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A texture descriptor based on the shape index and the accompanying curvedness measure is proposed, and it is evaluated for the automated analysis of astronomical image data. A representative sample of images of low-red shift galaxies from the Sloan Digital Sky Survey (SDSS) serves as a test bed. The goal of applying texture descriptors to these data is to extract novel information about galaxies, information which is often lost in more traditional analysis. In this study, we build a regression model for predicting a spectroscopic quantity, the specific star-formation rate (sSFR). As texture features we consider multi-scale gradient orientation histograms as well as multi-scale shape index histograms, which lead to a new descriptor. Our results show that we can successfully predict spectroscopic quantities from the texture in optical multi-band images. We successfully recover the observed bi-modal distribution of galaxies into quiescent and star-forming. The state-of-the-art for predicting the sSFR is a color-based physical model. We significantly improve its accuracy by augmenting the model with texture information. This study is the first step towards enabling the quantification of physical galaxy properties from imaging data alone.

Dust-enshrouded, starbursting, submillimeter galaxies (SMGs) at $z \geq 3$ have been proposed as progenitors of $z \geq 2$ compact quiescent galaxies (cQGs). To test this connection, we present a detailed spatially resolved study of the stars, dust and stellar mass in a sample of six submillimeter-bright starburst galaxies at $z \sim 4.5$. The stellar UV emission probed by HST is extended, irregular and shows evidence of multiple components. Informed by HST, we deblend Spitzer/IRAC data at rest-frame optical finding that the systems are undergoing minor mergers, with a typical stellar mass ratio of 1:6.5. The FIR dust continuum emission traced by ALMA locates the bulk of star formation in extremely compact regions (median $r_{\rm{e}} = 0.70 \pm 0.29$ kpc) and it is in all cases associated with the most massive component of the mergers (median $\log (M_{*}/M_{\odot}) = 10.49 \pm 0.32$). We compare spatially resolved UV slope ($\beta$) maps with the FIR dust continuum to study the infrared excess ($\rm{IRX} = L_{\rm{IR}}/L_{\rm{UV}}$)-$\beta$ relation. The SMGs display systematically higher $\rm{IRX}$ values than expected from the nominal trend, demonstrating that the FIR and UV emissions are spatially disconnected. Finally, we show that the SMGs fall on the mass-size plane at smaller stellar masses and sizes than cQGs at $z = 2$. Taking into account the expected evolution in stellar mass and size between $z = 4.5$ and $z = 2$ due to the ongoing starburst and mergers with minor companions, this is in agreement with a direct evolutionary connection between the two populations., Comment: Accepted by ApJ. 25 pages, 6 figures

We studied the molecular gas properties of AzTEC/C159, a star-forming disk galaxy at $z=4.567$. We secured $^{12}$CO molecular line detections for the $J=2\to1$ and $J=5\to4$ transitions using the Karl G. Jansky VLA and the NOEMA interferometer. The broad (FWHM$\sim750\,{\rm km\,s}^{-1}$) and tentative double-peaked profiles of both $^{12}$CO lines are consistent with an extended molecular gas reservoir, which is distributed in a rotating disk as previously revealed from [CII] 158$\mu$m line observations. Based on the $^{12}$CO(2$\to$1) emission line we derived $L'_{\rm{CO}}=(3.4\pm0.6)\times10^{10}{\rm \,K\,km\,s}^{-1}{\rm \,pc}^{2}$, that yields a molecular gas mass of $M_{\rm H_2 }(\alpha_{\rm CO}/4.3)=(1.5\pm0.3)\times 10^{11}{\rm M}_\odot$ and unveils a gas-rich system with $\mu_{\rm gas}(\alpha_{\rm CO}/4.3)\equiv M_{\rm H_2}/M_\star=3.3\pm0.7$. The extreme star formation efficiency (SFE) of AzTEC/C159, parametrized by the ratio $L_{\rm{IR}}/L'_{\rm{CO}}=(216\pm80)\, {\rm L}_{\odot}{\rm \,(K\,km\,s}^{-1}{\rm \,pc}^{2})^{-1}$, is comparable to merger-driven starbursts such as local ultra-luminous infrared galaxies (ULIRGs) and SMGs. Likewise, the $^{12}$CO(5$\to$4)/CO(2$\to$1) line brightness temperature ratio of $r_{52}= 0.55\pm 0.15$ is consistent with high excitation conditions, similar to that observed in SMGs. We constrained the value for the $L'_{\text{CO}}-{\rm H}_2$ mass conversion factor in AzTEC/C159, i.e. $\alpha_{\text{CO}}=3.9^{+2.7}_{-1.3}{\rm \,M}_{\odot}{\rm \,K}^{-1}{\rm \,km}^{-1}{\rm \,s\,pc}^{-2}$, that is consistent with a self-gravitating molecular gas distribution as observed in local star-forming disk galaxies. Cold gas streams from cosmological filaments might be fueling a gravitationally unstable gas-rich disk in AzTEC/C159, which breaks into giant clumps forming stars as efficiently as in merger-driven systems and generate high gas excitation., Comment: 10 pages, 4 figures, Accepted for publication in A&A

We present an analysis of 39 nuclei and their early-type hosts in the Virgo Cluster using ten broadband filters: F300W, F475W, F850LP, F160W, $u^*griz$, and $K_s$. We describe the Virgo Redux program, which provides high-resolution UV and NIR imaging. Combining this data with optical and NIR imaging from the ACS Virgo Cluster Survey and the Next Generation Virgo Cluster Survey, we estimate masses, metallicities and ages using simple stellar population (SSP) models. For 19 nuclei, we compare to SSP parameters derived from Keck and Gemini spectra and find reasonable agreement between the photometric and spectroscopic metallicity: the RMS scatter is 0.3 dex. We reproduce the nucleus-galaxy mass fraction of $0.33^{+0.09}_{-0.07}$ percent for galaxy stellar masses $10^{8.4}-10^{10.3} M_\odot$ with a typical precision of $\sim$35% for the nuclei masses. Based on available model predictions, there is no single preferred formation scenario for nuclei, suggesting that nuclei are formed stochastically through a mix of processes. Nuclei metallicities are statistically identical to those of their hosts, appearing $0.07 \pm 0.3$ dex more metal-rich on average -- although, omitting galaxies with unusual origins, nuclei are $0.20\pm0.28$ dex more metal-rich. Nuclei appear to be $0.56 \pm 0.12$ dex more metal rich than ultra-compact dwarf galaxies (UCDs) at fixed mass. We find no clear age difference between nuclei and their galaxies, with nuclei displaying a broad range of ages. Interestingly, we find that the most massive nuclei may be flatter and more closely aligned with the semi-major axes of their hosts, suggesting that they formed through predominantly dissipative processes., Comment: 46 pages, 25 figures, 9 tables. Accepted for publication in ApJ

