V. Le Brun

Publications (187)176.56 Total impact

• Article: Spot the difference. Impact of different selection criteria on early-type galaxies observed properties in zCOSMOS 20-k sample

  M. Moresco, L. Pozzetti, A. Cimatti, G. Zamorani, M. Bolzonella, F. Lamareille, M. Mignoli, E. Zucca, S. J. Lilly, C. M. Carollo, [......], C. Diener, A. M. Koekemoer, E. Le Floch, C. Lopez-Sanjuan, H. J. McCracken, P. Nair, P. Oesch, C. Scarlata, N. Scoville, N. Welikala
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  ABSTRACT: We present the analysis of photometric, spectroscopic and morphological properties for differently selected samples of early-type galaxies up to z=1 extracted from the zCOSMOS-20k spectroscopic survey. This analysis intends to explore the dependence of galaxy properties on the selection criterion adopted, to study the degree of contamination due to blue/star-forming/non-passive outliers, and to provide a comparison between different commonly used selection criteria. We extracted from the zCOSMOS-20k catalog 6 different samples of early-type galaxies based on morphology, optical colors, specific star formation rate, a best-fit to the observed spectral energy distribution, and a criterion combining morphological, spectroscopic and photometric informations. The "morphological" sample has the higher percentage of contamination in colors, specific star formation rate and presence of emission lines, while the "pure passive" sample is the purest, with properties mostly compatible with no star formation activity; however, it is also the less economic criterion in terms of information used. The best performing among the other criteria are the "photometric type" and the "sSFR", providing a percentage of contamination only slightly higher than the "pure passive" criterion (on average of a factor of ~2) but with absolute values of the properties of contaminants still compatible with a red, passively evolving population. We also provided two revised definitions of early type galaxies based on restframe color-color and color-mass criteria, that better reproduce the observed bimodalities. The analysis of the number densities shows evidences of mass-assembly downsizing, with galaxies at 10.25<log(M/Msun)<10.75 increasing their number by a factor ~2-4 from z=0.6 to z=0.2, by a factor ~2-3 from z=1 to z=0.2 at 10.75<log(M/Msun)<11, and by only ~10-50% from z=1 to z=0.2 at 11<log(M/Msun)<11.5.
  05/2013;
• Article: The VIMOS Public Extragalactic Redshift Survey (VIPERS). Galaxy clustering and redshift-space distortions at z=0.8 in the first data release

  S. de la Torre, L. Guzzo, J. A. Peacock, E. Branchini, A. Iovino, B. R. Granett, U. Abbas, C. Adami, S. Arnouts, J. Bel, [......], A. Burden, C. Di Porto, A. Marchetti, C. Marinoni, Y. Mellier, P. Monaco, R. C. Nichol, S. Phleps, M. Wolk, G. Zamorani
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  ABSTRACT: We present in this paper the general real- and redshift-space clustering properties of galaxies as measured in the first data release of the VIPERS survey. VIPERS is a large redshift survey designed to probe the distant Universe and its large-scale structure at 0.5 < z < 1.2. We describe in this analysis the global properties of the sample and discuss the survey completeness and associated corrections. This sample allows us to measure the galaxy clustering with an unprecedented accuracy at these redshifts. From the redshift-space distortions observed in the galaxy clustering pattern we provide a first measurement of the growth rate of structure at z = 0.8: f\sigma_8 = 0.47 +/- 0.08. This is completely consistent with the predictions of standard cosmological models based on Einstein gravity.
  03/2013;
• Article: The VIMOS Public Extragalactic Redshift Survey (VIPERS). Luminosity and stellar mass dependence of galaxy clustering at 0.5<z<1.1

  F. Marulli, M. Bolzonella, E. Branchini, I. Davidzon, S. de la Torre, B. R. Granett, L. Guzzo, A. Iovino, L. Moscardini, A. Pollo, [......], C. Di Porto, A. Marchetti, C. Marinoni, Y. Mellier, R. C. Nichol, J. A. Peacock, W. J. Percival, S. Phleps, M. Wolk, G. Zamorani
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  ABSTRACT: We investigate the dependence of galaxy clustering on luminosity and stellar mass, in the redshift range 0.5<z<1.1, using the first ~55000 redshifts from the VIMOS Public Extragalactic Redshift Survey (VIPERS). To characterize the spatial properties of the VIPERS galaxies, we measure the redshift-space two-point correlation functions (2PCF), and the projected correlation function, in samples covering different ranges of B-band absolute magnitudes and stellar masses. We consider both threshold and binned galaxy samples, with median B-band absolute magnitudes -21.6<MB-5log(h)<-19.5 and median stellar masses 9.8<log(M*[Msun/h^2] <10.7. We assess the real-space clustering in the data from the projected correlation function, that we model as a power-law in the range 0.2<r_p[Mpc/h]<20. Finally, we estimate the galaxy bias as a function of luminosity, stellar mass and redshift, assuming a flat {\Lambda} cold dark matter model to derive the dark matter 2PCF. We provide the best-fit parameters of the power-law model assumed for the real-space 2PCF - the correlation length and the slope - as well as the linear bias parameter, as a function of the B-band absolute magnitude, stellar mass and redshift. We find a clear luminosity and stellar mass dependence of the galaxy clustering at all redshifts, with more luminous and massive galaxies showing a higher clustering amplitude, in agreement with previous studies. Our analysis provides new constraints on galaxy formation models that now have to match the observed redshift evolution of the clustering of VIPERS galaxies in a broad range of luminosity and stellar masses, between z=0.5 and z=1.1.
  03/2013;
• Article: The VIMOS Public Extragalactic Redshift Survey (VIPERS). A Support Vector Machine classification of galaxies, stars and AGNs

