Mauro Giavalisco

Multiwavelength Study of Massive Galaxies at z~2. I. Star Formation and Galaxy Growth

Examining a sample of massive galaxies at 1.4 10 11 L⊙, show a mid-IR excess which is likely due to the presence of obscured active nuclei, as shown in a companion paper. There is a tight and roughly linear correlation between stellar mass and SFR for 24µm-detected galaxies. For a given mass, the SFR at z = 2 was larger by a factor of ∼ 4 and ∼ 30 relative to that in star forming galaxies at z = 1 and z = 0, respectively. Typical ultraluminous infrared galaxies (ULIRGs) at z = 2 are relatively ’transparent’ to ultraviolet light, and their activity is long lived ( > ∼ 400 Myr), unlike that in local ULIRGs and high redshift submillimeter-selected galaxies. ULIRGs are the common mode of star formation in massive galaxies at z = 2, and the high duty cycle suggests that major mergers are not the dominant trigger for this activity. Current galaxy formation models underpredict the normalization of the mass-SFR correlation by about a factor of 4, and the space density of ULIRGs by an order of magnitude, but give better agreement for z > 1.4 quiescent galaxies. Subject headings: galaxies: evolution — galaxies: formation — cosmology: observations — galaxies: starbursts — galaxies: high-redshift

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

We present the deepest 100 to 500 μm far-infrared observations obtained with the Herschel Space Observatory as part of the GOODS-Herschel key program, and examine the infrared (IR) 3–500 μm 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. We find that the ratio of total IR luminosity to rest-frame 8 μm 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 (  3 × 10^(10) L_⊙ kpc^(-2)) and a high specific star formation rate (i.e., starbursts). The rest-frame, UV-2700 A 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. Locally, 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) ~ 15–50 K), and an effective T_(dust)  ~ 31 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)~ 40 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.

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

TWO BRIGHT SUBMILLIMETER GALAXIES IN A z=4.05 PROTOCLUSTER IN GOODS-NORTH, AND ACCURATE RADIO-INFRARED PHOTOMETRIC REDSHIFTS

We present the serendipitous discovery of molecular gas CO emission lines with the IRAM Plateau de Bure interferometer coincident with two luminous submillimeter galaxies (SMGs) in the Great Observatories Origins Deep Survey North field (GOODS-N). The identification of the millimeter emission lines as CO[4-3] at z = 4.05 is based on the optical and near-IR photometric redshifts, radio-infrared photometric redshifts and Keck+DEIMOS optical spectroscopy. These two galaxies include the brightest submillimeter source in the field (GN20; S850µm = 20.3mJy, zCO = 4.055 ± 0.001) and its companion (GN20.2; S850µm = 9.9mJy, zCO = 4.051 ± 0.003). These are among the most distant submillimeter-selected galaxies reliably identified through CO emission and also some of the most luminous known. GN20.2 has a possible additional counterpart and a luminous AGN inside its primary counterpart revealed in the radio. Continuum emission of 0.3mJy at 3.3mm (0.65mm in the rest frame) is detected at 5� for GN20, the first dust continuum detection in an SMG at such long wavelength, unveiling a spectral energy distribution that is similar to local ultra luminous infrared galaxies. In terms of CO to bolometric luminosities, stella r mass and star formation rates (SFRs), these newly discovered z > 4 SMGs are similar to z ∼ 2 − 3 SMGs studied to date. These z ∼ 4 SMGs have much higher specific SFRs than typical B-band dropout Lyman break galaxi es at the same redshift. The stellar mass-SFR correlation for normal galaxies does not seem to evolve much further, between z ∼ 2 and z ∼ 4. A significant z = 4.05 spectroscopic redshift spike is observed in GOODS-N, and a strong spatial overdensity of B-band dropouts and IRAC selected z > 3.5 galaxies appears to be centered on the GN20 and GN20.2 galaxies. This suggests a proto-cluster structure with total mass ∼ 10 14 M⊙. Using photometry at mid-IR (24µm), submm (850µm) and radio (20cm) wavelengths, we show that reliable photometric redshifts (�z/(1+ z) ∼ 0.1) can be derived for SMGs over 1 < ∼ z < ∼ 4. This new photometric redshift technique has been used to provide a first estimate of the space density of 3.5 < z < 6 hyper-luminous starburst galaxies, and to show that they both contribute substantially to the SFR density at early epochs and that they can account for the presence of old galaxies at z ∼ 2 − 3. Many of these high-redshift starbursts will be within rea ch of Herschel. We find that the

