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Kocevski D.D.,University of California at Santa Cruz | Faber S.M.,University of California at Santa Cruz | Mozena M.,University of California at Santa Cruz | Koekemoer A.M.,US Space Telescope Science Institute | And 36 more authors.
Astrophysical Journal | Year: 2012

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 1042-44ergs-1) AGNs at 1.5 < z < 2.5 that are selected using the 4Ms 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. © 2012 The American Astronomical Society. All rights reserved.


Seymour N.,CSIRO | Seymour N.,University College London | Altieri B.,European Space Agency | De Breuck C.,European Southern Observatory | And 34 more authors.
Astrophysical Journal | Year: 2012

We present a detailed study of the infrared spectral energy distribution of the high-redshift radio galaxy MRC1138-26 at z = 2.156, also known as the Spiderweb Galaxy. By combining photometry from Spitzer, Herschel, and LABOCA we fit the rest-frame 5-300 μm emission using a two-component, starburst, and active galactic nucleus (AGN) model. The total infrared (8-1000 μm) luminosity of this galaxy is (1.97 ± 0.28) ×1013 L ⊙ with (1.17 ± 0.27) and (0.79 ± 0.09) ×1013 L ⊙ due to the AGN and starburst components, respectively. The high derived AGN accretion rate of 20% Eddington and the measured star formation rate (SFR) of 1390 ± 150 M ⊙yr-1 suggest that this massive system is in a special phase of rapid central black hole and host galaxy growth, likely caused by a gas-rich merger in a dense environment. The accretion rate is sufficient to power both the jets and the previously observed large outflow. The high SFR and strong outflow suggest that this galaxy could potentially exhaust its fuel for stellar growth in a few tens of Myr, although the likely merger of the radio galaxy with nearby satellites suggests that bursts of star formation may recur again on timescales of several hundreds of Myr. The age of the radio lobes implies the jet started after the current burst of star formation, and therefore we are possibly witnessing the transition from a merger-induced starburst phase to a radio-loud AGN phase. We also note tentative evidence for [C II]158 μm emission. This paper marks the first results from the Herschel Galaxy Evolution Project (Project HeRGÉ), a systematic study of the evolutionary state of 71 high-redshift, 1 < z < 5.2, radio galaxies. © 2012. The American Astronomical Society. All rights reserved.


Rawlings J.I.,University College London | Seymour N.,University College London | Seymour N.,CSIRO | Page M.J.,University College London | And 17 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

We present the mid-infrared (IR) spectra of seven of the most powerful radio-galaxies known to exist at 1.5


Guo Y.,University of Massachusetts Amherst | Giavalisco M.,University of Massachusetts Amherst | Cassata P.,University of Massachusetts Amherst | Ferguson H.C.,US Space Telescope Science Institute | And 16 more authors.
Astrophysical Journal | Year: 2012

A new set of color selection criteria (VJL) analogous with the BzK method is designed to select both star-forming galaxies (SFGs) and passively evolving galaxies (PEGs) at 2.3 ≲ z ≲ 3.5 by using rest-frame UV-optical (V - J versus J - L) colors. The criteria are thoroughly tested with theoretical stellar population synthesis models and real galaxies with spectroscopic redshifts to evaluate their efficiency and contamination. We apply the well-tested VJL criteria to the HST/WFC3 Early Release Science field and study the physical properties of selected galaxies. The redshift distribution of selected SFGs peaks at z ∼ 2.7, slightly lower than that of Lyman break galaxies at z ∼ 3. Comparing the observed mid-infrared fluxes of selected galaxies with the prediction of pure stellar emission, we find that our VJL method is effective at selecting massive dusty SFGs that are missed by the Lyman break technique. About half of the star formation in massive (Mstar > 1010 M⊙) galaxies at 2.3 ≲ z ≲ 3.5 is contributed by dusty (extinction E(B - V) > 0.4) SFGs, which, however, only account for ∼20% of the number density of massive SFGs. We also use the mid-infrared fluxes to clean our PEG sample and find that galaxy size can be used as a secondary criterion to effectively eliminate the contamination of dusty SFGs. The redshift distribution of the cleaned PEG sample peaks at z ∼ 2.5. We find six PEG candidates at z > 3 and discuss possible methods to distinguish them from dusty contamination. We conclude that at least part of our candidates are real PEGs at z ∼ 3, implying that these types of galaxies began to form their stars at z ≳ 5. We measure the integrated stellar mass density (ISMD) of PEGs at z ∼ 2.5 and set constraints on it at z > 3. We find that the ISMD grows by at least about a factor of 10 in 1Gyr at 3 < z <5 and by another factor of 10 in the next 3.5Gyr (1 < z < 3). © 2012 The American Astronomical Society. All rights reserved.