  K. Malek, A. Solarz, A. Pollo, A. Fritz, B. Garilli, M. Scodeggio, A. Iovino, B. R. Granett, U. Abbas, C. Adami, [......], A. Marchetti, C. Marinoni, Y. Mellier, L. Moscardini, R. C. Nichol, J. A. Peacock, W. J. Percival, S. Phleps, M. Wolk, G. Zamorani
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  ABSTRACT: The aim of this work is to develop a comprehensive method for classifying sources in large sky surveys and we apply the techniques to the VIMOS Public Extragalactic Redshift Survey (VIPERS). Using the optical (u*, g', r', i') and NIR data (z', Ks), we develop a classifier for identifying stars, AGNs and galaxies improving the purity of the VIPERS sample. Support Vector Machine (SVM) supervised learning algorithms allow the automatic classification of objects into two or more classes based on a multidimensional parameter space. In this work, we tailored the SVM for classifying stars, AGNs and galaxies, and applied this classification to the VIPERS data. We train the SVM using spectroscopically confirmed sources from the VIPERS and VVDS surveys. We tested two SVM classifiers and concluded that including NIR data can significantly improve the efficiency of the classifier. The self-check of the best optical + NIR classifier has shown a 97% accuracy in the classification of galaxies, 97 for stars, and 95 for AGNs in the 5-dimensional colour space. In the test on VIPERS sources with 99% redshift confidence, the classifier gives an accuracy equal to 94% for galaxies, 93% for stars, and 82% for AGNs. The method was applied to sources with low quality spectra to verify their classification, and thus increasing the security of measurements for almost 4 900 objects. We conclude that the SVM algorithm trained on a carefully selected sample of galaxies, AGNs, and stars outperforms simple colour-colour selection methods, and can be regarded as a very efficient classification method particularly suitable for modern large surveys.
  03/2013;
• Article: The VIMOS Public Extragalactic Redshift Survey (VIPERS). An unprecedented view of galaxies and large-scale structure at 0.5<z<1.2

  L. Guzzo, M. Scodeggio, B. Garilli, B. R. Granett, U. Abbas, C. Adami, S. Arnouts, J. Bel, M. Bolzonella, D. Bottini, [......], A. Burden, C. Di Porto, A. Marchetti, C. Marinoni, Y. Mellier, L. Moscardini, R. C. Nichol, W. J. Percival, S. Phleps, M. Wolk
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  ABSTRACT: We describe the construction and general features of VIPERS, the VIMOS Public Extragalactic Redshift Survey. This `Large Programme' has been using the ESO VLT with the aim of building a spectroscopic sample of ~100,000 galaxies with i_{AB}<22.5 and 0.5<z<1.5. The survey covers a total area of ~24 deg^2 within the CFHTLS-Wide W1 and W4 fields. VIPERS is designed to address a broad range of problems in large-scale structure and galaxy evolution, thanks to a unique combination of volume (~ 5 x 10^7 h^{-3} Mpc^3) and sampling rate (~ 40%), comparable to state-of-the-art surveys of the local Universe, together with extensive multi-band optical and near-infrared photometry. Here we present the survey design, the selection of the source catalogue and the development of the spectroscopic observations. We discuss in detail the overall selection function that results from the combination of the different constituents of the project. This includes the masks arising from the parent photometric sample and the spectroscopic instrumental footprint, together with the weights needed to account for the sampling and the success rates of the observations. Using the catalogue of 53,608 galaxy redshifts composing the forthcoming VIPERS Public Data Release 1 (PDR-1), we provide a first assessment of the quality of the spectroscopic data. Benefiting from the combination of size and detailed sampling of this dataset, we conclude by presenting a map showing in unprecedented detail the large-scale distribution of galaxies between 5 and 8 billion years ago. [abridged]
  03/2013;
• Article: HeII emitters in the VIMOS VLT Deep Survey: PopIII star formation or peculiar stellar populations in galaxies at 2<z<4.6?