We performed a comprehensive stacking analysis on ∼14, 200 quiescent galaxy (QG) candidates at z = 0–3 across mid-, far-infrared (MIR and FIR), and radio wavelengths. Identified via their rest- frame NUV ‑ r and r ‑ J colors, the QG candidates ({; ; ; M}; ; ; \star ={; ; ; 10}; ; ; 9.8{; ; ; {; ; ; {; ; ; --}; ; ; }; ; ; 12.2}; ; ; {; ; ; M}; ; ; ȯ ) have drastically different IR and radio properties depending on their 24 μm emission strength. The fraction of QG candidates with strong 24 μm emission (equivalent to inferred star formation rates SFR{; ; ; }; ; ; 24≥slant 100 {; ; ; M}; ; ; ȯ {; ; ; {; ; ; yr}; ; ; }; ; ; -1, hereafter “IR-bright”) increases with redshift and peaks at 15%, and their stacked MIPS 24 μm, Herschel (PACS and SPIRE) and VLA emissions are consistent with being star-forming galaxies (SFGs). In contrast, the majority of QG candidates are faint or undetected at 24 μm individually (i.e., SFR24 < 100 M⊙ yr‑1, hereafter “IR-faint”). Their low dust-obscured SFRs derived from Herschel stacking (SFRH ≲ 3, 15, 50 M⊙ yr‑1 out to z ∼ 1, 2, 3) are >2.5– 12.5× lower than compared to SFGs. This is consistent with the quiescence, as expected from their low unobscured SFRs, as inferred from modeling their ultraviolet-to-NIR photometry. The discrepancy between the LIR derived from stacking Herschel and 24 μm indicates that IR-faint QGs have dust SEDs that are different from those of SFGs. For the most massive ({; ; ; M}; ; ; \star ≥slant {; ; ; 10}; ; ; 11 {; ; ; M}; ; ; ȯ ) IR-faint QGs at z < 1.5, the stacked 1.4 GHz emission is in excess of that expected from other SFR indicators, suggesting a widespread presence of low-luminosity active galactic nuclei. Our results reaffirm the existence of a significant population of QGs out to z = 3, thus corroborating the need to quench star formation in galaxies at early epochs.

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We present new information on galaxies in the vicinity of luminous radio galaxies and quasars at z=4,5,6. These fields were previously found to contain overdensities of Lyman Break Galaxies (LBGs) or spectroscopic Lyman alpha emitters. We use HST and Spitzer data to infer stellar masses, and contrast our results with large samples of LBGs in more average environments as probed by the Great Observatories Origins Deep Survey (GOODS). The following results were obtained. First, LBGs in both overdense regions and in the field at z=4-5 lie on a very similar sequence in a z'-[3.6] versus [3.6] color-magnitude diagram. This is interpreted as a sequence in stellar mass (log[M*/Msun] = 9-11) in which galaxies become increasingly red due to dust and age as their star formation rate (SFR) increases. Second, the two radio galaxies are among the most massive objects (log[M*/Msun]~11) known to exist at z~4-5, and are extremely rare based on the low number density of such objects as estimated from the ~25x larger area GOODS survey. We suggest that the presence of these massive galaxies and supermassive black holes has been boosted through rapid accretion of gas or merging inside overdense regions. Third, the total stellar mass found in the z=4 ``proto-cluster'' TN1338 accounts for 4, based on a comparison with the massive X-ray cluster Cl1252 at z=1.2. Although future near-infrared observations should determine whether any massive galaxies are currently being missed, one possible explanation for this mass difference is that TN1338 evolves into a smaller cluster than Cl1252. This raises the interesting question of whether the most massive protocluster regions at z>4 remain yet to be discovered., The Astrophysical Journal, In Press (17 pages, 7 figures)

We study the stellar mass assembly of the Spiderweb Galaxy (MRC 1138-262), a massive z = 2.2 radio galaxy in a protocluster and the probable progenitor of a brightest cluster galaxy. Nearby protocluster galaxies are identified and their properties are determined by fitting stellar population models to their rest-frame ultraviolet to optical spectral energy distributions. We find that within 150 kpc of the radio galaxy the stellar mass is centrally concentrated in the radio galaxy, yet most of the dust-uncorrected, instantaneous star formation occurs in the surrounding low-mass satellite galaxies. We predict that most of the galaxies within 150 kpc of the radio galaxy will merge with the central radio galaxy by z = 0, increasing its stellar mass by up to a factor of ~ 2. However, it will take several hundred Myr for the first mergers to occur, by which time the large star formation rates are likely to have exhausted the gas reservoirs in the satellite galaxies. The tidal radii of the satellite galaxies are small, suggesting that stars and gas are being stripped and deposited at distances of tens of kpc from the central radio galaxy. These stripped stars may become intracluster stars or form an extended stellar halo around the radio galaxy, such as those observed around cD galaxies in cluster cores.

We present new constraints on the evolution of the early-type galaxy color-magnitude relation (CMR) based on deep near-infrared imaging of a galaxy protocluster at z=2.16 obtained using NICMOS on-board the Hubble Space Telescope. This field contains a spectroscopically confirmed space-overdensity of Lyman-alpha and H-alpha emitting galaxies which surrounds the powerful radio galaxy MRC 1138-262. Using these NICMOS data we identify a significant surface-overdensity (= 6.2x) of red J-H galaxies in the color-magnitude diagram (when compared with deep NICMOS imaging from the HDF-N and UDF). The optical-NIR colors of these prospective red-sequence galaxies indicate the presence of on-going dust-obscured star-formation or recently formed (

We present SIGAME (SImulator of GAlaxy Millimetre/submillimetre Emission), a new numerical code designed to simulate the 12CO rotational line emission spectrum of galaxies. Using sub-grid physics recipes to post-process the outputs of smoothed particle hydrodynamics (SPH) simulations, a molecular gas phase is condensed out of the hot and partly ionized SPH gas. The gas is subjected to far-UV radiation fields and cosmic ray ionization rates which are set to scale with the local star formation rate volume density. Level populations and radiative transport of the CO lines are solved with the 3-D radiative transfer code LIME. We have applied SIGAME to cosmological SPH simulations of three disc galaxies at z=2 with stellar masses in the range ~(0.5-2)x10^11 Msun and star formation rates ~40-140 Msun/yr. Global CO luminosities and line ratios are in agreement with observations of disc galaxies at z~2 up to and including J=3-2 but falling short of the few existing J=5-4 observations. The central 5 kpc regions of our galaxies have CO 3-2/1-0 and 7-6/1-0 brightness temperature ratios of ~0.55-0.65 and ~0.02-0.08, respectively, while further out in the disc the ratios drop to more quiescent values of ~0.5 and =3 and CO-to-H2 conversion factors [~=3.6 Msun*pc^2/(K km/s)]., 28 pages, 20 figures. Accepted for Publication in MNRAS. Substantial revisions from the previous version, including tests with model galaxies similar to the Milky Way. Improved figures and added tables