CANDELS: Constraining the AGN-Merger Connection with Host Morphologies at z 2

Using Hubble Space Telescope/WFC3 imaging taken as part of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey, we examine the role that major galaxy mergers play in triggering active galactic nucleus (AGN) activity at z ~ 2. Our sample consists of 72 moderate-luminosity (L X ~ 1042-44 erg s–1) AGNs at 1.5 < z < 2.5 that are selected using the 4 Ms Chandra observations in the Chandra Deep Field South, the deepest X-ray observations to date. Employing visual classifications, we have analyzed the rest-frame optical morphologies of the AGN host galaxies and compared them to a mass-matched control sample of 216 non-active galaxies at the same redshift. We find that most of the AGNs reside in disk galaxies (51.4+5.8 – 5.9%), while a smaller percentage are found in spheroids (27.8+5.8 – 4.6%). Roughly 16.7+5.3 – 3.5% of the AGN hosts have highly disturbed morphologies and appear to be involved in a major merger or interaction, while most of the hosts (55.6+5.6 – 5.9%) appear relatively relaxed and undisturbed. These fractions are statistically consistent with the fraction of control galaxies that show similar morphological disturbances. These results suggest that the hosts of moderate-luminosity AGNs are no more likely to be involved in an ongoing merger or interaction relative to non-active galaxies of similar mass at z ~ 2. The high disk fraction observed among the AGN hosts also appears to be at odds with predictions that merger-driven accretion should be the dominant AGN fueling mode at z ~ 2, even at moderate X-ray luminosities. Although we cannot rule out that minor mergers are responsible for triggering these systems, the presence of a large population of relatively undisturbed disk-like hosts suggests that the stochastic accretion of gas plays a greater role in fueling AGN activity at z ~ 2 than previously thought.

The Evolution of the Galaxy Rest-Frame Ultraviolet Luminosity Function Over the First Two Billion Years

We present a robust measurement and analysis of the rest-frame ultraviolet (UV) luminosity functions at z = 4-8. We use deep Hubble Space Telescope imaging over the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey/GOODS fields, the Hubble Ultra Deep Field, and the Hubble Frontier Field deep parallel observations near the Abell 2744 and MACS J0416.1-2403 clusters. The combination of these surveys provides an effective volume of 0.6-1.2 x 10(6) Mpc(3) over this epoch, allowing us to perform a robust search for faint (M-UV = -18) and bright (Muv \textless -21) high-redshift galaxies. We select candidate galaxies using a well-tested photometric redshift technique with careful screening of contaminants, finding a sample of 7446 candidate galaxies at 3.5 \textless z \textless 8.5, with \textgreater1000 galaxies at z approximate to 6-8. We measure both a stepwise luminosity function for candidate galaxies in our redshift samples, and a Schechter function, using a Markov Chain Monte Carlo analysis to measure robust uncertainties. At the faint end, our UV luminosity functions agree with previous studies, yet we find a higher abundance of UV-bright candidate galaxies at z \textgreater= 6. Our best-fit value of the characteristic magnitude MN is consistent with -21 at z \textgreater=, 5, which is different than that inferred based on previous trends at lower redshift, and brighter at similar to 2 sigma significance than previous measures at z = 6 and 7. At z = 8, a single power law provides an equally good fit to the UV luminosity function, while at z = 6 and 7 an exponential cutoff at the bright end is moderately preferred. We compare our luminosity functions to semi-analytical models, and find that the lack of evolution in M-UV(*) is consistent with models where the impact of dust attenuation on the bright end of the luminosity function decreases at higher redshift, although a decreasing impact of feedback may also be possible. We measure the evolution of the cosmic star-formation rate (SFR) density by integrating our observed luminosity functions to M-UV = -17, correcting for dust attenuation, and find that the SFR density declines proportionally to (1 +z)(-4.3 +/- 0 5) at z \textgreater 4, which is consistent with observations at z \textgreater= 9. Our observed luminosity functions are consistent with a reionization history that starts at z greater than or similar to 10, completes at z \textgreater 6, and reaches a midpoint (xH = 0.5) at 6.7 \textless z \textless9.4. Finally, using a constant cumulative number density selection and an empirically derived rising star-formation history, our observations predict that the abundance of bright z = 9 galaxies is likely higher than previous constraints, although consistent with recent estimates of bright z similar to 10 galaxies.

