Catálogo de publicaciones - libros

The development of primordial inhomogeneities into the non-linear regime and the formation of the first astrophysical objects within dark matter halos mark the transition from a simple, neutral, cooling universe – described by just a few parameters – to a messy ionized one – the realm of radiative, hydrodynamic, and star formation processes. The recent measurement by the satellite of a large optical depth to electron scattering implies that this transition must have begun very early, and that the universe was reionized at redshift =17± 5. It is an early generation of extremely metal-poor massive stars and/or ‘seed’ accreting black holes in subgalactic halos that may have generated the ultraviolet radiation and mechanical energy that reheated and reionized most of the hydrogen in the cosmos. The detailed thermal, ionization, and chemical enrichment history of the universe during the crucial formative stages around =10–20 depends on the power-spectrum of density fluctuations on small scales, the stellar initial mass function and star formation efficiency, a complex network of poorly understood ‘feedback’ mechanisms, and remains one of the crucial missing links in galaxy formation and evolution studies.

A combination of evidence is presented suggesting that the majority of the stars in today’s galaxies were born during a luminous infrared phase (LIRP) triggered by the local environment of galaxies. The CIRB is a fossil record of these LIRPs and therefore reflects the influence of triggered star formation through galaxy-galaxy interactions, including non merging tidal encounters. This scenario, in which galaxies experienced several LIRPs in their history, is consistent with the measured redshift evolution of the cosmic density of star formation rates and of stellar masses of galaxies.

Two of the most outstanding issues in modern astrophysics are what reionized the Universe and how did the first objects form. Observations of galaxies selected through the Lyman-Break technique indicate that UV photon output at the end of reionization was dominated by relatively faint low mass galaxies and not AGN.

We present a summary of optical/NIR deep surveys for very high- galaxies using the 8.2 m Subaru Telescope operated by National Astronomical Observatory of Japan. The prime focus mosaic CCD camera, Suprime-Cam, with a very wide field of view, 34× 27, allows us to carry out efficient optical deep surveys. In particular, the Subaru Deep Field project has provided us a number of Lyman emitters beyond =6. We discuss the star formation history in the early universe based on this project.

The unique high–resolution wide–field imaging capabilities of HST with ACS have allowed the characterization of galaxies at redshift 6, less than 1 Gyr from recombination. The dropout technique, applied to deep ACS , images in the RCDS 1252–2927, GOODS and UDF–Parallel fields has yielded large samples, allowing determination of their properties (e.g., size, color) and meaningful comparisons against lower redshift dropout samples. The use of cloning techniques has enabled us to control for many of the strong selection biases that affect the study of high redshift populations. A clear trend of size with redshift has been identified, and its impact on the luminosity density and star formation rate can be estimated. There is a significant, though modest, decrease in the star formation rate from redshifts ~ 2.5 out through ~ 6. The latest data also allow for the first robust determination of the luminosity function at ~ 6.

The Phoenix Deep Survey (PDS) is a multiwavelength survey based on deep 1.4 GHz radio observations used to identify a large sample of star forming galaxies to =1. Photometric redshifts are estimated for the optical counterparts to the radio-detected galaxies, and their uncertainties quantified by comparison with spectroscopic redshift measurements. The photometric redshift estimates and associated best-fitting spectral energy distributions are used in a stacking analysis exploring the mean radio properties of -band selected galaxies. Average flux densities of a few Jy are measured.

Recently, we conducted a Very Large Telescope (VLT) large program to search for forming clusters by looking for overdensities of Ly emitters around high redshift radio galaxies. In total seven proto-clusters were discovered, including a proto-cluster around the radio galaxy MRC 0316–257 at ~ 3.13. This structure has an excess of Ly emitters by a factor of 3 as compared to the field, and the derived mass is 2–5 × 10 M. The Ly emitters in the proto-cluster are on average bluer than Lyman Break Galaxies (LBGs). Also, the galaxies are faint (sub ) and small (half light radii < 1.7 kpc, which is smaller than the average size of LBGs). This might indicate that, at least a fraction of, Ly emitters could be young (~ 10 yr), nearly dust-free, forming galaxies.

Deep X-ray surveys have shown that the cosmic X-ray background (XRB) is largely due to the accretion onto supermassive black holes, integrated over the cosmic time. These surveys have resolved more than 90% of the X-ray background at ~ 1 keV and about 50% at 10 keV into discrete sources. Optical spectroscopic identifications show that the sources producing the bulk of the X-ray background are a mixture of unobscured (type-1) and obscured (type-2) AGNs, as predicted by the XRB population synthesis models. A class of highly luminous type-2 AGN, so called QSO-2s, has been detected in the deepest Chandra and XMM-Newton surveys. The fraction of type-2 AGN among all AGN, however, decreases significantly with luminosity. The new Chandra AGN redshift distribution peaks at much lower redshifts (~ 0.7) than that based on ROSAT data. The low redshift peak applies both to absorbed and unabsorbed AGN and is also seen in the 0.5–2 keV band alone. The new, preliminary X-ray luminosity function changes shape between low and high redshifts, confirming the luminosity-dependent density evolution model. The space density of Seyfert galaxies evolves much slower than that of QSOs.

Some recent results on the physical and statistical properties of nearby and distant AGN are presented. I first discuss the properties of “elusive” AGNs, i.e. obscured AGNs which do not show a Seyfert-like spectrum in the optical. Then I present preliminary results from a detailed study of the contribution of obscured AGNs and of their host galaxies to the infrared cosmic background. Finally I discuss an observational program aimed at investigating the properties of the most distant quasars, of their circumnuclear medium and the implications for their host galaxies.

The Subaru/XMM-Newton Deep Survey (SXDS) is a new deep Optical/X-ray survey, whose purpose is to provide an accurate census of the contents of the Universe without suffering from the biasing effects of large-scale structures. The SXDS is part of an ambitious project to obtain extensive multi-wavelength data across a ~1.3 deg region of sky. Other wavelength observations ranging from the X-ray to the radio are being pursued and/or planned through various facilities. These multiwavelength data would allow us a comprehensive approach to investigate the mass assembly history of galaxies over a broad span of cosmic history.