We present HST NICMOS+ACS and Spitzer IRAC+MIPS observations of 41 galaxies at 2

We examined the morphology-density relations for galaxy samples selected by luminosity and by mass in each of five massive X-ray clusters from z=0.023 to 0.83 for 674 spectroscopically-confirmed members. Rest-frame optical colors and visual morphologies were obtained primarily from Hubble Space Telescope images. Morphology-density relations (MDR) are derived in each cluster from a complete, luminosity-selected sample of 452 galaxies with a magnitude limit M_V < M^{*}_{V} + 1. The change in the early-type fraction with redshift matches previous work for massive clusters of galaxies. We performed a similar analysis, deriving MDRs for complete, mass-selected samples of 441 galaxies with a mass-limit of 10^{10.6} M_{\sun}. Our mass limit includes faint objects, the equivalent of =~1 mag below L^{*} for the red cluster galaxies, and encompasses =~70% of the stellar mass in cluster galaxies. The MDRs in the mass-selected sample at densities of Sigma > 50 galaxies Mpc^{-2} are similar to those in the luminosity-selected sample but show larger early-type fractions. However, the trend with redshift in the fraction of elliptical and S0 galaxies with masses > 10^{10.6} M_{\sun} differs significantly between the mass- and luminosity-selected samples. The clear trend seen in the early-type fraction from z=0 to z=~ 0.8 is not found in mass-selected samples. The early-type galaxy fraction changes much less, and is consistent with being constant at 92% +/- 4% at ��> 500 galaxies Mpc^{-2} and 83 +/- 3% at 50 < ��< 500 galaxies Mpc^{-2}. This suggests that galaxies of mass lower than > 10^{10.6} M_{\sun} play a significant role in the evolution of the early-type fraction in luminosity-selected samples. (Abstract abridged), 18 pages in emulate ApJ format, with 10 color figures, Accepted to ApJ. Version updated to reflect published version, includes new references and a correction to table 3

We report on deep spectroscopy, using LRIS on Keck I and FORS2 on the VLT, of a sample of 22 candidate z similar to 6 Lyman break galaxies (LBGs) selected by the (i)775 - z(850) > 1: 3 dropout criterion. Redshifts could be measured for eight objects. These redshifts are all in the range z = 5: 5-6.1, confirming the efficiency of the i775 - z850 color selection technique. Six of the confirmed galaxies show Ly alpha emission. Assuming that the 14 objects without redshifts are z similar to 6 LBGs that lack detectable Ly alpha emission lines, we infer that the fraction of Ly alpha-emitting LBGs with Ly alpha equivalent widths greater than 20 angstrom among z similar to 6 LBGs is approximate to 30%, similar to that found at z similar to 3. Every Ly alpha-emitting object in our sample is compact, with half-light radii r(hl) 97% confidence. We speculate that the small sizes of the Ly alpha-emitting LBGs are due to these objects being less massive than other LBGs at z similar to 6.

We present deep, continuum images of eleven high-redshift (0.811 ~ 2, and that the AGN phenomenon takes place within otherwise normal, perhaps passively evolving, galaxies. (abridged)

We present the main results from our 940 ksec observation of the Chandra Deep Field South (CDFS), using the source catalog described in an accompanying paper (Giacconi et al. 2001). We extend the measurement of source number counts to 5.5e-17 erg/cm^2/s in the soft 0.5-2 keV band and 4.5e-16 erg/cm^2/s in the hard 2-10 keV band. The hard band LogN-LogS shows a significant flattening (slope~=0.6) below ~1e-14 erg/cm^2/s, leaving at most 10-15% of the X-ray background (XRB) to be resolved, the main uncertainty lying in the measurement of the total flux of the XRB. On the other hand, the analysis in the very hard 5-10 keV band reveals a relatively steep LogN-LogS (slope ~=1.3) down to 1e-15 erg/cm^2/s. Together with the evidence of a progressive flattening of the average X-ray spectrum near the flux limit, this indicates that there is still a non negligible population of faint hard sources to be discovered at energies not well probed by Chandra, which possibly contribute to the 30 keV bump in the spectrum of the XRB. We use optical redshifts and identifications, obtained with the VLT, for one quarter of the sample to characterize the combined optical and X-ray properties of the CDFS sample. Different source types are well separated in a parameter space which includes X-ray luminosity, hardness ratio and R-K color. Type II objects, while redder on average than the field population, have colors which are consistent with being hosted by a range of galaxy types. Type II AGN are mostly found at z

We present the main results from our 940 ks observation of the Chandra Deep Field-South using the source catalog described in an accompanying paper by Giacconi et al. We extend the measurement of source number counts to 5.5 x 10(-17) ergs cm(-2) s(-1) in the soft 0.5-2 keV band and 4.5 x 10(-16) ergs cm(-2) s(-1) in the hard 2-10 keV band. The hard-band log N-log S shows a significant flattening (slope similar or equal to0.6) below approximate to10(-14) ergs cm(-2) s(-1), leaving at most 10%-15% of the X-ray background to be resolved, the main uncertainty lying in the measurement of the total flux of the X-ray background (XRB). On the other hand, the analysis in the very hard 5-10 keV band reveals a relatively steep log N-log S (slope similar or equal to1.3) down to 10(-15) ergs cm(-2) s(-1). Together with the evidence of a progressive flattening of the average X-ray spectrum near the flux limit, this indicates that there is still a nonnegligible population of faint hard sources to be discovered at energies not well probed by Chandra, which possibly contributes to the 30 keV bump in the spectrum of the XRB. We use optical redshifts and identifications, obtained with the Very Large Telescope, for one-quarter of the sample to characterize the combined optical and X-ray properties of the Chandra Deep Field-South sample. Different source types are well separated in a parameter space that includes X-ray luminosity, hardness ratio, and R-K color. Type II objects, while redder on average than the field population, have colors that are consistent with being hosted by a range of galaxy types. Type II active galactic nuclei are mostly found at z less than or similar to 1, in contrast with predictions based on active galactic nucleus population synthesis models, thus suggesting a revision of their evolutionary parameters.

We measure the merger fraction of massive galaxies using the UltraVISTA/COSMOS $Ks$-band selected catalog, complemented with the deeper, higher resolution 3DHST+CANDELS catalog selected in the HST/WFC3 $H$-band, presenting the largest mass-complete photometric merger sample up to $z\sim3$. We find that selecting mergers using the $H_{160}$-band flux ratio leads to an increasing merger fraction with redshift, while selecting mergers using the stellar mass ratio causes a diminishing redshift dependence. Defining major and minor mergers as having stellar mass ratios of 1:1 - 4:1 and 4:1 - 10:1 respectively, the results imply $\sim$1 major and $\lesssim$1 minor merger for an average massive (log$(M_{\star}/M_{\odot}) \geqslant 10.8$) galaxy during $z=0.1-2.5$. There may be an additional $\sim 0.5(0.3)$ major (minor) merger if we use the $H$-band flux ratio selection. The observed amount of major merging alone is sufficient to explain the observed number density evolution for the very massive (log$(M_{\star}/M_{\odot}) \geqslant 11.1$) galaxies. We argue that these very massive galaxies can put on a maximum of $6\%$ of stellar mass in addition to major and minor merging, so that their number density evolution remains consistent with observations. The observed number of major and minor mergers can increase the size of a massive quiescent galaxy by a factor of two at most. This amount of merging is enough to bring the compact quiescent galaxies formed at $z>2$ to lie at $1\sigma$ below the mean of the stellar mass-size relation as measured in some works (e.g. Newman et al. 2012), but additional mechanisms are needed to fully explain the evolution, and to be consistent with works suggesting stronger evolution (e.g. van der Wel et al. 2014)., Comment: Submitted to Astrophysical Journal. 25 pages, 12 figures, 6 tables