Bell E.F.,University of Michigan | Van Der Wel A.,Max Planck Institute for Astronomy | Papovich C.,Texas A&M University | Kocevski D.,University of California at Santa Cruz | And 21 more authors.
Astrophysical Journal | Year: 2012

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 × 10 10 M ⊙ from z = 2.2 to the present epoch, a time span of 10Gyr. 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 Sérsic index, and the population of concentrated galaxies grows similarly rapidly. We examine the joint distribution of star formation activity, Sérsic index, stellar mass, inferred velocity dispersion, and stellar surface density. Quiescence correlates poorly with stellar mass at all z < 2.2. Quiescence correlates well with Sérsic index at all redshifts. Quiescence correlates well with "velocity dispersion" and stellar surface density at 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 10Gyr, in qualitative agreement with the active galactic nucleus feedback paradigm. © 2012. The American Astronomical Society. All rights reserved.


Cassata P.,University of Massachusetts Amherst | Giavalisco M.,University of Massachusetts Amherst | Guo Y.,University of Massachusetts Amherst | Renzini A.,National institute for astrophysics | And 10 more authors.
Astrophysical Journal | Year: 2011

We report on the evolution of the number density and size of early-type galaxies (ETGs) from z ∼ 2 to z ∼ 0. We select a sample of 563 massive (M > 1010 M ⊙), passively evolving (specific star formation rate <10-2 Gyr-1), and morphologically spheroidal galaxies at 0 < z < 2.5, using the panchromatic photometry and spectroscopic redshifts available in the Great Observatories Origins Deep Surveys fields. We combine Advanced Camera for Surveys and Wide Field Camera 3 Hubble Space Telescope images to study the morphology of our galaxies in their optical rest frame in the entire 0 < z < 2.5 range. We find that throughout the explored redshift range the passive galaxies selected with our criteria have weak morphological K-correction, with size being slightly smaller in the optical than in the UV rest frame (by ∼20% and ∼10% at z > 1.2 and z < 1.2, respectively). We measure a significant evolution of the mass-size relation of ETGs, with a fractional increment that is almost independent of the stellar mass. ETGs formed at z > 1 appear to be preferentially small, and the evolution of the mass-size relation at z < 1 is driven by both the continuous size growth of the compact galaxies and the appearance of new ETGs with large sizes. We also find that the number density of all passive ETGs increases rapidly, by a factor of five, from z ∼ 2 to z ∼ 1, and then more mildly by another factor of 1.5 from z ∼ 1 to z ∼ 0. We interpret these results as evidence that the bulk of the ETGs are formed at 1 < z < 3 through a mechanism that leaves very compact remnants. At z < 1 the compact ETGs grow gradually in size, becoming normal-size galaxies, and at the same time new ETGs with normal-large sizes are formed. © 2011. The American Astronomical Society. All rights reserved.


