Publications (15)16.45 Total impact
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Article: Galaxy evolution in overdense environments at high redshift: passive early-type galaxies in a cluster at redshift 2
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ABSTRACT: We present a study of galaxy populations in the central region of the IRAC-selected, X-ray detected galaxy cluster Cl J1449+0856 at z=2. Based on a sample of spectroscopic and photometric cluster members, we investigate stellar populations and morphological structure of cluster galaxies over an area of ~0.7Mpc^2 around the cluster core. The cluster stands out as a clear overdensity both in redshift space, and in the spatial distribution of galaxies close to the center of the extended X-ray emission. The cluster core region (r<200 kpc) shows a clearly enhanced passive fraction with respect to field levels. However, together with a population of massive passive galaxies mostly with early-type morphologies, it also hosts massive actively star-forming, often highly dust-reddened sources. Close to the cluster center, a multi-component system of passive and star-forming galaxies could be the future BCG still assembling. We observe a clear correlation between passive stellar populations and an early-type morphology, in agreement with field studies at similar redshift. Passive early-type galaxies in this clusters are typically a factor 2-3 smaller than similarly massive early-types at z~0, but also on average larger by a factor ~2 than their field analogs at z~2, lending support to recent claims of an accelerated structural evolution in high-redshift dense environments. These results point towards the early formation of a population of massive galaxies, already evolved both in their structure and stellar populations, coexisting with still-actively forming massive galaxies in the central regions of young clusters 10 billion years ago.05/2013; -
Article: Newly-quenched galaxies as the cause for the apparent evolution in average size of the population
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ABSTRACT: We study the number densities of quenched early-type galaxies (Q-ETGs) of a given size in a homogeneous COSMOS sample over the interval 1 < z < 0.2. At 10^{10.5}<M<10^{11} Msun we see no change in the number density of compact Q-ETGs, while at >10^{11} Msun we find a decrease by 30-40%. In both mass bins, the increase of the median sizes of Q-ETGs with time is primarily caused by the addition of larger Q-ETGs. At all masses, compact Q-ETGs become redder towards later epochs, with a (U-V) color difference which is consistent with a passive evolution of their stellar populations. At all epochs, the larger Q-ETGs have colors that are systematically bluer than those of the more compact Q-ETGs, suggesting that the former are younger than the latter. The sizes and number of the star-forming galaxies over the same redshift interval also support the idea that new Q-ETGs are responsible for the growth in the median size of the population at a given mass. The sizes of the new members of the population appear to scale as roughly (1+z)^{-1}. We conclude that the dominant cause of the apparent size evolution of the population of Q-ETGs is the addition at later epochs of larger galaxies. [abridged]02/2013; -
Article: Mass assembly in quiescent and star-forming galaxies since z=4 from UltraVISTA
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ABSTRACT: We estimate the galaxy stellar mass function and stellar mass density for star-forming and quiescent galaxies with 0.2<z<4. We construct a deep K<24 sample of 220,000 galaxies selected using the UltraVISTA DR1 data release. Our analysis is based on precise 30-band photometric redshifts. By comparing these photometric redshifts with 10,800 spectroscopic redshifts from the zCOSMOS surveys, we find a precision of sigma((zp-zs)/(1+zs))=0.008 at i<22.5 and sigma((zp-zs)/(1+zs))=0.03 at 1.5<z<4. We derive the stellar mass function and correct for the Eddington bias. We find a mass-dependent evolution of the global and star-forming populations, with the low-mass end of the mass functions evolving more rapidly than the high-mass end. This mass-dependent evolution is a direct consequence of the star formation being quenched in galaxies more massive than M>10^10.7Msun. For the mass function of the quiescent galaxies, we do not find any significant evolution of the high-mass end at z<1; however we observe a clear flattening of the faint-end slope. From z~3 to z~1, their comoving stellar mass density increases by 1.6 dex between z~3 and z~1 and by less than 0.2 dex at z<1. Following Wilkins et al. (2008), we infer the star formation history from the mass density evolution. This inferred star formation history is in excellent agreement with SFR measurements at z<1.5, while we find differences of 0.2 dex at z>1.5 consistent with the expected uncertainties. We also develop a new method to infer the specific SFR from the mass function of star-forming galaxies. We find that the sSFR of 10^10Msun galaxies increases continuously in the redshift range 1<z<4. Finally, we compare our results with the semi-analytical model and find that these models overestimate the density of low mass quiescent galaxies by an order of magnitude, while the density of low-mass star-forming galaxies is successfully reproduced.01/2013; -
