arXiv subjects

S. Cristiani

Publications and source records attributed to S. Cristiani.

At least 163 records · Page 9Linked to original sources

arXiv2007

The CODEX-ESPRESSO experiment: cosmic dynamics, fundamental physics, planets and much more..

CODEX, a high resolution, super-stable spectrograph to be fed by the E-ELT, the most powerful telescope ever conceived, will for the first time provide the possibility of directly measuring the change of the expansion rate of the Universe with time and much more, from the variability of fundamental constants to the search for other earths. A study for the implementation at the VLT of a precursor of CODEX, dubbed ESPRESSO, is presently carried out by a collaboration including ESO, IAC, INAF, IoA Cambridge and Observatoire de Geneve. The present talk is focused on the cosmological aspects of the experiment.

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arXiv2007

Tracing the gas at redshift 1.7-3.5 with the Lyman-alpha forest: the FLO approach

[Abridged] We present FLO (From Lines to Over-densities), a new technique to reconstruct the hydrogen density field for the Lya forest lines observed in high resolution QSO spectra. The method is based on the hypothesis that the Lya lines arise in the low to intermediate density intergalactic gas and that the Jeans length is the typical size of the Lya absorbers. The reliability of FLO is tested against mock spectra obtained from cosmological simulations. The recovering algorithm gives satisfactory results in the range from the mean density to over-densities of ~30 and reproduces correctly the correlation function of the density field and the 1D power spectrum on scales between ~20 and 60 comoving Mpc. A sample of Lya forests from 22 high resolution QSO spectra is analysed, covering the redshift range 1.7<z<3.5. For each line of sight, we fit Voigt profiles to the lines of the Lya forest, providing the largest, homogeneous sample of fitted Lya lines ever studied. The line number density evolution with redshift follows a power-law relation: dn/dz=(166 +/- 4) [(1+z)/3.5]^{(2.8 +/- 0.2)} (1 sigma errors). The two-point correlation function of lines shows a signal up to separations of ~2 comoving Mpc; weak lines (log N(HI)<13.8) also show a significant clustering but on smaller scales (r<1.5 comoving Mpc). We estimate with FLO the hydrogen density field toward the 22 observed lines of sight. The redshift distribution of the average densities computed for each QSO is consistent with the cosmic mean hydrogen density in the analysed redshift range. The two-point correlation function and the 1D power spectrum of the delta field are estimated. The correlation function shows clustering signal up to ~4 comoving Mpc.

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arXiv2007

The Great Observatories Origins Deep Survey. VLT/FORS2 Spectroscopy in the GOODS-South Field: Part III

Aims. We present the full data set of the spectroscopic campaign of the ESO/GOODS program in the GOODS-South field, obtained with the FORS2 spectrograph at the ESO/VLT. Method. Objects were selected as candidates for VLT/FORS2 observations primarily based on the expectation that the detection and measurement of their spectral features would benefit from the high throughput and spectral resolution of FORS2. The reliability of the redshift estimates is assessed using the redshift-magnitude and color-redshift diagrams, and comparing the results with public data. Results. Including the third part of the spectroscopic campaign (12 masks) to the previous work (26 masks, Vanzella et al. 2005, 2006), 1715 spectra of 1225 individual targets have been analyzed. The actual spectroscopic catalog provides 887 redshift determinations. The typical redshift uncertainty is estimated to be sigma(z) ~ 0.001. Galaxies have been selected adopting different color criteria and using photometric redshifts. The resulting redshift distribution typically spans two domains: from z=0.5 to 2 and z=3.5 to 6.3. The reduced spectra and the derived redshifts are released to the community through the ESO web page http://www.eso.org/science/goods/

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arXiv2007

The red and blue galaxy populations in the GOODS field: evidence for an excess of red dwarfs

We study the evolution of the galaxy population up to z\sim3 as a function of its colour properties. In particular, luminosity functions and luminosity densities have been derived as a function of redshift for the blue/late and red/early populations. We use data from the GOODS-MUSIC catalogue which have typical magnitude limits z<26 and Ks<23.5 for most of the sample. About 8% of the galaxies have spectroscopic redshifts; the remaining have well calibrated photometric redshifts derived from the extremely wide multi-wavelength coverage in 14 bands (from the U band to the Spitzer 8 μm band). We have derived a catalogue of galaxies complete in rest-frame B-band, which has been divided in two subsamples according to their rest-frame U-V colour (or derived specific star formation rate, SSFR) properties. We confirm a bimodality in the U-V colour and SSFR of the galaxy sample up to z\sim 3. This bimodality is used to compute the LFs of the blue/late and red/early subsamples. The LFs of the blue/late and total samples are well represented by steep Schechter functions evolving in luminosity with increasing redshifts. The volume density of the LFs of the red/early populations decreases with increasing redshift. The shape of the red/early LFs shows an excess of faint red dwarfs with respect to the extrapolation of a flat Schechter function and can be represented by the sum of two Schechter functions. Our model for galaxy formation in the hierarchical clustering scenario, which also includes external feedback due to a diffuse UV background, shows a general broad agreement with the LFs of both populations, the larger discrepancies being present at the faint end for the red population. Hints on the nature of the red dwarf population are given on the basis of their stellar mass and spatial distributions.

