Ranga Ram Chary

UV CONTINUUM SLOPE AND DUST OBSCURATION FROM z ∼ 6 TO z ∼ 2: THE STAR FORMATION RATE DENSITY AT HIGH REDSHIFT*

We provide a systematic measurement of the rest-frame UV continuum slope β over a wide range in redshift (z ~ 2-6) and rest-frame UV luminosity (0.1 L^*_(z = 3) to 2 L^*_(z = 3)) to improve estimates of the star formation rate (SFR) density at high redshift. We utilize the deep optical and infrared data (Advanced Camera for Surveys/NICMOS) over the Chandra Deep Field-South and Hubble Deep Field-North Great Observatories Origins Deep Survey fields, as well as the UDF for our primary UBVi dropout Lyman Break Galaxy sample. We also use strong lensing clusters to identify a population of very low luminosity, high-redshift dropout galaxies. We correct the observed distributions for both selection biases and photometric scatter. We find that the UV-continuum slope of the most luminous galaxies is substantially redder at z ~ 2-4 than it is at z ~ 5-6 (from ~–2.4 at z ~ 6 to ~–1.5 at z ~ 2). Lower luminosity galaxies are also found to be bluer than higher luminosity galaxies at z ~ 2.5 and z ~ 4. We do not find a large number of galaxies with βs as red as –1 in our dropout selections at z ~ 4, and particularly at z ≳ 5, even though such sources could be readily selected from our data (and also from Balmer Break Galaxy searches at z ~ 4). This suggests that star-forming galaxies at z ≳ 5 almost universally have very blue UV-continuum slopes, and that there are not likely to be a substantial number of dust-obscured galaxies at z ≳ 5 that are missed in dropout searches. Using the same relation between UV-continuum slope and dust extinction as has been found to be appropriate at both z ~ 0 and z ~ 2, we estimate the average dust extinction of galaxies as a function of redshift and UV luminosity in a consistent way. As expected, we find that the estimated dust extinction increases substantially with cosmic time for the most UV luminous galaxies, but remains small (≾2 times) at all times for lower luminosity galaxies. Because these same lower luminosity galaxies dominate the luminosity density in the UV continuum, the overall dust extinction correction remains modest at all redshifts and the evolution of this correction with redshift is only modest. We include the contribution from ultra-luminous IR galaxies in our SFR density estimates at z ~ 2-6, but find that they contribute only ~20% of the total at z ~ 2.5 and ≾10% at z ≳ 4.

Multiwavelength Study of Massive Galaxies at z ~ 2. II. Widespread Compton-thick Active Galactic Nuclei and the Concurrent Growth of Black Holes and Bulges

Approximately 20‐30% of 1.4 6.2 keV. The stacked X-ray spectrum rises steeply at > 10 keV, suggesting that these sources host Compton-thick Active Galactic Nuclei (AGNs) with column densities NH > ∼ 10 24 cm −2 and an average, unobscured X-ray luminosity L2−8keV ≈(1‐4) × 10 43 erg s −1 . Their sky density (∼ 3200 deg −2 ) and space density (∼ 2.6 × 10 −4 Mpc −3 ) are twice those of X-ray detected AGNs at z ≈ 2, and much larger than those of previously-known Compton thick sources at similar redshifts. The mid-IR excess galaxies are part of the long sought-after population of distant heavily obscured AGNs predicted by synthesis models of the X-ray background. The fraction of mid-IR excess objects increases with galaxy mass, reaching ∼ 50‐60% for M ∼ 10 11 M⊙, an effect likely connected with downsizing in galaxy formation. The ratio of the inferred black hole growth rate from these Compton-thick sources to the global star formation rate at z = 2 is similar to the mass ratio of black holes to stars in local s pheroids, implying concurrent growth of both within the precursors of today’s massive galaxies. Subject headings:galaxies: evolution — galaxies: formation — galaxies: active — X-rays: galaxies

Reliable Identification of Compton-thick Quasars at z ≈ 2: Spitzer Mid-Infrared Spectroscopy of HDF-oMD49

