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Alberto Cappi

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I am astronomer at INAF - OAS Bologna.

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Papers by Alberto Cappi

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Preliminary Results from the ESO Slice Project (ESP)

arXiv (Cornell University), 1995

We present the first results of a galaxy redshift survey, ESO Slice Project (ESP), we are accompl... more We present the first results of a galaxy redshift survey, ESO Slice Project (ESP), we are accomplishing as an ESO Key-Project over about 40 square degrees in a region near the South Galactic Pole. The limiting magnitude is b J = 19.4. Up to now ∼ 85% of the observations has been completed and ∼ 65% of the data has been reduced providing ∼ 2000 galaxy redshifts. We present some preliminary results concerning the large scale galaxy distribution, the luminosity function and the properties of galaxies with spectra showing prominent emission lines.

VizieR Online Data Catalog: Galaxy clusters in the CFHTLS (Grove+, 2009)

VIPERS view of the star formation history of early-type galaxies

Photonics Applications in Astronomy, Communications, Industry, and High-Energy Physics Experiments 2015, 2015

We present studies over the relations between stellar mass, redshift and star formation history f... more We present studies over the relations between stellar mass, redshift and star formation history for a high quality sample of early-type galaxies (ETGs) observed by the by the VIMOS Public Extragalactic Redshift Survey (VIPERS). VIPERS is an ongoing Large Programme to map in detail the large-scale distribution of galaxies at 0:5 < z < 1:2 with a unique volume (24 deg2) and sampling rate (~= 45%). At this redshift, IPERS fills a unique niche in galaxy surveys, provides a exceptional opportunity to study galaxies and their evolution at an epoch when the Universe had approximately half its current age. VIPERS data set will become the z ~ 1 equivalent of current state-of-the-art local (z < 0:2) surveys, allowing us to compare measurements at these two different epochs on equal statistical footing. The final sample of this survey is going to reach nearly 100,000 galaxies. As the VIPERS sample contains ~ 15% of the early type galaxies with known spectroscopic redshift and a very good quality spectra, it is a perfect sample to study the star formation history based on their spectroscopic features. We show that there the age of stellar population is depending on the stellar mass and the redshift. Our results show that lower mass galaxies have young stellar populations, while higher mass ETGs are populated with old stars. This suggest that marginal star formation occurs in massive galaxies. This result is consistent with the observations of the local Universe. Moreover, this trend of evolution is preserved for the different redshift range between 0:4 < z < 1:2.

VizieR Online Data Catalog: VIPERS. Searching for cosmic voids (Micheletti+, 2014)

File voidtable.dat lists the catalogue of maximal spheres detected in VIPERS samples, divided in ... more File voidtable.dat lists the catalogue of maximal spheres detected in VIPERS samples, divided in groups corresponding to individual voids. The spheres have been associated with larger under-densities using the percolation method, thus we associate each sphere with a void index indicating larger empty regions. The identification number (ID) of each maximal sphere is composed of: the VIPERS field to which the maximal sphere belongs (W1 or W4); a void index indicating to which void the sphere belongs (VVVV); a sphere index SSSS indicating the size rank of the sphere relative to other maximal spheres within the same void. The complete ID has the form Wx.VVVV.SSSS. For each maximal sphere of our catalogue we provide: its ID; its comoving radius in Mpc (Ωm=0.27, ΩΛ=0.73 and H0=70km/s/Mpc); the right ascension, declination, and redshift of the sphere's centre; a p-value giving the detection significance with respect to a Poisson distribution. Voids are sorted in ascending redshift order, with the redshift associated with each void being that of its largest sphere. Spheres inside each void are sorted in decreasing radius order. (1 data file)

An optical group catalog to z = 1 from the zcosmos 10 k sample

We present a galaxy group catalogue spanning the redshift range 0.1 z 1 in the ∼ 1.7 deg 2 COSMOS... more We present a galaxy group catalogue spanning the redshift range 0.1 z 1 in the ∼ 1.7 deg 2 COSMOS field, based on the first ∼ 10, 000 zCOSMOS spectra. The performance of both the Friendsof-Friends (FOF) and Voronoi-Delaunay-Method (VDM) approaches to group identification has been extensively explored and compared using realistic mock catalogues. We find that the performance improves substantially if groups are found by progressively optimizing the group-finding parameters for successively smaller groups, and that the highest fidelity catalogue, in terms of completeness and purity, is obtained by combining the independently created FOF and VDM catalogues. The final completeness and purity of this catalogue, both in terms of the groups and of individual members, compares favorably with recent results in the literature. The current group catalogue contains 102 groups with N ≥ 5 spectroscopically confirmed members, with a further ∼ 700 groups with 2 ≤ N ≤ 4. Most of the groups can be assigned a velocity dispersion and a dark-matter mass derived from the mock catalogues, with quantifiable uncertainties. The fraction of zCOSMOS galaxies in groups is about 25% at low redshift and decreases toward ∼ 15% at z ∼ 0.8. The zCOSMOS group catalogue is broadly consistent with that expected from the semi-analytic evolution model underlying the mock catalogues. Not least, we show that the number density of groups with a given intrinsic richness increases from redshift z ∼ 0.8 to the present, consistent with the hierarchical growth of structure.

Biasing and High‐Order Statistics from the Southern‐Sky Redshift Survey

The Astrophysical Journal, 1999

THE zCOSMOS 20k GROUP CATALOG

The Astrophysical Journal, 2012

We present an optical group catalog between 0.1 z 1 based on 16,500 high-quality spectroscopic re... more 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 have only 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.

The Very Large Telescope Visible Multi‐Object Spectrograph Mask Preparation Software

Publications of the Astronomical Society of the Pacific, 2005

Object Spectrograph) is a multiobject imaging spectrograph installed at the VLT (Very large Teles... more Object Spectrograph) is a multiobject imaging spectrograph installed at the VLT (Very large Telescope) at the ESO (European Southern Observatory) Paranal Observatory that is especially suited for survey work. VIMOS is characterized by its very high multiplexing factor: it is possible to take up to 800 spectra with 10Љ long slits in a single exposure. To fully exploit its multiplexing potential, we designed and implemented a dedicated software tool: the VIMOS Mask Preparation Software (VMMPS), which allows the astronomer to select the objects to be spectroscopically observed, and provides for automatic slit positioning and slit number maximization within the instrumental constraints. The output of VMMPS is used to manufacture the slit masks to be mounted in the instrument for spectroscopic observations.

