On the Search for Quasar Light Echoes

TL;DR

This paper proposes a method to detect quasar light echoes in the intergalactic medium using Ly-alpha forest spectra, enabling insights into quasar lifetimes and luminosities through simulations and observational data analysis.

Contribution

It introduces a novel technique to identify and analyze quasar light echoes in spectra, tested with simulations, to constrain quasar properties and lifetimes.

Findings

Bright quasar echoes could be detectable in current data.

Approximately 10 echoes may be found in a 1 deg^2 survey at 95% confidence.

Null results could still inform quasar luminosity and lifetime models.

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 Ly-alpha 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 Ly-alpha 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 square degree area, we expect that ~10 echoes from quasars with B band luminosities L_B=3x10^45 ergs/s exist that could be found at 95% confidence, assuming a quasar lifetime of ~10^7 yr. Even a null result from such a search would have interesting implications for our understanding of quasar luminosities and lifetimes.