Manymodelsthatseektoexplaintheoriginof theunresolvedX-raybackgroundpredictthatCompton-thickactive galactic nuclei (AGNs) are ubiquitous at high redshift. However, few distant Compton-thick AGNs have been reliably identified to date. Here we present Spitzer IRS spectroscopy and 3.6Y70 � m photometry of a z ¼ 2:211 optically identified AGN (HDF-oMD49) that is formally undetected in the 2 Ms Chandra Deep FieldYNorth (CDF-N) survey.TheSpitzerIRSspectrumandspectralenergydistributionof thisobjectisAGNdominated,andacomparison of the energetics at X-ray wavelengths to those derived from mid-infrared (mid-IR) and optical spectroscopy shows that the AGN is intrinsically luminous (L2Y10 keV � 3 ;10 44 ergs s � 1 ) but heavily absorbed by Compton-thick material (NH 310 24 cm � 2 ); i.e., this object is a Compton-thick quasar. Adopting the same approach that we applied to HDF-oMD49, we found a further six objects at z � 2Y2:5 in the literature that are also X-ray weak/undetected but haveevidenceforAGNactivityfromopticaland/ormid-IRspectroscopy,andshowthatallof thesesourcesarelikely to be Compton-thick quasars with L2Y10 keV >10 44 ergs s � 1 . On the basis of the definition of Daddi et al., these Compton-thick quasars would be classified as mid-IR excess galaxies, and our study provides the first spectroscopic confirmation of Compton-thick AGN activity in a subsample of these z � 2 mid-IR-bright galaxies. Using the four objects that lie in the CDF-N field, we estimate the space density of reliably identified Compton-thick quasars [� � (0:7Y2:5) ;10 � 5 Mpc � 3 for L2Y10 keV >10 44 ergs s � 1 objects at z � 2Y2:5] and show that Compton-thick accretion was probably as ubiquitous as unobscured accretion in the distant universe. Subject headingg galaxies: active — galaxies: high-redshift — infrared: galaxies — ultraviolet: galaxies — X-rays: galaxies

BALANCING THE ENERGY BUDGET BETWEEN STAR FORMATION AND ACTIVE GALACTIC NUCLEI IN HIGH-REDSHIFT INFRARED LUMINOUS GALAXIES

We present deep Spitzer mid-infrared spectroscopy, along with 16, 24, 70, and 850 μm photometry, for 22 galaxies located in the Great Observatories Origins Deep Survey-North (GOODS-N) field. The sample spans a redshift range of 0.6 ≾ z ≾ 2.6, 24 μm flux densities between ~0.2 and 1.2 mJy, and consists of submillimeter galaxies (SMGs), X-ray or optically selected active galactic nuclei (AGNs), and optically faint (z_(AB) > 25 mag) sources. We find that infrared (IR; 8-1000 μm) luminosities derived by fitting local spectral energy distributions (SEDs) with 24 μm photometry alone are well matched to those when additional mid-infrared spectroscopic and longer wavelength photometric data are used for galaxies having z ≾ 1.4 and 24 μm-derived IR luminosities typically ≾ 3 × 10^(12) L_⊙. However, for galaxies in the redshift range between 1.4 ≾ z ≾ 2.6, typically having 24-μm-derived IR luminosities ≳ 3 × 10^(12) L_⊙, IR luminosities are overestimated by an average factor of ~5 when SED fitting with 24 μm photometry alone. This result arises partly due to the fact that high-redshift galaxies exhibit aromatic feature equivalent widths that are large compared to local galaxies of similar luminosities. Using improved estimates for the IR luminosities of these sources, we investigate whether their infrared emission is found to be in excess relative to that expected based on extinction-corrected UV star formation rates (SFRs), possibly suggesting the presence of an obscured AGN. Through a spectral decomposition of mid-infrared spectroscopic data, we are able to isolate the fraction of IR luminosity arising from an AGN as opposed to star formation activity. This fraction is only able to account for ~30% of the total IR luminosity among the entire sample and ~35% of the excess IR emission among these sources, on average, suggesting that AGNs are not the dominant cause of the inferred mid-infrared excesses in these systems. Of the sources identified as having mid-infrared excesses, half are accounted for by using proper bolometric corrections while half show the presence of obscured AGNs. This implies sky and space densities for Compton-thick AGNs of ~1600 deg^(–2) and ~1.3 × 10^(–4) Mpc^(–3), respectively. We also note that IR luminosities derived from SED fitting the mid-infrared and 70 μm broadband photometry agree within ~50% to those values estimated using the additional mid-infrared spectroscopic and submillimeter data. An inspection of the far-infrared (FIR)-radio correlation shows no evidence for evolution over this redshift range. However, we find that the SMGs have IR/radio ratios which are a factor of ~3 lower, on average, than what is measured for star-forming galaxies in the local universe.