Three billion years after the big bang (at redshift z=2), half of the most massive galaxies were already old, quiescent systems with little to no residual star formation and extremely compact with stellar mass densities at least an order of magnitude larger than in low redshift ellipticals, their descendants. Little is known about how they formed, but their evolved, dense stellar populations suggest formation within intense, compact starbursts 1-2 Gyr earlier (at 3, Comment: ApJ (in press)

RXJ0848.6+4453 (Lynx W) at redshift 1.27 is part of the Lynx Supercluster of galaxies. Our analysis of stellar populations and star formation history in the cluster covers 24 members and is based on deep optical spectroscopy from Gemini North and imaging data from HST. Focusing on the 13 bulge-dominated galaxies for which we can determine central velocity dispersions, we find that these show a smaller evolution of sizes and velocity dispersions than reported for field galaxies and galaxies in poorer clusters. The galaxies in RXJ0848.6+4453 populate the Fundamental Plane similar to that found for lower redshift clusters with a zero point offset corresponding to an epoch of last star formation at z_form= 1.95+-0.2. The spectra of the galaxies in RXJ0848.6+4453 are dominated by young stellar populations at all galaxy masses and in many cases show emission indicating low level on-going star formation. The average age of the young stellar populations (estimated from H-zeta) is consistent with a major star formation episode 1-2 Gyr prior, which in turn agrees with z_form=1.95. Galaxies dominated by young stellar populations are distributed throughout the cluster. We speculate that low level star formation has not yet been fully quenched in the center of this cluster may be because the cluster is significantly poorer than other clusters previously studied at similar redshifts, which appear to have very little on-going star formation in their centers., Comment: Accepted for publication in Astronomical Journal. High-resolution figures available from the first author by request

Data in astronomy is rapidly growing with upcoming surveys producing 30 TB of images per night. Highly informative spectra are too expensive to measure for each detected object, hence ways of reliably estimating physical properties from images alone are paramount. The objective of this work is to test whether a “big data ready” k-nearest neighbour regression can successfully estimate the specific star formation rate (sSFR) from colours of low-redshift galaxies. The nearest neighbour algorithm achieves a root mean square error (RMSE) of 0.30, outperforming the state-of-the-art astronomical model achieving a RMSE of 0.36.

We present deep, near-infrared HST/WFC3 grism spectroscopy and imaging for a sample of 14 galaxies at z~2 selected from a mass-complete photometric catalog in the COSMOS field. By combining the grism observations with photometry in 30 bands, we derive accurate constraints on their redshifts, stellar masses, ages, dust extinction and formation redshifts. We show that the slope and scatter of the z~2 mass-size relation of quiescent galaxies is consistent with the local relation, and confirm previous findings that the sizes for a given mass are smaller by a factor of two to three. Finally, we show that the observed evolution of the mass-size relation of quiescent galaxies between z=2 and 0 can be explained by quenching of increasingly larger star-forming galaxies, at a rate dictated by the increase in the number density of quiescent galaxies with decreasing redshift. However, we find that the scatter in the mass-size relation should increase in the quenching-driven scenario in contrast to what is seen in the data. This suggests that merging is not needed to explain the evolution of the median mass-size relation of massive galaxies, but may still be required to tighten its scatter, and explain the size growth of individual z=2 galaxies quiescent galaxies., 16 pages, 8 figures, accepted for publication in the Astrophysical Journal

We have used the SINFONI near-infrared integral field unit on the VLT to resolve the optical emission line structure of one of the brightest (L~1e44 erg/s) and nearest (z=2.38) of all Lya blobs (LABs). The target, known in the literature as object 'B1' (Francis et al. 1996), lies at a redshift where the main optical emission lines are accessible in the observed near-infrared. We detect luminous [OIII]4959,5007A and Ha emission with a spatial extent of at least 32x40 kpc (4"x5"). The dominant optical emission line component shows relatively broad lines (600-800 km/s, FWHM) and line ratios consistent with AGN-photoionization. The new evidence for AGN photoionization, combined with previously detected CIV and luminous, warm infrared emission, suggest that B1 is the site of a hidden quasar. This is confirmed by the fact that [OII] is relatively weak compared to [OIII] (extinction-corrected [OIII]/[OII] of about 3.8), which is indicative of a high, Seyfert-like ionization parameter. From the [OIII] luminosity we infer a bolometric AGN luminosity of ~3e46 erg/s, and further conclude that the obscured AGN may be Compton-thick given existing X-ray limits. The large line widths observed are consistent with clouds moving within the narrow line region of a luminous QSO. The AGN scenario is capable of producing sufficient ionizing photons to power the Lya, even in the presence of dust. By performing a census of similar objects in the literature, we find that virtually all luminous LABs harbor obscured quasars. Based on simple duty-cycle arguments, we conclude that AGN are the main drivers of the Lya in LABs rather than the gravitational heating and subsequent cooling suggested by cold stream models. We also conclude that the empirical relation between LABs and overdense environments at high redshift must be due to a more fundamental correlation between AGN (or massive galaxies) and environment., Comment: ApJ, Accepted (12 pages, 6 figures, 2 tables)

International audience; We present the mid-infrared (IR) spectra of seven of the most powerful radio-galaxies known to exist at 1.5 < z < 2.6. The radio emission of these sources is dominated by the AGN with 500 MHz luminosities in the range 1027.8-1029.1 W Hz-1. The AGN signature is clearly evident in the mid-IR spectra; however, we also detect polycyclic aromatic hydrocarbon emission, indicative of prodigious star formation at a rate of up to ˜1000 M&sun; yr-1. Interestingly, we observe no significant correlation between AGN power and star formation in the host galaxy. We also find most of these radio galaxies to have weak 9.7 mum silicate absorption features (tau9.7 mum < 0.8) which implies that their mid-IR obscuration is predominantly due to the dusty torus that surrounds the central engine, rather than the host galaxy. The tori are likely to have an inhomogeneous distribution with the obscuring structure consisting of individual clouds. We estimate that these radio galaxies have already formed the bulk of their stellar mass and appear to lie at a stage in their evolution where the obscured AGN dominates the energy output of the system but star formation is also prevalent.