Article: Deep near-infrared spectroscopy of passively evolving galaxies at z>1.4
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ABSTRACT: [Abridged] We present the results of new near-IR spectroscopic observations of passive galaxies at z>1.4 in a concentration of BzK-selected galaxies in the COSMOS field. The observations have been conducted with Subaru/MOIRCS, and have resulted in absorption lines and/or continuum detection for 18 out of 34 objects. This allows us to measure spectroscopic redshifts for a sample almost complete to K(AB)=21. COSMOS photometric redshifts are found in fair agreement overall with the spectroscopic redshifts, with a standard deviation of ~0.05; however, ~30% of objects have photometric redshifts systematically underestimated by up to ~25%. We show that these systematic offsets in photometric redshifts can be removed by using these objects as a training set. All galaxies fall in four distinct redshift spikes at z=1.43, 1.53, 1.67 and 1.82, with this latter one including 7 galaxies. SED fits to broad-band fluxes indicate stellar masses in the range of ~4-40x10^10Msun and that star formation was quenched ~1 Gyr before the cosmic epoch at which they are observed. The spectra of several individual galaxies have allowed us to measure their Hdelta_F and Dn4000 indices, which confirms their identification as passive galaxies, as does a composite spectrum resulting from the coaddition of 17 individual spectra. The effective radii of the galaxies have been measured on the HST/ACS F814W image, confirming the coexistence at these redshifts of passive galaxies which are substantially more compact than their local counterparts with others that follow the local size-stellar mass relation. For the galaxy with best S/N spectrum we were able to measure a velocity dispersion of 270+/-105 km/s, indicating that this galaxy lies closely on the virial relation given its stellar mass and effective radius.06/2012; -
Article: A mature cluster with X-ray emission at z=2.07
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ABSTRACT: We report evidence of a fully established galaxy cluster at z=2.07, consisting of a ~20sigma overdensity of red, compact spheroidal galaxies spatially coinciding with extended X-ray emission detected with XMM-Newton. We use VLT VIMOS and FORS2 spectra and deep Subaru, VLT and Spitzer imaging to estimate the redshift of the structure from a prominent z=2.07 spectroscopic redshift spike of emission-line galaxies, concordant with the accurate 12-band photometric redshifts of the red galaxies. Using NICMOS and Keck AO observations, we find that the red galaxies have elliptical morphologies and compact cores. While they do not form a tight red sequence, their colours are consistent with that of a >1.3$~Gyr population observed at z~2.1. From an X-ray luminosity of .2*10^43 erg s^-1 and the stellar mass content of the red galaxy population, we estimate a halo mass of 5.3-8*10^13 Msun, comparable to the nearby Virgo cluster. These properties imply that this structure could be the most distant, mature cluster known to date and that X-ray luminous, elliptical-dominated clusters are already forming at substantially earlier epochs than previously known. Comment: 14 pages, 12 figures; accepted for publication in Astronomy & Astrophysics11/2010; -
Article: A z = 1.82 Analog of Local Ultra-massive Elliptical Galaxies
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ABSTRACT: We present observations of a very massive galaxy at z = 1.82 that show that its morphology, size, velocity dispersion, and stellar population properties are fully consistent with those expected for passively evolving progenitors of today's giant ellipticals. These findings are based on a deep optical rest-frame spectrum obtained with the Multi-Object InfraRed Camera and Spectrograph on the Subaru Telescope of a high-z passive galaxy candidate (pBzK) from the COSMOS field, for which we accurately measure its redshift of z = 1.8230 and obtain an upper limit on its velocity dispersion σ < 326 km s–1. By detailed stellar population modeling of both the galaxy broadband spectral energy distribution and the rest-frame optical spectrum, we derive a star formation-weighted age and formation redshift of t sf 1-2 Gyr and z form 2.5-4, and a stellar mass of M (3-4) × 1011 M ☉. This is in agreement with a virial mass limit of M vir < 7 × 1011 M ☉, derived from the measured σ value and stellar half-light radius, as well as with the dynamical mass limit based on the Jeans equations. In contrast to previously reported super-dense passive galaxies at z ~ 2, the present galaxy at z = 1.82 appears to have both size and velocity dispersion similar to early-type galaxies in the local universe with similar stellar mass. This suggests that z ~ 2 massive and passive galaxies may exhibit a wide range of properties, then possibly following quite different evolutionary histories from z ~ 2 to z = 0.The Astrophysical Journal Letters 04/2010; 715(1):L6. · 5.53 Impact Factor -
Article: High‐redshift elliptical galaxies: are they (all) really compact?
