Probing quasar lifetimes with proximate 21-centimetre absorption in the diffuse intergalactic medium at redshifts z ≥ 6

Abstract

Enhanced ionizing radiation in close proximity to redshift z ≳ 6 quasars creates short windows of intergalactic Lyα transmission blueward of the quasar Lyα emission lines. The majority of these Lyα near-zones are consistent with quasars that have optically/UV bright lifetimes of $t_{\rm Q}\sim 10^{5}-10^{7}\rm \, yr$. However, lifetimes as short as $t_{\rm Q}\lesssim 10^{4}\rm \, yr$ appear to be required by the smallest Lyα near-zones. These short lifetimes present an apparent challenge for the growth of $\sim 10^{9}\rm \, M_{\odot }$ black holes at z ≳ 6. Accretion over longer timescales is only possible if black holes grow primarily in an obscured phase, or if the quasars are variable on timescales comparable to the equilibriation time for ionized hydrogen. Distinguishing between very young quasars and older quasars that have experienced episodic accretion with Lyα absorption alone is challenging, however. We therefore predict the signature of proximate 21-cm absorption around z ≳ 6 radio-loud quasars. For modest pre-heating of intergalactic hydrogen by the X-ray background, where the spin temperature $T_{\rm S} \lesssim 10^{2}\rm \, K$ prior to any quasar heating, we find proximate 21-cm absorption should be observable in the spectra of radio-loud quasars. The extent of the proximate 21-cm absorption is sensitive to the integrated lifetime of the quasar. Evidence for proximate 21-cm absorption from the diffuse intergalactic medium within $2-3\rm \, pMpc$ of a (radio-loud) quasar would be consistent with a short quasar lifetime, $t_{\rm Q}\lesssim 10^{5}\rm \, yr$, and would provide a complementary constraint on models for high redshift black hole growth.