arXiv subjects
Publications and source records attributed to S. Cristiani.
We present progress results of a new survey for bright QSOs (V<14.5, R<15.4, B_J<15.2) covering the whole sky at high galactic latitudes, |b|>30. The surface density of QSOs brighter than B_J=14.8 turns out to be 2.9 +- 0.8 10^{-3} deg^{-2}. The optical Luminosity Function at 0.04 < z < 0.3 shows significant departures from the standard pure luminosity evolution, providing new insights in the modelling of the QSO phenomenon.
We present and compare in this paper new photometric redshift catalogs of the galaxies in three public fields: the NTT Deep Field, the HDF-N and the HDF-S. Photometric redshifts have been obtained for thewhole sample, by adopting a $χ^2$ minimization technique on a spectral library drawn from the Bruzual and Charlot synthesis models, with the addition of dust and intergalactic absorption. The accuracy, determined from 125 galaxies with known spectroscopic redshifts, is $σ_z\sim 0.08 (0.3)$ in the redshift intervals $z=0-1.5 (1.5-3.5)$. The global redshift distribution of I-selected galaxies shows a distinct peak at intermediate redshifts, z~0.6 at I_{AB}<26 and z~0.8 at I_{AB}<27.5 followed by a tail extending to z~6. We also present for the first time the redshift distribution of the total IR-selected sample to faint limits ($Ks \leq 21$ and $J\leq22$). It is found that the number density of galaxies at 1.25 5 candidates in the HDF filter set and that the 4 brightest candidates at $z>5$ in the HDF-S are indeed most likely M stars. (ABRIDGED)
We present progress results of a new survey for bright QSOs (V<14.5, R<15.4, B_J<15.2) covering the whole sky at |b|>30. The surface density of QSOs brighter than B_J=14.8 turns out to be $2.9 \pm 0.8 \cdot 10^{-3} deg^{-2}$. The optical Luminosity Function at 0.04 < z < 0.3 shows significant departures from the standard pure luminosity evolution, providing new insights in the modelling of the QSO phenomenon.
We report on high-resolution observations ($\Re \simeq 45000$) of the Hubble Deep Field South QSO J2233-606 obtained with the VLT UV-Visual Echelle Spectrograph (UVES). We present spectral data for the wavelength region $3050 < λ< 10000$ Å. The $S/N$ ratio of the final spectrum is about 50 per resolution element at 4000 Å, 90 at 5000 Å, 80 at 6000 Å, 40 at 8000 Å. Redshifts, column densities and Doppler widths of the absorption features have been determined with Voigt-profile fitting. A total of 621 lines have been measured. In particular 270 Ly-alpha lines, 41 Ly-beta and 24 systems containing metal lines have been identified. Together with other data in the literature, the present spectrum confirms that the evolution of the number density of Ly-alpha lines with $\log N($\huno$) > 14$ has an upturn at $z \sim 1.4-1.6$.
We report on low-resolution multi-object spectroscopy of 30 faint targets (R \~ 24-25) in the HDF-S and AXAF deep field obtained with the VLT Focal Reducer/low dispersion Spectrograph (FORS1). Eight high-redshift galaxies with 2.75< z < 4 have been identified. The spectroscopic redshifts are in good agreement with the photometric ones with a dispersion $σ_z = 0.07$ at z<2 and $σ_z = 0.16$ at z>2. The inferred star formation rates of the individual objects are moderate, ranging from a few to a few tens solar masses per year. Five out of the eight high-z objects do not show prominent emission lines. One object has a spectrum typical of an AGN. In the AXAF field two relatively close pairs of galaxies have been identified, with separations of 8.7 and 3.1 proper Mpc and mean redshifts of 3.11 and 3.93, respectively.
This paper presents the first results of a survey for bright quasars (V < 14.5 and R<15.4) covering the North Hemisphere at galactic latitudes |b|>30. The photometric database is derived from the GSC and USNO catalogs. Quasars are identified on the basis of their X-ray emission measured in the ROSAT All Sky Survey. The surface density of quasars brighter than 15.5 mag turns out to be $9 \pm 1 \cdot 10^{-3} deg^{-2}$, about 3 times higher than that estimated by the PG survey. The quasar optical Luminosity Function (LF) at $0.04 < z \le 0.3$ is computed and shown to be consistent with a Luminosity Dependent Luminosity Evolution of the type derived by La Franca and Cristiani (1997) in the range $0.3 < z \le 2.2$. The predictions of semi-analytical models of hierarchical structure formation agree remarkably well with the present observations.
