E. Giallongo

Measuring and modelling the redshift evolution of clustering: the Hubble Deep Field North

ABSTRA C T The evolution of galaxy clustering from za 0t oz . 4:5 is analysed using the angular correlation function and the photometric redshift distribution of galaxies brighter than IAB < 28:5 in the Hubble Deep Field North. The reliability of the photometric redshift estimates is discussed on the basis of the available spectroscopic redshifts, comparing different codes and investigating the effects of photometric errors. The redshift bins in which the clustering properties are measured are then optimized to take into account the uncertainties of the photometric redshifts. The results show that the comoving correlation length r0 has a small decrease in the range 0 & z & 1 followed by an increase at higher z. We compare these results with the theoretical predictions of a variety of cosmological models belonging to the general class of Cold Dark Matter scenarios, including Einstein‐de Sitter models, an open model and a flat model with non-zero cosmological constant. Comparison with the expected mass clustering evolution indicates that the observed high-redshift galaxies are biased tracers of the dark matter with an effective bias b strongly increasing with redshift. Assuming an Einstein‐de Sitter universe, we obtain b . 2: 5a tz . 2 and b . 5a tz . 4. These results support theoretical scenarios of biased galaxy formation in which the galaxies observed at high redshift are preferentially located in more massive haloes. Moreover, they suggest that the usual parameterization of the clustering evolution as jOr; zUajOr; 0UO1 1 zU 2O31eU is not a good description for any value of e . Comparison of the clustering amplitudes that we measured at z . 3 with those reported by Adelberger et al. and Giavalisco et al., based on a different selection, suggests that the clustering depends on the abundance of the objects: more abundant objects are less clustered, as expected in the paradigm of hierarchical galaxy formation. The strong clustering and high bias measured at z . 3 are consistent with the expected density of massive haloes predicted in the frame of the various cosmologies considered here. At z . 4, the strong clustering observed in the Hubble Deep Field requires a significant fraction of massive haloes to be already formed by that epoch. This feature could be a discriminant test for the cosmological parameters if confirmed by future observations.

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

Aims. We present a high quality multiwavelength (from 0.3 to 8.0 µm) catalog of the large and deep area in the GOODS Southern Field covered by the deep near-IR observations obtained with the ESO VLT. Methods. 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 µm), 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. Results. The final catalog is made up of 14 847 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.

Star formation and mass assembly in high redshift galaxies

Aims. The goal of this work is to infer the star formation properties and the mass assembly process of high redshift (0.3 ≤ z 0.3, the star formation rate is correlated well with stellar mass, and this relationship seems to steepen with redshift if one relies on IR-based estimates of the SFR; b) the contribution to the global SFRD by massive galaxies increases with redshift up to � 2.5, more rapidly than for galaxies of lower mass, but appears to flatten at higher z; c) despite this increase, the most important contributors to the SFRD at any z are galaxies of about, or immediately lower than, the characteristic stellar mass;

Unveiling Obscured Accretion in the Chandra Deep Field-South

We make use of deep HST, VLT, Spitzer, and Chandra data on the Chandra Deep Field-South to constrain the number of Compton-thick AGNs in this field. We show that sources with high 24 μm-to-optical flux ratios and red colors form a distinct source population, and that their infrared luminosity is dominated by AGN emission. Analysis of the X-ray properties of these extreme sources shows that most of them (80% ± 15%) are indeed likely to be highly obscured, Compton-thick AGNs. The number of infrared-selected, Compton-thick AGNs with 5.8 μm luminosity higher than 1044.2 ergs s−1 turns out to be similar to that of X-ray-selected, unobscured, and moderately obscured AGNs with 2-10 keV luminosity higher than 1043 ergs s−1 in the redshift bin 1.2-2.6. This factor of 2 source population is exactly what is needed to solve the discrepancies between model predictions and X-ray AGN selection.