BeppoSAXtemperature maps of galaxy clusters in the Corona Borealis supercluster: A2061, A2067 and A2124

Monthly Notices of the Royal Astronomical Society, 2004

In this paper we present the analysis of BeppoSAX observations of the cluster pairs A2061-A2067 a... more In this paper we present the analysis of BeppoSAX observations of the cluster pairs A2061-A2067 and A2122-A2124, located in the Corona Borealis supercluster, which have been selected as candidate merging clusters. The aim of this work is to study the physics of the intracluster medium and to look for the possible presence of merging signatures. We derived the global temperatures and abundances and the temperature profiles and maps for these clusters. We do not find any significant evidence of interaction between the clusters forming the pairs, but we detect a candidate shock inside A2061. On the basis of the X-ray and optical properties of this cluster we propose a scenario in which a group is falling inside A2061. This interaction is in the phase in which the cores have not yet met and in which the formation of a shock is expected.

The COSMOS density field: a reconstruction using both weak lensing and galaxy distributions

Monthly Notices of the Royal Astronomical Society, 2012

The COSMOS field has been the subject of a wide range of observations, with a number of studies f... more 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 ± 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.

$Research paper thumbnail of The [O iii] emission line luminosity function of optically selected type-2 AGN from zCOSMOS$^{\rm,}$$

The [O iii] emission line luminosity function of optically selected type-2 AGN from zCOSMOS$^{\rm,}$

Astronomy and Astrophysics, 2010

Aims. We present a catalog of 213 type-2 AGN selected from the zCOSMOS survey. The selected sampl... more Aims. We present a catalog of 213 type-2 AGN selected from the zCOSMOS survey. The selected sample covers a wide redshift range (0.15 < z < 0.92) and is deeper than any other previous study, encompassing the luminosity range 10 5.5 L < L [OIII] < 10 9.1 L. We explore the intrinsic properties of these AGN and the relation to their X-ray emission (derived from the XMM-COSMOS observations). We study their evolution by computing the [O iii]λ5007 Å line luminosity function (LF) and we constrain the fraction of obscured AGN as a function of luminosity and redshift. Methods. The sample was selected on the basis of the optical emission line ratios, after applying a cut to the signal-to-noise ratio (S /N) of the relevant lines. We used the standard diagnostic diagrams ([O iii]/Hβ versus [N ii]/Hα and [O iii]/Hβ versus [S ii]/Hα) to isolate AGN in the redshift range 0.15 < z < 0.45 and the diagnostic diagram [O iii]/Hβ versus [O ii]/Hβ to extend the selection to higher redshift (0.5 < z < 0.92). Results. Combining our sample with one drawn from SDSS, we found that the best description of the evolution of type-2 AGN is a luminositydependent density evolution model. Moreover, using the type-1 AGN LF we were able to constrain the fraction of type-2 AGN to the total (type-1 + type-2) AGN population. We found that the type-2 fraction decreases with luminosity, in agreement with the most recent results, and shows signs of a slight increase with redshift. However, the trend with luminosity is visible only after combining the SDSS+zCOSMOS samples. From the COSMOS data points alone, the type-2 fraction seems to be quite constant with luminosity.

K+a galaxies in the zCOSMOS survey

Astronomy and Astrophysics, 2009

Aims. The identities of the main processes triggering and quenching star-formation in galaxies re... more Aims. The identities of the main processes triggering and quenching star-formation in galaxies remain unclear. A key stage in evolution, however, appears to be represented by post-starburst galaxies. To investigate the prevalence of these galaxies and their impact on galaxy evolution in general, we initiated a multiwavelength study of galaxies with k + a spectral features in the well-studied COSMOS field. Methods. We examine a mass-selected sample of k + a galaxies in the COSMOS field at z = 0.48−1.2 using the spectroscopic zCOSMOS sample. To classify galaxies in their k + a phase, we use a spectroscopic criterion, based on the amplitude of the Hδ absorption line and the absence of the [OII] emission line. We develop our analysis for a well-defined sample by imposing stringent confidence levels on the spectroscopic redshifts and spectral measurements. We compare our results for two mass-selected samples of star-forming and quiescent galaxies selected using a purely spectral classification scheme from the 10 000 zCOSMOS catalogue (i.e. , based on measurements of 4000 Å break and EW[OII]). Results. In our mass-limited sample, k+a galaxies occupy the brightest tail of the luminosity distribution. They are as massive as quiescent galaxies and populate the green valley in the colour versus luminosity (or stellar mass) distribution. A small percentage (<8%) of these galaxies have radio and/or X-ray counterparts (implying an upper limit to the SFR of ∼8 M yr −1). Over the entire redshift range explored, the class of post-starburst galaxies is morphologically a heterogeneous population with a similar incidence of bulge-dominated and disky galaxies. This distribution does not vary with the strength of the Hδ absorption line but instead with stellar mass in a way reminiscent of the well-known mass-morphology relation. The results about the incidence of asymmetries and the concentration of the light distribution derived from HST/ACS images imply that this galaxy population possibly represents an intermediate stage of galaxy evolution. Although k + a galaxies are also found in underdense regions, they appear to reside typically in a similarly rich environment as quiescent galaxies on a physical scale of ∼2-8 Mpc, and in groups they show a morphological early-to-late type ratio similar to the quiescent galaxy class. With the current data set, we do not find evidence of statistical significant evolution in either the number/mass density of k + a galaxies at intermediate redshift with respect to the local values, or the spectral properties, although more solid results on this and other aspects will be obtained following the completion of the survey. Conclusions. Several mechanisms related and unrelated to the environment are at work in quenching star-formation activity in galaxies on short timescales (<1 Gyr). Those galaxies, which are affected by a sudden quenching of their star-formation activity, may increase the stellar mass of the red-sequence by up to a non-negligible level of ∼10%.

The dominant role of mergers in the size evolution of massive early-type galaxies sincez ~ 1

Astronomy & Astrophysics, 2012

Aims. The role of galaxy mergers in massive galaxy evolution, and in particular to mass assembly ... more Aims. The role of galaxy mergers in massive galaxy evolution, and in particular to mass assembly and size growth, remains an open question. In this paper we measure the merger fraction and rate, both minor and major, of massive early-type galaxies (M ≥ 10 11 M) in the COSMOS field, and study their role in mass and size evolution. Methods. We used the 30-band photometric catalogue in COSMOS, complemented with the spectroscopy of the zCOSMOS survey, to define close pairs with a separation on the sky plane 10 h −1 kpc ≤ r p ≤ 30 h −1 kpc and a relative velocity Δv ≤ 500 km s −1 in redshift space. We measured both major (stellar mass ratio μ ≡ M ,2 /M ,1 ≥ 1/4) and minor (1/10 ≤ μ < 1/4) merger fractions of massive galaxies, and studied their dependence on redshift and on morphology (early types vs. late types). Results. 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-type galaxies (ETGs) is higher by a factor of three than that for late-type galaxies (LTGs), and both are nearly constant with redshift. The fraction of major mergers for massive LTGs evolves faster (n LT MM ∼ 4) than for ETGs (n ET MM = 1.8). Conclusions. Our results show that massive ETGs 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 μ ≥ 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 (μ < 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 the velocity dispersion evolution of these galaxies. We conclude from these results, and after exploring all the possible uncertainties in our picture, 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 (μ < 1/4) events.