We estimate the galaxy stellar mass function and stellar mass density for star-forming and quiescent galaxies with 0.210^10.7Msun. For the mass function of the quiescent galaxies, we do not find any significant evolution of the high-mass end at z1.5 consistent with the expected uncertainties. We also develop a new method to infer the specific star formation rate from the mass function of star-forming galaxies. We find that the specific star formation rate of 10^10Msun galaxies increases continuously in the redshift range 1, Comment: 20 pages, 17 figures, Accepted for publication in A&A

We present dynamical and structural scaling relations of quiescent galaxies at z=2, including the dynamical mass-size relation and the first constraints on the fundamental plane (FP). The backbone of the analysis is a new, very deep VLT/X-shooter spectrum of a massive, compact, quiescent galaxy at z=2.0389. We detect the continuum between 3700-22000A and several strong absorption features (Balmer series, Ca H+K, G-band), from which we derive a stellar velocity dispersion of 318 +/- 53 km/s. We perform detailed modeling of the continuum emission and line indices and derive strong simultaneous constraints on the age, metallicity, and stellar mass. The galaxy is a dusty (A_V=0.77 (+0.36,-0.32)) solar metallicity (log(Z/Zsun) = 0.02 (+0.20,-0.41)) post starburst galaxy, with a mean luminosity weighted log(age/yr) of 8.9 +/- 0.1. The galaxy formed the majority of its stars at z>3 and currently has little or no ongoing star formation. We compile a sample of three other z~2 quiescent galaxies with measured velocity dispersions, two of which are also post starburst like. Their dynamical mass-size relation is offset significantly less than the stellar mass-size relation from the local early type relations, which we attribute to a lower central dark matter fraction. Recent cosmological merger simulations qualitatively agree with the data, but can not fully account for the evolution in the dark matter fraction. The z~2 FP requires additional evolution beyond passive stellar aging, to be in agreement with the local FP. The structural evolution predicted by the cosmological simulations is insufficient, suggesting that additional, possibly non-homologous structural evolution is needed., Re-submitted to ApJ after implementing the comments of the referee

Using a mass-selected ($M_{\star} \ge 10^{11} M_{\odot}$) sample of 198 galaxies at 0 < z < 3.0 with HST/NICMOS $H_{160}$-band images from the COSMOS survey, we find evidence for the evolution of the pair fraction above z ~ 2, an epoch in which massive galaxies are believed to undergo significant structural and mass evolution. We observe that the pair fraction of massive galaxies is 0.15 \pm 0.08 at 1.7 < z < 3.0, where galaxy pairs are defined as massive galaxies having a companion of flux ratio from 1:1 to 1:4 within a projected separation of 30 kpc. This is slightly lower, but still consistent with the pair fraction measured previously in other studies, and the merger fraction predicted in halo-occupation modelling. The redshift evolution of the pair fraction is described by a power law F(z) = (0.07 \pm 0.04) * (1+z) ^ (0.6 \pm 0.5). The merger rate is consistent with no redshift evolution, however it is difficult to constrain due to the limited sample size and the high uncertainties in the merging timescale. Based on the merger rate calculation, we estimate that a massive galaxy undergoes on average 1.1 \pm 0.5 major merger from z = 3 to 0. The observed merger fraction is sufficient to explain the number density evolution of massive galaxies, but insufficient to explain the size evolution. This is a hint that mechanism(s) other than major merging may be required to increase the sizes of the massive, compact quiescent galaxies from z ~ 2 to 0., Comment: 13 pages, 5 figures, Accepted for publication in the Astrophysical Journal

We report the discovery of a significant excess of candidate Halpha emitters (HAEs) in the field of the radio galaxy 4C 23.56 at z=2.483. Using the MOIRCS near-infrared imager on the Subaru Telescope we found 11 candidate emission-line galaxies to a flux limit of ~7.5 10^-17 erg s-1 cm-2, which is about 5 times excess from the expected field counts with ~3-sigma significance. Three of these are spectroscopically confirmed as redshifted Halpha at z=2.49. The distribution of candidate emitters on the sky is tightly confined to a 1.2-Mpc-radius area at z=2.49, locating 4C 23.56 at the western edge of the distribution. Analysis of the deep Spitzer MIPS 24 mu m imaging shows that there is also an excess of faint MIPS sources. All but two of the 11 HAEs are also found in the MIPS data. The inferred star-formation rate (SFR) of the HAEs based on the extinction-corrected Halpha luminosity (median SFR >~100 M_solar yr-1) is similar to those of HAEs in random fields at z~2. On the other hand, the MIPS-based SFR for the HAEs is on average 3.6 times larger, suggesting the existence of the star-formation significanly obscured by dust. The comparison of the Halpha-based star-formation activities of the HAEs in the 4C 23.56 field to those in another proto-cluster around PKS 1138-262 at z=2.16 reveals that the latter tend to have fainter Halpha emission despite similar K-band magnitudes. This suggests that star-formation may be suppressed in the PKS 1138-262 protocluster relative to the 4C 23.56 protocluster. This difference among the HAEs in the two proto-clusters at z > 2 may imply that some massive cluster galaxies are just forming at these epochs with some variation among clusters., 29 pages, 13 figures, to be published in PASJ Subaru Special Issue (2011 Mar.)

We study the environments of 6 radio galaxies at 2.2 < z < 2.6 using wide-field near-infrared images. We use colour cuts to identify galaxies in this redshift range, and find that three of the radio galaxies are surrounded by significant surface overdensities of such galaxies. The excess galaxies that comprise these overdensities are strongly clustered, suggesting they are physically associated. The colour distribution of the galaxies responsible for the overdensity are consistent with those of galaxies that lie within a narrow redshift range at z ~ 2.4. Thus the excess galaxies are consistent with being companions of the radio galaxies. The overdensities have estimated masses in excess of 10^14 solar masses, and are dense enough to collapse into virizalised structures by the present day: these structures may evolve into groups or clusters of galaxies. A flux-limited sample of protocluster galaxies with K < 20.6 mag is derived by statistically subtracting the fore- and background galaxies. The colour distribution of the protocluster galaxies is bimodal, consisting of a dominant blue sequence, comprising 77 +/- 10% of the galaxies, and a poorly populated red sequence. The blue protocluster galaxies have similar colours to local star-forming irregular galaxies (U -V ~ 0.6), suggesting most protocluster galaxies are still forming stars at the observed epoch. The blue colours and lack of a dominant protocluster red sequence implies that these cluster galaxies form the bulk of their stars at z < 3.

Venemans et al. (2005) found evidence for an overdensity of Ly-alpha emission line galaxies associated with the radio galaxy MRC 0316-257 at z=3.13 indicating the presence of a massive protocluster. Here, we present the results of a search for additional star-forming galaxies and AGN within the protocluster. Narrow-band infrared imaging was used to select candidate [O III] emitters in a 1.1 x 1.1 Mpc^2 region around the radio galaxy. Thirteen candidates have been detected. Four of these are among the previously confirmed sample of Ly-alpha galaxies, and an additional three have been confirmed through follow-up infrared spectroscopy. The three newly confirmed objects lie within a few hundred km/s of each other, but are blueshifted with respect to the radio galaxy and Ly-alpha emitters by ~2100 km/s. Although the sample is currently small, our results indicate that the radio-selected protocluster is forming at the centre of a larger, ~60 co-moving Mpc super-structure. On the basis of an HST/ACS imaging study we calculate dust-corrected star-formation rates and investigate morphologies and sizes of the [O III] candidate emitters. From a comparison of the star formation rate derived from UV-continuum and [O III] emission, we conclude that at least two of the [O III] galaxies harbour an AGN which ionized the O+ gas., Comment: 11 pages, 6 figures. Accepted for publication in MNRAS

