The evolution of cold neutral gas and the star formation history

Abstract

There is a well known disparity between the evolution the star formation rate density, ψ * , and the abundance of neutral hydrogen (H I), the raw material for star formation. Recently, however, we have shown that ψ * may be correlated with the fraction of cool atomic gas, as traced through the 21-cm absorption of H I. This is expected since star formation requires cold (T ∼ 10 K) gas and so this could address the issue of why the star formation rate density does not trace the bulk atomic gas. The data are, however, limited to redshifts of z < ∼ 2, where both ψ * and the cold gas fraction exhibit a similar steep climb from the present day (z = 0), and so it is unknown whether the cold gas fraction follows the same decline as ψ * at higher redshift. In order to address this, we have used unpublished archival observations of 21-cm absorption in high redshift damped Lyman-α absorption systems to increase the sample at z > ∼ 2. The data suggest that the cold gas fraction does exhibit a decrease, although this is significantly steeper than ψ * at z ∼ 3. This is, however, degenerate with the extents of the absorbing galaxy and the background continuum emission and upon removing these, via canonical evolution models, we find the mean spin temperature of the gas to be T spin ≈ 3000 K, compared to the ≈ 2000 K expected from the ψ * T spin ≈ 100 M ⊙ yr −1 Mpc −3 K fit at z < ∼ 2. These temperatures are consistent with the observed high neutral hydrogen column densities, which require T < ∼ 4000 K in order for the gas not to be highly ionised.

Super

“…It is particularly important to study the impact that such galactic-scale AGN-driven outflows may have on their host galaxies at z ∼ 2 where both the volume-averaged star formation rate and the black hole growth rate peak (e.g. Shankar et al 2009;Madau & Dickinson 2014;Curran 2019;Wilkins et al 2019;Tacconi et al 2020). Tremendous progress has been made through integral field unit (IFU) spectroscopy, which provides spatially resolved information on the structure and extension of the outflows (e.g.…”

FLASH early science – discovery of an intervening H i 21-cm absorber from an ASKAP survey of the GAMA 23 field

“…Rafelski et al 2012;Cooke et al 2015;De Cia et al 2018) lacked sufficient coolants in the gas to form a significant fraction of CNM via fine structure line cooling. Recently, by explicitly modelling the source covering fraction as a function of angular diameter distance, Curran (2019Curran ( , 2017 used literature searches for redshifted 21-cm absorption to show that the spin temperature has evolved with the star formation rate history of the Universe. However, we caution that this uses an evolutionary model for the covering fraction of radio sources that would mimic any perceived evolution in the spin temperature and so future model-independent methods are required to verify such a claim.…”

The evolution of neutral hydrogen over the past 11 Gyr via HI 21 cm absorption

“…Absorbers at higher redshifts will suffer from geometric effects more and lower the incidence of the absorber covering the background source (Curran 2012). Like-wise, spin temperature may also be systematically higher at higher redshifts, decreasing the measured absorber optical depth with increasing redshift Srianand et al 2012;Roy et al 2013;Curran 2019). Neither the spin temperature of the absorber nor the covering factor of the background source is constrained for most H 21 cm absorption systems.…”

“…This requires the assumption that Lyα and 21 cm absorption trace the same sight line, which may not always be valid (see, e.g., Kanekar et al 2007). The few intervening absorbers with observations of both 21 cm absorption and Lyα absorption reveal a broad range in the measured spin temperature of the absorbing gas, from 100 K to nearly 10,000 K (e.g., Wolfe & Briggs 1981;York et al 2007;Roy et al 2013;Kanekar et al 2014;Dutta et al 2017;Curran 2019). Combining our large intervening H 21 cm absorption study with prior Lyα and 21 cm emission studies potentially provides the ability to constrain the largely uncertain spin temperature and covering factor of the absorbers through means of comparison with prior measurements of f (N HI , X) and Ω HI .…”