Cosmic histories of star formation and reionization: An analysis with a power-law approximation

With a simple power-law approximation of high-redshift ($\gtrsim3.5$) star formation history, i.e., $\dot{\rho}_*(z)\propto [(1+z)/4.5]^{-\alpha}$, we investigate the reionization of intergalactic medium (IGM) and the consequent Thomson scattering optical depth for cosmic microwave background (CMB) photons. A constraint on the evolution index $\alpha$ is derived from the CMB optical depth measured by the {\it Wilkinson Microwave Anisotropy Probe} (WMAP) experiment, which reads $\alpha\approx2.18\lg{\mathscr{N}_{\gamma}}-3.89$, where the free parameter $\mathscr{N}_\gamma$ is the number of the escaped ionizing ultraviolet photons per baryon. Moreover, the redshift for full reionization, $z_f$, can also be expressed as a function of $\alpha$ as well as $\mathscr{N}_{\gamma}$. By further taking into account the implication of the Gunn-Peterson trough observations to quasars for the full reionization redshift, i.e., $6\lesssim z_f \lesssim7$, we obtain $0.3\lesssim\alpha\lesssim1.3$ and $80\lesssim\mathscr{N}_{\gamma}\lesssim230$. For a typical number of $\sim4000$ of ionizing photons released per baryon of normal stars, the fraction of these photons escaping from the stars, $f_{\rm esc}$, can be constrained to within the range of $(2.0-5.8)$.Comment: 10 pages, 4 figures, accepted for publication in JCA

Modeling the neutral hydrogen distribution in the post-reionization Universe: intensity mapping

We model the distribution of neutral hydrogen (HI) in the post-reionization era and investigate its detectability in 21 cm intensity mapping with future radio telescopes like the Square Kilometer array (SKA). We rely on high resolution hydrodynamical N-body simulations that have a state-of-the-art treatment of the low density photoionized gas in the inter-galactic medium (IGM). The HI is assigned a-posteriori to the gas particles following two different approaches: a halo-based method in which HI is assigned only to gas particles residing within dark matter halos; a particle-based method that assigns HI to all gas particles using a prescription based on the physical properties of the particles. The HI statistical properties are then compared to the observational properties of Damped Lyman-alpha Absorbers (DLAs) and of lower column density systems and reasonable good agreement is found for all the cases. Among the halo-based method, we further consider two different schemes that aim at reproducing the observed properties of DLAs by distributing HI inside halos: one of this results in a much higher bias for DLAs, in agreement with recent observations, which boosts the 21 cm power spectrum by a factor similar to 4 with respect to the other recipe. Furthermore, we quantify the contribution of HI in the diffuse IGM to both Omega(HI) and the HI power spectrum finding to be subdominant in both cases. We compute the 21 cm power spectrum from the simulated HI distribution and calculate the expected signal for both SKAl-mid and SKAl-low configurations at 2.4 = 2.4 with SKAl-mid would instead require a much longer observation time to achieve a comparable SNR level. \ua9 2014 IOP Publishing Ltd and Sissa Medialab srl

The large-scale cross-correlation of Damped Lyman alpha systems with the Lyman alpha forest: first measurements from BOSS

We present the first measurement of the large-scale cross-correlation of Lyα forest absorption and Damped Lyman α systems (DLA), using the 9th Data Release of the Baryon Oscillation Spectroscopic Survey (BOSS). The cross-correlation is clearly detected on scales up to 40h-1Mpc and is well fitted by the linear theory prediction of the standard Cold Dark Matter model of structure formation with the expected redshift distortions, confirming its origin in the gravitational evolution of structure. The amplitude of the DLA-Lyα cross-correlation depends on only one free parameter, the bias factor of the DLA systems, once the Lyα forest bias factors are known from independent Lyα forest correlation measurements. We measure the DLA bias factor to be bD = (2.17±0.20)βF0.22, where the Lyα forest redshift distortion parameter βF is expected to be above unity. This bias factor implies a typical host halo mass for DLAs that is much larger than expected in present DLA models, and is reproduced if the DLA cross section scales with halo mass as Mhα, with α = 1.1±0.1 for βF = 1. Matching the observed DLA bias factor and rate of incidence requires that atomic gas remains extended in massive halos over larger areas than predicted in present simulations of galaxy formation, with typical DLA proper sizes larger than 20 kpc in host halos of masses ~ 1012Msun. We infer that typical galaxies at z simeq 2 to 3 are surrounded by systems of atomic clouds that are much more extended than the luminous parts of galaxies and contain ~ 10% of the baryons in the host halo