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arXiv2007

A Photometrically Detected Forming Cluster of Galaxies at Redshift 1.6 in the GOODS Field

We report the discovery of a localized overdensity at z~1.6 in the GOODS-South Field, presumably a poor cluster in the process of formation. The three-dimensional galaxy density has been estimated on the basis of well calibrated photometric redshifts from the multiband photometric GOODS-MUSIC catalog using the (2+1)D technique. The density peak is embedded in the larger scale overdensity of galaxies known to exist at z=1.61 in the area. The properties of the member galaxies are compared to those of the surrounding field and we found that the two populations are significantly different supporting the reality of the structure. The reddest galaxies, once evolved according to their best fit models, have colors consistent with the red sequence of lower redshift clusters. The estimated M_200 total mass of the cluster is in the range 1.3 x 10^14 - 5.7x 10^14 Msun, depending on the assumed bias factor b. An upper limit for the 2-10 keV X-ray luminosity, based on the 1Ms Chandra observations, is L_X=0.5 x 10^43 erg s^-1, suggesting that the cluster has not yet reached the virial equilibrium.

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arXiv2006

Lyman Break Galaxies in the VLT/FORS2 spectroscopic campaign in the GOODS-S field

We present initial results from our ongoing campaign of spectroscopic identifications of Lyman--break galaxies (LBGs) at z ~ 4, 5 and 6 with FORS2 at the ESO VLT. The sample of LBGs is selected from the HST/ACS images in the GOODS southern field (CDF--S), and here we discuss the spectroscopic properties of 103 LBGs that we have cofirmed in the redshift interval 3.5 < z < 6.3, of which 34 at z>5 and 27 between 5.5 and 6.3. As for LBGs at lower redshifts, we find that LBGs with LyA in emission are characterized by weaker intestellar absorption lines and bluer UV continua than galaxies whose LyA is observed in absorption. No correlation between the LyA properties and the apparent elongation of the UV morphology of the galaxies is observed, suggesting that geometrical effects are not the cause of the spectral differences. A trend of increasing LyA equivalent width with decreasing luminosity is also observed. We also present new measures of the UV luinosity function of LBGs.

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arXiv2006

The Luminosity Function of high-redshift QSOs - A combined analysis of GOODS and SDSS

Aims: In this work the luminosity function of QSOs is measured in the redshift range 3.5 3.5. Conclusions: The estimated space density evolution of QSOs indicates a suppression of the formation and/or feeding of Supermassive Black Holes at these redshifts. The QSO contribution to the UV background is insufficient to ionize the IGM at 3.5<z<5.2.

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arXiv2006

Tomography of the intergalactic medium with Ly-alpha forests in close QSO pairs

We study the three-dimensional distribution of non virialised matter at z~2 using high resolution spectra of QSO pairs and simulated spectra drawn from cosmological hydrodynamical simulations. We have collected the largest sample of QSO pairs ever observed with UVES at the ESO-VLT, with angular separations between ~1 and 14 arcmin. The observed correlation functions of the transmitted flux in the HI Lyman alpha forest along and transverse to the lines of sight are in good agreement implying that the distortions in redshift space due to peculiar velocities are small. The clustering signal is significant up to velocity separations of ~200 km/s, or about 3 h^{-1} comoving Mpc. The regions at lower overdensity (rho/ < 6.5) are still clustered but on smaller scales (Delta v < 100 km/s). The observed and simulated correlation functions are compatible at the 3 sigma level. A better concordance is obtained when only the low overdensity regions are selected for the analysis or when the effective optical depth of the simulated spectra is increased artificially, suggesting a deficiency of strong lines in the simulated spectra. We found that also a lower value of the power-law index of the temperature-density relation for the Lyman alpha forest gas improves the agreement between observed and simulated results. If confirmed, this would be consistent with other observations favouring a late HeII reionization epoch (at z~3). We remark the detection of a significant clustering signal in the cross correlation coefficient at a transverse velocity separation Delta v_{\perp} ~500 km/s whose origin needs further investigation.