The VLA-VIRMOS Deep Field

Astronomy & Astrophysics, 2003

We have conducted a deep survey (rms noise 17 µJy) with the Very Large Array (VLA) at 1.4 GHz, wi... more We have conducted a deep survey (rms noise 17 µJy) with the Very Large Array (VLA) at 1.4 GHz, with a resolution of 6 arcsec, of a 1 deg 2 region included in the VIRMOS VLT Deep Survey. In the same field we already have multiband photometry down to I AB = 25, and spectroscopic observations will be obtained during the VIRMOS VLT survey. The homogeneous sensitivity over the whole field has allowed to derive a complete sample of 1054 radio sources (5σ limit). We give a detailed description of the data reduction and of the analysis of the radio observations, with particular care to the effects of clean bias and bandwidth smearing, and of the methods used to obtain the catalogue of radio sources. To estimate the effect of the resolution bias on our observations we have modelled the effective angular-size distribution of the sources in our sample and we have used this distribution to simulate a sample of radio sources. Finally we present the radio count distribution down to 0.08 mJy derived from the catalogue. Our counts are in good agreement with the best fit derived from earlier surveys, and are about 50% higher than the counts in the HDF. The radio count distribution clearly shows, with extremely good statistics, the change in the slope for the sub-mJy radio sources.

The SWIRE-VVDS-CFHTLS surveys: stellar mass assembly over the last 10 Gyr. Evidence for a major build up of the red sequence betweenz= 2 andz= 1

Astronomy & Astrophysics, 2007

We present an analysis of the stellar mass growth over the last 10 Gyr (z ≤ 2) using a unique lar... more We present an analysis of the stellar mass growth over the last 10 Gyr (z ≤ 2) using a unique large sample of galaxies selected at 3.6 µm. We have assembled accurate photometric and spectroscopic redshifts for ∼21 200 and 1500 galaxies, respectively, with F(3.6 µm) ≥ 9.0 µJy by combining data from Spitzer-SWIRE IRAC, the VIMOS VLT Deep Survey (VVDS), UKIDSS and very deep optical CFHTLS photometry. We split our sample into quiescent (red) and active (blue) galaxies on the basis of an SED fitting procedure that we have compared with the strong rest-frame color bimodality (NUV − r) ABS. The present sample contains ∼4400 quiescent galaxies. Our measurements of the K-rest frame luminosity function and luminosity density evolution support the idea that a large fraction of galaxies is already assembled at z ∼ 1.2, with almost 80% and 50% of the active and quiescent populations already in place, respectively. Based on the analysis of the evolution of the stellar mass-to-light ratio (in K-band) for the spectroscopic sub-sample, we derive the stellar mass density for the entire sample. We find that the global evolution of the stellar mass density is well reproduced by the star formation rate derived from UV based measurements when an appropriate dust correction is applied, which supports the idea of an initial mass function that is on average universal. Over the last 8 Gyr (z ≤ 1.2) we observe that the stellar mass density of the active population shows a modest mass growth rate (ρ ∼ 0.005(±0.005) M /Mpc 3 /yr), consistent with a constant stellar mass density, ρ active ∼ 3.1 × 10 8 M /Mpc 3. In contrast, an increase by a factor of ∼2 for the quiescent population over the same timescale is observed. As a consequence, the growth of the stellar mass in the quiescent population must be due to the shutoff of star formation in active galaxies that migrate into the quiescent population. We estimate this stellar mass flux to beρ A→Q ∼ 0.017(±0.004) M /Mpc 3 /yr, which balances the major fraction of new stars born according to our best SFR estimate (ρ = 0.025(±0.003) M /Mpc 3 /yr). From z = 2 to z = 1.2, we observe a major build-up of the quiescent population with an increase by a factor of ∼10 in stellar mass (a mass growth rate of ∼0.063 M /Mpc 3 /yr). This rapid evolution suggests that we are observing the epoch when, for the first time in the history of the universe, an increasing fraction of galaxies end their star formation activity and start to build up the red sequence.

The VIRMOS deep imaging survey

Astronomy & Astrophysics, 2003

In this paper we describe in detail the reduction, preparation and reliability of the photometric... more In this paper we describe in detail the reduction, preparation and reliability of the photometric catalogues which comprise the CFH12K-VIRMOS deep field. This region, consisting of four contiguous pointings of the CFH12K wide-field mosaic camera, covers a total area of 1.2 deg 2. The survey reaches a limiting magnitude of B AB ∼ 26.5, V AB ∼ 26.2, R AB ∼ 25.9 and I AB ∼ 25.0 (corresponding to the point at which our recovery rate for simulated point-like sources sources falls below 50%). In total the survey contains 90 729 extended sources in the magnitude range 18.0 < I AB < 24.0. We demonstrate our catalogues are free from systematic biases and are complete and reliable down these limits. By comparing our galaxy number counts to previous wide-field CCD surveys, we estimate that the upper limit on bin-to-bin systematic photometric errors for the I− limited sample is ∼10% in this magnitude range. We estimate that 68% of the catalogues sources have absolute per coordinate astrometric uncertainties less than |∆α| ∼ 0.38 and |∆δ| ∼ 0.32. Our internal (filter-to-filter) per coordinate astrometric uncertainties are |∆α| ∼ 0.08 and |∆δ| ∼ 0.08. We quantify the completeness of our survey in the joint space defined by object total magnitude and peak surface brightness. We also demonstrate that no significant positional incompleteness effects are present in our catalogues to I AB < 24.0. Finally, we present numerous comparisons between our catalogues and published literature data: galaxy and star counts, galaxy and stellar colours, and the clustering of both point-like and extended populations. In all cases our measurements are in excellent agreement with literature data to I AB < 24.0. This combination of depth and areal coverage makes this multi-colour catalogue a solid foundation to select galaxies for follow-up spectroscopy with VIMOS on the ESO-VLT and a unique database to study the formation and evolution of the faint galaxy population to z ∼ 1 and beyond.