Cassata P.,University of Massachusetts Amherst | Giavalisco M.,University of Massachusetts Amherst | Guo Y.,University of Massachusetts Amherst | Ferguson H.,US Space Telescope Science Institute | And 14 more authors.
Astrophysical Journal Letters | Year: 2010

We present near-IR images, obtained with the Hubble Space Telescope and the WFC3/IR camera, of six passive and massive galaxies at redshift 1.3 < z < 2.4 (specific star formation rate <10-2 Gyr-1; stellar mass M ∼ 1011 M ⊙), selected from the Great Observatories Origins Deep Survey. These images, which have a spatial resolution of ∼ 1.5 kpc, provide the deepest view of the optical rest-frame morphology of such systems to date. We find that the light profile of these galaxies is regular and well described by a Sérsic model with index typical of today's spheroids. Their size, however, is generally much smaller than today's early types of similar stellar masses, with four out of six galaxies having re ∼ 1 kpc or less, in quantitative agreement with previous similar measures made at rest-frame UV wavelengths. The images reach limiting surface brightness μ ∼ 26.5magarcsec-2 in the F160W bandpass; yet, there is no evidence of a faint halo in the galaxies of our sample, even in their stacked image. We also find that these galaxies have very weak "morphological k-correction" between the rest-frame UV (from the Advanced Camera for Surveys z band) and the rest-frame optical (WFC3 H band): the Sérsic index, physical size, and overall morphology are independent or only mildly dependent on the wavelength, within the errors. © 2010. The American Astronomical Society.


Guo Y.,University of Massachusetts Amherst | Giavalisco M.,University of Massachusetts Amherst | Cassata P.,University of Massachusetts Amherst | Ferguson H.C.,US Space Telescope Science Institute | And 9 more authors.
Astrophysical Journal | Year: 2011

We report the detection of color gradients in six massive (stellar mass (M star) > 1010 M ) and passively evolving (specific star formation rate <10-11 yr-1) galaxies at redshift 1.3 < z < 2.5 identified in the Hubble Ultra Deep Field using ultra-deep Hubble Space Telescope (HST) Advanced Camera for Surveys and WFC3/IR images. After carefully matching the different point-spread functions, we obtain color maps and multi-band optical/near-IR photometry (BVizYJH) in concentric annuli, from the smallest resolved radial distance (1.7kpc) up to several times the H-band effective radius. We find that the inner regions of these galaxies have redder rest-frame UV-optical colors (U- V, U- B, and B- V) than the outer parts. The slopes of the color gradient have no obvious dependence on the redshift and on the stellar mass of the galaxies. They do mildly depend, however, on the overall dust obscuration (E(B - V)) and rest-frame (U- V) color, with more obscured or redder galaxies having steeper color gradients. The z 2 color gradients are also steeper than those of local early-type ones. The gradient of a single parameter (age, extinction, or metallicity) cannot fully explain the observed color gradients. Fitting the spatially resolved HST seven-band photometry to stellar population synthesis models, we find that, regardless of assumptions on the metallicity gradient, the redder inner regions of the galaxies have slightly higher dust obscuration than the bluer outer regions, implying that dust partly contributes to the observed color gradients, although the magnitude depends on the assumed extinction law. Due to the age-metallicity degeneracy, the derived age gradient depends on the assumptions for the metallicity gradient. We discuss the implications of a number of assumptions for metallicity gradients on the formation and evolution of these galaxies. We find that the evolution of the mass-size relationship from z 2 to the present cannot be driven by in situ extended star formation, which implies that accretion or merger is mostly responsible for the growth of their stellar mass and size. The lack of a correlation between the strength of the color gradient and the stellar mass argues against the metallicity gradient predicted by the monolithic-collapse scenario, which would require significant major mergers to evolve into the one observed at the present. © 2011. The American Astronomical Society. All rights reserved..