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ABSTRACT: We investigate the properties of 12 ultramassive passively evolving early-type galaxies (ETGs) at zphot > 1.4 in the COSMOS 2 deg2 field. These 12 ETGs were selected as pBzKs, have accurate 1.4 ≲zphot≲ 1.7, high Sérsic index profiles typical of ellipticals, no detection at 24 μm, resulting in a complete ETG sample at M* > 2.5 × 1011 M⊙ (Chabrier initial mass function). Contrary to the previous claims, the half-light radii estimated in very high signal-to-noise ratio imaging data from Hubble Space Telescope+Advanced Camera for Surveys (ACS) are found to be large for most of the sample, consistent with local ellipticals. If the high-redshift ETGs with M* < 2.5 × 1011 M⊙ are really small in size and compact as reported in the previous studies, our result may suggest a ‘downsizing’ scenario, whereby the most massive ETGs reach their final structure earlier and faster than the lower mass ones. However, simulating galaxies with morphological properties fixed to those of local ETGs with the same stellar mass show that the few compact galaxies that we still recover in our sample can be understood in terms of fluctuations due to noise preventing the recovery of the extended low surface brightness haloes in the light profile. Such haloes, typical of Sérsic profiles, extending even up to 40 kpc, are indeed seen in our sample.Monthly Notices of the Royal Astronomical Society 01/2010; 401(2):933 - 940. · 4.90 Impact Factor -
Article: MAMBO 1.2 mm Observations of BzK-selected Star-forming Galaxies at z ~ 2
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ABSTRACT: We present MAMBO 1.2 mm observations of five BzK-preselected vigorous starburst galaxies at z ~ 2. Two of these were detected at more than 99.5% confidence levels, with 1.2 mm fluxes around 1.5 mJy. These millimeter fluxes imply vigorous activity with star formation rates (SFRs) of ≈500-1500 M☉ yr-1, which were also confirmed by detections at 24 μm with the Multiband Imaging Photometer on Spitzer (MIPS). The two detected galaxies are the ones in the sample with the highest SFRs estimated from the rest-frame UV, and their far-IR-derived and UV-derived SFRs agree reasonably well. This is different from local ultraluminous infrared galaxies (ULIRGs) and high-z submillimeter/millimeter-selected galaxies, for which the UV is reported to underestimate SFRs by factors of 10-100, but similar to the average BzK ULIRG at z ~ 2. The two galaxies detected at 1.2 mm are brighter in K than the typical NIR counterparts of MAMBO and SCUBA sources, implying a significantly different K-band-to-submillimeter/millimeter flux ratio as well. This suggests a scenario in which z ~ 2 galaxies, after their rapid (sub)millimeter-bright phase that is opaque to optical/UV light, evolve into a longer lasting phase of K-band-bright and massive objects. Targeting the most UV-active BzK galaxies could yield substantial detection rates at submillimeter/millimeter wavelengths.The Astrophysical Journal 12/2008; 637(1):L5. · 6.02 Impact Factor -
Article: Discovery of an Evolved Galaxy Cluster at z>2
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ABSTRACT: Over a 1,000-arcmin^2 field imaged with the Spitzer/IRAC, we have discovered a candidate galaxy cluster at z>2 with extremely red, evolved galaxies concentrated within an area of only 30 arcsec in diameter. The cluster candidate shows an extended x-ray emission with L_{X}(0.1-2.4 keV) ˜eq 10^{44} erg s^{-1} likely to be associated with cluster potential. Among the galaxies with meaningful photometric redshifts, 20 galaxies in this region are consistent with photometric redshift of z_{phot}=2.15±0.2. Their rest-frame optical colors are consistent with those of red-sequence galaxies with 1-3 Gyr stellar populations, reaching (z-K)_{AB}˜eq 4. Compared with the passively evolving field galaxies at similar redshifts, these colors are typically ˜eq1 magnitude redder, indicating the presence of a strong environmental effect on the evolution of stellar populations at high redshift.09/2008; 399:363. -
Article: Kinemetry of SINS High-Redshift Star-Forming Galaxies: Distinguishing Rotating Disks from Major Mergers