Merging history of three bimodal clusters

Astronomy & Astrophysics, 2010

We present a combined X-ray and optical analysis of three bimodal galaxy clusters selected as mer... more We present a combined X-ray and optical analysis of three bimodal galaxy clusters selected as merging candidates at z ∼ 0.1. These targets are part of MUSIC (MUlti-Wavelength Sample of Interacting Clusters), which is a general project designed to study the physics of merging clusters by means of multi-wavelength observations. Observations include spectro-imaging with XMM-Newton EPIC camera, multi-object spectroscopy (260 new redshifts), and wide-field imaging at the ESO 3.6 m and 2.2 m telescopes. We build a global picture of these clusters using X-ray luminosity and temperature maps together with galaxy density and velocity distributions. Idealized numerical simulations were used to constrain the merging scenario for each system. We show that A2933 is very likely an equal-mass advanced pre-merger ∼200 Myr before the core collapse, while A2440 and A2384 are post-merger systems (∼450 Myr and ∼1.5 Gyr after core collapse, respectively). In the case of A2384, we detect a spectacular filament of galaxies and gas spreading over more than 1 h −1 Mpc, which we infer to have been stripped during the previous collision. The analysis of the MUSIC sample allows us to outline some general properties of merging clusters: a strong luminosity segregation of galaxies in recent post-mergers; the existence of preferential axes-corresponding to the merging directions-along which the BCGs and structures on various scales are aligned; the concomitance, in most major merger cases, of secondary merging or accretion events, with groups infalling onto the main cluster, and in some cases the evidence of previous merging episodes in one of the main components. These results are in good agreement with the hierarchical scenario of structure formation, in which clusters are expected to form by successive merging events, and matter is accreted along large-scale filaments.

The VVDS type-1 AGN sample: the faint end of the luminosity function

Astronomy & Astrophysics, 2007

In a previous paper (Gavignaud et al. 2006, A&A, 457, 79), we presented the type-1 Active Galacti... more In a previous paper (Gavignaud et al. 2006, A&A, 457, 79), we presented the type-1 Active Galactic Nuclei (AGN) sample obtained from the first epoch data of the VIMOS-VLT Deep Survey (VVDS). The sample consists of 130 faint, broad-line AGN with redshift up to z = 5 and 17.5 < I AB < 24.0, selected on the basis of their spectra. The sample is thus free of the morphological and color selection biases, that lead to significant incompleteness in the optical surveys of faint AGN. In this paper we present the measurement of the Optical Luminosity Function up to z = 3.6 derived from this sample, we compare our results with previous results from brighter samples both at low and at high redshift and finally, through the estimate of the bolometric luminosity function, we compare them also with the results from X-ray and mid-IR selected samples. Our data, more than one magnitude fainter than previous optical surveys, allow us to constrain the faint part of the luminosity function up to high redshift. A comparison of our data with the 2dF sample at low redshift (1 < z < 2.1) shows that the VVDS data can not be well fitted with the PLE models derived by previous samples. Qualitatively, this appears to be due to the fact that our data suggest the presence of an excess of faint objects at low redshift (1.0 < z < 1.5) with respect to these models. By combining our faint VVDS sample with the large sample of bright AGN extracted from the SDSS DR3 (Richards et al. 2006b, AJ, 131, 2766) and testing a number of different evolutionary models, we find that the model which better represents the combined luminosity functions, over a wide range of redshift and luminosity, is a luminosity dependent density evolution (LDDE) model, similar to those derived from the major Xsurveys. Such a parameterization allows the redshift of the AGN space density peak to change as a function of luminosity and explains the excess of faint AGN that we find at 1.0 < z < 1.5. On the basis of this model we find, for the first time from the analysis of optically selected samples, that the peak of the AGN space density shifts significantly towards lower redshift going to lower luminosity objects. The position of this peak moves from z ∼ 2.0 for M B < −26.0 to z ∼ 0.65 for −22 < M B < −20. This result, already found in a number of X-ray selected samples of AGN, is consistent with a scenario of "AGN cosmic downsizing", in which the density of more luminous AGN, possibly associated to more massive black holes, peaks earlier in the history of the Universe (i.e. at higher redshift), than that of low luminosity ones, which reaches its maximum later (i.e. at lower redshift).

Comparison of the VIMOS-VLT Deep Survey with the Munich semi-analytical model

Astronomy & Astrophysics, 2012

Aims. Our aim is to perform the same colour−density analysis on galaxy mock samples as was carrie... more Aims. Our aim is to perform the same colour−density analysis on galaxy mock samples as was carried out on a 5 h −1 Mpc scale using the VIMOS-VLT Deep Survey (VVDS), and to compare the results from these mock samples with observed data. This allows us to test galaxy evolution in the model and to understand the relation between the studied environment and the underlying dark matter distribution. Methods. We used galaxy mock catalogues with the same flux limits as the VVDS-Deep (I AB ≤ 24) survey (Cmocks), constructed using a semi-analytic model for galaxy evolution applied to the Millennium Simulation. From each Cmock, we extracted a subsample of galaxies mimicking the VVDS observational strategy (Omocks). We then computed the B-band luminosity function LF and the colour−density relation in the mock samples using the same methods as employed for the VVDS data. Results. We find that the B-band LF in mock samples roughly agrees with the observed LF, but at 0.2 < z < 0.8 the faint-end slope of the model LF is steeper than the observed one. Computing the LF for early-and late-type galaxies separately, we show that mock samples have an excess of faint early-type galaxies and of bright late-type galaxies compared with the data. We find that the colour−density relation in Omocks agrees excellently with that in Cmocks. This suggests that the VVDS observational strategy does not introduce any severe bias to the observed colour−density relation. At z ∼ 0.7, the colour−density relation in mock samples agrees qualitatively with observations, with red galaxies residing preferentially in high densities. However, the strength of the colour−density relation in mock samples does not vary within 0.2 < z < 1.5, while the observed relation flattens with increasing redshift and possibly inverts at z ∼ 1.3. We argue that the lack of evolution in the colour−density relation in the model cannot be due only to inaccurate prescriptions for the evolution of satellite galaxies, but indicates that the treatment of the central galaxies has also to be revised. Conclusions. The reversal of the colour−density relation can be explained by wet mergers between young galaxies, producing a starburst event. This should be seen on group scales, where mergers are frequent, with possibly some residual trend on larger scales. This residual is found in observations at z = 1.5 on a scale of ∼5 h −1 Mpc, but not in the model, suggesting that the treatment of physical processes influencing both satellites and central galaxies in models should be revised. A detailed analysis would be desirable on small scales as well, which requires flux limits fainter than those of the VVDS data.