The K20 survey - I. Disentangling old and dusty star-forming galaxies in the ERO population

We present the results of VLT optical spectroscopy of a complete sample of 78 EROs with R-Ks\geq5 over a field of 52 arcmin^2. About 70% of the 45 EROs with Ks\leq19.2 have been spectroscopically identified with old passively evolving and dusty star-forming galaxies at 0.7<z<1.5. The two classes are about equally populated and for each of them we present and discuss the average spectrum. From the old ERO average spectrum and for Z=Z_{\odot} we derive a minimum age of \sim 3 Gyr, corresponding to a formation redshift of z_f \gtsima 2.4. PLE models with such formation redshifts well reproduce the density of old EROs (consistent with being passively evolving ellipticals), whereas the predictions of the current hierarchical merging models are lower than the observed densities by large factors (up to an order of magnitude). From the average spectrum of the star-forming EROs we estimate a substantial dust extinction with E(B-V) \gtsima 0.5. The star formation rates, corrected for the average reddening, suggest a significant contribution from EROs to the cosmic star-formation density at z \sim 1.

Faint AGNs at z > 4 in the CANDELS GOODS-S Field: Looking for Contributors to the Reionization of the Universe

Context. Establishing the number of faint active galactic nuclei (AGNs) at z = 4-6 is crucial to understanding their cosmological importance as main contributors to the reionization of the Universe. Aims. In order to derive the AGN contribution to the cosmological ionizing emissivity we have selected faint AGN candidates at z \textgreater 4 in the CANDELS GOODS-South field, which is one of the deepest fields with extensive multiwavelength coverage from Chandra, HST, Spitzer, and various ground-based telescopes. Methods. We have adopted a relatively novel criterion. As a first step, high redshift galaxies are selected in the NIR H band down to very faint levels (H \textless= 27) using reliable photometric redshifts. At z \textgreater 4 this corresponds to a selection criterion based on the galaxy rest-frame UV flux. AGN candidates are then picked up from this parent sample if they show X-ray fluxes above a threshold of F-X similar to 1.5 x 10(-17) erg cm(-2) s(-1) (0.5-2 keV), corresponding to a probability of spurious detections of 2 x 10(-4) in the deep X-ray 4 Ms Chandra image. Results. We have found 22 AGN candidates at z \textgreater 4 and we have derived the first estimate of the UV luminosity function in the redshift interval 4 \textless z \textless 6.5 and absolute magnitude interval -22.5 less than or similar to M-1450 less than or similar to -18.5 typical of local Seyfert galaxies. The faint end of the derived luminosity function is about two to four magnitudes fainter at z similar to 4-6 than that derived from previous UV surveys. We estimated ionizing emissivities and hydrogen photoionization rates in the same redshift interval under reasonable assumptions and after discussion of possible caveats, the most important being the large uncertainties involved in the estimate of photometric redshift for sources with featureless, almost power-law SEDs and/or low average escape fraction of ionizing photons from the AGN host galaxies. Both effects could, in principle, significantly reduce the estimated average volume densities and/or ionizing emissivities, especially at the highest redshifts. Conclusions. At z = 4-6.5 we argue that, under reasonable evaluations of possible biases, the probed AGN population can produce photoionization rates consistent with that required to keep the intergalactic medium observed in the Lyman-alpha forest of high redshift QSO spectra highly ionized, providing an important contribution to the cosmic reionization.

The Abundance of Distant and Extremely Red Galaxies: The Role of AGN Feedback in Hierarchical Models

We investigate the effect of AGN feedback associated with the bright QSO phase on the color distribution of galaxies from z = 0 up to z = 4. To this aim, we insert a blast-wave model of AGN feedback in our semianalytic model of galaxy formation, which includes the growth of supermassive black holes and the AGN activity triggered by interactions of the host galaxies. The AGN feedback is directly related to the impulsive, luminous quasar phase. We test our model by checking the consistency of its results against (1) the QSO luminosity functions from z = 0 to 4, and (2) the observed local relation between the black hole mass mBH and the mass of the host galaxy. At low redshift the inclusion of AGN feedback enhances the number of red bright galaxies so that the color distribution of Mr 1.5) galaxies; at 0.5 2.5.

