High redshift AGNs and HI reionisation: limits from the unresolved X-ray background

The rapidly declining population of bright quasars at z~3 appears to make an increasingly small contribution to the ionising background at the HI Lyman limit. It is then generally though that massive stars in (pre-)galactic systems may provide the additional ionising flux needed to complete HI reionisation by z>6. A galaxy dominated background, however, may require that the escape fraction of Lyman continuum radiation from high redshift galaxies is as high as 10%, a value somewhat at odds with (admittedly scarce) observational constraints. High escape fractions from dwarf galaxies have been advocated, or, alternatively, a so-far undetected (or barely detected) population of unobscured, high-redshift faint AGNs. Here we question the latter hypothesis, and show that such sources, to be consistent with the measured level of the unresolved X-ray background at z=0, can provide a fraction of the HII filling factor not larger than 13% by z=6. The fraction rises to <27% in the somewhat extreme case of a constant comoving redshift evolution of the AGN emissivity. This still calls for a mean escape fraction of ionising photons from high-z galaxies >10%.Comment: A&A Letter, accepted (4 pages, 2 figures

Extragalactic Background Light: new constraints from the study of the photon-photon absorption on blazar spectra

The study of the Extragalactic Background Light (EBL) is crucial to understand many astrophysical problems (as the formation of first stars, the evolution of galaxies and the role of dust emission). At present, one of the most powerful ways to put constraints on EBL is represented by the study of the photon-photon absorption on gamma-ray spectra of TeV blazars. Adopting this method, we found that, if the only contribution to the optical and Near Infrared (NIR) background is given by galaxies, the spectrum of the blazar H1426+428 cannot be fitted. To reproduce the observational data of H1426+428 a Near Infrared excess with respect to galaxy counts is required, with amplitude consistent with both the Matsumoto et al. (2000) data with Kelsall's model of zodiacal light (ZL) subtraction and the DIRBE data with Wright's model of ZL subtraction. The derived constraints on the optical EBL are weaker, because the experimental errors on blazar data are still bigger than the differences among various optical EBL models. In the mid-infrared the SPITZER measurement at 24 micron provides the best fit of the blazar spectrum.Comment: 8 pages, 5 figures, to appear in Proceedings of "Baryons in Dark Matter Halos", 5-9 October 2004, Novigrad, Croati

Unresolved X-ray background: clues on galactic nuclear activity at z>6

We study, by means of dedicated simulations of massive black hole build-up, the possibility to constraint the existence and nature of the AGN population at z>6 with available and planned X-ray and near infrared space telescopes. We find that X-ray deep-field observations can set important constraints to the faint-end of the AGN luminosity function at very high redshift. Planned X-ray telescopes should be able to detect AGN hosting black holes with masses down to >10^5 Msun (i.e., X-ray luminosities in excess of 10^42 erg s^-1), and can constrain the evolution of the population of massive black hole at early times (6<z<10). We find that this population of AGN should contribute substantially (~25%) to the unresolved fraction of the cosmic X-ray background in the 0.5-10 keV range, and that a significant fraction (~3-4%) of the total background intensity would remain unaccounted even after future X-ray observations. As byproduct, we compute the expected UV background from AGN at z>6 and we discuss the possible role of AGN in the reionization of the Universe at these early epochs, showing that AGN alone can provide enough ionizing photons only in the (improbable) case of an almost completely homogeneous inter-galactic medium. Finally, we show that super-Eddington accretion, suggested by the observed QSOs at z>6, must be a very rare event, confined to black holes living in the highest density peaks.Comment: 9 pages, 7 figures, MNRAS in pres