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arXiv2006

The Galaxy Mass Function up to z=4 in the GOODS-MUSIC sample: into the epoch of formation of massive galaxies

The goal of this work is to measure the evolution of the Galaxy Stellar Mass Function and of the resulting Stellar Mass Density up to redshift ~4, in order to study the assembly of massive galaxies in the high redshift Universe. We have used the GOODS-MUSIC catalog, containing ~3000 Ks-selected galaxies with multi-wavelength coverage extending from the U band to the Spitzer 8 micron band, of which 27% have spectroscopic redshifts and the remaining fraction have accurate photometric redshifts. On this sample we have applied a standard fitting procedure to measure stellar masses. We compute the Galaxy Stellar Mass Function and the resulting Stellar Mass Density up to redshift ~4, taking into proper account the biases and incompleteness effects. Within the well known trend of global decline of the Stellar Mass Density with redshift, we show that the decline of the more massive galaxies may be described by an exponential timescale of ~6 Gyrs up to z~1.5, and proceeds much faster thereafter, with an exponential timescale of ~0.6 Gyrs. We also show that there is some evidence for a differential evolution of the Galaxy Stellar Mass Function, with low mass galaxies evolving faster than more massive ones up to z~1-1.5 and that the Galaxy Stellar Mass Function remains remarkably flat (i.e. with a slope close to the local one) up to z~1-1.3. The observed behaviour of the Galaxy Stellar Mass Function is consistent with a scenario where about 50% of present-day massive galaxies formed at a vigorous rate in the epoch between redshift 4 and 1.5, followed by a milder evolution until the present-day epoch.

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arXiv2006

On the radio properties of the highest redshift quasars

We present deep radio observations of the most distant complete quasar sample drawn from the Sloan Digital Sky Survey. Combining our new data with those from literature we obtain a sample which is ~100 per cent complete down to S_1.4GHz = 60 μJy over the redshift range 3.8 < z < 5. The fraction of radio detections is relatively high (~43 per cent), similar to what observed locally in bright optical surveys. Even though the combined radio and optical properties of quasars remain overall unchanged from z ~ 5 to the local Universe, there is some evidence for a slight over-abundance of radio-loud sources at the highest redshifts when compared with the lower-z regime. Exploiting the deep radio VLA observations we present the first attempt to directly derive the radio luminosity function of bright quasars at z ~ 4. The unique depth -- both in radio and optical -- allows us to thoroughly explore the population of optically bright FR~II quasars up to z ~ 5 and opens a window on the behaviour of the brightest FR~I sources. A close investigation of the space density of radio loud quasars also suggests a differential evolution, with the more luminous sources showing a less pronounced cut-off at high z when compared with the less luminous ones.

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arXiv2006

The Great Observatories Origins Deep Survey VLT/FORS2 Spectroscopy in the GOODS-South Field: Part II

We present the second campaign of the ESO/GOODS program of spectroscopy of faint galaxies in the GOODS-South field. Objects were selected as candidates for VLT/FORS2 observations primarily based on the expectation that the detection and measurement of their spectral features would benefit from the high throughput and spectral resolution of FORS2. The reliability of the redshift estimates is assessed using the redshift-magnitude and color-redshift diagrams and comparing the results with public data. 807 spectra of 652 individua targets have been obtained in service mode with the FORS2 spectrograph at the ESO/VLT, providing 501 redshift determinations. The typical redshift uncertainty is estimated to be sigma_z ~ 0.001. Galaxies have been selected adopting three different color criteria and using the photometric redshifts.The resulting redshift distribution typically spans two redshift domains: from z=0.5 to 2 and z=3.5 to 6.2. In particular, 94 B435-,V606-,i775 "dropout" Lyman break galaxies have been observed, yielding redshifts for 65 objects in the interval 3.4<z<6.2. Three sources have been serendipitously discovered in the redshift interval 4.8<z<5.5. Together with the previous release, 930 sources have now been observed and 724 redshift determinations have been carried out. The reduced spectra and the derived redshifts are released to the community through the ESO web page http://www.eso.org/science/goods/. Large scale structures are clearly detected at z=0.666, 0.734, 1.096, 1.221, 1.300, and 1.614. A sample of 34 sources with tilted [OII]3727 emission has been identified, 32 of them in the redshift range 0.9<z<1.5.