  P. Cassata, O. Le Fevre, S. Charlot, T. Contini, O. Cucciati, B. Garilli, G. Zamorani, C. Adami, S. Bardelli, V. Le Brun, B. Lemaux, D. Maccagni, A. Pollo, L. Pozzetti, L. Tresse, D. Vergani, A. Zanichelli, E. Zucca
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  ABSTRACT: The aim of this work is to identify HeII emitters at 2<z<4.6 and to constrain the source of the hard ionizing continuum that powers the HeII emission. We have assembled a sample of 352 galaxies with a high quality spectroscopic redshift at 2<z<4.6 from the VVDS survey, and we have identified 40 HeII1640A emitters. We study their spectral properties, measuring the fluxes, equivalent widths (EW) and FWHM for most relevant lines. About 10% of galaxies at z~3 show HeII in emission, with rest frame equivalent widths EW0~1-7A, equally distributed between galaxies with Lya in emission or in absorption. We find 12 high-quality HeII emitters with unresolved HeII line (FWHM_0<1200km/s), 13 high-quality emitters with broad He II emission (FWHM_0>1200km/s), 3 AGN, and an additional 12 possible HeII emitters. The properties of the individual broad emitters are in agreement with expectations from a W-R model. On the contrary, the properties of the narrow emitters are not compatible with such model, neither with predictions of gravitational cooling radiation produced by gas accretion. Rather, we find that the EW of the narrow HeII line emitters are in agreement with expectations for a PopIII star formation, if the episode of star formation is continuous, and we calculate that a PopIII SFR of 0.1-10 Mo yr-1 only is enough to sustain the observed HeII flux. We conclude that narrow HeII emitters are either powered by the ionizing flux from a stellar population rare at z~0 but much more common at z~3, or by PopIII star formation. As proposed by Tornatore et al. (2007), incomplete ISM mixing may leave some small pockets of pristine gas at the periphery of galaxies from which PopIII may form, even down to z~2 or lower. If this interpretation is correct, we measure at z~3 a SFRD in PopIII stars of 10^6Mo yr^-1 Mpc^-3 qualitatively comparable to the value predicted by Tornatore et al. (2007).
  12/2012;
• Article: The Colors of Central and Satellite Galaxies in zCOSMOS out to z ~ 0.8 and Implications for Quenching

  C. Knobel, S. J. Lilly, K. Kovac, Y. Peng, T. J. Bschorr, C. M. Carollo, T. Contini, J. -P. Kneib, O. Le Fevre, V. Mainieri, [......], A. Cappi, A. Cimatti, G. Coppa, A. M. Koekemoer, C. Lopez-Sanjuan, H. J. McCracken, M. Moresco, P. Nair, L. Pozzetti, N. Welikala
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  ABSTRACT: We examine the red fraction of central and satellite galaxies in the large zCOSMOS group catalog out to z ~ 0.8 correcting for both the incompleteness in stellar mass and for the less than perfect purities of the central and satellite samples. We show that, at all masses and at all redshifts, the fraction of satellite galaxies that have been quenched, i.e., are red, is systematically higher than that of centrals, as seen locally in the Sloan Digital Sky Survey (SDSS). The satellite quenching efficiency, which is the probability that a satellite is quenched because it is a satellite rather than a central, is, as locally, independent of stellar mass. Furthermore, the average value is about 0.5, which is also very similar to that seen in the SDSS. We also construct the mass functions of blue and red centrals and satellites and show that these broadly follow the predictions of the Peng et al. analysis of the SDSS groups. Together, these results indicate that the effect of the group environment in quenching satellite galaxies was very similar when the universe was about a half its present age, as it is today.
  11/2012;
• Article: Proto-groups at 1.8<z<3 in the zCOSMOS-deep sample

  C. Diener, S. J. Lilly, C. Knobel, G. Zamorani, G. Lemson, P. Kampczyk, N. Scoville, C. M. Carollo, T. Contini, J. -P. Kneib, [......], A. Cappi, A. Cimatti, G. Coppa, A. M. Koekemoer, C. López-Sanjuan, H. J. McCracken, M. Moresco, P. Nair, L. Pozzetti, N. Welikala
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  ABSTRACT: We identify 42 candidate groups lying between 1.8<z<3.0 from a sample of 3502 galaxies with spectroscopic redshifts in the zCOSMOS-deep redshift survey within the same redshift interval. These systems contain three to five spectroscopic galaxies that lie within 500kpc in projected distance (in physical space) and within 700km/s in velocity. Based on extensive analysis of mock catalogues that have been generated from the Millennium simulation, we examine the likely nature of these systems at the time of observation, and what they will evolve into down to the present epoch. Although few of the "member" galaxies are likely to reside in the same halo at the epoch we observe them, 50% of the systems will eventually bring them all into the same halo, and almost all (93%) will have at least part of the member galaxies in the same halo by the present epoch. Most of the candidate groups can therefore be described as "proto-groups". An estimate of the overdensities is also consistent with the idea that these systems are being seen at the start of the assembly process. We also examine present-day haloes and ask whether their progenitors would have been seen amongst our candidate groups. For present-day haloes between 10^14-10^15Msun/h, 35% should have appeared amongst our candidate groups, and this would have risen to 70% if our survey had been fully-sampled, so we can conclude that our sample can be taken as representative of a large fraction of such systems. There is a clear excess of massive galaxies above 10^10Msun around the locations of the candidate groups in a large independent COSMOS photo-z sample, but we see no evidence in this latter data for any colour differentiation with respect to the field. This is however consistent with the idea that such differentiation arises in satellite galaxies, as indicated at z<1, if the candidate groups are indeed only starting to be assembled.
  10/2012;
• Article: The cosmic evolution of oxygen and nitrogen abundances in star-forming galaxies over the past 10 Gyrs