First Stars Contribution to the Near Infrared Background Fluctuations

We show that the emission from the first, metal-free stars inside Population III objects (PopIIIs) are needed to explain the level of fluctuations in the Near Infrared Background (NIRB) recently discovered by Kashlinsky et al. (2002), at least at the shortest wavelengths. Clustering of (unresolved) Pop IIIs can in fact account for the entire signal at almost all the ~1-30 arcsec scales probed by observations in the J band. Their contribution fades away at shorter frequencies and becomes negligible in the K band. ``Normal'', highly clustered, ~3 galaxies undergoing intense star-formation such as those found in the Hubble Deep Fields can 'fill in' this gap and provide for the missing signal. It is in fact found that their contribution to the intensity fluctuations is the dominant one at lambda=2.17 mum, while it gradually looses importance in the H and J bands. The joint contribution from these two populations of cosmic objects is able, within the errors, to reproduce the observed power spectrum in the whole Near Infrared range on small angular scales (theta < 200" for Pop III protogalaxies). Signals on larger scales detected by other experiments instead require the presence of more local sources.Comment: 6 pages, 4 figures, submitted to MNRA

The Gamma Ray Burst Luminosity Function in the Light of the Swift 2-year Data

We compute the luminosity function (LF) and the formation rate of long gamma ray bursts (GRBs) by fitting the observed differential peak flux distribution obtained by the BATSE satellite in three different scenarios: i) GRBs follow the cosmic star formation and their LF is constant in time; ii) GRBs follow the cosmic star formation but the LF varies with redshift; iii) GRBs form preferentially in low-metallicity environments. We find that the differential peak flux number counts obtained by BATSE and by Swift can be reproduced using the same LF and GRB formation rate, indicating that the two satellites are observing the same GRB population. We then check the resulting redshift distributions in the light of Swift 2-year data, focusing in particular on the relatively large sample of GRBs detected at z>2.5. We show that models in which GRBs trace the cosmic star formation and are described by a constant LF are ruled out by the number of high-z Swift detections. This conclusion does not depend on the redshift distribution of bursts that lack of optical identification, nor on the existence of a decline in star formation rate at z>2, nor on the adopted faint-end of the GRB LF. Swift observations can be explained by assuming that the LF varies with redshift and/or that GRB formation is limited to low-metallicity environments.Comment: 7 pages, 3 figures, ApJ Letter in pres

Constraints on the Accretion History of Massive Black Holes from Faint X-ray Counts

We investigate how hierarchical models for the co-evolution of the massive black hole (MBH) and AGN population can reproduce the observed faint X-ray counts. We find that the main variable influencing the theoretical predictions is the Eddington ratio of accreting sources. We compare three different models proposed for the evolution of AGN Eddington ratio, f_Edd: constant f_Edd=1, f_Edd decreasing with redshift, and f_Edd depending on the AGN luminosity, as suggested by simulations of galactic mergers including BHs and AGN feedback. We follow the full assembly of MBHs and host halos from early times to the present in a LambdaCDM cosmology. AGN activity is triggered by halo major mergers and MBHs accrete mass until they satisfy the observed correlation with velocity dispersion. We find that all three models can reproduce fairly well the total faint X-ray counts. The redshift distribution is however poorly matched in the first two models. The Eddington ratios suggested by merger simulations predicts no turn-off of the faint end of the AGN optical luminosity function at redshifts z>=1 down to very low luminosity.Comment: MNRAS in press, 7 pages, 8 figure

