# AGN feedback & the Lyα Forest

I showed that AGN feedback could have a measurable effect on the statistics of the low redshift Lyman-Alpha Forest. ADS link

When UV light is emitted from a distant quasar it is, invariably, eclipsed by intervening clouds of neutral Hydrogen gas. This gas leaves an absorption fingerprint that is, due to the cosmological expansion of the universe, redshifted to different wavelengths along the spectrum. The collection of Lyman-α rep absorption features that have been redshifted in this way in these spectra is referred to as the Lyman-α Forest. The column density distribution (CDD) is a statistic that summarizes the number of absorbers of a given column density along an average line of sight.

In 2014, the observed values of the CDD were substantially lower than those that were predicted by simulations– i.e. there was almost 5x as much neutral Hydrogen gas in the simulations as there is in the real universe. This led to a so-called “Photon Under-Production Crisis,” wherein the problem was assumed that with a larger ionizing flux the neutral Hydrogen could be ionized to a state that would not contribute to the Lyman-α Forest. In response to this hypothesis, I led a study that sought to understand whether AGN feedback has the potential to solve the Photon Under-Production Crisis” by both ionizing and heating gas. Using the Illustris cosmological simulation we found relatively good agreement with the most recent HST COS observations. Using other simulations with variations of the AGN feedback model of different strengths, we showed that the offset in the CDD found in previous studies can be overcome by changing the strength of the AGN feedback in concert with the ionizing ultraviolet background. We conclude that between the two there is likely sufficient freedom within the observations constraining the UVB and the strength of AGN feedback to resolve the Photon Under-Production Crisis.

For more details, see the paper.

I also helped with a follow-up paper using the results of Illustris-TNG