Dating individual quasars with the He ii proximity effect

Abstract

ABSTRACT Constraints on the time-scales of quasar activity are key to understanding the formation and growth of supermassive black holes (SMBHs), quasar triggering mechanisms, and possible feedback effects on their host galaxies. However, observational estimates of this so-called quasar lifetime are highly uncertain (tQ ∼ 104–109 yr), because most methods are indirect and involve many model-dependent assumptions. Direct evidence of earlier activity is gained from the higher ionization state of the intergalactic medium (IGM) in the quasar environs, observable as enhanced Ly α transmission in the so-called proximity zone. Due to the ∼30 Myr equilibration time-scale of He ii in the z ∼ 3 IGM, the size of the He ii proximity zone depends on the time the quasar had been active before our observation ton ≤ tQ, enabling up to ±0.2 dex precise measurements of individual quasar on-times that are comparable to the e-folding time-scale tS ∼ 44 Myr of SMBH growth. Here we present the first statistical sample of 13 quasars whose accurate and precise systemic redshifts allow for measurements of sufficiently precise He ii quasar proximity zone sizes between ≃2 and ≃15 proper Mpc from science-grade Hubble Space Telescope (HST) spectra. Comparing these sizes to predictions from cosmological hydrodynamical simulations post-processed with 1D radiative transfer, we infer a broad range of quasar on-times from ton ≲ 1 Myr to ton > 30 Myr that does not depend on quasar luminosity, black hole mass, or Eddington ratio. These results point to episodic quasar activity over a long duty cycle, but do not rule out substantial SMBH growth during phases of radiative inefficiency or obscuration.