A combination of ground-based (NTT and VLT) and HST (HDF-N and HDF-S) public imaging surveys have been used to collect a sample of 1712 I-selected and 319 $K\leq 21$ galaxies. Photometric redshifts have been obtained for all these galaxies. The results have been compared with the prediction of an analytic rendition of the current CDM hierarchical models for galaxy formation. We focus in particular on two observed quantities: the galaxy redshift distribution at K<21 and the evolution of the UV luminosity density. The derived photometric redshift distribution is in agreement with the hierarchical CDM prediction, with a fraction of only 5% of galaxies detected at z>2. This result strongly supports hierarchical scenarios where present-day massive galaxies are the result of merging processes. The observed UV luminosity density in the I-selected sample is confined within a factor of 4 over the whole range 0 3. CDM models in $Λ$-dominated universe are in better agreement at 3<z<4.5, but predict a pronounced peak at z~1.5 and a drop by a factor of 8 from z=1.5 to z=4 that is not observed in the data. We conclude that improvements are required in the treatment of the physical processes directly related to the SFR, e.g. the starbust activity in merger processes and/or different feedback to the star formation activity.
Spectra, position, magnitudes and colors are presented for 485 faint (B<20.5) emission line objects selected with the ultraviolet-excess (UVX) criterion on a area of 24.6 sq. deg in the South Galactic Pole. The objects were selected from the analysis of pixel-to-pixel stacking of COSMOS scans of UKST U, J and R plates. The candidates were observed with the Meudon-ESO Fiber Optics System (MEFOS) at the ESO 3.6m telescope. 429 type 1 AGNs have been identified (373 in the redshift range 0.3<z<2.2). This sample has allowed the measure of a difference on the QSO clustering evolution in comparison with that found for galaxies (La Franca et al 1998). The region is part of the ESO Imaging Survey (EIS) and of the 2dF QSO redshift survey.
The results of the FORS and ISAAC Science Verification of the FORS and ISAAC instruments at the VLT ANTU/UT1 are described. The following observations have been carried out: 1) the Cluster Deep Field MS1008.1-1224 2) the Antlia dwarf spheroidal Galaxy 3) multiple Object Spectroscopy of Lyman break galaxies in the AXAF and Hubble Deep Field South 4) ISAAC IR Spectroscopy of a gravitationally magnified galaxy at z=2.72. The data have been made public for the ESO community and, in the case of HDF-S, worldwide.
Together with the CMB, the three sources of information that astronomers have at high redshift as probes of the formation and evolution of the LSS are QSOs, galaxies and absorbers observed in the spectrum of distant background objects. In this contribution I try to give a hint of historical perspective, following how the technological advances have driven the emphasis from one class to another, in order to show what are the likely forthcoming milestones.
We present spectroscopic observations of galaxies lying within 1' from the QSO J2233-606 in the HDF-S. Several are found to be coincident in redshift with absorption line systems seen in the HST spectrum of the QSO. We detect a new quasar with z=1.3360 at only 45" angular separation from this QSO. This quasar pair is thus an ideal target for QSO environment studies: indeed, strong HI absorptions are seen at z(abs) = 1.3367 in the QSO spectrum, consistent with neutral hydrogen column densities log N(HI) > 16. A bright galaxy at 4"7 from the QSO is at z = 0.570, corresponding also to HI absorption in the QSO's spectrum. Other galaxies are also discussed.
QSOs and radio-galaxies, together with the CMB, ``normal'' galaxies and clusters, represent the main source of information about the origin and evolution of the Large Scale Structure. They can be used either directly, as tracers of the density peaks, or as cosmic lighthouses, probing the Universe along the line of sight with their conspicuous flow of photons. The amplitude of the QSO clustering has been measured to be $r_o \simeq 6 \hmpc$ at $z\sim 2$ and appears to increase with redshift. These properties are consistent with a scenario in which the quasars, with a lifetime of $t \sim 10^8 yr$, sparsely sample halos of mass greater than $M_{min} \sim 10^{12}-10^{13} h^{-1}$ M$_{\odot}$. Radio-galaxies are found to cluster on similar scales. The study of QSO and radio-galaxy environments is in qualitative agreement with their clustering properties. Absorption-line systems observed in high-resolution spectra of QSOs show structure, including voids/underdensities and overdensities, from the smallest scales to tens of Mpc. QSO pairs, groups and lenses allow us to get a 3-D glimpse of the absorbers, providing invaluable information about their size, ionization, mass. Great progress in this field is expected from the new instrumentation coming into operation at the 8-m class telescopes.
We present a deep BVrI multicolor catalog of galaxies in a 5.62 sq.arcmin field 80 arcsec south of the high redshift (z=4.7) quasar BR 1202-0725, derived from observations with the direct CCD camera SUSI at the ESO NTT. The formal 5$σ$ magnitude limits (in 2 x FWHM apertures) are 26.9, 26.5, 25.9 and 25.3 in B, V, r and I respectively. Counts, colors for the star and galaxy samples are discussed and a comparison with a deep HST image in the I band is presented. The percentage of merged or blended galaxies in the SUSI data to this magnitude limit is estimated to be not higher than 1%. At the same galactic latitude of the HDF but pointing toward the galactic center, the star density in this field is found to be ~3 times higher, with ~20% of the objects with V-I > 3.0. Reliable colors have been measured for galaxies selected down to r = 26. The choice of the optical filters has been optimized to define a robust multicolor selection of galaxies at 3.8 <= z <= 4.4. Within this interval the surface density of galaxy candidates with r < 26 in this field is 2.7 +/- 0.4 arcmin^-2 corresponding to a comoving density of Star Formation Rate at 3.8 <= z <= 4.4 of 10^{-2.00} - 10^{-1.82} h^3 Mo /yr/ Mpc^3.
