Witnessing the formation of a brightest cluster galaxy at z>2

We present deep observations taken with the HST Advanced Camera for Surveys of the central massive galaxy in a forming cluster at z=2.2. The galaxy hosting the powerful radio source MRC 1138-262 is associated with one of the most extensive merger systems known in the early universe. Our HST/ACS image shows many star-forming galaxies merging within a ~200 kpc region that emits both diffuse line emission and continuum in the rest-frame UV. Because this galaxy lives in an overdense environment, it represents a rare view of a brightest cluster galaxy in formation at z>2 which may serve as a testbed for predictions of massive cluster galaxy formation.Comment: Contribution to the proceedings of "The Fate of Gas in Galaxies", Dwingeloo, July 2006, with 2 colour figures. To appear in New Astronomy Reviews, Vol. 51 (2007), eds. Morganti, Oosterloo, Villar-Martin & van Gorko

Proto-clusters in the Lambda CDM Universe

We compare the highly clustered populations of very high redshift galaxies with proto-clusters identified numerically in a standard $\Lambda$CDM universe ($\Omega_0=0.3, \lambda_0=0.7$) simulation. We evolve 256^3 dark matter particles in a comoving box of side 150h^{-1}Mpc. By the present day there are 63 cluster sized objects of mass in excess of 10^{14}h^{-1}Mo in this box. We trace these clusters back to higher redshift finding that their progenitors at z=4--5 are extended regions of typically 20--40 Mpc (comoving) in size, with dark halos of mass in excess of 10^{12}h^{-1}Mo and are overdense by typically 1.3--13 times the cosmological mean density. Comparison with the observation of Lyman alpha emitting (LAEs) galaxies at z=4.86 and at z=4.1 indicates that the observed excess clustering is consistent with that expected for a proto-cluster region if LAEs typically correspond to massive dark halos of more than 10^{12}h^{-1}Mo. We give a brief discussion on the relation between high redshift concentration of massive dark halos and present day rich clusters of galaxies.Comment: 4 pages, 5 figures, Accepted for publication in ApJ Letter

On the Gas Surrounding High Redshift Galaxy Clusters

Francis & Hewett (1993) identified two 10-Mpc scale regions of the high redshift universe that were seemingly very overdense in neutral hydrogen. Subsequent observations showed that at least one of these gas-rich regions enveloped a cluster of galaxies at redshift 2.38. We present improved observations of the three background QSOs with sightlines passing within a few Mpc of this cluster of galaxies. All three QSOs show strong neutral hydrogen absorption at the cluster redshift, suggesting that this cluster (and perhaps all high redshift clusters) may be surrounded by a ~5 Mpc scale region containing ~ 10^12 solar masses of neutral gas. If most high redshift clusters are surrounded by such regions, we show that the gas must be in the form of many small ( 0.03 cm^-3) clouds, each of mass < 10^6 solar masses. These clouds are themselves probably gathered into > 20 kpc sized clumps, which may be galaxy halos or protogalaxies. If this gas exists, it will be partially photoionised by the UV background. We predict the diffuse Ly-alpha flux from this photoionisation, and place observational limits on its intensity.Comment: 19 pages, 5 figures. Accepted for publication in PAS

Can the intergalactic medium cause a rapid drop in Lyman alpha emission at z>6?

The large cross-section of the Lyman alpha (Lya) line makes it a sensitive probe of the ionization state of the intergalactic medium (IGM). Here we present the most complete study to date of the IGM Lya opacity, and its application to the redshift evolution of the 'Lya fraction', i.e. the fraction of color-selected galaxies with a detectable Lya emission line. We use a tiered approach, which combines large-scale semi-numeric simulations of reionization with moderate-scale hydrodynamic simulations of the ionized IGM. This allows us to simultaneously account for evolution in both: (i) the opacity from an incomplete (patchy) reionization, parameterized by the filling factor of ionized regions, Q_HII; and (ii) the opacity from self-shielded systems in the ionized IGM, parameterized by the average photo-ionization rate inside HII regions, \Gamma. In contrast to recent empirical models, attenuation from patchy reionization has a unimodal distribution along different sightlines, while attenuation from self-shielded systems is more bimodal. We quantify the average IGM transmission in our (Q_HII, \Gamma) parameter space, which can easily be used to interpret new data sets. Using current observations, we predict that the Lya fraction cannot drop by more than a factor of ~2 with IGM attenuation alone, even for HII filling factors as low as Q_HII>0.1. Larger changes in the Lya fraction could result from a co-evolution with galaxy properties. Marginalizing over \Gamma, we find that current observations constrain Q_HII < 0.6 at z=7 [68% confidence level (C.L.)]. However, all of our parameter space is consistent with observations at 95% C.L., highlighting the need for larger observational samples at z >= 6.Comment: 12 pages, 10 figures, MNRAS submitte