  E. Perez-Montero, T. Contini, F. Lamareille, C. Maier, C. M. Carollo, J. P. Kneib, O. Le Fevre, S. Lilly, V. Mainieiri, A. Renzini, [......], R. Pello, Y. Peng, V. Presotto, E. Ricciardelli, J. D. Silverman, M. Tanaka, L. A. M. Tasca, L. Tresse, D. Vergani, E. Zucca
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  ABSTRACT: The chemical evolution of galaxies on a cosmological timescale is still a matter of debate despite the increasing number of available data provided by spectroscopic surveys of star-forming galaxies at different redshifts. The fundamental relations involving metallicity, such as the mass-metallicity relation (MZR) or the fundamental-metallicity relation, give controversial results about the reality of evolution of the chemical content of galaxies at a given stellar mass. In this work we shed some light on this issue using the completeness reached by the 20k bright sample of the zCOSMOS survey and using for the first time the nitrogen-to-oxygen ratio (N/O) as a tracer of the gas phase chemical evolution of galaxies that is independent of the star formation rate. Emission-line galaxies both in the SDSS and 20k zCOSMOS bright survey were used to study the evolution from the local Universe of the $MZR up to a redshift of 1.32 and the relation between stellar mass and nitrogen-to-oxygen ratio (MNOR) up to a redshift of 0.42 using the N2S2 parameter. All the physical properties derived from stellar continuum and gas emission-lines, including stellar mass, star formation rates, metallicity and N/O, were calculated in a self-consistent way over the full redshift range. We confirm the trend to find lower metallicities in galaxies of a given stellar mass in a younger Universe. This trend is even observed when taking possible selection effects into account that are due to the observed larger median star formation rates for galaxies at higher redshifts. We also find a significant evolution of the MNOR up to z = 0.4. Taking the slope of the O/H vs. N/O relation into account for the secondary-nitrogen production regime, the observed evolution of the MNOR is consistent with the trends found for both the MZR and its equivalent relation using new expressions to reduce its dependence on SFR.
  10/2012;
• Article: The VIMOS Public Extragalactic Redshift Survey (VIPERS): spectral classification through Principal Component Analysis

  A. Marchetti, B. R. Granett, L. Guzzo, A. Fritz, B. Garilli, M. Scodeggio, U. Abbas, C. Adami, S. Arnouts, M. Bolzonella, [......], C. Marinoni, Y. Mellier, L. Moscardini, R. C. Nichol, J. A. Peacock, W. J. Percival, S. Phleps, C. Schimd, M. Wolk, G. Zamorani
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  ABSTRACT: We develop a Principal Component Analysis aimed at classifying a sub-set of 27,350 spectra of galaxies in the range 0.4 < z < 1.0 collected by the VIMOS Public Extragalactic Redshift Survey (VIPERS). We apply an iterative algorithm to simultaneously repair parts of spectra affected by noise and/or sky residuals, and reconstruct gaps due to rest-frame transformation, and obtain a set of orthogonal spectral templates that span the diversity of galaxy types. By taking the three most significant components, we find that we can describe the whole sample without contamination from noise. We produce a catalogue of eigen-coefficients and template spectra that will be part of future VIPERS data releases. Our templates effectively condense the spectral information into two coefficients that can be related to the age and star formation rate of the galaxies. We examine the spectrophotometric types in this space and identify early, intermediate, late and starburst galaxies.
  07/2012;
• Article: The zCOSMOS 20k Group Catalog

  C. Knobel, S. J. Lilly, A. Iovino, K. Kovac, T. J. Bschorr, V. Presotto, P. A. Oesch, P. Kampczyk, C. M. Carollo, T. Contini, [......], A. Cappi, A. Cimatti, G. Coppa, A. M. Koekemoer, C. López-Sanjuan, H. J. McCracken, M. Moresco, P. Nair, L. Pozzetti, N. Welikala
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  ABSTRACT: We present an optical group catalog between 0.1 < z < 1 based on 16,500 high-quality spectroscopic redshifts in the completed zCOSMOS-bright survey. The catalog published herein contains 1498 groups in total and 192 groups with more than five observed members. The catalog includes both group properties and the identification of the member galaxies. Based on mock catalogs, the completeness and purity of groups with three and more members should be both about 83% with respect to all groups that should have been detectable within the survey, and more than 75% of the groups should exhibit a one-to-one correspondence to the "real" groups. Particularly at high redshift, there are apparently more galaxies in groups in the COSMOS field than expected from mock catalogs. We detect clear evidence for the growth of cosmic structure over the last seven billion years in the sense that the fraction of galaxies that are found in groups (in volume-limited samples) increases significantly with cosmic time. In the second part of the paper, we develop a method for associating galaxies that only have photo-z to our spectroscopically identified groups. We show that this leads to improved definition of group centers, improved identification of the most massive galaxies in the groups, and improved identification of central and satellite galaxies, where we define the former to be galaxies at the minimum of the gravitational potential wells. Subsamples of centrals and satellites in the groups can be defined with purities up to 80%, while a straight binary classification of all group and non-group galaxies into centrals and satellites achieves purities of 85% and 75%, respectively, for the spectroscopic sample.
  06/2012;
• Article: A group-galaxy cross-correlation function analysis in zCOSMOS

