Evolution of neutral oxygen during the epoch of reionization and its use in estimating the neutral hydrogen fraction

Abstract

We use synthetic sightlines drawn through snapshots of the Technicolor Dawn simulations to explore how the statistics of neutral oxygen (O i) absorbers respond to hydrogen reionization. The ionization state of the circumgalactic medium (CGM) initially roughly tracks that of the intergalactic medium, but beginning at z = 8 the CGM grows systematically more neutral owing to self-shielding. Weak absorbers trace diffuse gas that lies farther from halos, hence they are ionized first, whereas stronger systems are less sensitive to reionization. The overall O i covering fraction decreases slowly with time owing to competition between ongoing enrichment and gradual encroachment of ionization fronts into increasingly overdense gas. While the declining covering fraction is partially offset by continued formation of new halos, the ionization of the diffuse gas causes the predicted line-of-sight incidence rate of O i absorbers to decline abruptly at the overlap epoch, in qualitative agreement with observations. In comparison to the recently-observed equivalent width (EW) distribution at z ≈ 6, the simulations underproduce systems with EW ≥ 0.1Å, although they reproduce weaker systems with EW ≥ 0.05Å. By z ≈ 5, the incidence of EW < 0.1Å systems are overproduced, consistent with previous indications that the simulated ionizing background is too weak at z < 6. The summed column densities of Si ii and Si iv trace the total oxygen column, and hence the ratio of the O i and Si ii + Si iv comoving mass densities traces the progress of reionization. This probe may prove particularly useful in the regime where x H i > 10%.

The mean free path of ionizing photons at 5 < z < 6: evidence for rapid evolution near reionization

“…A number of observations now suggest that reionization had a midpoint around 𝑧 ∼7-8 and ended near 𝑧 ∼ 6, or even later. These include (i) the electron optical depth to CMB photons (Planck Collaboration et al 2020), (ii) the decline in Ly𝛼 emission ★ E-mail: george.becker@ucr.edu from galaxies at 𝑧 > 6 (e.g., Jung et al 2020;Morales et al 2021, and references therein), (iii) large-scale opacity fluctuations in the Ly𝛼 forest at 𝑧 < 6 (Fan et al 2006;Becker et al 2015;Bosman et al 2018;Eilers et al 2018;Yang et al 2020), (iv) the association of large Ly𝛼 troughs at 𝑧 ∼ 5.7 with galaxy underdensities (Becker et al 2018;Kashino et al 2020), (v) Ly𝛼 damping wings seen in the spectra of 𝑧 ∼ 7 QSOs (Mortlock et al 2011;Greig et al 2017Greig et al , 2019Davies et al 2018;Bañados et al 2018;Wang et al 2020), (vi) the thermal history of the IGM at 𝑧 > 5 (Boera et al 2019;Walther et al 2019;Gaikwad et al 2020), and (vii) the evolution in the number density of neutral metal absorbers near 𝑧 ∼ 6 (Becker et al 2011(Becker et al , 2019Cooper et al 2019;Doughty & Finlator 2019). If reionization did indeed end near or below 𝑧 = 6 then the mean free path at 𝑧 < 6, even in fully ionized regions, should increase rapidly with time as the absorbers are evaporated or pressure smoothed from photoionization heating.…”

The mean free path of ionizing photons at 5 &lt; z &lt; 6: evidence for rapid evolution near reionization

“…galaxy underdensities (Becker et al 2018;Kashino et al 2020), (v) Ly α damping wings seen in the spectra of z ∼ 7 QSOs (Mortlock et al 2011;Greig et al 2017;Bañados et al 2018;Davies et al 2018;Greig, Mesinger & Bañados 2019;Wang et al 2020), (vi) the thermal history of the IGM at z > 5 (Boera et al 2019;Walther et al 2019;Gaikwad et al 2020), and (vii) the evolution in the number density of neutral metal absorbers near z ∼ 6 (Becker et al 2011(Becker et al , 2019Cooper et al 2019;Doughty & Finlator 2019). If reionization did end near or below z = 6, then the mean free path at z < 6 should increase rapidly with time as large H II bubbles merge and the last remaining neutral islands are ionized (e.g.…”

Evolution of CIV Absorbers I. The Cosmic Incidence

“…The TD simulations model galaxy growth and the baryon cycle, while self-consistently modeling an evolving multifrequency, spatially-inhomogeneous UVB. Though the TD simulations accurately reproduce observations of the galaxy stellar mass function, the co-moving path density of Cii and Siiv absorbers, and the mean transmission in the Ly α forest, at z ≥ 5 they underproduce the dN/dX of strong C iv absorbers (Finlator et al 2018;Doughty & Finlator 2019;Finlator et al 2020). Similar results have been reported for the Eagle simulations (Rahmati et al 2016).…”