The quasar Q1208+1011 (z_{em}=3.8) is the second highest redshift double quasar ever detected. Several indications point toward it being a gravitational lensed system, although a definitive proof is still lacking. We present new evidence of its lensed nature based on the weakness of the ``proximity effect'' measured in the high resolution Lyman absorption spectrum of the QSO. A luminosity amplification as large as 22 has been derived from this analysis. Indications on the redshift of the lensing galaxy can be obtained from the analysis of the intervening heavy element absorption systems discovered in the QSO high resolution spectrum. On statistical and dynamical grounds a MgII system present at z=1.13 appears as the most likely candidate for the lensing galaxy. We compare the observed parameters with a simple isothermal model for the lens to derive the properties of the lensing galaxy. The resulting magnification factor is smaller, although marginally consistent with that derived by the analysis of the proximity effect.
Spectra of the QSO pair Q0307-195A,B have been obtained in the Ly-alpha Forest (3660 - 3930 A) and C IV (4720 - 4850 A) regions with a FWHM resolution between 0.7 and 0.5 A. 46 lines have been detected in the spectrum of object A while 36 in the spectrum of object B, of them 29 and 20 were identified as Ly-alpha absorptions respectively. The present observations have been supplemented with data of comparable quality on other 7 QSO pairs available in the literature to give an enlarged sample of 217 Ly-alpha lines with rest equivalent width W_o > 0.3 A. The analysis of the hits (i.e. when an absorption line appears in both QSO spectra) and misses (i.e. when a line is seen in any of the QSO spectra, but no line is seen in the other), carried out with an improved statistical approach, indicates that the absorbers have typically a large size (h = H_0/100 km/s/Mpc, q_0 = 0.5): R = 362 h^{-1} kpc, with 95 % confidence limits 298< R <426 h^{-1} kpc and R = 412 h^{-1} kpc, with 95 % confidence limits 333< R <514 h^{-1} kpc for the radius in idealized spherical and disc geometries, respectively. The present data do not allow to establish any correlation of the typical inferred size with the proper separation or with the redshift of the pairs. The correlation between the observed equivalent widths of the absorbers in the adjacent lines of sight becomes poorer and poorer with increasing proper separation. A disc geometry with a column density profile N(r) \propto (r/R_0)^{-gamma}, gamma=4, is found to reasonably reproduce the data with R_0 ~ 100-200 h^{-1} kpc, but also spherical clouds with the same column density profile and a power-law distribution of radii may give a satisfactory representation of the observations.
We present a deep BVrIK multicolor catalog of galaxies in the field of the high redshift (z=4.7) quasar BR 1202-0725. Reliable colors have been measured for galaxies selected down to R=25. The choice of the optical filters has been optimized to define a robust multicolor selection of galaxies at 3.8 1.5. At variance with brighter surveys (I<22.5) there is a tail in the distribution towards high redshifts up to z~4. The luminosity function at z~0.6 shows a steepening for M_B>-19. The observed cosmological ultraviolet luminosity density is computed in the overall redshift interval z=0.3-4.5 reaching a value 2x10^{19} W/Hz/Mpc^3 at z~0.8. We have derived in a homogeneous way, using the GISSEL libraries, the physical parameters connected with the fitted spectral energy distributions. The bulk of the blue intermediate redshift population with z=0.4-1 mostly consists of very young star-forming galaxies with a median starburst age of the order of a few 10^8 yr and typical mass in luminous stars ~2x10^8 Mo.
Recent results have questioned the description of the QSO luminosity function in terms of a pure luminosity evolution and call for a luminosity dependent luminosity evolution. Measurements of the QSO clustering amplitude and evolution allow further distinguishing among the various physical scenarios proposed to interpret the QSO phenomenon. The general properties of the QSO population would arise naturally if quasars are short-lived events connected to a characteristic halo mass $\sim 5 \cdot 10^{12}$ M$_\odot$. This is the typical mass of groups of galaxies in which the interactions triggering the QSO activity preferentially take place.
The evolution of QSO clustering is investigated with a new sample of 388 QSOs with 0.3<z<2.2, B<20.5 and M_B<-23. Evidence is found for an increase of the clustering amplitude with increasing redshift. These measurements allow to further distinguish among the various physical scenarios proposed to interpret the QSO phenomenon. A single population model is inconsistent with the observations. The general properties of the QSO population would arise naturally if quasars are short-lived events connected to a characteristic halo mass ~5x10^{12} M_sun.