Observing the very low-surface brightness dwarfs in a deep field in the VIRGO cluster: constraints on Dark Matter scenarios

We report the discovery of 11 very faint (r< 23), low surface brightness ({\mu}_r< 27 mag/arcsec^2) dwarf galaxies in one deep field in the Virgo cluster, obtained by the prime focus cameras (LBC) at the Large Binocular Telescope (LBT). These extend our previous sample to reach a total number of 27 galaxies in a field of just of 0.17 deg^2 located at a median distance of 390 kpc from the cluster center. Their association with the Virgo cluster is supported by their separate position in the central surface brightness - total magnitude plane with respect to the background galaxies of similar total magnitude. For a significant fraction (26\%) of the sample the association to the cluster is confirmed by spectroscopic follow-up. We show that the mere abundance of satellite galaxies corresponding to our observed number in the target field provides extremely tight constraints on Dark Matter models with suppressed power spectrum compared to the Cold Dark Matter case, independently of the galaxy luminosity distribution. In particular, requiring the observed number of satellite galaxies not to exceed the predicted abundance of Dark Matter sub-halos yields a limit m_X >3 keV at 1-{\sigma} and m_X > 2.3 keV at 2-{\sigma} confidence level for the mass of thermal Warm Dark Matter particles. Such a limit is competitive with other limits set by the abundance of ultra-faint satellite galaxies in the Milky Way, is completely independent of baryon physics involved in galaxy formation, and has the potentiality for appreciable improvements with next observations. We extend our analysis to Dark Matter models based on sterile neutrinos, showing that our observations set tight constraints on the combination of sterile neutrino mass m_{\nu} and mixing parameter sin^2(2{\theta}). We discuss the robustness of our results with respect to systematics.Comment: Accepted for publication in Astronomy & Astrophysic

Evolution in the Escape Fraction of Ionizing Photons and the Decline in Strong Lya Emission from z>6 Galaxies

The rapid decline in the number of strong Lyman Alpha (Lya) emitting galaxies at z > 6 provides evidence for neutral hydrogen in the IGM, but is difficult to explain with plausible models for reionization. We demonstrate that the observed reduction in Lya flux from galaxies at z > 6 can be explained by evolution in the escape fraction of ionizing photons, f_esc. We find that the median observed drop in the fraction of galaxies showing strong Lya emission, as well as the observed evolution of the Lya luminosity function both follow from a small increase in f_esc of Delta f_esc ~ 0.1 from f_esc ~ 0.6 at z ~ 6. This high escape fraction may be at odds with current constraints on the ionising photon escape fraction, which favor smaller values of f_esc < 20%. However, models that invoke a redshift evolution of f_ esc that is consistent with these constraints can suppress the z~7 Lya flux to the observed level, if they also include a small evolution in global neutral fraction of Delta x_HI ~ 0.2. Thus, an evolving escape fraction of ionising photons can be a plausible part of the explanation for evolution in the Lya emission of high redshift galaxies. More generally, our analysis also shows that the drop in the Lya fraction is quantitatively consistent with the observed evolution in the Lya luminosity functions of Lya Emitters.Comment: 9 pages, 3 figures. Accepted for publication in MNRAS. Added references & brief discussion on line shape evolutio

AGNs as main contributors to the UV ionizing emissivity at high redshifts: predictions from a Lambda-CDM model with linked AGN/galaxy evolution

We have evaluated the contribution of the AGN population to the ionization history of the Universe based on a semi-analytic model of galaxy formation and evolution in the CDM cosmological scenario. The model connects the growth of black holes and of the ensuing AGN activity to galaxy interactions. In the model we have included a self consistent physical description of the escape of ionizing UV photons; this is based on the blast-wave model for the AGN feedback we developed in a previous paper to explain the distribution of hydrogen column densities in AGNs of various redshifts and luminosities, due to absorption by the host galaxy gas. The model predicts UV luminosity functions for AGNs which are in good agreement with those derived from the observations especially at low and intermediate redshifts (z=3). At higher redshifts (z>5) the model tends to overestimate the data at faint luminosities. Critical biases both in the data and in the model are discussed to explain such apparent discrepancies. The predicted hydrogen photoionization rate as a function of redshift is found to be consistent with that derived from the observations. All that suggests to reconsider the role of the AGNs as the main driver of the ionization history of the Universe.Comment: ApJ accepte