On the Search for Quasar Light Echoes

Abstract

The UV radiation from a quasar leaves a characteristic pattern in the distribution of ionized hydrogen throughout the surrounding space. This pattern or light echo propagates through the intergalactic medium at the speed of light, and can be observed by its imprint on the Lyα forest spectra of background sources. As the echo persists after the quasar has switched off, it offers the possibility of searching for dead quasars, and constraining their luminosities and lifetimes. We outline a technique to search for and characterize these light echoes. To test the method, we create artificial Lyα forest spectra from cosmological simulations at z=3, apply light echoes and search for them. We show how the simulations can also be used to quantify the significance level of any detection. We find that light echoes from the brightest quasars could be found in observational data. With absorption-line spectra of 100 redshift z~3-3.5 quasars or galaxies in a 1 deg2 area, we expect that ~10 echoes from quasars with B-band luminosities LB=3×1045 ergs-1 exist that could be found at 95% confidence, assuming a quasar lifetime of ~107 yr. Even a null result from such a search would have interesting implications for our understanding of quasar luminosities and lifetimes.