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arXiv2006

The GOODS-MUSIC sample: a multicolour catalog of near-IR selected galaxies in the GOODS-South field

We present a high quality multiwavelength (from 0.3 to 8.0 micron) catalog of the large and deep area in the GOODS Southern Field covered by the deep near-IR observations obtained with the ESO VLT. The catalog is entirely based on public data: in our analysis, we have included the F435W, F606W, F775W and F850LP ACS images, the JHKs VLT data, the Spitzer data provided by IRAC instrument (3.6, 4.5, 5.8 and 8.0 micron), and publicly available U-band data from the 2.2ESO and VLT-VIMOS. We describe in detail the procedures adopted to obtain this multiwavelength catalog. In particular, we developed a specific software for the accurate "PSF-matching" of space and ground-based images of different resolution and depth (ConvPhot), of which we analyse performances and limitations. We have included both z-selected, as well as Ks-selected objects, yielding a unique, self-consistent catalog. The largest fraction of the sample is 90% complete at z~26 or Ks~23.8 (AB scale). Finally, we cross-correlated our data with all the spectroscopic catalogs available to date, assigning a spectroscopic redshift to more than 1000 sources. The final catalog is made up of 14847 objects, at least 72 of which are known stars, 68 are AGNs, and 928 galaxies with spectroscopic redshift (668 galaxies with reliable redshift determination). We applied our photometric redshift code to this data set, and the comparison with the spectroscopic sample shows that the quality of the resulting photometric redshifts is excellent, with an average scatter of only 0.06. The full catalog, which we named GOODS-MUSIC (MUltiwavelength Southern Infrared Catalog), including the spectroscopic information, is made publicly available, together with the software specifically designed to this end.

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arXiv2006

The Clustering Evolution of Distant Red Galaxies in the GOODS-MUSIC Sample

We use the GOODS-MUSIC sample, a catalog of ~3000 Ks-selected galaxies based on VLT and HST observation of the GOODS-South field with extended multi-wavelength coverage (from 0.3 to 8 micron) and accurate estimates of the photometric redshifts to select 179 DRGs with J-Ks>1.3 in an area of 135 sq. arcmin. We first show that the J-Ks>1.3 criterion selects a rather heterogeneous sample of galaxies, going from the targeted high-redshift luminous evolved systems, to a significant fraction of lower redshift (1<z<2) and less luminous dusty starbursts. These low-redshift DRGs are significantly less clustered than higher-z DRGs. With the aid of extreme and simplified theoretical models of clustering evolution we show that it is unlikely that the two samples are drawn from the same population observed at two different stages of evolution. High-z DRGs likely represent the progenitors of the more massive and more luminous galaxies in the local Universe and might mark the regions that will later evolve into structures of intermediate mass, like groups or small galaxy clusters. Low-z DRGs, on the other hand, will likely evolve into slightly less massive field galaxies.

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arXiv2006

Cosmic evolution of the galaxy mass and luminosity functions by morphological type from multi-wavelength data in the CDF-South

We constrain the evolution of the galaxy mass and luminosity functions from the analysis of (public) multi-wavelength data in the Chandra Deep Field South (CDFS) area, obtained from the GOODS and other projects, and including very deep high-resolution imaging by HST/ACS. Our reference catalogue of faint high-redshift galaxies, which we have thoroughly tested for completeness and reliability, comes from a deep (S(3.6micron)>1 microJy) image by IRAC on the Spitzer Observatory. These imaging data in the field are complemented with extensive optical spectroscopy by the ESO VLT/FORS2 and VIMOS spectrographs, while deep K-band VLT/ISAAC imaging is also used to derive further complementary statistical constraints and to assist the source identification and SED analysis. We have selected a highly reliable IRAC 3.6micron sub-sample of 1478 galaxies with S(3.6)>10microJy, 47% of which have spectroscopic redshift, while for the remaining objects both COMBO-17 and Hyperz are used to estimate the photometric redshift. This very extensive dataset is exploited to assess evolutionary effects in the galaxy luminosity and stellar mass functions, while luminosity/density evolution is further constrained with the number counts and redshift distributions. The deep ACS imaging allows us to differentiate these evolutionary paths by morphological type, which our simulations show to be reliable at least up to z=1.5 for the two main early- (E/S0) and late-type (Sp/Irr) classes. These data, as well as our direct estimate of the stellar mass function above M=10^(10)M_sun for the spheroidal subclass, consistently evidence a progressive dearth of such objects to occur starting at z=0.7, paralleled by an increase in luminosity. (abridged)

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arXiv2005

Probing the evolution of the near-IR luminosity function of galaxies to z ~ 3 in the Hubble Deep Field South