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Preliminary Results from the ESO Slice Project (ESP)

arXiv (Cornell University), 1995

We present the first results of a galaxy redshift survey, ESO Slice Project (ESP), we are accompl... more We present the first results of a galaxy redshift survey, ESO Slice Project (ESP), we are accomplishing as an ESO Key-Project over about 40 square degrees in a region near the South Galactic Pole. The limiting magnitude is b J = 19.4. Up to now ∼ 85% of the observations has been completed and ∼ 65% of the data has been reduced providing ∼ 2000 galaxy redshifts. We present some preliminary results concerning the large scale galaxy distribution, the luminosity function and the properties of galaxies with spectra showing prominent emission lines.

VizieR Online Data Catalog: Galaxy clusters in the CFHTLS (Grove+, 2009)

VIPERS view of the star formation history of early-type galaxies

Photonics Applications in Astronomy, Communications, Industry, and High-Energy Physics Experiments 2015, 2015

We present studies over the relations between stellar mass, redshift and star formation history f... more We present studies over the relations between stellar mass, redshift and star formation history for a high quality sample of early-type galaxies (ETGs) observed by the by the VIMOS Public Extragalactic Redshift Survey (VIPERS). VIPERS is an ongoing Large Programme to map in detail the large-scale distribution of galaxies at 0:5 < z < 1:2 with a unique volume (24 deg2) and sampling rate (~= 45%). At this redshift, IPERS fills a unique niche in galaxy surveys, provides a exceptional opportunity to study galaxies and their evolution at an epoch when the Universe had approximately half its current age. VIPERS data set will become the z ~ 1 equivalent of current state-of-the-art local (z < 0:2) surveys, allowing us to compare measurements at these two different epochs on equal statistical footing. The final sample of this survey is going to reach nearly 100,000 galaxies. As the VIPERS sample contains ~ 15% of the early type galaxies with known spectroscopic redshift and a very good quality spectra, it is a perfect sample to study the star formation history based on their spectroscopic features. We show that there the age of stellar population is depending on the stellar mass and the redshift. Our results show that lower mass galaxies have young stellar populations, while higher mass ETGs are populated with old stars. This suggest that marginal star formation occurs in massive galaxies. This result is consistent with the observations of the local Universe. Moreover, this trend of evolution is preserved for the different redshift range between 0:4 < z < 1:2.

VizieR Online Data Catalog: VIPERS. Searching for cosmic voids (Micheletti+, 2014)

File voidtable.dat lists the catalogue of maximal spheres detected in VIPERS samples, divided in ... more File voidtable.dat lists the catalogue of maximal spheres detected in VIPERS samples, divided in groups corresponding to individual voids. The spheres have been associated with larger under-densities using the percolation method, thus we associate each sphere with a void index indicating larger empty regions. The identification number (ID) of each maximal sphere is composed of: the VIPERS field to which the maximal sphere belongs (W1 or W4); a void index indicating to which void the sphere belongs (VVVV); a sphere index SSSS indicating the size rank of the sphere relative to other maximal spheres within the same void. The complete ID has the form Wx.VVVV.SSSS. For each maximal sphere of our catalogue we provide: its ID; its comoving radius in Mpc (Ωm=0.27, ΩΛ=0.73 and H0=70km/s/Mpc); the right ascension, declination, and redshift of the sphere's centre; a p-value giving the detection significance with respect to a Poisson distribution. Voids are sorted in ascending redshift order, with the redshift associated with each void being that of its largest sphere. Spheres inside each void are sorted in decreasing radius order. (1 data file)

An optical group catalog to z = 1 from the zcosmos 10 k sample

We present a galaxy group catalogue spanning the redshift range 0.1 z 1 in the ∼ 1.7 deg 2 COSMOS... more We present a galaxy group catalogue spanning the redshift range 0.1 z 1 in the ∼ 1.7 deg 2 COSMOS field, based on the first ∼ 10, 000 zCOSMOS spectra. The performance of both the Friendsof-Friends (FOF) and Voronoi-Delaunay-Method (VDM) approaches to group identification has been extensively explored and compared using realistic mock catalogues. We find that the performance improves substantially if groups are found by progressively optimizing the group-finding parameters for successively smaller groups, and that the highest fidelity catalogue, in terms of completeness and purity, is obtained by combining the independently created FOF and VDM catalogues. The final completeness and purity of this catalogue, both in terms of the groups and of individual members, compares favorably with recent results in the literature. The current group catalogue contains 102 groups with N ≥ 5 spectroscopically confirmed members, with a further ∼ 700 groups with 2 ≤ N ≤ 4. Most of the groups can be assigned a velocity dispersion and a dark-matter mass derived from the mock catalogues, with quantifiable uncertainties. The fraction of zCOSMOS galaxies in groups is about 25% at low redshift and decreases toward ∼ 15% at z ∼ 0.8. The zCOSMOS group catalogue is broadly consistent with that expected from the semi-analytic evolution model underlying the mock catalogues. Not least, we show that the number density of groups with a given intrinsic richness increases from redshift z ∼ 0.8 to the present, consistent with the hierarchical growth of structure.

Biasing and High‐Order Statistics from the Southern‐Sky Redshift Survey

The Astrophysical Journal, 1999

THE zCOSMOS 20k GROUP CATALOG

The Astrophysical Journal, 2012

We present an optical group catalog between 0.1 z 1 based on 16,500 high-quality spectroscopic re... more 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 have only 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.

The Very Large Telescope Visible Multi‐Object Spectrograph Mask Preparation Software

Publications of the Astronomical Society of the Pacific, 2005

Object Spectrograph) is a multiobject imaging spectrograph installed at the VLT (Very large Teles... more Object Spectrograph) is a multiobject imaging spectrograph installed at the VLT (Very large Telescope) at the ESO (European Southern Observatory) Paranal Observatory that is especially suited for survey work. VIMOS is characterized by its very high multiplexing factor: it is possible to take up to 800 spectra with 10Љ long slits in a single exposure. To fully exploit its multiplexing potential, we designed and implemented a dedicated software tool: the VIMOS Mask Preparation Software (VMMPS), which allows the astronomer to select the objects to be spectroscopically observed, and provides for automatic slit positioning and slit number maximization within the instrumental constraints. The output of VMMPS is used to manufacture the slit masks to be mounted in the instrument for spectroscopic observations.