  C. Knobel, S. J. Lilly, C. M. Carollo, T. Contini, J. -P. Kneib, O. Le Fevre, V. Mainieri, A. Renzini, M. Scodeggio, G. Zamorani, [......], A. Cappi, A. Cimatti, G. Coppa, A. M. Koekemoer, C. López-Sanjuan, H. J. McCracken, M. Moresco, P. Nair, L. Pozzetti, N. Welikala
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  ABSTRACT: We present a group-galaxy cross-correlation analysis using a group catalog produced from the 16,500 spectra from the optical zCOSMOS galaxy survey. Our aim is to perform a consistency test in the redshift range 0.2 < z < 0.8 between the clustering strength of the groups and mass estimates that are based on the richness of the groups. We measure the linear bias of the groups by means of a group-galaxy cross-correlation analysis and convert it into mass using the bias-mass relation for a given cosmology, checking the systematic errors using realistic group and galaxy mock catalogs. The measured bias for the zCOSMOS groups increases with group richness as expected by the theory of cosmic structure formation and yields masses that are reasonably consistent with the masses estimated from the richness directly, considering the scatter that is obtained from the 24 mock catalogs. An exception are the richest groups at high redshift (estimated to be more massive than 10^13.5 M_sun), for which the measured bias is significantly larger than for any of the 24 mock catalogs (corresponding to a 3-sigma effect), which is attributed to the extremely large structure that is present in the COSMOS field at z ~ 0.7. Our results are in general agreement with previous studies that reported unusually strong clustering in the COSMOS field.
  06/2012;
• Article: The COSMOS Density Field: A Reconstruction Using Both Weak Lensing and Galaxy Distributions

  A. Amara, S. Lilly, K. Kovac, J. Rhodes, R. Massey, G. Zamorani, C. M. Carollo, T. Contini, J. -P. Kneib, O. Le Fevre, [......], A. Cappi, A. Cimatti, G. Coppa, A. Koekoemoer, C. Lopez-Sanjuan, H. J. McCracken, M. Moresco, P. Nair, L. Pozzetti, N. Welikala
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  ABSTRACT: The COSMOS field has been the subject of a wide range of observations, with a number of studies focusing on reconstructing the 3D dark matter density field. Typically, these studies have focused on one given method or tracer. In this paper, we reconstruct the distribution of mass in the COSMOS field out to a redshift z=1 by combining Hubble Space Telescope weak lensing measurements with zCOSMOS spectroscopic measurements of galaxy clustering. The distribution of galaxies traces the distribution of mass with high resolution (particularly in redshift, which is not possible with lensing), and the lensing data empirically calibrates the mass normalisation (bypassing the need for theoretical models). Two steps are needed to convert a galaxy survey into a density field. The first step is to create a smooth field from the galaxy positions, which is a point field. We investigate four possible methods for this: (i) Gaussian smoothing, (ii) convolution with truncated isothermal sphere, (iii) fifth nearest neighbour smoothing and (iv) a muliti-scale entropy method. The second step is to rescale this density field using a bias prescription. We calculate the optimal bias scaling for each method by comparing predictions from the smoothed density field with the measured weak lensing data, on a galaxy-by-galaxy basis. In general, we find scale-independent bias for all the smoothing schemes, to a precision of 10%. For the nearest neighbour smoothing case, we find the bias to be 2.51\pm 0.25. We also find evidence for a strongly evolving bias, increasing by a factor of ~3.5 between redshifts 0<z<0.8. We believe this strong evolution can be explained by the fact that we use a flux limited sample to build the density field.
  05/2012;
• Source

  Available from: Christian Maier

  Article: The dominant role of mergers in the size evolution of massive early-type galaxies since z ~ 1