We present results from an extensive spectroscopic survey, carried out with VLT FORS, and from an extensive multiwavelength imaging data set from the HST Advanced Camera for Surveys and ground based facilities, of the cluster of galaxies RDCS J1252.9-2927. We have spectroscopically confirmed 38 cluster members in the redshift range 1.22 < z < 1.25. A cluster median redshift of z=1.237 and a rest-frame velocity dispersion of 747^{+74}_{-84} km/s are obtained. Using the 38 confirmed redshifts, we were able to resolve, for the first time at z > 1, kinematic structure. The velocity distribution, which is not Gaussian at the 95% confidence level, is consistent with two groups that are also responsible for the projected east-west elongation of the cluster. The groups are composed of 26 and 12 galaxies with velocity dispersions of 486^{+47}_{-85} km/s and 426^{+57}_{-105} km/s, respectively. The elongation is also seen in the intracluster gas and the dark matter distribution. This leads us to conclude that RDCS J1252.9-2927 has not yet reached a final virial state. We extend the analysis of the color-magnitude diagram of spectroscopic members to more than 1 Mpc from the cluster center. The scatter and slope of non-[OII]-emitting cluster members in the near-IR red sequence is similar to that seen in clusters at lower redshift. Furthermore, most of the galaxies with luminosities greater than ~ K_s*+1.5 do not show any [OII], indicating that these more luminous, redder galaxies have stopped forming stars earlier than the fainter, bluer galaxies. Our observations provide detailed dynamical and spectrophotometric information on galaxies in this exceptional high-redshift cluster, delivering an in-depth view of structure formation at this epoch only 5 Gyr after the Big Bang., 29 pages. 16 figures. ApJ accepted. Tables 2,3 and 5, figure 1 and the full figure 5 will be available in the paper and electronic editions from ApJ. v2: minor corrections to the abstract and text to match the Journal's version

We present deep, high angular-resolution HST/NICMOS imaging in the Hubble Deep Field South (HDF-S), focusing on a subset of 14 Distant Red Galaxies (DRGs) at z ~ 2.5 galaxies that have been pre-selected to have J-K > 2.3. We find a clear trend between the rest-frame optical sizes of these sources and their luminosity-weighted stellar ages as inferred from their broad-band spectral energy distributions (SEDs). Galaxies whose SEDs are consistent with being dusty and actively star forming generally show extended morphologies in the NICMOS images (r_e >~ 2 kpc), while the 5 sources which are not vigorously forming stars are extremely compact (r_e, Comment: Accepted to The Astrophysical Journal, 8 pages, 5 figures

We present deep HST/ACS observations in g,r,i,z towards the z=4.1 radio galaxy TN J1338-1942 and its overdensity of >30 spectroscopically confirmed Lya emitters (LAEs). We select 66 g-band dropouts to z=27, 6 of which are also a LAE. Although our color-color selection results in a relatively broad redshift range centered on z=4.1, the field of TN J1338-1942 is richer than the average field at the >5 sigma significance, based on a comparison with GOODS. The angular distribution is filamentary with about half of the objects clustered near the radio galaxy, and a small, excess signal (2 sigma) in the projected pair counts at separations of 10^14 Msun structure, confirming that it is possible to find and study cluster progenitors in the linear regime at z>4., Accepted for publication in ApJ (33 pages, 19 figures, emulateapj). v3 includes updated methods, discussion and referencing

We present the evolution of the luminosity-size and stellar mass-size relations of luminous (L_V>3.4x10^10h_70^-2L_sun) and of massive (M_*>3x10^10h_70^-2M_sun) galaxies in the last ~11 Gyr. We use very deep near-infrared images of the Hubble Deep Field-South and the MS1054-03 field in the J_s, H and K_s bands from FIRES to retrieve the sizes in the optical rest-frame for galaxies with z>1. We combine our results with those from GEMS at 0.2, Comment: Accepted for publication in ApJ. The new version includes several improvements: much accurate size estimations and a better completeness and robustness analysis. Tables of data are included. 29 pages and 14 figures (one low resolution)

We analyse the cluster color-magnitude relation (CMR) for early-type galaxies in two of the richer clusters in the z ~ 0.9 supercluster system to derive average ages and formation redshifts for the early-type galaxy population. Both clusters were observed with the Advanced Camera for Surveys aboard the {\it Hubble Space Telescope} through the F606W and F814W filters, which brackets the rest-frame 4000 ��break at the cluster redshifts of z\sim 0.9. We fit the zeropoint and slope of the red cluster sequence, and model the scatter about this relation to estimate average galaxy ages and formation redshifts. We find intrinsic scatters of 0.038-0.053 mag in ($V_{606}-I_{814}$) for the E and E+S0 populations, corresponding to average ages of 3.5-3.7 Gyr and formation redshifts z_{f}=2.4-2.6. We find at least one significant difference between the Cl1604+4304 and Cl1604+4321 early-type CMRs. Cl1604+4321, the less X-ray luminous and massive of the two, lacks bright L^* ellipticals. We combine the galaxy samples to fit a composite CMR down to 0.15L^*, and find that the slope of the combined cluster CMR is significantly steeper than for RX J0152.7-1357 but consistent with MS 1054-03, both at similar redshift. The slope of the Cl1604 CMR at the bright end (L > 0.5L^*) is flatter and consistent with the CMR slopes found for other high redshift clusters. We find evidence for increasing scatter with increasing magnitude along the early-type CMR, consistent with a 'downsizing' scenario, indicating younger mean ages with decreasing galaxy mass., replaced to fix bizarre shift in vertical centering, ApJ accepted

We present a deep image of the radio galaxy MRC 1138-262 taken with the Hubble Space Telescope (HST) at a redshift of z = 2.2. The galaxy is known to have properties of a cD galaxy progenitor and be surrounded by a 3 Mpc-sized structure, identified with a protocluster. The morphology shown on the new deep HST/ACS image is reminiscent of a spider's web. More than 10 individual clumpy features are observed, apparently star-forming satellite galaxies in the process of merging with the progenitor of a dominant cluster galaxy 11 Gyr ago. There is an extended emission component, implying that star formation was occurring over a 50 times 40 kpc region at a rate of more than 100 M_sun/yr. A striking feature of the newly named ``Spiderweb galaxy'' is the presence of several faint linear galaxies within the merging structure. The dense environments and fast galaxy motions at the centres of protoclusters may stimulate the formation of these structures, which dominate the faint resolved galaxy populations in the Hubble Ultra Deep Field. The new image provides a unique testbed for simulations of forming dominant cluster galaxies., Comment: 5 pages, 2 figures (reduced to grayscale); ApJ Letters

