A Fundamental Test for Galaxy Formation Models: Matching the Lyman-$\alpha$ Absorption Profiles of Galactic Halos over Three Decades in Distance

TL;DR

This paper compares hydrodynamical simulations with observations of Lyman-alpha absorption around galaxies to test galaxy formation models across a wide range of scales, highlighting the importance of gas physics and the potential for precise discrimination between models.

Contribution

It introduces a method to constrain galaxy formation physics by matching simulated and observed Lyman-alpha absorption profiles over three decades in distance.

Findings

Simulations agree with observations beyond 2 Mpc, validating the ΛCDM model for the IGM.

Significant differences between models and observations on 20 kpc to 2 Mpc scales.

Lyman-alpha absorption primarily probes the temperature-density relationship of circumgalactic gas.

Abstract

Galaxy formation depends critically on the physical state of gas in the circumgalactic medium (CGM) and its interface with the intergalactic medium (IGM), determined by the complex interplay between inflows from the IGM and outflows from supernovae or AGN feedback. The average Lyman-alpha (Ly-a) absorption profile around galactic halos represents a powerful tool to probe their gaseous environments. We compare predictions from Illustris and Nyx hydrodynamical simulations with the observed absorption around foreground quasars, damped Ly-a systems, and Lyman-break galaxies. We show how large-scale BOSS and small-scale quasar pair measurements can be combined to precisely constrain the absorption profile over three decades in transverse distance 20kpc$\lesssim b\lesssim$20Mpc. Far from galaxies $\gtrsim2$Mpc, the simulations converge to the same profile and provide a reasonable match to the observations. This asymptotic agreement arises because the $\Lambda$CDM model successfully describes the ambient IGM, and represents a critical advantage of studying the mean absorption profile. However, significant differences between the simulations, and between simulations and observations are present on scales 20kpc$\lesssim b\lesssim$2Mpc, illustrating the challenges of accurately modeling and resolving galaxy formation physics. It is noteworthy that these differences are observed as far out as $\sim2$Mpc, indicating that the `sphere-of-influence' of galaxies could extend to approximately $\sim7$ times the halo virial radius ($\sim100$kpc). Current observations are very precise on these scales and can thus strongly discriminate between different galaxy formation models. We demonstrate that the Ly-a absorption profile is primarily sensitive to the underlying temperature-density relationship of diffuse gas around galaxies, and argue that it thus provides a fundamental test of galaxy formation models.