  C. López-Sanjuan, O. Le Fèvre, O. Ilbert, L. A. M. Tasca, C. Bridge, O. Cucciati, P. Kampczyk, L. Pozzetti, C. K. Xu, C. M. Carollo, [......], A. Cappi, A. Cimatti, G. Coppa, A. Koekemoer, C. T. Liu, M. Moresco, P. Nair, P. Oesch, K. Schawinski, N. Welikala
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  ABSTRACT: In this paper we measure the merger fraction and rate, both minor and major, of massive early-type galaxies (M_star >= 10^11 M_Sun) in the COSMOS field, and study their role in mass and size evolution. We use the 30-band photometric catalogue in COSMOS, complemented with the spectroscopy of the zCOSMOS survey, to define close pairs with a separation 10h^-1 kpc <= r_p <= 30h-1 kpc and a relative velocity Delta v <= 500 km s^-1. We measure both major (stellar mass ratio mu = M_star,2/M_star,1 >= 1/4) and minor (1/10 <= mu < 1/4) merger fractions of massive galaxies, and study their dependence on redshift and on morphology. The merger fraction and rate of massive galaxies evolves as a power-law (1+z)^n, with major mergers increasing with redshift, n_MM = 1.4, and minor mergers showing little evolution, n_mm ~ 0. When split by their morphology, the minor merger fraction for early types is higher by a factor of three than that for spirals, and both are nearly constant with redshift. Our results show that massive early-type galaxies have undergone 0.89 mergers (0.43 major and 0.46 minor) since z ~ 1, leading to a mass growth of ~30%. We find that mu >= 1/10 mergers can explain ~55% of the observed size evolution of these galaxies since z ~ 1. Another ~20% is due to the progenitor bias (younger galaxies are more extended) and we estimate that very minor mergers (mu < 1/10) could contribute with an extra ~20%. The remaining ~5% should come from other processes (e.g., adiabatic expansion or observational effects). This picture also reproduces the mass growth and velocity dispersion evolution of these galaxies. We conclude from these results that merging is the main contributor to the size evolution of massive ETGs at z <= 1, accounting for ~50-75% of that evolution in the last 8 Gyr. Nearly half of the evolution due to mergers is related to minor (mu < 1/4) events.
  02/2012;
• Source

  Available from: Christian Maier

  Article: Improved constraints on the expansion rate of the Universe up to z~1.1 from the spectroscopic evolution of cosmic chronometers

  M. Moresco, A. Cimatti, Raul Jimenez, L. Pozzetti, G. Zamorani, M. Bolzonella, J. Dunlop, F. Lamareille, M. Mignoli, H. Pearce, [......], S. Foucaud, W. Hartley, M. Kamionkowski, A. M. Koekemoer, C. López-Sanjuan, H. J. McCracken, P. Nair, P. Oesch, A. Stanford, N. Welikala
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  ABSTRACT: We present new improved constraints on the Hubble parameter H(z) in the redshift range 0.15 < z < 1.1, obtained from the differential spectroscopic evolution of early-type galaxies as a function of redshift. We extract a large sample of early-type galaxies (\sim11000) from several spectroscopic surveys, spanning almost 8 billion years of cosmic lookback time (0.15 < z < 1.42). We select the most massive, red elliptical galaxies, passively evolving and without signature of ongoing star formation. Those galaxies can be used as standard cosmic chronometers, as firstly proposed by Jimenez & Loeb (2002), whose differential age evolution as a function of cosmic time directly probes H(z). We analyze the 4000 {\AA} break (D4000) as a function of redshift, use stellar population synthesis models to theoretically calibrate the dependence of the differential age evolution on the differential D4000, and estimate the Hubble parameter taking into account both statistical and systematical errors. We provide 8 new measurements of H(z) (see Tab. 4), and determine its change in H(z) to a precision of 5-12% mapping homogeneously the redshift range up to z \sim 1.1; for the first time, we place a constraint on H(z) at z \neq 0 with a precision comparable with the one achieved for the Hubble constant (about 5-6% at z \sim 0.2), and covered a redshift range (0.5 < z < 0.8) which is crucial to distinguish many different quintessence cosmologies. These measurements have been tested to best match a \Lambda CDM model, clearly providing a statistically robust indication that the Universe is undergoing an accelerated expansion. This method shows the potentiality to open a new avenue in constrain a variety of alternative cosmologies, especially when future surveys (e.g. Euclid) will open the possibility to extend it up to z \sim 2.
  01/2012;
• Source

  Available from: Christian Maier

  Article: A journey from the outskirts to the cores of groups I: Color- and mass-segregation in 20K-zCOSMOS groups

