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The lifetime of quasars is fundamental for understanding the growth of supermassive black holes, and is an important ingredient in models of the reionization of the intergalactic medium. However, despite various attempts to determine quasar lifetimes, current estimates from a variety of methods are uncertain by orders of magnitude. This work combines cosmological hydrodynamical simulations and 1D radiative transfer to investigate the structure and evolution of the He II Ly$alpha$ proximity zones around quasars at $z simeq 3-4$. We show that the time evolution in the proximity zone can be described by a simple analytical model for the approach of the He II fraction $x_{rm HeII}left( t right)$ to ionization equilibrium, and use this picture to illustrate how the transmission profile depends on the quasar lifetime, quasar UV luminosity, and the ionization state of helium in the ambient IGM (i.e. the average He II fraction, or equivalently the metagalactic He II ionizing background). A significant degeneracy exists between the lifetime and the average He II fraction, however the latter can be determined from measurements of the He II Ly$alpha$ optical depth far from quasars, allowing the lifetime to be measured. We advocate stacking existing He II quasar spectra at $zsim 3$, and show that the shape of this average proximity zone profile is sensitive to lifetimes as long as $sim 30$ Myr. At higher redshift $zsim 4$ where the He II fraction is poorly constrained, degeneracies will make it challenging to determine these parameters independently. Our analytical model for He II proximity zones should also provide a useful description of the properties of H I proximity zones around quasars at $z simeq 6-7$.
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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 (t_Q~10^4-10^9 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$alpha$ transmission in the so-called proximity zone. Due to the ~30 Myr equilibration time-scale of HeII in the z~3 IGM, the size of the HeII proximity zone depends on the time the quasar had been active before our observation t_on<t_Q, enabling up to $pm$0.2 dex precise measurements of individual quasar on-times that are comparable to the e-folding time-scale t_S~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 HeII 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 one-dimensional radiative transfer, we infer a broad range of quasar on-times from t_on<1 Myr to t_on>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.
Quasars and active galactic nuclei (AGN) are significant contributors to the metagalactic ionizing background at redshifts z < 3. Recent HST/COS composite spectra of AGN find a harder flux distribution in the Lyman continuum, F_nu ~ nu^{-alpha_s} (alpha_s = 1.41+/-0.15) compared to previous studies. This index appears to be inconsistent with observed He II / H I absorption ratios (eta) in the Lya forest. We explore effects of internal AGN absorption in the He II (4 ryd) continuum using an analytic source-function model of the ionizing background in which the emissivity (j_nu) arises from quasars, reprocessed by the opacity (kappa_nu) of the intervening Lya forest and distinct AGN escape fractions f_esc(HI) and f_esc(HeII) at 1~ryd and 4~ryd, respectively. We also examine H I and He II photoelectric heating from the reprocessed background, whose spectral index (alpha_b > alpha_s) depends on alpha_s, the H I column density slope beta, and the ratio R = f_esc(HI) / f_esc(HeII). We compare the model to Lya absorption lines of He II and H I toward the quasar HE~2347-4342. Internal AGN absorption with f_esc(HeII) = 0.6-0.8 but f_esc(HI) = 1 would increase the index by Delta-alpha_b = 0.3-1.0, corresponding to eta = 60-200 for beta = 1.5-1.6, in agreement with HST/COS observations at z = 2.5-2.9. The observed range of ratios, eta < 200, constrains alpha_b < 3.4 and f_esc(HeII) > 0.4. Individual AGN with softer spectra, alpha_s > 1.7, and more internal He II absorption could produce a few absorbers with eta > 300 in proximity to AGN transverse to the sight line.
The lifetime of quasars can be estimated by means of their proximity zone sizes, which are regions of enhanced flux bluewards of the Lyman-$alpha$ emission line observed in the rest-frame UV spectra of high-redshift quasars, because the intergalactic gas has a finite response time to the quasars radiation. We estimate the effective lifetime of the high-redshift quasar population from the composite transmitted flux profile within the proximity zone region of a sample of $15$ quasars at $5.8leq zleq 6.6$ with precise systemic redshifts, and similar luminosities, i.e. $-27.6leq M_{1450}leq-26.4$, and thus a similar instantaneous ionizing power. We develop a Bayesian method to infer the effective lifetime from the composite spectrum, including robust estimates of various sources of uncertainty on the spectrum. We estimate an effective lifetime of the quasar population as a whole of $log_{10}(t_{Q}/{yr}) = 5.7^{+0.5 (+0.8)}_{-0.3 (-0.5)}$ given by the median and $68$th ($95$th) percentile of the posterior probability distribution. While our result is consistent with previous quasar lifetime studies, it poses significant challenges on the current model for the growth of supermassive black holes (SMBHs) located in the center of the quasars host galaxies, which requires that quasar lifetimes are more than an order of magnitude longer.
We constrain the average episodic quasar lifetime (as in steady-state accretion) using two statistics of quasars that are recently turned off (i.e., dimmed by a large factor): 1) the fraction of turned-off quasars in a statistical sample photometrically observed over an extended period (e.g., $Delta t=20$ yrs); 2) the fraction of massive galaxies that show orphan broad MgII emission, argued to be short-lived echoes of recently turned-off quasars. The two statistics constrain the average episodic quasar lifetime to be hundreds to thousands of years. Much longer (or shorter) episodic lifetimes are strongly disfavored by these observations. This average episodic lifetime is broadly consistent with the infall timescale (viscous time) in the standard accretion disk model for quasars, suggesting that quasar episodes are governed by accretion disk physics rather than by the gas supply on much larger scales. Compared with the cumulative quasar lifetime of $sim 10^6-10^8,$yrs constrained from quasar clustering and massive black hole demographics, our results suggest that there are $sim 10^3-10^5$ episodes of quasar accretion during the assembly history of the supermassive black hole. Such short episodes should be clustered over intervals of $sim 10^4,$yrs to account for the sizes of ionized narrow-line regions in quasars. Our statistical argument also dictates that there will always be a small fraction of extreme variability quasars caught in state transitions over multi-year observing windows, despite the much longer episodic lifetime. These transitions could occur in a rather abrupt fashion during non-steady accretion.
We investigate the strength of ultraviolet Fe II emission in fainter quasars compared with brighter quasars for 1.0 <= z <= 1.8, using the SDSS (Sloan Digital Sky Survey) DR7QSO catalogue and spectra of Schneider et al., and the SFQS (SDSS Faint Quasar Survey) catalogue and spectra of Jiang et al. We quantify the strength of the UV Fe II emission using the W2400 equivalent width of Weymann et al., which is defined between two rest-frame continuum windows at 2240-2255 and 2665-2695 Ang. The main results are the following. (1) We find that for W2400 >~ 25 Ang. there is a universal (i.e. for quasars in general) strengthening of W2400 with decreasing intrinsic luminosity, L3000. (2) In conjunction with previous work by Clowes et al., we find that there is a further, differential, strengthening of W2400 with decreasing L3000 for those quasars that are members of Large Quasar Groups (LQGs). (3) We find that increasingly strong W2400 tends to be associated with decreasing FWHM of the neighbouring Mg II {lambda}2798 broad emission line. (4) We suggest that the dependence of W2400 on L3000 arises from Ly{alpha} fluorescence. (5) We find that stronger W2400 tends to be associated with smaller virial estimates from Shen et al. of the mass of the central black hole, by a factor ~ 2 between the ultrastrong emitters and the weak. Stronger W2400 emission would correspond to smaller black holes that are still growing. The differential effect for LQG members might then arise from preferentially younger quasars in the LQG environments.
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