggests that previous estimates of the faint end of the Lbol LF may have underestimated the steepness of the faint-end slope at L_(bol) < 10^(12) L⊙. Our multiwavelength constraints on the global SFRD indicate that approximately one-third of the present-day stellar mass density was formed in subultraluminous galaxies between redshifts z = 1.9–3.4.

Ultraviolet to Mid-Infrared Observations of Star-forming Galaxies at z ~ 2: Stellar Masses and Stellar Populations*

We present the broadband UV through mid-infrared spectral energy distributions (SEDs) of a sample of 72 spectroscopically confirmed star-forming galaxies at z = 2.30 ? 0.3. Located in a 72 arcmin2 field centered on the bright background QSO, HS 1700+643, these galaxies were preselected to lie at z ~ 2 solely on the basis of their rest-frame UV colors and luminosities and should be representative of UV-selected samples at high redshift. In addition to deep ground-based photometry spanning from 0.35 to 2.15 ?m, we make use of Spitzer IRAC data, which probe the rest-frame near-IR at z ~ 2. The range of stellar populations present in the sample is investigated with simple, single-component stellar population synthesis models. The inability to constrain the form of the star formation history limits our ability to determine the parameters of extinction, age, and star formation rate without using external multiwavelength information. Emphasizing stellar mass estimates, which are much less affected by these uncertainties, we find log M*/M? = 10.32 ? 0.51 for the sample. The addition of Spitzer IRAC data as a long-wavelength baseline reduces stellar mass uncertainties by a factor of 1.5-2 relative to estimates based on optical-Ks photometry alone. However, the total stellar mass estimated for the sample is remarkably insensitive to the inclusion of IRAC data. We find correlations between stellar mass and rest-frame R band (observed Ks) and rest-frame 1.4 ?m (observed 4.5 ?m) luminosities, although with significant scatter. Even at rest-frame 1.4 ?m, the mass-to-light ratio varies by a factor of 15 indicating that even the rest-frame near-IR, when taken alone, is a poor indicator of stellar mass in star-forming galaxies at z ~ 2. Allowing for the possibility of episodic star formation, we find that typical galaxies in our sample could contain up to 3 times more stellar mass in an old underlying burst than what was inferred from single-component modeling. In contrast, mass estimates for the most massive galaxies in the sample (M* > 1011 M?) are fairly insensitive to the method used to model the stellar population. Galaxies in this massive tail, which are also the oldest objects in the sample, could plausibly evolve into the passive galaxies discovered at z ~ 1.7 with near-IR selection techniques. In the general framework of hierarchical galaxy formation and mergers, which implies episodic star formation histories, galaxies at high redshift may pass in and out of UV-selected and near-IR color-selected samples as they evolve from phases of active star formation to quiescence and back again.

The Spatial clustering of star-forming galaxies at redshifts 1.4 <~ z <~ 3.5

We analyzed the spatial distribution of 28,500 photometrically selected galaxies with magnitude 23.5 < AB < 25.5 and redshift 1.4 < z < 3.5 in 21 fields with a total area of 0.81 deg2. The galaxies were divided into three subsamples, with mean redshifts = 1.7, 2.2, and 2.9, according to their UnG colors. Combining the galaxies measured angular clustering with redshift distributions inferred from 1600 spectroscopic redshifts, we find comoving correlation lengths at the three redshifts of r0 = 4.5 ± 0.6, 4.2 ± 0.5, and 4.0 ± 0.6 h-1 Mpc, respectively, and infer a roughly constant correlation function slope of γ = 1.6 ± 0.1. We derive similar numbers from the 1600 object spectroscopic sample itself with a new statistic, K, that is insensitive to many possible systematics. Galaxies that are bright in (λrest ~ 1500-2500 A) cluster more strongly than fainter galaxies at z = 2.9 and 2.2 but not, apparently, at z = 1.7. Comparison to a numerical simulation that is consistent with recent WMAP observations suggests that galaxies in our samples are associated with dark matter halos of mass 1011.2-1011.8 (z = 2.9), 1011.8-1012.2 (z = 2.2), and 1011.9-1012.3 M☉ (z = 1.7) and that a small fraction of the halos contain more than one galaxy that satisfies our selection criteria. Adding recent observations of galaxy clustering at z ~ 0 and ~1 to the simulation results, we conclude that the typical object in our samples will evolve into an elliptical galaxy by redshift z = 0 and will already have an early-type spectrum by redshift z = 1. We comment briefly on the implied relationship between galaxies in our survey and those selected with other techniques.

Star formation and extinction in redshift z~2 galaxies: inferences from spitzer mips observations

We use very deep Spitzer MIPS 24 ?m observations to examine the bolometric luminosities (Lbol) and UV extinction properties of more than 200 spectroscopically identified, optically selected (UnG) z ~ 2 galaxies, supplemented with near-IR-selected (BzK and DRG) and submillimeter galaxies at similar redshifts, in the GOODS-N field. Focusing on redshifts 1.5 1012 L?, with a mean LIR 2 ? 1011 L?. Using 24 ?m observations as an independent probe of dust extinction, we find that, as in the local universe, the obscuration LIR/L1600 is strongly dependent on Lbol and ranges in value from <1 to ~1000 within the sample considered. However, the obscuration is generally ~10 times smaller at a given Lbol at z ~ 2 than at z ~ 0. We show that the values of LIR and obscuration inferred from the UV spectral slope ? generally agree well with the values inferred from L5-8.5 ?m for Lbol < 1012 L?. Using the specific SFRs of galaxies as a proxy for cold gas fraction, we find a wide range in the evolutionary state of galaxies at z ~ 2, from galaxies that have just begun to form stars to those that have already accumulated most of their stellar mass and are about to become, or already are, passively evolving.