CANDELS: The Progenitors of Compact Quiescent Galaxies at z 2

We combine high-resolution Hubble Space Telescope/WFC3 images with multi-wavelength photometry to track the evolution of structure and activity of massive (M > 1010 M ?) galaxies at redshifts z = 1.4-3 in two fields of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey. We detect compact, star-forming galaxies (cSFGs) whose number densities, masses, sizes, and star formation rates (SFRs) qualify them as likely progenitors of compact, quiescent, massive galaxies (cQGs) at z = 1.5-3. At z 2, cSFGs present SFR = 100-200 M ? yr?1, yet their specific star formation rates (sSFR ~ 10?9?yr?1) are typically half that of other massive SFGs at the same epoch, and host X-ray luminous active galactic nuclei (AGNs) 30?times (~30%) more frequently. These properties suggest that cSFGs are formed by gas-rich processes (mergers or disk-instabilities) that induce a compact starburst and feed an AGN, which, in turn, quench the star formation on dynamical timescales (few 108?yr). The cSFGs are continuously being formed at z = 2-3 and fade to cQGs down to z ~ 1.5. After this epoch, cSFGs are rare, thereby truncating the formation of new cQGs. Meanwhile, down to z = 1, existing cQGs continue to enlarge to match local QGs in size, while less-gas-rich mergers and other secular mechanisms shepherd (larger) SFGs as later arrivals to the red sequence. In summary, we propose two evolutionary tracks of QG formation: an early (z 2), formation path of rapidly quenched cSFGs fading into cQGs that later enlarge within the quiescent phase, and a late-arrival (z 2) path in which larger SFGs form extended QGs without passing through a compact state.

CANDELS: THE EVOLUTION OF GALAXY REST-FRAME ULTRAVIOLET COLORS FROM z = 8 TO 4

We study the evolution of galaxy rest-frame ultraviolet (UV) colors in the epoch 4 z 8. We use new wide-field near-infrared data in the Great Observatories Origins Deep Survey-South field from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey, Hubble Ultra Deep Field (HUDF) 2009, and Early Release Science programs to select galaxies via photometric redshift measurements. Our sample consists of 2812 candidate galaxies at z 3.5, including 113 at z 7-8. We fit the observed spectral energy distribution to a suite of synthetic stellar population models and measure the value of the UV spectral slope (?) from the best-fit model spectrum. We run simulations to show that this measurement technique results in a smaller scatter on ? than other methods, as well as a reduced number of galaxies with catastrophically incorrect ? measurements (i.e., ?? > 1). We find that the median value of ? evolves significantly from ?1.82+0.00 ? 0.04 at z?= 4 to ?2.37+0.26 ? 0.06 at z?= 7. Additionally, we find that faint galaxies at z?= 7 have ? = ?2.68+0.39 ? 0.24 (~ ?2.4 after correcting for observational bias); this is redder than previous claims in the literature and does not require exotic stellar populations (e.g., very low metallicities or top-heavy initial mass functions) to explain their colors. This evolution can be explained by an increase in dust extinction, from low amounts at z?= 7 to A V ~ 0.5?mag at z?= 4. The timescale for this increase is consistent with low-mass asymptotic giant branch stars forming the bulk of the dust. We find no significant (<2?) correlation between ? and M UV when measuring M UV at a consistent rest-frame wavelength of 1500??. This is particularly true at bright magnitudes, though our results do show evidence for a weak correlation at faint magnitudes when galaxies in the HUDF are considered separately, hinting that dynamic range in sample luminosities may play a role. We do find a strong correlation between ? and the stellar mass at all redshifts, in that more massive galaxies exhibit redder colors. The most massive galaxies in our sample have similarly red colors at each redshift, implying that dust can build up quickly in massive galaxies and that feedback is likely removing dust from low-mass galaxies at z ? 7. Thus, the stellar-mass?metallicity relation, previously observed up to z ~ 3, may extend out to z?= 7-8.