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ABSTRACT: We present a simple set of kinematic criteria that can distinguish between galaxies dominated by ordered rotational motion and those involved in major merger events. Our criteria are based on the dynamics of the warm ionized gas (as traced by H-alpha) within galaxies, making this analysis accessible to high-redshift systems, whose kinematics are primarily traceable through emission features. Using the method of kinemetry (developed by Krajnovic and co-workers), we quantify asymmetries in both the velocity and velocity dispersion maps of the warm gas, and the resulting criteria enable us to empirically differentiate between non-merging and merging systems at high redshift. We apply these criteria to 11 of our best-studied rest-frame UV/optical-selected z~2 galaxies for which we have near infrared integral field spectroscopic data from SINFONI on the VLT. Of these 11 systems, we find that >50% have kinematics consistent with a single rotating disk interpretation, while the remaining systems are more likely undergoing major mergers. This result, combined with the short formation timescales of these systems, provides evidence that rapid, smooth accretion of gas plays a significant role in galaxy formation at high redshift. Comment: Accepted for publication in the Astrophysical Journal. 24 pages, 14 figures02/2008; -
Article: A wide area survey for high-redshift massive galaxies. I. Number counts and clustering of BzKs and EROs
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ABSTRACT: We have combined deep BRIz' imaging over 2x940 arcmin^2 fields obtained with the Suprime-Cam on the Subaru telescope with JKs imaging with the SOFI camera at the New Technology Telescope to search for high-redshift massive galaxies. K-band selected galaxies have been identified over an area of ~920 arcmin^2 to K_Vega=19.2, of which 320 arcmin^2 are complete to K_Vega=20. The BzK selection technique was used to obtain complete samples of ~500 candidate massive star-forming galaxies (sBzKs) and ~160 candidate massive, passively-evolving galaxies (pBzKs), both at 1.4<z<2.5. With the (R-K)_Vega > 5 criterion we also identified ~850 extremely red objects (EROs). The surface density of sBzKs and pBzKs is found to 1.20+/-0.05 arcmin^{-2} and 0.38+/-0.03 arcmin^{-2}, respectively. Both sBzKs and pBzKs are strongly clustered, at a level at least comparable to that of EROs, with pBzKs appearing more clustered than sBzKs. We estimate the reddening, star formation rates (SFRs) and stellar masses (M_*) of the sBzKs, confirming that to K_Vega~20 median values are M_*~10^{11}M_sun, SFR 190M_sun yr^{-1}, and E(B-V)~0.44. The most massive sBzKs are also the most actively star-forming, an effect which can be seen as a manifestation of downsizing at early epochs. The space density of massive pBzKs at z~1.4-2 is 20%+/-7% that of similarly massive early-type galaxies at z~0, and similar to that of sBzKs of the same mass. We argue that star formation quenching in these sBzKs will result in nearly doubling the space density of massive early-type galaxies, thus matching their local density. Comment: 19 pages, 13 figures, accepted by ApJ. While checking the proofs we became aware of a material mistake of non-trivial scientific relevance. In the original it was reported that the comoving volume density of passive BzK-selected galaxies with =1.7 and more massive than 10^{11}M_sun was 45%+/-15% of the local number density of similarly massive early-type galaxies. This fraction actually turns out to be 20%+/-7%. Section 6.4, point 5 in section 7, and the abstract have been modified accordingly10/2005; -
Article: MAMBO 1.2 mm observations of BzK-selected star-forming galaxies at z ~ 2
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Article: The SINS Survey: SINFONI integral field spectroscopy of z ~ 2 star-forming galaxies
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ABSTRACT: We present the Spectroscopic Imaging survey in the near-infrared (near-IR) with SINFONI (SINS) of high-redshift galaxies. With 80 objects observed and 63 detected in at least one rest-frame optical nebular emission line, mainly Halpha, SINS represents the largest survey of spatially resolved gas kinematics, morphologies, and physical properties of star-forming galaxies at z ~ 1-3. We describe the selection of the targets, the observations, and the data reduction. We then focus on the "SINS Halpha sample," consisting of 62 rest-UV/optically selected sources at 1.3 < z < 2.6 for which we targeted primarily the Halpha and [N II] emission lines. Only ââ°Ë30% of this sample had previous near-IR spectroscopic observations. The galaxies were drawn from various imaging surveys with different photometric criteria; as a whole, the SINS Halpha sample covers a reasonable representation of massive M sstarf gsim 1010 M sunstar-forming galaxies at z ââ°Ë 