The K20 survey. III. Photometric and spectroscopic properties of the sample

The K20 survey is an ESO VLT optical and near-infrared spectroscopic survey aimed at obtaining spectral infor- mation and redshifts of a complete sample of about 550 objects to Ks 20:0 over two independent fields with a total area of 52 arcmin 2 . In this paper we discuss the scientific motivation of such a survey, we describe the photometric and spectroscopic properties of the sample, and we release the Ks-band photometric catalog. Extensive simulations showed that the sample is photometrically highly complete to Ks= 20. The observed galaxy counts and the R Ks color distribution are consistent with literature results. We observed spectroscopically 94% of the sample, reaching a spectroscopic redshift identification complete- ness of 92% to Ks 20:0 for the observed targets, and of 87% for the whole sample (i.e. counting also the unobserved targets). Deep spectroscopy was complemented with multi-band deep imaging in order to derive tested and reliable photometric red- shifts for the galaxies lacking spectroscopic redshifts. The results show a very good agreement between the spectroscopic and the photometric redshifts with = 0:01 and with a dispersion ofz= 0:09. Using both the spectroscopic and the photometric redshifts, we reached an overall redshift completeness of about 98%. The size of the sample, the redshift complete- ness, the availability of high quality photometric redshifts and multicolor spectral energy distributions make the K20 survey database one of the most complete samples available to date for constraining the currently competing scenarios of galaxy formation and for a variety of other galaxy evolution studies.

New Observations of z ~ 7 Galaxies: Evidence for a Patchy Reionization

We present new results from our search for z ~ 7 galaxies from deep spectroscopic observations of candidate z dropouts in the CANDELS fields. Despite the extremely low flux limits achieved by our sensitive observations, only two galaxies have robust redshift identifications, one from its Lyα emission line at z = 6.65, the other from its Lyman break, i.e., the continuum discontinuity at the Lyα wavelength consistent with a redshift of 6.42 but with no emission line. In addition, for 23 galaxies we present deep limits in the Lyα equivalent width derived from the nondetections in ultradeep observations. Using this new data as well as previous samples, we assemble a total of 68 candidate z ~ 7 galaxies with deep spectroscopic observations, of which 12 have a line detection. With this much enlarged sample we can place solid constraints on the declining fraction of Lyα emission in z ~ 7 Lyman-break galaxies compared to z ~ 6, both for bright and faint galaxies. Applying a simple analytical model, we show that the present data favor a patchy reionization process rather than a smooth one.

The physical properties of the Lyα forest at z > 1.5★

Combining a new, increased data set of eight quasi-stellar objects (QSOs) covering the Lyα forest at redshifts 1.5 < z < 3.6 from VLT/UVES observations with previously published results, we have investigated the properties of the Lyα forest at 1.5 < z <4. With the six QSOs covering the Lyα forest at 1.5 < z < 2.5, we have extended previous studies in this redshift range. In particular, we have concentrated on the evolution of the line number density and the clustering of the Lyα forest at z≤ 2.5, where the Lyα forest starts to show some inhomogeneity from sightline to sightline. We have fitted Voigt profiles to the Lyα absorption lines as in previous studies, and have, for two QSOs with zem∼ 2.4, fitted Lyα and higher order of Lyman lines down to 3050 A simultaneously. This latter approach has been taken in order to study the Lyβ forest at z∼ 2.2 and the higher H i column density Lyα forest in the Lyβ forest region. For a given NH I range, the Lyα forest at 1.5 1014 cm−2, there is a variation in the line number density from sightline to sightline at z < 2.5. This variation is stronger for higher column density systems, probably due to more gravitationally evolved structures at lower z. The mean H i opacity is at 1.5 < z < 4. HST observations show evidence for slower evolution of at z < 1. For NH i= 1012.5–15 cm−2, the differential column density distribution function, f(NH i), can be best fitted by f(NH I∝NH i−β with β≈ 1.5 for 1.5 < z < 4. When combined with HST observations, the exponent β increases as z decreases at 0 < z < 4 for NH i= 1013–17 cm−2. The correlation strength of the step optical depth correlation function shows the strong evolution from 〈z〉= 3.3 to 〈z〉= 2.1, although there is a large scatter along different sightlines. The analyses of the Lyβ forest at z∼ 2.2 are, in general, in good agreement with those of the Lyα forest.