The Herschel view of the dominant mode of galaxy growth from z = 4 to the present day

We present an analysis of the deepest Herschel images in four major extragalactic fields GOODS–North, GOODS–South, UDS, and COSMOS obtained within the GOODS–Herschel and CANDELS–Herschel key programs. The star formation picture provided by a total of 10 497 individual far-infrared detections is supplemented by the stacking analysis of a mass complete sample of 62 361 star-forming galaxies from the Hubble Space Telescope (HST) H band-selected catalogs of the CANDELS survey and from two deep ground-based Ks band-selected catalogs in the GOODS–North and the COSMOS-wide field to obtain one of the most accurate and unbiased understanding to date of the stellar mass growth over the cosmic history. We show, for the first time, that stacking also provides a powerful tool to determine the dispersion of a physical correlation and describe our method called “scatter stacking”, which may be easily generalized to other experiments. The combination of direct UV and far-infrared UV-reprocessed light provides a complete census on the star formation rates (SFRs), allowing us to demonstrate that galaxies at z = 4 to 0 of all stellar masses (M∗) follow a universal scaling law, the so-called main sequence of star-forming galaxies. We find a universal close-to-linear slope of the log 10(SFR)–log 10(M∗) relation, with evidence for a flattening of the main sequence at high masses (log 10(M∗/M⊙) > 10.5) that becomesless prominent with increasing redshift and almost vanishes by z ≃ 2. This flattening may be due to the parallel stellar growth of quiescent bulges in star-forming galaxies, which mostly happens over the same redshift range. Within the main sequence, we measure a nonvarying SFR dispersion of 0.3 dex: at a fixed redshift and stellar mass, about 68% of star-forming galaxies form stars at a universal rate within a factor 2. The specific SFR (sSFR = SFR/M∗) of star-forming galaxies is found to continuously increase from z = 0 to 4. Finally we discuss the implications of our findings on the cosmic SFR history and on the origin of present-day stars: more than two-thirds of present-day stars must have formed in a regime dominated by the “main sequence” mode. As a consequence we conclude that, although omnipresent in the distant Universe, galaxy mergers had little impact in shaping the global star formation history over the last 12.5 billion years.

What Turns Galaxies Off? The Different Morphologies of Star-forming and Quiescent Galaxies since z ~ 2 from CANDELS

We use HST/WFC3 imaging from the CANDELS Multi-Cycle Treasury Survey, in conjunction with the Sloan Digital Sky Survey, to explore the evolution of galactic structure for galaxies with stellar masses >3 ? 1010 M ? from z = 2.2 to the present epoch, a time span of 10?Gyr. We explore the relationship between rest-frame optical color, stellar mass, star formation activity, and galaxy structure. We confirm the dramatic increase from z = 2.2 to the present day in the number density of non-star-forming galaxies above 3 ? 1010 M ? reported by others. We further find that the vast majority of these quiescent systems have concentrated light profiles, as parameterized by the S?rsic index, and the population of concentrated galaxies grows similarly rapidly. We examine the joint distribution of star formation activity, S?rsic index, stellar mass, inferred velocity dispersion, and stellar surface density. Quiescence correlates poorly with stellar mass at all z 1.3, and somewhat less well at lower redshifts. Yet, there is significant scatter between quiescence and galaxy structure: while the vast majority of quiescent galaxies have prominent bulges, many of them have significant disks, and a number of bulge-dominated galaxies have significant star formation. Noting the rarity of quiescent galaxies without prominent bulges, we argue that a prominent bulge (and perhaps, by association, a supermassive black hole) is an important condition for quenching star formation on galactic scales over the last 10?Gyr, in qualitative agreement with the active galactic nucleus feedback paradigm.