PopIII signatures in the spectra of PopII/I GRBs

We investigate signatures of population III (PopIII) stars in the metal-enriched environment of GRBs originating from population II-I (PopII/I) stars by using abundance ratios derived from numerical simulations that follow stellar evolution and chemical enrichment. We find that at $z>10$ more than $10$ of PopII/I GRBs explode in a medium previously enriched by PopIII stars (we refer to them as GRBII$\rightarrow$III). Although the formation of GRBII$\rightarrow$III is more frequent than that of pristine PopIII GRBs (GRBIIIs), we find that the expected GRBII$\rightarrow$III observed rate is comparable to that of GRBIIIs, due to the usually larger luminosities of these latter. GRBII$\rightarrow$III events take place preferentially in small proto-galaxies with stellar masses $\rm M_\star \sim 10^{4.5} - 10^7\,\rm M_\odot$, star formation rates $\rm SFR \sim 10^{-3}-10^{-1}\,\rm M_\odot/yr$ and metallicities $Z \sim 10^{-4}-10^{-2}\,\rm Z_\odot$. On the other hand, galaxies with $Z < 10^{-2.8}\,\rm Z_\odot$ are dominated by metal enrichment from PopIII stars and should preferentially host GRBII$\rightarrow$III. Hence, measured GRB metal content below this limit could represent a strong evidence of enrichment by pristine stellar populations. We discuss how to discriminate PopIII metal enrichment on the basis of various abundance ratios observable in the spectra of GRBs' afterglows. By employing such analysis, we conclude that the currently known candidates at redshift $z\simeq 6$ -- i.e. GRB 050904 \cite[][]{2006Natur.440..184K} and GRB 130606A \cite[][]{2013arXiv1312.5631C} -- are likely not originated in environments pre-enriched by PopIII stars.Comment: 9 pages, 7 figures; MNRAS accepte

Professional Learning Communities in the Expanded Learning Field

This white paper uses twelve evaluation reports of the Professional Learning Community (PLC) initiatives, as well as interviews with PLC participants and facilitators, to better understand how the PLC model is used in the Expanded Learning field, to demonstrate the benefits to participating staff and expanded learning programs, and to share best practices for youth-serving organizations interested in using PLCs

Ultra-faint high-redshift galaxies in the Frontier Fields

By combining cosmological simulations with Frontier Fields project lens models we find that, in the most optimistic case, galaxies as faint as $m \approx 33 - 34$ (AB magnitude at $1.6 \rm \mu m$) can be detected in the Frontier Fields. Such faint galaxies are hosted by dark matter halos of mass $\sim10^9 M_\odot$ and dominate the ionizing photon budget over currently observed bright galaxies, thus allowing for the first time the investigation of the dominant reionization sources. In addition, the observed number of these galaxies can be used to constrain the role of feedback in suppressing star formation in small halos: for example, if galaxy formation is suppressed in halos with circular velocity $v_c < 50$ km s$^{-1}$, galaxies fainter than $m=31$ should not be detected in the FFs.Comment: 5 pages, 7 figures, accepted for publication in MNRAS Letter

Counts of high-redshift GRBs as probe of primordial non-Gaussianities

We propose to use high-redshift long $\gamma$-ray bursts (GRBs) as cosmological tools to constrain the amount of primordial non-Gaussianity in the density field. By using numerical, N-body, hydrodynamic, chemistry simulations of different cosmological volumes with various Gaussian and non-Gaussian models, we self-consistently relate the cosmic star formation rate density to the corresponding GRB rate. Assuming that GRBs are fair tracers of cosmic star formation, we find that positive local non-Gaussianities, described in terms of the non-linear parameter, \fnl, might boost significantly the GRB rate at high redshift, $z \gg 6$. Deviations with respect to the Gaussian case account for a few orders of magnitude if \fnl$\sim 1000$, one order of magnitude for \fnl$\sim 100$, and a factor of $\sim 2$ for \fnl$\sim 50$. These differences are found only at large redshift, while at later times the rates tend to converge. Furthermore, a comparison between our predictions and the observed GRB data at $z > 6$ allows to exclude large negative \fnl, consistently with previous works. Future detections of any long GRB at extremely high redshift ($z\sim 15-20$) could favor non-Gaussian scenarios with positive \fnl. More stringent constraints require much larger high-$z$ GRB complete samples, currently not available in literature. By distinguishing the contributions to the GRB rate from the metal-poor population III regime, and the metal-enriched population II-I regime, we conclude that the latter is a more solid tracer of the underlying matter distribution, while the former is strongly dominated by feedback mechanisms from the first, massive, short-lived stars, rather than by possible non-Gaussian fluctuations. This holds quite independently of the assumed population III initial mass function.Comment: 12 pages; MNRAS in press. Chi ha paura muore tutti i giorn