The first measurement of the quasar lifetime distribution

Abstract

Understanding the growth of the supermassive black holes (SMBH) powering luminous quasars, their co-evolution with host galaxies, and impact on the surrounding intergalactic medium (IGM) depends sensitively on the duration of quasar accretion episodes. Unfortunately, this time-scale, known as the quasar lifetime, tQ, is still uncertain by orders of magnitude (tQ & 0.01 Myr - 1Gyr). However, the extent of the He II Ly α proximity zones in the absorption spectra of zqso ~3-4 quasars constitutes a unique probe, providing sensitivity to lifetimes up to ~30 Myr. Our recent analysis of 22 archival Hubble Space Telescope He II proximity zone spectra reveals a surprisingly broad range of emission time-scales, indicating that some quasars turned on ≤1Myr ago, whereas others have been shining for ≥30 Myr. Determining the underlying quasar lifetime distribution (QLD) from proximity zone measurements is a challenging task owing to: (1) the limited sensitivity of individual measurements; (2) random sampling of the quasar light curves; (3) density fluctuations in the quasar environment; and (4) the inhomogeneous ionization state of He II in a reionizing IGM.We combine a seminumerical He II reionization model, hydrodynamical simulations post-processed with ionizing radiative transfer, and a novel statistical framework to infer the QLD from an ensemble of proximity zone measurements. Assuming a lognormalQLD, we infer amean (log10(tQ/Myr)) = 0.22+0.22-0.25 and standard deviation σlog10tQ = 0.80+0.37-0.27. Our results allow us to estimate the probability of detecting very young quasars with tQ ≤ 0.1Myr from their proximity zone sizes yielding p(≤0.1Myr) = 0.19+0.11-0.09, which is broadly consistent with recent determination at z ~ 6.