1.5-2.5, with some bias toward bluer systems compared to pure K-selected samples due to the requirement of secure optical redshift. The sample spans 2 orders of magnitude in stellar mass and in absolute and specific star formation rates, with median values ââ°Ë3 à 1010 M sun, ââ°Ë70 M sun yr--1, and ââ°Ë3 Gyr--1. The ionized gas distribution and kinematics are spatially resolved on scales ranging from ââ°Ë1.5 kpc for adaptive optics assisted observations to typically ââ°Ë4-5 kpc for seeing-limited data. The Halpha morphologies tend to be irregular and/or clumpy. About one-third of the SINS Halpha sample galaxies are rotation-dominated yet turbulent disks, another one-third comprises compact and velocity dispersion-dominated objects, and the remaining galaxies are clear interacting/merging systems; the fraction of rotation-dominated systems increases among the more massive part of the sample. The Halpha luminosities and equivalent widths suggest on average roughly twice higher dust attenuation toward the H II regions relative to the bulk of the stars, and comparable current and past-averaged star formation rates. Based on observations obtained at the Very Large Telescope (VLT) of the European Southern Observatory, Paranal, Chile (ESO Programme IDs 070.A-0229, 070.B-0545, 073.B-9018, 074.A-9011, 075.A-0466, 076.A-0527, 077.A-0576, 078.A-0055, 078.A-0600, 079.A-0341, 080.A-0330, 080.A-0635, and 080.A-0339).The Astrophysical Journal, v.706, 1364-1428 (2009). -
Article: High-redshift elliptical galaxies: are they (all) really compact?
Monthly Notices of the Royal Astronomical Society, v.401, 933-940 (2010). -
Article: Very high gas fractions and extended gas reservoirs in z = 1.5 disk galaxies
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ABSTRACT: We present evidence for very high gas fractions and extended molecular gas reservoirs in normal, near-infrared-selected (BzK) galaxies at z ~ 1.5. Our results are based on multi-configuration CO[2-1] observations obtained at the IRAM Plateau de Bure Interferometer. All six star-forming galaxies observed were detected at high significance. High spatial resolution observations resolve the CO emission in four of them, implying sizes of the gas reservoirs of order of 6-11 kpc and suggesting the presence of ordered rotation. The galaxies have UV morphologies consistent with clumpy, unstable disks, and UV sizes that are consistent with those measured in CO. The star formation efficiencies are homogeneously low within the sample and similar to those of local spiralsââ¬âthe resulting gas depletion times are ~0.5 Gyr, much higher than what is seen in high-z submillimeter galaxies and quasars. The CO luminosities can be predicted to within 0.15 dex from the observed star formation rates (SFRs) and stellar masses, implying a tight correlation of the gas mass with these quantities. We use new dynamical models of clumpy disk galaxies to derive dynamical masses for our sample. These models are able to reproduce the peculiar spectral line shapes of the CO emission. After accounting for the stellar and dark matter masses, we derive molecular gas reservoirs with masses of (0.4-1.2)Ãâ1011 M sun. The implied conversion (CO luminosity-to-gas mass) factor is very high: ñCO = 3.6 ñ 0.8, consistent with a Galactic conversion factor but 4 times higher than that of local ultra-luminous IR galaxies that is typically used for high-redshift objects. The gas mass in these galaxies is comparable to or larger than the stellar mass, and the gas accounts for an impressive 50%-65% of the baryons within the galaxies' half-light radii. We are thus witnessing truly gas-dominated galaxies at z ~ 1.5, a finding that explains the high specific SFRs observed for z > 1 galaxies. The BzK galaxies can be viewed as scaled-up versions of local disk galaxies, with low-efficiency star formation taking place inside extended, low-excitation gas disks. These galaxies are markedly different than local ULIRGs and high-z submillimeter galaxies and quasars, where higher excitation and more compact gas is found.The Astrophysical Journal, v.713, 686-707 (2010).
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Institutions
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2010
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Université Paris Diderot - Paris 7
Paris, Ile-de-France, France
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2008
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Yonsei University
Seoul, Seoul, South Korea
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