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

Measuring the Redshift Evolution of Clustering: the Hubble Deep Field South

We present an analysis of the evolution of galaxy clustering in the redshift interval 0<z<4.5 in the HDF-S. The HST optical data are combined with infrared ISAAC/VLT observations, and photometric redshifts are used for all the galaxies brighter than I_AB<27.5. The clustering signal is obtained in different redshift bins using two different approaches: a standard one, which uses the best redshift estimate of each object, and a second one, which takes into account the redshift probability function of each object. This second method makes it possible to improve the information in the redshift intervals where contamination from objects with insecure redshifts is important. With both methods, we find that the clustering strength up to z~3.5 in the HDF-S is consistent with the previous results in the HDF-N. While at redshift lower than z~1 the HDF galaxy population is un/anti-biased (b<1) with respect to the underlying dark matter, at high redshift the bias increases up to b~2-3, depending on the cosmological model. These results support previous claims that, at high redshift, galaxies are preferentially located in massive haloes, as predicted by the biased galaxy formation scenario. The impact of cosmic errors on our analyses has been quantified, showing that errors in the clustering measurements in the HDF surveys are indeed dominated by shot-noise in most regimes. Future observations with instruments like the ACS on HST will improve the S/N by at least a factor of two and more detailed analyses of the errors will be required. In fact, pure shot-noise will give a smaller contribution with respect to other sources of errors, such as finite volume effects or non-Poissonian discreteness effects.Comment: 17 pages Latex, with 12 PostScript figures, Accepted for publication in MNRA

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

The Spectral Slope and Escape Fraction of Bright Quasars at z∼3.8z \sim 3.8: the Contribution to the Cosmic UV Background

We use a sample of 1669 QSOs ($r<20.15$, $3.6<z<4.0$) from the BOSS survey to study the intrinsic shape of their continuum and the Lyman continuum photon escape fraction (f$_{esc}$), estimated as the ratio between the observed flux and the expected intrinsic flux (corrected for the intergalactic medium absorption) in the wavelength range 865-885 \AA\ rest-frame. Modelling the intrinsic QSO continuum shape with a power-law, $F_{\lambda}\propto\lambda^{-\gamma}$, we find a median $\gamma=1.30$ (with a dispersion of $0.38$, no dependence on the redshift and a mild intrinsic luminosity dependence) and a mean f$_{esc}=0.75$ (independent of the QSO luminosity and/or redshift). The f$_{esc}$ distribution shows a peak around zero and a long tail of higher values, with a resulting dispersion of $0.7$. If we assume for the QSO continuum a double power-law shape (also compatible with the data) with a break located at $\lambda_{\rm br}=1000$ \AA\ and a softening $\Delta\gamma=0.72$ at wavelengths shorter than $\lambda_{\rm br}$, the mean f$_{esc}$ rises to $=0.82$. Combining our $\gamma$ and f$_{esc}$ estimates with the observed evolution of the AGN luminosity function (LF) we compute the AGN contribution to the UV ionizing background (UVB) as a function of redshift. AGN brighter than one tenth of the characteristic luminosity of the LF are able to produce most of it up $z\sim 3$, if the present sample is representative of their properties. At higher redshifts a contribution of the galaxy population is required. Assuming an escape fraction of Lyman continuum photons from galaxies between $5.5$ and $7.6\%$, independent of the galaxy luminosity and/or redshift, a remarkably good fit to the observational UVB data up to $z\sim 6$ is obtained. At lower redshift the extrapolation of our empirical estimate agrees well with recent UVB observations, dispelling the so-called Photon Underproduction Crisis.Comment: 8 pages, 9 figures, MNRAS accepte

The Asiago-ESO/RASS QSO Survey II. The Southern Sample

This is the second paper of a series describing the Asiago-ESO/RASS QSO survey, a project aimed at the construction of an all-sky statistically well-defined sample of very bright QSOs (B_J 30^{\circ}). The area covered by the survey is 5660 sq. deg. Spectroscopy for the 137 still unidentified objects has been obtained. The total number of AGN turns out to be 111, 63 of which are new identifications. The properties of the selection are discussed. The completeness and the success rate for this survey at the final stage are 63% and 46%, respectively

Data Analysis Software for the ESPRESSO Science Machine

ESPRESSO is an extremely stable high-resolution spectrograph which is currently being developed for the ESO VLT. With its groundbreaking characteristics it is aimed to be a "science machine", i.e., a fully-integrated instrument to directly extract science information from the observations. In particular, ESPRESSO will be the first ESO instrument to be equipped with a dedicated tool for the analysis of data, the Data Analysis Software (DAS), consisting in a number of recipes to analyze both stellar and quasar spectra. Through the new ESO Reflex GUI, the DAS (which will implement new algorithms to analyze quasar spectra) is aimed to get over the shortcomings of the existing software providing multiple iteration modes and full interactivity with the data.Comment: 5 pages, 2 figures; proceedings of ADASS XXI

Beacons into the Cosmic Dark Ages: Boosted transmission of Lyα\alpha from UV bright galaxies at z≳7z \gtrsim 7

Recent detections of Lyman alpha (Ly$\alpha$) emission from $z>7.5$ galaxies were somewhat unexpected given a dearth of previous non-detections in this era when the intergalactic medium (IGM) is still highly neutral. But these detections were from UV bright galaxies, which preferentially live in overdensities which reionize early, and have significantly Doppler-shifted Ly$\alpha$ line profiles emerging from their interstellar media (ISM), making them less affected by the global IGM state. Using a combination of reionization simulations and empirical ISM models we show, as a result of these two effects, UV bright galaxies in overdensities have $>2\times$ higher transmission through the $z\sim7$ IGM than typical field galaxies, and this boosted transmission is enhanced as the neutral fraction increases. The boosted transmission is not sufficient to explain the observed high Ly$\alpha$ fraction of $M_\mathrm{UV} \lesssim -22$ galaxies (Stark et al. 2017), suggesting Ly$\alpha$ emitted by these galaxies must be stronger than expected due to enhanced production and/or selection effects. Despite the bias of UV bright galaxies to reside in overdensities we show Ly$\alpha$ observations of such galaxies can accurately measure the global neutral hydrogen fraction, particularly when Ly$\alpha$ from UV faint galaxies is extinguished, making them ideal candidates for spectroscopic follow-up into the cosmic Dark Ages.Comment: 6 pages, 5 figures. Accepted for publication in ApJ