Abstract
ABSTRACT We interpret observations of intergalactic low ionization metal absorption systems at redshifts z ≳ 5 in terms of pressure-confined clouds. We find clouds confined by the expected pressure of galactic haloes with masses $11\lt \log M_h/h^{-1}\, \mathrm{M}_\odot \lt 12$ provide a good description of the column density ratios between low ionization metal absorbers. Some of the ratios, however, require extending conventional radiative transfer models of irradiated slabs to spherical (or cylindrical) clouds to allow for lines of sight passing outside the cores of the clouds. Moderate depletion of silicon on to dust grains is also indicated in some systems. The chemical abundances inferred span the range between solar and massive-star-dominated stellar populations as may arise in starburst galaxies. The typical H i column densities matching the data correspond to damped Ly α absorbers (DLAs) or sub-DLAs, with sizes of 40 pc to 3 kpc, gas masses 3.5 < log Mc/M⊙ < 8 and metallicites $0.001\!-\!0.01\, \mathrm{Z}_\odot$. Such systems continue to pose a challenge for galaxy-scale numerical simulations to reproduce.
Highlights
Cosmic reionization remains one of the paramount unsolved problems of modern cosmology
In agreement with previous modelling, we find that the low ionisation metal column density ratios require high neutral hydrogen column densities typical of Damped Lymanα Absorbers (DLAs)
The metal absorption lines measured in sub-DLA and DLA systems at z 5 may provide important clues to the origin and nature of galaxies in the first billion years in the Universe
Summary
Cosmic reionization remains one of the paramount unsolved problems of modern cosmology. Observations of high redshift Quasi-Stellar Objects (QSOs) show the intergalactic medium (IGM) was reionized by z = 5 (Becker et al 2015; Bosman et al 2018). Measurements of the Cosmic Microwave Background (CMB) require the IGM to have been ionised no earlier than z 8.4, and suggest a midpoint reionization epoch of zre = 7.7 ± 0.7 (Planck Collaboration 2018). Held to be star-forming galaxies, current evidence shows that the ionising emissivity of observed sources at z > 5 still falls short of what is required (Bouwens et al 2015; Finkelstein et al 2019). Some studies suggest that galaxies hosting active galactic nuclei contribute significantly to the ultraviolet background near the end of the reionization epoch at z < 7
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