ALBERT

Description: We investigate the recent claim of ‘photon underproduction crisis’ by Kollmeier et al. which suggests that the known sources of ultraviolet (UV) radiation may not be sufficient to generate the inferred H i photoionization rate ( $\Gamma _{\rm H\,\small {i}}$ ) in the low-redshift intergalactic medium. Using the updated QSO emissivities from the recent studies and our cosmological radiative transfer code developed to estimate the UV background, we show that the QSO contributions to $\Gamma _{\rm H\,\small {i}}$ is higher by a factor ~2 as compared to the previous estimates. Using self-consistently computed combinations of star formation rate density and dust attenuation, we show that a typical UV escape fraction of 4 per cent from star-forming galaxies should be sufficient to explain the inferred $\Gamma _{\rm H\,\small {i}}$ by Kollmeier et al. Interestingly, we find that the contribution from QSOs alone can explain the recently inferred $\Gamma _{\rm H\,\small {i}}$ by Shull et al. which used the same observational data but different simulation. Therefore, we conclude that the crisis is not as severe as it was perceived before and there seems no need to look for alternate explanations such as low luminosity hidden QSOs or decaying dark matter particles.

Description: Using our cosmological radiative transfer code, we study the implications of the updated quasi-stellar object (QSO) emissivity and star formation history for the escape fraction ( f esc ) of hydrogen ionizing photons from galaxies. We estimate the f esc that is required to reionize the Universe and to maintain the ionization state of the intergalactic medium in the post-reionization era. At z  〉 5.5, we show that a constant f esc of 0.14–0.22 is sufficient to reionize the Universe. At z  〈 3.5, consistent with various observations, we find that f esc can have values from 0 to 0.05. However, a steep rise in f esc , of at least a factor of ~3, is required between z  = 3.5 and 5.5. It results from a rapidly decreasing QSO emissivity at z  〉 3 together with a nearly constant measured H  i photoionization rates at 3 〈  z  〈 5. We show that this requirement of a steep rise in f esc over a very short time can be relaxed if we consider the contribution from a recently found large number density of faint QSOs at z ≥ 4. In addition, a simple extrapolation of the contribution of such QSOs to high- z suggests that QSOs alone can reionize the Universe. This implies, at z  〉 3.5, that either the properties of galaxies should evolve rapidly to increase the f esc or most of the low-mass galaxies should host massive black holes and sustain accretion over a prolonged period. These results motivate a careful investigation of theoretical predictions of these alternate scenarios that can be distinguished using future observations. Moreover, it is also very important to revisit the measurements of H  i photoionization rates that are crucial to the analysis presented here.

Description: We report the detection of two O vi absorbers at z = 0.416 14 and 0.419 50 (| v | = 710 km s –1 ), towards SBS 0957+599. Both absorbers are multiphase systems tracing substantial reservoirs of warm baryons. The low- and intermediate-ionization metals in the z = 0.416 14 absorber are consistent with an origin in photoionized gas. O vi has a velocity structure different from other metal species. Ly α shows the presence of a broad feature. The linewidths for O vi and the broad Ly α suggest T = 7.1 x 10 5  K. This warm medium is probing a baryonic column, which is an order of magnitude more than the total hydrogen in the cooler photoionized gas. The second absorber is detected only in H i and O vi . Here a temperature of 4.6 x 10 4  K supports O vi originating in a low-density photoionized gas. A broad component is seen in Ly α, offset from O vi . The temperature in the broad Ly α is T 2.1 x 10 5  K. The absorbers reside in a galaxy overdensity region with seven spectroscopically identified galaxies within ~10 Mpc and v ~ 1000 km s –1 of the z = 0.416 14 absorber, and two galaxies inside a similar separation from the z = 0.419 50 absorber. The distribution of galaxies relative to the absorbers suggests that the line of sight could be intercepting a large-scale filament connecting galaxy groups, or the extended halo of a sub- L * galaxy. Though kinematically proximate, the two absorbers reaffirm the diversity in the physical conditions of low red-shift O vi systems and the galactic environments they inhabit.