BeppoSAXtemperature maps of galaxy clusters in the Corona Borealis supercluster: A2061, A2067 and A2124

Monthly Notices of the Royal Astronomical Society, 2004

In this paper we present the analysis of BeppoSAX observations of the cluster pairs A2061-A2067 a... more In this paper we present the analysis of BeppoSAX observations of the cluster pairs A2061-A2067 and A2122-A2124, located in the Corona Borealis supercluster, which have been selected as candidate merging clusters. The aim of this work is to study the physics of the intracluster medium and to look for the possible presence of merging signatures. We derived the global temperatures and abundances and the temperature profiles and maps for these clusters. We do not find any significant evidence of interaction between the clusters forming the pairs, but we detect a candidate shock inside A2061. On the basis of the X-ray and optical properties of this cluster we propose a scenario in which a group is falling inside A2061. This interaction is in the phase in which the cores have not yet met and in which the formation of a shock is expected.

The COSMOS density field: a reconstruction using both weak lensing and galaxy distributions

Monthly Notices of the Royal Astronomical Society, 2012

The COSMOS field has been the subject of a wide range of observations, with a number of studies f... more 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 ± 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.

$Research paper thumbnail of The [O iii] emission line luminosity function of optically selected type-2 AGN from zCOSMOS$^{\rm,}$$

The [O iii] emission line luminosity function of optically selected type-2 AGN from zCOSMOS$^{\rm,}$

Astronomy and Astrophysics, 2010

Aims. We present a catalog of 213 type-2 AGN selected from the zCOSMOS survey. The selected sampl... more Aims. We present a catalog of 213 type-2 AGN selected from the zCOSMOS survey. The selected sample covers a wide redshift range (0.15 < z < 0.92) and is deeper than any other previous study, encompassing the luminosity range 10 5.5 L < L [OIII] < 10 9.1 L. We explore the intrinsic properties of these AGN and the relation to their X-ray emission (derived from the XMM-COSMOS observations). We study their evolution by computing the [O iii]λ5007 Å line luminosity function (LF) and we constrain the fraction of obscured AGN as a function of luminosity and redshift. Methods. The sample was selected on the basis of the optical emission line ratios, after applying a cut to the signal-to-noise ratio (S /N) of the relevant lines. We used the standard diagnostic diagrams ([O iii]/Hβ versus [N ii]/Hα and [O iii]/Hβ versus [S ii]/Hα) to isolate AGN in the redshift range 0.15 < z < 0.45 and the diagnostic diagram [O iii]/Hβ versus [O ii]/Hβ to extend the selection to higher redshift (0.5 < z < 0.92). Results. Combining our sample with one drawn from SDSS, we found that the best description of the evolution of type-2 AGN is a luminositydependent density evolution model. Moreover, using the type-1 AGN LF we were able to constrain the fraction of type-2 AGN to the total (type-1 + type-2) AGN population. We found that the type-2 fraction decreases with luminosity, in agreement with the most recent results, and shows signs of a slight increase with redshift. However, the trend with luminosity is visible only after combining the SDSS+zCOSMOS samples. From the COSMOS data points alone, the type-2 fraction seems to be quite constant with luminosity.

K+a galaxies in the zCOSMOS survey

Astronomy and Astrophysics, 2009

Aims. The identities of the main processes triggering and quenching star-formation in galaxies re... more Aims. The identities of the main processes triggering and quenching star-formation in galaxies remain unclear. A key stage in evolution, however, appears to be represented by post-starburst galaxies. To investigate the prevalence of these galaxies and their impact on galaxy evolution in general, we initiated a multiwavelength study of galaxies with k + a spectral features in the well-studied COSMOS field. Methods. We examine a mass-selected sample of k + a galaxies in the COSMOS field at z = 0.48−1.2 using the spectroscopic zCOSMOS sample. To classify galaxies in their k + a phase, we use a spectroscopic criterion, based on the amplitude of the Hδ absorption line and the absence of the [OII] emission line. We develop our analysis for a well-defined sample by imposing stringent confidence levels on the spectroscopic redshifts and spectral measurements. We compare our results for two mass-selected samples of star-forming and quiescent galaxies selected using a purely spectral classification scheme from the 10 000 zCOSMOS catalogue (i.e. , based on measurements of 4000 Å break and EW[OII]). Results. In our mass-limited sample, k+a galaxies occupy the brightest tail of the luminosity distribution. They are as massive as quiescent galaxies and populate the green valley in the colour versus luminosity (or stellar mass) distribution. A small percentage (<8%) of these galaxies have radio and/or X-ray counterparts (implying an upper limit to the SFR of ∼8 M yr −1). Over the entire redshift range explored, the class of post-starburst galaxies is morphologically a heterogeneous population with a similar incidence of bulge-dominated and disky galaxies. This distribution does not vary with the strength of the Hδ absorption line but instead with stellar mass in a way reminiscent of the well-known mass-morphology relation. The results about the incidence of asymmetries and the concentration of the light distribution derived from HST/ACS images imply that this galaxy population possibly represents an intermediate stage of galaxy evolution. Although k + a galaxies are also found in underdense regions, they appear to reside typically in a similarly rich environment as quiescent galaxies on a physical scale of ∼2-8 Mpc, and in groups they show a morphological early-to-late type ratio similar to the quiescent galaxy class. With the current data set, we do not find evidence of statistical significant evolution in either the number/mass density of k + a galaxies at intermediate redshift with respect to the local values, or the spectral properties, although more solid results on this and other aspects will be obtained following the completion of the survey. Conclusions. Several mechanisms related and unrelated to the environment are at work in quenching star-formation activity in galaxies on short timescales (<1 Gyr). Those galaxies, which are affected by a sudden quenching of their star-formation activity, may increase the stellar mass of the red-sequence by up to a non-negligible level of ∼10%.

The dominant role of mergers in the size evolution of massive early-type galaxies sincez ~ 1

Astronomy & Astrophysics, 2012

Aims. The role of galaxy mergers in massive galaxy evolution, and in particular to mass assembly ... more Aims. The role of galaxy mergers in massive galaxy evolution, and in particular to mass assembly and size growth, remains an open question. In this paper we measure the merger fraction and rate, both minor and major, of massive early-type galaxies (M ≥ 10 11 M) in the COSMOS field, and study their role in mass and size evolution. Methods. We used the 30-band photometric catalogue in COSMOS, complemented with the spectroscopy of the zCOSMOS survey, to define close pairs with a separation on the sky plane 10 h −1 kpc ≤ r p ≤ 30 h −1 kpc and a relative velocity Δv ≤ 500 km s −1 in redshift space. We measured both major (stellar mass ratio μ ≡ M ,2 /M ,1 ≥ 1/4) and minor (1/10 ≤ μ < 1/4) merger fractions of massive galaxies, and studied their dependence on redshift and on morphology (early types vs. late types). Results. 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-type galaxies (ETGs) is higher by a factor of three than that for late-type galaxies (LTGs), and both are nearly constant with redshift. The fraction of major mergers for massive LTGs evolves faster (n LT MM ∼ 4) than for ETGs (n ET MM = 1.8). Conclusions. Our results show that massive ETGs 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 μ ≥ 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 (μ < 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 the velocity dispersion evolution of these galaxies. We conclude from these results, and after exploring all the possible uncertainties in our picture, 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 (μ < 1/4) events.