AN ACCOUNTING OF THE DUST-OBSCURED STAR FORMATION AND ACCRETION HISTORIES OVER THE LAST ∼11 BILLION YEARS

We report on an accounting of the star-formation- and accretion-driven energetics of 24 μm-detected sources in the Great Observatories Origins Deep Survey-North field. For sources having infrared (IR; 8-1000 μm) luminosities ≳3 × 10^(12) L_☉ when derived by fitting local spectral energy distributions (SEDs) to 24 μm photometry alone, we find these IR luminosity estimates to be a factor of ~4 times larger than those estimated when the SED fitting includes additional 16 and 70 μm data (and in some cases mid-IR spectroscopy and 850 μm data). This discrepancy arises from the fact that high-luminosity sources at z ≫ 0 appear to have far- to mid-IR ratios, as well as aromatic feature equivalent widths, typical of lower luminosity galaxies in the local universe. Using our improved estimates for IR luminosity and active galactic nucleus (AGN) contributions, we investigate the evolution of the IR luminosity density versus redshift arising from star formation and AGN processes alone. We find that, within the uncertainties, the total star-formation-driven IR luminosity density is constant between 1.15 ≲ z ≲ 2.35, although our results suggest a slightly larger value at z ≳ 2. AGNs appear to account for ≲18% of the total IR luminosity density integrated between 0 ≲ z ≲ 2.35, contributing ≲25% at each epoch. Luminous infrared galaxies (LIRGs; 10^(11) L_☉ ≤ L_(IR) < 10^(12) L_☉) appear to dominate the star formation rate density along with normal star-forming galaxies (L_(IR) < 10^(11) L_☉) between 0.6 ≲ z ≲ 1.15. Once beyond z gsim 2, the contribution from ultraluminous infrared galaxies (L IR ≥ 10^(12) L_☉) becomes comparable with that of LIRGs. Using our improved IR luminosity estimates, we find existing calibrations for UV extinction corrections based on measurements of the UV spectral slope typically overcorrect UV luminosities by a factor of ~2, on average, for our sample of 24 μm-selected sources; accordingly we have derived a new UV extinction correction more appropriate for our sample.

The Hubble Space Telescope GOODS NICMOS Survey: overview and the evolution of massive galaxies at 1.5< z< 3

We present the details and early results from a deep near-infrared survey utilizing the NICMOS instrument on the Hubble Space Telescope centred around massive M_* > 10^(11) M_⊙ galaxies at 1.7 10^(11) M_⊙, whereby we find an increase of a factor of 8 between z= 3 and 1.5, demonstrating that this is an epoch when massive galaxies establish most of their stellar mass. We also provide an overview of the evolutionary properties of these galaxies, such as their merger histories, and size evolution.

Galaxy Evolution in Overdense Environments at High Redshift: Passive Early-type Galaxies in a Cluster at z ~ 2

We present a study of galaxy populations in the central region of the IRAC-selected, X-ray-detected galaxy cluster Cl J1449+0856 at z = 2. Based on a sample of spectroscopic and photometric cluster members, we investigate stellar populations and the morphological structure of cluster galaxies over an area of ~0.7 Mpc^2 around the cluster core. The cluster stands out as a clear overdensity both in redshift space and in the spatial distribution of galaxies close to the center of the extended X-ray emission. The cluster core region (r < 200 kpc) shows a clearly enhanced passive fraction with respect to field levels. However, together with a population of massive, passive galaxies mostly with early-type morphologies, the cluster core also hosts massive, actively star-forming, often highly dust reddened sources. Close to the cluster center, a multi-component system of passive and star-forming galaxies could represent the future brightest cluster galaxy still forming. We observe a clear correlation between passive stellar populations and an early-type morphology, in agreement with field studies at similar redshift. Passive early-type galaxies in this cluster are typically a factor of 2-3 smaller than similarly massive early types at z ~ 0. On the other hand, these same objects are on average larger by a factor of ~2 than field early-types at similar redshift, lending support to recent claims of an accelerated structural evolution in high-redshift dense environments. These results point toward the early formation of a population of massive galaxies, already evolved both in their structure and stellar populations, coexisting with still actively forming massive galaxies in the central regions of young clusters 10 billion years ago.