[Abridged] We present the rest-frame Js-band and Ks-band luminosity function of a sample of about 300 galaxies selected in the HDF-S at Ks<23 (Vega). We use calibrated photometric redshift together with spectroscopic redshift for 25% of the sample. The sample has allowed to probe the evolution of the LF in the three redshift bins [0;0.8), [0.8;1.9) and [1.9;4) centered at the median redshift z_m ~ [0.6,1.2,3]. The values of alpha we estimate are consistent with the local value and do not show any trend with redshift. We do not see evidence of evolution from z=0 to z_m ~ 0.6 suggesting that the population of local bright galaxies was already formed at z<0.8. On the contrary, we clearly detect an evolution of the LF to z_m ~ 1.2 characterized by a brightening of M* and by a decline of phi*. To z_m ~ 1.2 M* brightens by about 0.4-0.6 mag and phi* decreases by a factor 2-3. This trend persists, even if at a less extent, down to z_m ~ 3 both in the Js-band and in the Ks-band LF. The decline of the number density of bright galaxies seen at z>0.8 suggests that a significant fraction of them increases their stellar mass at 1 3. Thus, our results suggest that the assembly of high-mass galaxies is spread over a large redshift range and that the increase of their stellar mass has been very efficient also at very high redshift at least for a fraction of them.

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arXiv2005

Probing 3-D matter distribution at z~2 with QSO multiple lines of sight

We investigate the 3-D matter distribution at z~2 with high resolution (R ~ 40000) spectra of QSO pairs and groups obtained with the UVES spectrograph at ESO VLT. Our sample is unique for the number density of objects and the variety of separations, between 0.5 and 7 proper Mpc. We compute the real space cross-correlation function of the Lyman-alpha forest transmitted fluxes. There is a significant clustering signal up to ~2 proper Mpc, which is still present when absorption lines with high column density (log N > 13.8) are excluded.

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arXiv2005

The K20 survey. VII. The spectroscopic catalogue: spectral properties and evolution of the galaxy population

The K20 survey is a near infrared-selected, deep (Ks < 20) redshift survey targeting galaxies in two independent regions of the sky, the CDFS and the q0055-2659 field. The total Ks-selected sample includes 545 objects. Optical spectra for 525 of them have been obtained, providing 501 spectroscopic identifications (including 12 type-1 AGN and 45 stars). This paper describes the final K20 spectroscopic catalogue, along with the technique used to determine redshifts, measure the spectral features and characterize the spectra. The classification of the galaxy spectra has been performed according to a simple parametric recipe that uses the equivalent widths of the two main emission lines (O[II] and Halpha+[N II]) and two continuum indices (the 4000A break index, and a near-UV color index, C(28-39)). We defined three main spectroscopic classes: red early-type galaxies, blue emission-line galaxies and the intermediate galaxies, which show emission lines but a red continuum. More than 95% of the examined galaxies is included in one of these spectral types and a composite spectrum is built for each of the three galaxy classes. The blue emission-line and the early-type galaxies have been divided in redshift bins, and the corresponding composite spectra have been built, in order to investigate the evolution of the spectral properties of the K20 galaxies with redshift.

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arXiv2004

The B-Band Luminosity Function of Red and Blue Galaxies up to z=3.5

We have explored the redshift evolution of the luminosity function of red and blue galaxies up to $z=3.5$. This was possible joining a deep I band composite galaxy sample, which includes the spectroscopic K20 sample and the HDFs samples, with the deep $H_{AB}=26$ and $K_{AB}=25$ samples derived from the deep NIR images of the Hubble Deep Fields North and South, respectively. About 30% of the sample has spectroscopic redshifts and the remaining fraction well-calibrated photometric redshifts. This allowed to select and measure galaxies in the rest-frame blue magnitude up to $z\sim 3$ and to derive the redshift evolution of the B-band luminosity function of galaxies separated by their rest-frame $U-V$ color or specific (i.e. per unit mass) star-formation rate. The class separation was derived from passive evolutionary tracks or from their observed bimodal distributions. Both distributions appear bimodal at least up to $z\sim 2$ and the locus of red/early galaxies is clearly identified up to these high redshifts. Both luminosity and density evolutions are needed to describe the cosmological behaviour of the red/early and blue/late populations. The density evolution is greater for the early population with a decrease by one order of magnitude at $z\sim 2-3$ with respect to the value at $z\sim 0.4$. The luminosity densities of the early and late type galaxies with $M_B<-20.6$ appear to have a bifurcation at $z>1$. Indeed while star-forming galaxies slightly increase or keep constant their luminosity density, "early" galaxies decrease in their luminosity density by a factor $\sim 5-6$ from $z\sim 0.4$ to $z\sim 2.5-3$. A comparison with one of the latest versions of the hierarchical CDM models shows a broad agreement with the observed number and luminosity density evolutions of both populations.

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