Mapping Quasar Light Echoes in 3D with Lyα Forest Tomography

Abstract

The intense radiation emitted by luminous quasars dramatically alters the ionization state of their surrounding IGM. This so-called proximity effect extends out to tens of Mpc, and manifests as large coherent regions of enhanced Lyman-α (Lyα) forest transmission in absorption spectra of background sightlines. Here we present a novel method based on Lyα forest tomography, which is capable of mapping these quasar 'light echoes' in three dimensions. Using a dense grid (10-100) of faint (m r ≈ 24.7 mag) background galaxies as absorption probes, one can measure the ionization state of the IGM in the vicinity of a foreground quasar, yielding detailed information about the quasar's radiative history and emission geometry. An end-to-end analysis -combining cosmological hydrodynamical simulations post-processed with a quasar emission model, realistic estimates of galaxy number densities, and instrument + telescope throughput -is conducted to explore the feasibility of detecting quasar light echoes. We present a new fully Bayesian statistical method that allows one to reconstruct quasar light echoes from thousands of individual low S/N transmission measurements. Armed with this machinery, we undertake an exhaustive parameter study and show that light echoes can be convincingly detected for luminous (M 1450 < −27.5 mag corresponding to m 1450 < 18.4 mag at z 3.6) quasars at redshifts 3 < z QSO < 5, and that a relative precision better than 20 % on the quasar age can be achieved for individual objects, for the expected range of ages between 1 Myr and 100 Myr. The observational requirements are relatively modest -moderate resolution (R 750) multi object spectroscopy at low S/N > 5 is sufficient, requiring three hour integrations using existing instruments on 8m class telescopes.

LATIS: The Lyα Tomography IMACS Survey

“…Although opacity fluctuations in the IGM arise mainly from density fluctuations, they are also affected by inhomogeneities in the ionizing radiation field, which are most profound in the vicinity of quasars. Schmidt et al (2019) showed that radiation from hyperluminous quasars might erase the flux deficit that otherwise would be produced by the surrounding matter overdensity. IGM tomography can then be used to trace quasar light echoes, providing a novel way to age date individual systems.…”

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

“…Quasar flickering can thus create an ionization "echo" in its surroundings. It has long been proposed to use this effect to accurately time the past radiative activity of luminous quasars by using the Lyα opacity toward background sources at small impact parameters (Adelberger 2004;Hennawi et al 2006;Visbal & Croft 2008;Schmidt et al 2019). We are now in a position to attempt such a measurement in practice.…”

Searching for the shadows of giants – II. The effect of local ionization on the Ly α absorption signatures of protoclusters at redshift z ∼ 2.4

“…In our local sources model we have furthermore assumed isotropic AGN emission, and have ignored non-equilibrium ionization and light traveltime effects (e.g. Schmidt et al 2019). Finally, we have adopted a simple model for the distribution of LAEs in the transmission maps that does not include the effect of the local IGM opacity on Ly α emitter visibility.…”

“…Similarly, non-equilibrium photoionization and light traveltime effects due to flickering AGN emission may also result in gas that is less highly ionized (e.g. Oppenheimer & Schaye 2013;Schmidt et al 2019).…”