  V. Presotto, A. Iovino, M. Scodeggio, O. Cucciati, C. Knobel, M. Bolzonella, P. Oesch, A. Finoguenov, M. Tanaka, K. Kovač, [......], L. Barnes, R. Bordoloi, A. Cappi, A. Cimatti, G. Coppa, A. M. Koekemoer, H. J. McCracken, M. Moresco, P. Nair, N. Welikala
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  ABSTRACT: Using the group catalog obtained from zCOSMOS spectroscopic data and the complementary photometric data from the COSMOS survey, we explore segregation effects occurring in groups of galaxies at intermediate/high redshifts. We built two composite groups at intermediate (0.2 <= z <= 0.45) and high (0.45 < z <= 0.8) redshifts, and we divided the corresponding composite group galaxies into three samples according to their distance from the group center. We explored how galaxy stellar masses and colors - working in narrow bins of stellar masses - vary as a function of the galaxy distance from the group center. We found that the most massive galaxies in our sample (Log(M_gal/M_sun) >= 10.6) do not display any strong group-centric dependence of the fractions of red/blue objects. For galaxies of lower masses (9.8 <= Log(M_gal/M_sun) <= 10.6) there is a radial dependence in the changing mix of red and blue galaxies. This dependence is most evident in poor groups, whereas richer groups do not display any obvious trend of the blue fraction. Interestingly, mass segregation shows the opposite behavior: it is visible only in rich groups, while poorer groups have a a constant mix of galaxy stellar masses as a function of radius. We suggest a simple scenario where color- and mass-segregation originate from different physical processes. While dynamical friction is the obvious cause for establishing mass segregation, both starvation and galaxy-galaxy collisions are plausible mechanisms to quench star formation in groups at a faster rate than in the field. In poorer groups the environmental effects are caught in action superimposed to secular galaxy evolution. Their member galaxies display increasing blue fractions when moving from the group center to more external regions, presumably reflecting the recent accretion history of these groups.
  01/2012;
• Article: Environmental effects in the interaction and merging of galaxies in zCOSMOS

  P. Kampczyk, S. J. Lilly, L. de Ravel, O. Le Fèvre, M. Bolzonella, C. M. Carollo, C. Diener, C. Knobel, K. Kovac, C. Maier, [......], L. Guzzo, J. Kartaltepe, C. Marinoni, H. J. McCracken, P. Memeo, B. Meneux, P. Oesch, C. Porciani, L. Pozzetti, R. Scaramella
  [show abstract] [hide abstract]
  ABSTRACT: (Abridged) We analyze the environments and galactic properties (morphologies and star-formation histories) of a sample of 153 close kinematic pairs in the redshift range 0.2 < z < 1 identified in the zCOSMOS-bright 10k spectroscopic sample of galaxies. Correcting for projection effects, the fraction of close kinematic pairs is three times higher in the top density quartile than in the lowest one. This translates to a three times higher merger rate because the merger timescales are shown, from mock catalogues based on the Millennium simulation, to be largely independent of environment once the same corrections for projection is applied. We then examine the morphologies and stellar populations of galaxies in the pairs, comparing them to control samples that are carefully matched in environment so as to remove as much as possible the well-known effects of environment on the properties of the parent population of galaxies. Once the environment is properly taken into account in this way, we find that the early-late morphology mix is the same as for the parent population, but that the fraction of irregular galaxies is boosted by 50-75%, with a disproportionate increase in the number of irregular-irregular pairs (factor of 4-8 times), due to the disturbance of disk galaxies. Future dry-mergers, involving elliptical galaxies comprise less than 5% of all close kinematic pairs. In the closest pairs, there is a boost in the specific star-formation rates of star-forming galaxies of a factor of 2-4, and there is also evidence for an increased incidence of post star-burst galaxies. Although significant for the galaxies involved, the "excess" star-formation associated with pairs represents only about 5% of the integrated star-formation activity in the parent sample. Although most pair galaxies are in dense environments, the effects of interaction appear to be largest in the lower density environments.
  12/2011;
• Source

  Available from: Henry Joy McCracken

  Article: The power spectrum from the angular distribution of galaxies in the CFHTLS-Wide fields at redshift ~0.7

  B. R. Granett, L. Guzzo, J. Coupon, S. Arnouts, P. Hudelot, O. Ilbert, H. J. McCracken, Y. Mellier, C. Adami, J. Bel, [......], B. Meneux, L. Paioro, M. Polletta, A. Pollo, M. Scodeggio, H. Schlagenhaufer, L. Tasca, R. Tojeiro, D. Vergani, A. Zanichelli
  [show abstract] [hide abstract]
  ABSTRACT: We measure the real-space galaxy power spectrum on large scales at redshifts 0.5 to 1.2 using optical colour-selected samples from the CFHT Legacy Survey. With the redshift distributions measured with a preliminary ~14000 spectroscopic redshifts from the VIMOS Public Extragalactic Redshift Survey (VIPERS), we deproject the angular distribution and directly estimate the three-dimensional power spectrum. We use a maximum likelihood estimator that is optimal for a Gaussian random field giving well-defined window functions and error estimates. This measurement presents an initial look at the large-scale structure field probed by the VIPERS survey. We measure the galaxy bias of the VIPERS-like sample to be b_g=1.38 +- 0.05 (sigma_8=0.8) on scales k<0.2h/mpc averaged over 0.5<z<1.2. We further investigate three photometric redshift slices, and marginalising over the bias factors while keeping other LCDM parameters fixed, we find the matter density Omega_m=0.30+-0.06.
  11/2011;
• Source

  Available from: Peter L. Capak

  Article: Dissecting Photometric Redshift for Active Galactic Nucleus Using XMM- and Chandra-COSMOS Samples