著者人数：44名, Accepted: 2005-11-22, 資料番号: SA1000634000

We present a comparison of late-type galaxies (Sa and later) in intermediate redshift clusters and the field using ACS imaging of four cluster fields: CL0152-1357, CL1056-0337 (MS1054), CL1604+4304, and CL1604+4321. Concentration, asymmetry, and clumpiness parameters are calculated for each galaxy in blue (F606W or F625W) and red (F775W or F814W) filters. Galaxy half-light radii, disk scale lengths, color gradients, and overall color are compared. We find marginally significant differences in the asymmetry distributions of spiral and irregular galaxies in the X-ray luminous and X-ray faint clusters. The massive clusters contain fewer galaxies with large asymmetries. The physical sizes of the cluster and field populations are similar; no significant differences are found in half-light radii or disk scale lengths. The most significant difference is in rest-frame $U-B$ color. Late-type cluster galaxies are significantly redder, $\sim 0.3$ magnitudes at rest-frame $U-B$, than their field counterparts. Moreover, the intermediate-redshift cluster galaxies tend to have blue inward color gradients, in contrast to the field galaxies, but similar to late-type galaxies in low redshift clusters. These blue inward color gradients are likely to be the result of enhanced nuclear star formation rates relative to the outer disk. Based on the significant rest-frame color difference, we conclude that late-type cluster members at $z\sim0.9$ are not a pristine infalling field population; some difference in past and/or current star formation history is already present. This points to high redshift ``groups'', or filaments with densities similar to present-day groups, as the sites where the first major effects of environment are imprinted., updated title

We have obtained a high S/N (22.3 hr integration) UV continuum VLT FORS2 spectrum of an extremely bright (z_850 = 24.3) z = 5.515 +/- 0.003 starforming galaxy (BD38) in the field of the z = 1.24 cluster RDCS 1252.9-2927. This object shows substantial continuum (0.41 +/- 0.02 \muJy at \lambda1300) and low-ionization interstellar absorption features typical of LBGs at lower redshift (z ~ 3); this is the highest redshift LBG confirmed via metal absorption spectral features. The equivalent widths of the absorption features are similar to z ~ 3 strong Ly\alpha absorbers. No noticeable Ly\alpha emission was detected (F 5. -- Abstract Abridged, Comment: Accepted to ApJ Letters. 4 pages in emulate ApJ format, 3 color figures, 1 with lower resolution. Full resolution file available at http://physics.ucsc.edu/~cdow/bd38/bd38.pdf

We present new measurements of the galaxy luminosity function (LF) and its dependence on local galaxy density, color, morphology, and clustocentric radius for the massive z=0.83 cluster MS1054-0321. Our analyses are based on imaging performed with the ACS onboard the HST in the F606W, F775W and F850LP passbands and extensive spectroscopic data obtained with the Keck LRIS. Our main results are based on a spectroscopically selected sample of 143 cluster members with morphological classifications derived from the ACS observations. Our three primary findings are (1) the faint-end slope of the LF is steepest in the bluest filter, (2) the LF in the inner part of the cluster (or highest density regions) has a flatter faint-end slope, and (3) the fraction of early-type galaxies is higher at the bright end of the LF, and gradually decreases toward fainter magnitudes. These characteristics are consistent with those in local galaxy clusters, indicating that, at least in massive clusters, the common characteristics of cluster LFs are established at z=0.83. We also find a 2sigma deficit of intrinsically faint, red galaxies (i-z>0.5, Mi>-19) in this cluster. This trend may suggest that faint, red galaxies (which are common in z30 Mpc^-2, coinciding with the environment where the galaxy star formation rate and the morphology-density relation start to appear. (abridged), ApJ in press, references updated

The cosmological star formation rate in the combined Chandra Deep Fields North and South is derived from our X-Ray Luminosity Function for Galaxies in these Deep Fields. Mild evolution is seen up to redshift order unity with SFR ~ (1 + z)^{2.7}. This is the first directly observed normal star-forming galaxy X-ray luminosity function (XLF) at cosmologically interesting redshifts (z>0). This provides the most direct measure yet of the X-ray derived cosmic star-formation history of the Universe. We make use of Bayesian statistical methods to classify the galaxies and the two types of AGN, finding the most useful discriminators to be the X-ray luminosity, X-ray hardness ratio, and X-ray to optical flux ratio. There is some residual AGN contamination in the sample at the bright end of the luminosity function. Incompleteness slightly flattens the XLF at the faint end of the luminosity function. The XLF has a lognormal distribution and agrees well with the radio and infrared luminosity functions. However, the XLF does not agree with the Schechter luminosity function for the H-alpha LF indicating that additional and different physical processes may be involved in the establishment of the lognormal form of the XLF. The agreement of our star formation history points with the other star formation determinations in different wavebands (IR, Radio, H-alpha) gives an interesting constraint on the IMF, and X-rays may be measuring directly the binary star formation history of the Universe. X-ray studies will continue to be useful for probing the star formation history of the universe by avoiding problems of obscuration. Star formation may therefore be measured in more detail by deep surveys with future x-ray missions., Accepted for publication in ApJ. 19 pages with 10 figures formatted with emulateapj. Version with B/W only figures available at http://www.pha.jhu.edu/~ptak/papers

Observations from the VLT and the HST show that the most luminous radio galaxies are excellent tracers of protoclusters at z > 2. Several on-going projects to find and study distant protoclusters are reviewed. After summarising the macro-properties of the identified clusters, the detection of various different galaxy populations are described. Finally, preliminary results on the properties of the protocluster members are presented and remarks are made on the galaxy sizes and morphologies. The Lyα - emitting protocluster galaxies are smaller than field Lyman break galaxies at the same redshift.

Based on the photometry of 10 near-UV, optical, and near-infrared bands of the Chandra Deep Field South, we estimate the photometric redshifts for 342 X-ray sources, which constitute ~99% of all the detected X-ray sources in the field. The models of spectral energy distribution are based on galaxies and a combination of power-law continuum and emission lines. Color information is useful for source classifications: Type-I AGN show non-thermal spectral features that are distinctive from galaxies and Type-II AGN. The hardness ratio in X-ray and the X-ray-to-optical flux ratio are also useful discriminators. Using rudimentary color separation techniques, we are able to further refine our photometric redshift estimations. Among these sources, 137 have reliable spectroscopic redshifts, which we use to verify the accuracy of photometric redshifts and to modify the model inputs. The average relative dispersion in redshift distribution is ~8%, among the most accurate for photometric surveys. The high reliability of our results is attributable to the high quality and broad coverage of data as well as the applications of several independent methods and a careful evaluation of every source. We apply our redshift estimations to study the effect of redshift on broadband colors and to study the redshift distribution of AGN. Our results show that both the hardness ratio and U-K color decline with redshift, which may be the result of a K-correction. The number of Type-II AGN declines significantly at z>2 and that of galaxies declines at z>1. However, the distribution of Type-I AGN exhibits less redshift dependence. As well, we observe a significant peak in the redshift distribution at z=0.6. We demonstrate that our photometric redshift estimation produces a reliable database for the study of X-ray luminosity of galaxies and AGN., 40 pages, 11 figures. Accepted for publication in the Astrophysical Journal

