Time-dependent behaviour of quasar proximity zones at z ∼ 6

Abstract

Since the discovery of z ∼ 6 quasars two decades ago, studies of their Lyα-transparent proximity zones have largely focused on their utility as a probe of cosmic reionization. But even when in a highly ionized intergalactic medium, these zones provide a rich laboratory for determining the timescales that govern quasar activity and the concomitant growth of their supermassive black holes. In this work, we use a suite of 1D radiative transfer simulations of quasar proximity zones to explore their timedependent behaviour for activity timescales from ∼ 10 3 -10 8 years. The sizes of the simulated proximity zones, as quantified by the distance at which the smoothed Lyα transmission drops below 10% (denoted R p ), are in excellent agreement with observations, with the exception of a handful of particularly small zones that have been attributed to extremely short 10 4 lifetimes. We develop a physically motivated semi-analytic model of proximity zones which captures the bulk of their equilibrium and non-equilibrium behaviour, and use this model to investigate how quasar variability on < ∼ 10 5 year timescales is imprinted on the distribution of observed proximity zone sizes. We show that large variations in the ionizing luminosity of quasars on timescales of < ∼ 10 4 years are disfavored based on the good agreement between the observed distribution of R p and our model prediction based on "lightbulb" (i.e. steady constant emission) light curves.

Three Lyα Emitting Galaxies within a Quasar Proximity Zone at z ∼ 5.8

“…timescales (see also Eilers et al 2017Eilers et al , 2020Davies et al 2020). While many uncertainties remain, such findings seem to align with the above picture.…”

Detecting and Characterizing Young Quasars. I. Systemic Redshifts and Proximity Zone Measurements

“…It requires at least 16 e-foldings, i.e., 7×10 8 yr, in order to grow a billion-solar-mass black hole from an initial stellar remnant black hole seed with M seed ∼100 M e , even if they accrete continuously at the Eddington limit (e.g., Volonteri 2010Volonteri , 2012. However, it is currently unknown whether quasars obey this exponential light curve, or if other physics related to the triggering of quasar activity and the supply of fuel complicate this simple picture, giving rise to much more complex light curves (e.g., Di Matteo et al 2005;Hopkins et al 2005;Springel et al 2005;Novak et al 2011;Davies et al 2020). These timescales required for the growth of SMBHs are comparable to the age of the universe at z6.…”

“…We recently showed how the extent of the proximity zones around high-redshift quasars provides a new and independent constraint on the lifetime of quasars (Eilers et al 2017a(Eilers et al , 2018bDavies et al 2019;Khrykin et al 2019). These regions of enhanced transmitted flux within the Lyα forest in the immediate vicinity of the quasars have been ionized by the quasar's intense radiation itself (e.g., Bajtlik et al 1988;Haiman & Cen 2001;Wyithe et al 2005;Bolton & Haehnelt 2007a;Lidz et al 2007;Bolton et al 2011;Keating et al 2015) and are sensitive to the lifetime of the quasars because intergalactic gas has a finite response time to the quasars' radiation (e.g., Khrykin et al 2016;Eilers et al 2017a;Davies et al 2020). The equilibration timescale t eq describes the time when the IGM has reached ionization equilibrium with the ionizing photons emitted by the quasar, i.e., » Gt eq H 1 I , where Γ H I denotes the total photoionization rate from the quasar as well as the ultraviolet background.…”

“…However, the quasar's radiation dominates the radiation field within the proximity zone, which has been observationally defined as the location at which the smoothed, continuum-normalized transmitted flux drops below the 10% level (Fan et al 2006). A photoionization rate of Γ H I ≈10 −12 s −1 from the quasar's radiation at the "edge" of the proximity zone at z≈6 leads to an equilibration timescale of t eq ≈3×10 4 yr (Davies et al 2020).…”

SMBH seeds from dissipative dark matter

“…However, even in these models, an Eddington-limit accretion has to be sustained for most of the lifetime of the seeds, which implies a very high duty-cycle of SMBHs in the early Universe. Thus, such a scenario is hard to reconcile with some of the massive quasars at z 6 with low measured Eddington ratios [26,27] as well as the short quasar lifetimes (∼ 10 4−5 yr) found in observations of quasar proximity zones at z ∼ 6 [28][29][30][31][32].…”