  M. Salvato, O. Ilbert, G. Hasinger, A. Rau, F. Civano, G. Zamorani, M. Brusa, M. Elvis, C. Vignali, H. Aussel, [......], D. B. Sanders, M. Scodeggio, N. Scoville, P. Shopbell, J. Silvermann, Y. Taniguchi, L. Tasca, L. Tresse, J. R. Trump, E. Zucca
  [show abstract] [hide abstract]
  ABSTRACT: In this paper, we release accurate photometric redshifts for 1692 counterparts to Chandra sources in the central square degree of the Cosmic Evolution Survey (COSMOS) field. The availability of a large training set of spectroscopic redshifts that extends to faint magnitudes enabled photometric redshifts comparable to the highest quality results presently available for normal galaxies. We demonstrate that morphologically extended, faint X-ray sources without optical variability are more accurately described by a library of normal galaxies (corrected for emission lines) than by active galactic nucleus (AGN) dominated templates, even if these sources have AGN-like X-ray luminosities. Preselecting the library on the bases of the source properties allowed us to reach an accuracy \sigma _{\Delta z/(1+z_{spec})}\sim 0.015 with a fraction of outliers of 5.8% for the entire Chandra-COSMOS sample. In addition, we release revised photometric redshifts for the 1735 optical counterparts of the XMM-detected sources over the entire 2 deg2 of COSMOS. For 248 sources, our updated photometric redshift differs from the previous release by Δz > 0.2. These changes are predominantly due to the inclusion of newly available deep H-band photometry (H AB = 24 mag). We illustrate once again the importance of a spectroscopic training sample and how an assumption about the nature of a source together, with the number and the depth of the available bands, influences the accuracy of the photometric redshifts determined for AGN. These considerations should be kept in mind when defining the observational strategies of upcoming large surveys targeting AGNs, such as eROSITA at X-ray energies and the Australian Square Kilometre Array Pathfinder Evolutionary Map of the Universe in the radio band. Based on observations by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the National Aeronautics Space Administration under contract NAS8-03060. Also based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA Inc., under NASA contract NAS 5-26555. Also based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under NASA contract 1407. Also based on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan; the XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA; the European Southern Observatory under Large Program 175.A-0839, Chile; the Kitt Peak National Observatory, Cerro Tololo Inter-American Observatory and the National Optical Astronomy Observatory, which are operated by the Association of Universities for Research in Astronomy, Inc. (AURA), under cooperative agreement with the National Science Foundation; the Canada-France-Hawaii Telescope with MegaPrime/MegaCam operated as a joint project by the CFHT Corporation, CEA/DAPNIA, the NRC and CADC of Canada, the CNRS of France, TERAPIX, and the University of Hawaii.
  The Astrophysical Journal 11/2011; 742:61. · 6.02 Impact Factor
• Source

  Available from: V. Mainieri

  Article: The Impact of Galaxy Interactions on Active Galactic Nucleus Activity in zCOSMOS

  J. D. Silverman, P. Kampczyk, K. Jahnke, R. Andrae, S. J. Lilly, M. Elvis, F. Civano, V. Mainieri, C. Vignali, G. Zamorani, [......], E. Zucca, M. Brusa, N. Cappelluti, A. Comastri, A. Finoguenov, H. Fu, R. Gilli, H. Hao, L. C. Ho, and M. Salvato
  [show abstract] [hide abstract]
  ABSTRACT: Close encounters between galaxies are expected to be a viable mechanism, as predicted by numerical simulations, by which accretion onto supermassive black holes can be initiated. To test this scenario, we construct a sample of 562 galaxies (M * > 2.5 × 1010 M ☉) in kinematic pairs over the redshift range 0.25 < z < 1.05 that are more likely to be interacting than a well-matched control sample of 2726 galaxies not identified as being in a pair, both from the zCOSMOS 20k spectroscopic catalog. Galaxies that harbor an active galactic nucleus (AGN) are identified on the basis of their X-ray emission (L 0.5-10 keV > 2 × 1042 erg s–1) detected by Chandra. We find a higher fraction of an AGN in galaxies in pairs relative to isolated galaxies of similar stellar mass. Our result is primarily due to an enhancement of AGN activity, by a factor of 1.9 (observed) and 2.6 (intrinsic), for galaxies in pairs of projected separation less than 75 kpc and line-of-sight velocity offset less than 500 km s–1. This study demonstrates that close kinematic pairs are conducive environments for black hole growth, either indicating a causal physical connection or an inherent relation, such as, to enhanced star formation. In the Appendix, we describe a method for estimating the intrinsic fractions of galaxies (either in pairs or the field) hosting an AGN with confidence intervals, and an excess fraction in pairs. We estimate that 17.8+8.4 –7.4% of all moderate-luminosity AGN activity takes place within galaxies undergoing early stages of interaction that leaves open the question as to what physical processes are responsible for fueling the remaining ~80% that may include late-stage mergers.
  The Astrophysical Journal 11/2011; 743(1):2. · 6.02 Impact Factor