Talia M.,University of Bologna | Mignoli M.,National institute for astrophysics | Cimatti A.,University of Bologna | Kurk J.,Max Planck Institute for Extraterrestrial Physics | And 12 more authors.
Astronomy and Astrophysics | Year: 2012

Aims. We use rest-frame UV spectroscopy to investigate the properties related to large-scale gas outflow, and both the dust extinction and star-formation rates (SFRs) of a sample of z ~ 2 star-forming galaxies from the Galaxy Mass Assembly ultradeep Spectroscopic Survey (GMASS). Methods. Dust extinction is estimated from the rest-frame UV continuum slope and used to obtain dust-corrected SFRs for the galaxies in the sample. A composite spectrum is created by averaging all the single spectra of our sample, and the equivalent widths and centroids of the absorption lines associated with the interstellar medium are measured. We then calculate the velocity offsets of these lines relative to the composite systemic velocity, which is obtained from photospheric stellar absorption lines and nebular emission lines. Finally, to investigate correlations between galaxy UV spectral characteristics and galaxy general properties, the sample is divided into two bins that are equally populated, according to the galaxy properties of stellar mass, color excess, and SFR. A composite spectrum for each group of galaxies is then created, and both the velocity offsets and the equivalent widths of the interstellar absorption lines are measured. Results. For the entire sample, we derive a mean value of the continuum slope β= -1.11 ± 0.44 (rms). For each galaxy, we calculate the dust extinction from the UV spectrum and then use this to correct the flux measured at 1500 (rest-frame), before converting the corrected UV flux into a SFR. We find that our galaxies have an average SFR of SFR= 52 ± 48 Mo yr -1 (rms) and that there is a positive correlation between SFR and stellar mass, in agreement with other works, the logarithmic slope of the relation being 1.10 ± 0.10. We discover that the low-ionization absorption lines associated with the interstellar medium measured in the composite spectrum, are blueshifted with respect to the rest frame of the system, which indicates that there is outflowing gas with typical velocities of about -100 kms -1. Finally, investigating the correlations between the galaxy UV spectral characteristics and general galaxy properties, we find a possible correlation between the equivalent width of the interstellar absorption lines and SFR, stellar mass, and color excess similar to that previously reported to hold at different redshifts. © ESO, 2012.


Kurk J.,Max Planck Institute for Physics | Kurk J.,Max Planck Institute for Astronomy | Kurk J.,National institute for astrophysics | Cimatti A.,National institute for astrophysics | And 14 more authors.
Astronomy and Astrophysics | Year: 2013

Context. Ultra-deep imaging of small parts of the sky has revealed many populations of distant galaxies, providing insight into the early stages of galaxy evolution. Spectroscopic follow-up has mostly targeted galaxies with strong emission lines at z > 2 or concentrated on galaxies at z < 1. Aims. The populations of both quiescent and actively star-forming galaxies at 1 < z < 2 are still under-represented in our general census of galaxies throughout the history of the Universe. In the light of galaxy formation models, however, the evolution of galaxies at these redshifts is of pivotal importance and merits further investigation. In addition, photometry provides only limited clues about the nature and evolutionary status of these galaxies.We therefore designed a spectroscopic observing campaign of a sample of both massive, quiescent and star-forming galaxies at z > 1.4. Methods. To determine redshifts and physical properties, such as metallicity, dust content, dynamical masses, and star formation history, we performed ultra-deep spectroscopy with the red-sensitive optical spectrograph FORS2 at the Very Large Telescope. We first constructed a sample of objects, within the CDFS/GOODS area, detected at 4.5 μm, to be sensitive to stellar mass rather than star formation intensity. The spectroscopic targets were selected with a photometric redshift constraint (z > 1.4) and magnitude constraints (BAB < 26, IAB < 26.5), which should ensure that these are faint, distant, and fairly massive galaxies. Results. We present the sample selection, survey design, observations, data reduction, and spectroscopic redshifts. Up to 30 h of spectroscopy of 174 spectroscopic targets and 70 additional objects enabled us to determine 210 redshifts, of which 145 are at z > 1.4. The redshift distribution is clearly inhomogeneous with several pronounced redshift peaks. From the redshifts and photometry, we deduce that the BzK selection criteria are efficient (82%) and suffer low contamination (11%). Several papers based on the GMASS survey show its value for studies of galaxy formation and evolution. We publicly release the redshifts and reduced spectra. In combination with existing and on-going additional observations in CDFS/GOODS, this data set provides a legacy for future studies of distant galaxies. © ESO 2012.

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