The VLA-VIRMOS Deep Field

Astronomy & Astrophysics, 2003

We have conducted a deep survey (rms noise 17 µJy) with the Very Large Array (VLA) at 1.4 GHz, wi... more We have conducted a deep survey (rms noise 17 µJy) with the Very Large Array (VLA) at 1.4 GHz, with a resolution of 6 arcsec, of a 1 deg 2 region included in the VIRMOS VLT Deep Survey. In the same field we already have multiband photometry down to I AB = 25, and spectroscopic observations will be obtained during the VIRMOS VLT survey. The homogeneous sensitivity over the whole field has allowed to derive a complete sample of 1054 radio sources (5σ limit). We give a detailed description of the data reduction and of the analysis of the radio observations, with particular care to the effects of clean bias and bandwidth smearing, and of the methods used to obtain the catalogue of radio sources. To estimate the effect of the resolution bias on our observations we have modelled the effective angular-size distribution of the sources in our sample and we have used this distribution to simulate a sample of radio sources. Finally we present the radio count distribution down to 0.08 mJy derived from the catalogue. Our counts are in good agreement with the best fit derived from earlier surveys, and are about 50% higher than the counts in the HDF. The radio count distribution clearly shows, with extremely good statistics, the change in the slope for the sub-mJy radio sources.

The SWIRE-VVDS-CFHTLS surveys: stellar mass assembly over the last 10 Gyr. Evidence for a major build up of the red sequence betweenz= 2 andz= 1

Astronomy & Astrophysics, 2007

We present an analysis of the stellar mass growth over the last 10 Gyr (z ≤ 2) using a unique lar... more We present an analysis of the stellar mass growth over the last 10 Gyr (z ≤ 2) using a unique large sample of galaxies selected at 3.6 µm. We have assembled accurate photometric and spectroscopic redshifts for ∼21 200 and 1500 galaxies, respectively, with F(3.6 µm) ≥ 9.0 µJy by combining data from Spitzer-SWIRE IRAC, the VIMOS VLT Deep Survey (VVDS), UKIDSS and very deep optical CFHTLS photometry. We split our sample into quiescent (red) and active (blue) galaxies on the basis of an SED fitting procedure that we have compared with the strong rest-frame color bimodality (NUV − r) ABS. The present sample contains ∼4400 quiescent galaxies. Our measurements of the K-rest frame luminosity function and luminosity density evolution support the idea that a large fraction of galaxies is already assembled at z ∼ 1.2, with almost 80% and 50% of the active and quiescent populations already in place, respectively. Based on the analysis of the evolution of the stellar mass-to-light ratio (in K-band) for the spectroscopic sub-sample, we derive the stellar mass density for the entire sample. We find that the global evolution of the stellar mass density is well reproduced by the star formation rate derived from UV based measurements when an appropriate dust correction is applied, which supports the idea of an initial mass function that is on average universal. Over the last 8 Gyr (z ≤ 1.2) we observe that the stellar mass density of the active population shows a modest mass growth rate (ρ ∼ 0.005(±0.005) M /Mpc 3 /yr), consistent with a constant stellar mass density, ρ active ∼ 3.1 × 10 8 M /Mpc 3. In contrast, an increase by a factor of ∼2 for the quiescent population over the same timescale is observed. As a consequence, the growth of the stellar mass in the quiescent population must be due to the shutoff of star formation in active galaxies that migrate into the quiescent population. We estimate this stellar mass flux to beρ A→Q ∼ 0.017(±0.004) M /Mpc 3 /yr, which balances the major fraction of new stars born according to our best SFR estimate (ρ = 0.025(±0.003) M /Mpc 3 /yr). From z = 2 to z = 1.2, we observe a major build-up of the quiescent population with an increase by a factor of ∼10 in stellar mass (a mass growth rate of ∼0.063 M /Mpc 3 /yr). This rapid evolution suggests that we are observing the epoch when, for the first time in the history of the universe, an increasing fraction of galaxies end their star formation activity and start to build up the red sequence.

The VIRMOS deep imaging survey

Astronomy & Astrophysics, 2003

In this paper we describe in detail the reduction, preparation and reliability of the photometric... more In this paper we describe in detail the reduction, preparation and reliability of the photometric catalogues which comprise the CFH12K-VIRMOS deep field. This region, consisting of four contiguous pointings of the CFH12K wide-field mosaic camera, covers a total area of 1.2 deg 2. The survey reaches a limiting magnitude of B AB ∼ 26.5, V AB ∼ 26.2, R AB ∼ 25.9 and I AB ∼ 25.0 (corresponding to the point at which our recovery rate for simulated point-like sources sources falls below 50%). In total the survey contains 90 729 extended sources in the magnitude range 18.0 < I AB < 24.0. We demonstrate our catalogues are free from systematic biases and are complete and reliable down these limits. By comparing our galaxy number counts to previous wide-field CCD surveys, we estimate that the upper limit on bin-to-bin systematic photometric errors for the I− limited sample is ∼10% in this magnitude range. We estimate that 68% of the catalogues sources have absolute per coordinate astrometric uncertainties less than |∆α| ∼ 0.38 and |∆δ| ∼ 0.32. Our internal (filter-to-filter) per coordinate astrometric uncertainties are |∆α| ∼ 0.08 and |∆δ| ∼ 0.08. We quantify the completeness of our survey in the joint space defined by object total magnitude and peak surface brightness. We also demonstrate that no significant positional incompleteness effects are present in our catalogues to I AB < 24.0. Finally, we present numerous comparisons between our catalogues and published literature data: galaxy and star counts, galaxy and stellar colours, and the clustering of both point-like and extended populations. In all cases our measurements are in excellent agreement with literature data to I AB < 24.0. This combination of depth and areal coverage makes this multi-colour catalogue a solid foundation to select galaxies for follow-up spectroscopy with VIMOS on the ESO-VLT and a unique database to study the formation and evolution of the faint galaxy population to z ∼ 1 and beyond.

