The impact of local stellar radiation on the H i column density distribution

Abstract

It is often assumed that local sources of ionizing radiation have little impact on the distribution of neutral hydrogen in the post-reionization Universe. While this is a good assumption for the intergalactic medium, analytic arguments suggest that local sources may typically be more important than the meta-galactic background radiation for high column density absorbers (N HI > 10 17 cm −2 ). We post-process cosmological, hydrodynamical simulations with accurate radiation transport to investigate the impact of local stellar sources on the column density distribution function of neutral hydrogen. We demonstrate that the limited numerical resolution and the simplified treatment of the interstellar medium (ISM) that are typical of the current generation of cosmological simulations provide significant challenges, but that many of the problems can be overcome by taking two steps. First, using ISM particles rather than stellar particles as sources results in a much better sampling of the source distribution, effectively mimicking higher-resolution simulations. Second, by rescaling the source luminosities so that the amount of radiation escaping into the intergalactic medium agrees with that required to produce the observed background radiation, many of the results become insensitive to errors in the predicted fraction of the radiation that escapes the immediate vicinity of the sources. By adopting this strategy and by drastically varying the assumptions about the structure of the unresolved ISM, we conclude that we can robustly estimate the effect of local sources for column densities N HI ≪ 10 21 cm −2 . However, neither the escape fraction of ionizing radiation nor the effect of local sources on the abundance of N HI 10 21 cm −2 systems can be predicted with confidence. We find that local stellar radiation is unimportant for N HI ≪ 10 17 cm −2 , but that it can affect Lyman Limit and Damped Lyα systems. For 10 18 < N HI < 10 21 cm −2 the impact of local sources increases with redshift. At z = 5 the abundance of absorbers is substantially reduced for N HI ≫ 10 17 cm −2 , but at z = 0 the effect only becomes significant for N HI 10 21 cm −2 .

Cosmic distribution of highly ionized metals and their physical conditions in the EAGLE simulations

“…In addition, the neglect of inhomogeneity in the UVB radiation close to local sources of ionization radiation can be considered as another caveat. We note, however, that local sources are only thought to be important for absorbers as rare as Lyman limit systems (e.g., Schaye 2006;Rahmati et al 2013b) and will therefore possibly only affect absorbers with the highest ion column densities. However, as Oppenheimer & Schaye (2013b) showed, non-equilibrium effects in the proximity of the fossil AGNs can enhance the ion fraction of higher ionization stages such as Ovi and Neviii while reducing the abundances of lower ionization ions such as Civ.…”

Witnessing galaxy assembly in an extended z≈3 structure

“…As discussed in the literature (e.g. Fumagalli et al 2011;Rahmati et al 2013), while local sources of radiation may alter the cross section of individual absorbers, extended DLAs next to galaxies persist even after including local sources. This implies that associations between DLAs and galaxies in simulations are generally robust even without inclusion of local sources during the radiative transfer post-processing.…”

“…While local radiation is clearly relevant to model in detail the particle-by-particle ionization state of the gas and its emissivity near galaxies, in practice, the inclusion of radiative transfer effects from local sources is subject to uncertainties related to the unresolved structure of the interstellar medium within large cosmological simulations. These uncertainties are particularly significant at high column densities (N HI 10 21 cm −2 ; Rahmati et al 2013), which are relevant for this work.…”

Neutral hydrogen in galaxy clusters: impact of AGN feedback and implications for intensity mapping

“…We interpolate to obtain the value of the above parameters for redshifts not covered by their Table A1. We notice that we are neglecting radiation from local sources (Miralda-Escudé 2005;Schaye 2006;Rahmati et al 2013b) and local X-ray radiation from the hot intracluster medium (ICM) (Kannan et al 2015) when computing the mass in neutral hydrogen. In Sec.…”