We report the discovery of large scale structures of X-ray sources in the 1Msec observation of the Chandra Deep Field South. Two main structures appear as narrow (Delta_z < 0.02) spikes in the source redshift distribution at z=0.67 and z=0.73, respectively. Their angular distribution spans a region at least ~ 17 arcmin wide, corresponding to a physical size of 7.3 h_{70}^{-1} Mpc at a redshift of z ~ 0.7 (Omega_m=0.3, Omega_{Lambda}=0.7). These spikes are populated by 19 sources each, which are mainly identified as Active Galactic Nuclei (AGN). Two sources in each spike are extended in X-rays, corresponding to galaxy groups/clusters embedded in larger structures. The X-ray source redshift distribution shows other spikes, the most remarkable at z=1.04, 1.62 and 2.57. This is one of the first evidences for large scale structure traced by X-ray sources and for spatial clustering of X-ray selected AGN. The X-ray data have been complemented with the spectroscopic data from the K20 near infrared survey (Cimatti et al. 2002), which covers ~1/10 of the X-ray field. Also in this survey the source redshift distribution shows several spikes. Two narrow structures at z=0.67 and z=0.73 (again with Delta_z ~ 0.02) are the most significant features, containing 24 and 47 galaxies, respectively. While the K20 structure at z=0.73 is dominated by a standard galaxy cluster with a significant concentration around a central cD galaxy and morphological segregation, the galaxies at z=0.67 constitute a loose structure rather uniformly distributed along the K20 field.[abridged], 18 pages, 3 figures. Accepted by The Astrophysical Journal. A high resolution version of Fig.2 can be found at http://www.arcetri.astro.it/~gilli/f2_col.eps

We present quantitative morphological analyses of 37 HST/WFPC2 counterparts of X-ray sources in the 1 Ms Chandra Deep Field-South (CDFS). We investigate: 1) 1-D surface brightness profiles via isophotal ellipse fitting; 2) 2-D, PSF- convolved, bulge+disk+nucleus profile-fitting; 3) asymmetry and concentration indices compared with all ~3000 sources in our three WFPC2 fields; and 4) near- neighbor analyses comparing local environments of X-ray sources versus the field control sample. Significant nuclear point-source optical components appear in roughly half of the resolved HST/WFPC2 counterparts, showing a narrow range of F_X/F_{opt,nuc} consistent with the several HST-unresolved X-ray sources (putative type-1 AGN) in our fields. We infer roughly half of the HST/WFPC2 counterparts host unobscured AGN, which suggests no steep decline in the type-1/type-2 ratio out to the redshifts z~0.5-1 typical of our sources. The concentration indices of the CDFS counterparts are clearly larger on average than those of the field distribution, at 5-sigma, suggesting that the strong correlation between central black hole mass and host galaxy properties (including concentration index) observed in nearby galaxies is already evident by z~0.5-1. By contrast, the asymmetry index distribution of the 21 resolved CDFS sources at I, 30 pages, incl. 8 figures; accepted for publication in the Astrophysical Journal

We present HST/WFPC2 observations of a well-defined sample of 40 X-ray sources with X-ray fluxes above the detection threshold of the full 1 Msec Chandra Deep Field South (CDFS). The sensitivity and spatial resolution of our HST observations are sufficient to detect the optical counterparts of 37 of the X-ray sources, yielding information on their morphologies and environments. In this paper we extend the results obtained in our previous study on the 300 ks CDFS X-ray data (Schreier et al. 2001, Paper I). Specifically, we show that the optical counterparts to the X-ray sources are divided into two distinct populations: 1) an optically faint group with relatively blue colors, similar to the faint blue field galaxy population, and 2) an optically brighter group, including resolved galaxies with average colors significantly redder than the corresponding bright field galaxy population. The brighter objects comprise a wide range of types, including early and late type galaxies, starbursts, and AGN. By contrast, we show that the faint blue X-ray population are most consistent with being predominantly Type 2 AGN of low to moderate luminosity, located at higher redshifts (z ~ 1 - 2). This conclusion is supported by luminosity function models of the various classes of objects. Hence, the combination of deep X-ray data with the high spatial resolution of HST are for the first time allowing us to probe the faint end of the AGN luminosity function at cosmologically interesting redshifts.

We present improved constraints on the formation time scale of massive cluster galaxies based on rest-frame optical spectra of galaxies in a forming cluster located at z=2.16. The spectra are obtained with MOIRCS on the Subaru telescope with an integration time of ~7 hours. We achieve accurate redshift measurements by fitting SEDs using the spectra and broad-band photometry simultaneously, allowing us to identify probable cluster members. Clusters at low redshifts are dominated by quiescent galaxies, but we find that quiescent galaxies and star forming galaxies co-exist in this z=2 system. Interestingly, the quiescent galaxies form a weak red sequence in the process of forming. By stacking the spectra of star forming galaxies, we observe strong emission lines such as [OII] and [OIII] and we obtain a tentative hint of AGN activities in these galaxies. On the other hand, the stacked spectrum of the quiescent galaxies reveals a clear 4000A break with a possible CaII H+K absorption feature and strong emission lines such as [OII] are absent in the spectrum, confirming the quiescent nature of these galaxies. We then perform detailed spectral analyses of the stacked spectrum, which suggest that these massive quiescent galaxies formed at redshifts between 3 and 4 on a time scale of, Comment: 17 pages, 14 figures, accepted for publication in ApJ

We quantify the presence of active galactic nuclei (AGNs) in a mass-complete (M * > 5 ? 1010 M ?) sample of 123 star-forming and quiescent galaxies at 1.5 ? z ? 2.5, using X-ray data from the 4 Ms Chandra Deep Field-South (CDF-S) survey. 41% ? 7% of the galaxies are detected directly in X-rays, 22% ? 5% with rest-frame 0.5-8 keV luminosities consistent with hosting luminous AGNs (L 0.5-8 keV > 3 ? 1042 erg s?1). The latter fraction is similar for star-forming and quiescent galaxies, and does not depend on galaxy stellar mass, suggesting that perhaps luminous AGNs are triggered by external effects such as mergers. We detect significant mean X-ray signals in stacked images for both the individually non-detected star-forming and quiescent galaxies, with spectra consistent with star formation only and/or a low-luminosity AGN in both cases. Comparing star formation rates inferred from the 2-10 keV luminosities to those from rest-frame IR+UV emission, we find evidence for an X-ray excess indicative of low-luminosity AGNs. Among the quiescent galaxies, the excess suggests that as many as 70%-100% of these contain low- or high-luminosity AGNs, while the corresponding fraction is lower among star-forming galaxies (43%-65%). Our discovery of the ubiquity of AGNs in massive, quiescent z ~ 2 galaxies provides observational support for the importance of AGNs in impeding star formation during galaxy evolution.

We present a study of protoclusters associated with high redshift radio galaxies. We imaged MRC1017-220 (z=1.77) and MRC0156-252 (z=2.02) using the near-infrared wide-field (7.5'x7.5') imager VLT/HAWK-I in the Y, H and Ks bands. We present the first deep Y-band galaxy number counts within a large area (200 arcmin2). We then develop a purely near-infrared colour selection technique to isolate galaxies at 1.6, Comment: accepted for publication in A&A, 16 pages, 13 figures, 3 tables