THE SWIFT GRB HOST GALAXY LEGACY SURVEY. II. REST-FRAME NEAR-IR LUMINOSITY DISTRIBUTION AND EVIDENCE FOR A NEAR-SOLAR METALLICITY THRESHOLD

We present rest-frame near-IR (NIR) luminosities and stellar masses for a large and uniformly selected population of gamma-ray burst (GRB) host galaxies using deep Spitzer Space Telescope imaging of 119 targets from the Swift GRB Host Galaxy Legacy Survey spanning 0.03 < z < 6.3, and we determine the effects of galaxy evolution and chemical enrichment on the mass distribution of the GRB host population across cosmic history. We find a rapid increase in the characteristic NIR host luminosity between z ~ 0.5 and z ~ 1.5, but little variation between z ~ 1.5 and z ~ 5. Dust-obscured GRBs dominate the massive host population but are only rarely seen associated with low-mass hosts, indicating that massive star-forming galaxies are universally and (to some extent) homogeneously dusty at high redshift while low-mass star-forming galaxies retain little dust in their interstellar medium. Comparing our luminosity distributions with field surveys and measurements of the high-z mass–metallicity relation, our results have good consistency with a model in which the GRB rate per unit star formation is constant in galaxies with gas-phase metallicity below approximately the solar value but heavily suppressed in more metal-rich environments. This model also naturally explains the previously reported excess in the GRB rate beyond z gsim 2; metals stifle GRB production in most galaxies at z < 1.5 but have only minor impact at higher redshifts. The metallicity threshold we infer is much higher than predicted by single-star models and favors a binary progenitor. Our observations also constrain the fraction of cosmic star formation in low-mass galaxies undetectable to Spitzer to be small at z < 4.

Spitzer infrared spectrometer 16μm observations of the GOODS fields

We present Spitzer 16μm imaging of the Great Observatories Origins Deep Survey (GOODS) fields. We survey n150 arcmin^2 in each of the two GOODS fields (North and South), to an average 3σ depth of 40 and 65 μJy, nrespectively. We detect ~1300 sources in both fields combined. We validate the photometry using the 3–24μm nspectral energy distribution of stars in the fields compared to Spitzer spectroscopic templates. Comparison with nISOCAM and AKARI observations in the same fields shows reasonable agreement, though the uncertainties are nlarge. We provide a catalog of photometry, with sources cross-correlated with available Spitzer, Chandra, and nHubble Space Telescope data. Galaxy number counts show good agreement with previous results from ISOCAM nand AKARI with improved uncertainties. We examine the 16–24μm flux ratio and find that for most sources it nlies within the expected locus for starbursts and infrared luminous galaxies. A color cut of S_(16)/S_(24) > 1.4 selects nmostly sources which lie at 1.1 < z < 1.6, where the 24μm passband contains both the redshifted 9.7 μm silicate nabsorption and the minimum between polycyclic aromatic hydrocarbon emission peaks. We measure the integrated ngalaxy light of 16μm sources and find a lower limit on the galaxy contribution to the extragalactic background nlight at this wavelength to be 2.2 ± 0.2 nW m^(−2) sr^(−1).

Measuring PAH Emission in Ultradeep Spitzer IRS Spectroscopy of High Redshift IR Luminous Galaxies

The study of the dominant population of high-redshift IR-luminous galaxies (1011-1012 L☉ at 1 < z < 3), requires observation of sources at the ~0.1 mJy level in the mid-IR. We present the deepest spectra taken to date with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. We targeted two faint (f24 ~ 0.15 mJy) sources in the Southern GOODS field at z = 1.09 and z = 2.69. Spectra of the lower redshift target were taken in the observed-frame 8-21 μm range, while the spectrum of the higher redshift target covered 21-37 μm. We also present the spectra of two secondary sources within the slit. We detect strong PAH emission in all four targets, and compare the spectra to those of local galaxies observed by the IRS. The z = 1.09 source appears to be a typical, star-formation-dominated IR-luminous galaxy, while the z = 2.69 source is a composite source with strong star formation and a prominent AGN. The IRAC colors of this source show no evidence of rest-frame near-IR stellar photospheric emission. We demonstrate that an AGN that contributes only a small (~10%) fraction of the bolometric luminosity can produce enough hot dust emission to overwhelm the near-IR photospheric emission from stars. Such sources would be excluded from photometric surveys that rely on the near-IR bump to identify starbursts, leading to an underestimate of the star formation rate density.