Merging history of three bimodal clusters

Astronomy & Astrophysics, 2010

We present a combined X-ray and optical analysis of three bimodal galaxy clusters selected as mer... more We present a combined X-ray and optical analysis of three bimodal galaxy clusters selected as merging candidates at z ∼ 0.1. These targets are part of MUSIC (MUlti-Wavelength Sample of Interacting Clusters), which is a general project designed to study the physics of merging clusters by means of multi-wavelength observations. Observations include spectro-imaging with XMM-Newton EPIC camera, multi-object spectroscopy (260 new redshifts), and wide-field imaging at the ESO 3.6 m and 2.2 m telescopes. We build a global picture of these clusters using X-ray luminosity and temperature maps together with galaxy density and velocity distributions. Idealized numerical simulations were used to constrain the merging scenario for each system. We show that A2933 is very likely an equal-mass advanced pre-merger ∼200 Myr before the core collapse, while A2440 and A2384 are post-merger systems (∼450 Myr and ∼1.5 Gyr after core collapse, respectively). In the case of A2384, we detect a spectacular filament of galaxies and gas spreading over more than 1 h −1 Mpc, which we infer to have been stripped during the previous collision. The analysis of the MUSIC sample allows us to outline some general properties of merging clusters: a strong luminosity segregation of galaxies in recent post-mergers; the existence of preferential axes-corresponding to the merging directions-along which the BCGs and structures on various scales are aligned; the concomitance, in most major merger cases, of secondary merging or accretion events, with groups infalling onto the main cluster, and in some cases the evidence of previous merging episodes in one of the main components. These results are in good agreement with the hierarchical scenario of structure formation, in which clusters are expected to form by successive merging events, and matter is accreted along large-scale filaments.

The VVDS type-1 AGN sample: the faint end of the luminosity function

Astronomy & Astrophysics, 2007

In a previous paper (Gavignaud et al. 2006, A&A, 457, 79), we presented the type-1 Active Galacti... more In a previous paper (Gavignaud et al. 2006, A&A, 457, 79), we presented the type-1 Active Galactic Nuclei (AGN) sample obtained from the first epoch data of the VIMOS-VLT Deep Survey (VVDS). The sample consists of 130 faint, broad-line AGN with redshift up to z = 5 and 17.5 < I AB < 24.0, selected on the basis of their spectra. The sample is thus free of the morphological and color selection biases, that lead to significant incompleteness in the optical surveys of faint AGN. In this paper we present the measurement of the Optical Luminosity Function up to z = 3.6 derived from this sample, we compare our results with previous results from brighter samples both at low and at high redshift and finally, through the estimate of the bolometric luminosity function, we compare them also with the results from X-ray and mid-IR selected samples. Our data, more than one magnitude fainter than previous optical surveys, allow us to constrain the faint part of the luminosity function up to high redshift. A comparison of our data with the 2dF sample at low redshift (1 < z < 2.1) shows that the VVDS data can not be well fitted with the PLE models derived by previous samples. Qualitatively, this appears to be due to the fact that our data suggest the presence of an excess of faint objects at low redshift (1.0 < z < 1.5) with respect to these models. By combining our faint VVDS sample with the large sample of bright AGN extracted from the SDSS DR3 (Richards et al. 2006b, AJ, 131, 2766) and testing a number of different evolutionary models, we find that the model which better represents the combined luminosity functions, over a wide range of redshift and luminosity, is a luminosity dependent density evolution (LDDE) model, similar to those derived from the major Xsurveys. Such a parameterization allows the redshift of the AGN space density peak to change as a function of luminosity and explains the excess of faint AGN that we find at 1.0 < z < 1.5. On the basis of this model we find, for the first time from the analysis of optically selected samples, that the peak of the AGN space density shifts significantly towards lower redshift going to lower luminosity objects. The position of this peak moves from z ∼ 2.0 for M B < −26.0 to z ∼ 0.65 for −22 < M B < −20. This result, already found in a number of X-ray selected samples of AGN, is consistent with a scenario of "AGN cosmic downsizing", in which the density of more luminous AGN, possibly associated to more massive black holes, peaks earlier in the history of the Universe (i.e. at higher redshift), than that of low luminosity ones, which reaches its maximum later (i.e. at lower redshift).

Comparison of the VIMOS-VLT Deep Survey with the Munich semi-analytical model

Astronomy & Astrophysics, 2012

Aims. Our aim is to perform the same colour−density analysis on galaxy mock samples as was carrie... more Aims. Our aim is to perform the same colour−density analysis on galaxy mock samples as was carried out on a 5 h −1 Mpc scale using the VIMOS-VLT Deep Survey (VVDS), and to compare the results from these mock samples with observed data. This allows us to test galaxy evolution in the model and to understand the relation between the studied environment and the underlying dark matter distribution. Methods. We used galaxy mock catalogues with the same flux limits as the VVDS-Deep (I AB ≤ 24) survey (Cmocks), constructed using a semi-analytic model for galaxy evolution applied to the Millennium Simulation. From each Cmock, we extracted a subsample of galaxies mimicking the VVDS observational strategy (Omocks). We then computed the B-band luminosity function LF and the colour−density relation in the mock samples using the same methods as employed for the VVDS data. Results. We find that the B-band LF in mock samples roughly agrees with the observed LF, but at 0.2 < z < 0.8 the faint-end slope of the model LF is steeper than the observed one. Computing the LF for early-and late-type galaxies separately, we show that mock samples have an excess of faint early-type galaxies and of bright late-type galaxies compared with the data. We find that the colour−density relation in Omocks agrees excellently with that in Cmocks. This suggests that the VVDS observational strategy does not introduce any severe bias to the observed colour−density relation. At z ∼ 0.7, the colour−density relation in mock samples agrees qualitatively with observations, with red galaxies residing preferentially in high densities. However, the strength of the colour−density relation in mock samples does not vary within 0.2 < z < 1.5, while the observed relation flattens with increasing redshift and possibly inverts at z ∼ 1.3. We argue that the lack of evolution in the colour−density relation in the model cannot be due only to inaccurate prescriptions for the evolution of satellite galaxies, but indicates that the treatment of the central galaxies has also to be revised. Conclusions. The reversal of the colour−density relation can be explained by wet mergers between young galaxies, producing a starburst event. This should be seen on group scales, where mergers are frequent, with possibly some residual trend on larger scales. This residual is found in observations at z = 1.5 on a scale of ∼5 h −1 Mpc, but not in the model, suggesting that the treatment of physical processes influencing both satellites and central galaxies in models should be revised. A detailed analysis would be desirable on small scales as well, which requires flux limits fainter than those of the VVDS data.