# Clustering of MgII absorption line systems around massive galaxies: an important constraint on feedback processes in galaxy formation

## TL;DR

This study investigates MgII absorption line clustering around massive galaxies, revealing extensive high-velocity gas likely driven out by AGN feedback, challenging existing models and constraining galaxy formation theories.

## Contribution

It provides new observational constraints on the distribution and velocity of MgII absorbers, highlighting the need for feedback processes in galaxy formation models.

## Key findings

- Excess MgII extends out to 20 Mpc around massive galaxies.
- High velocity MgII absorbers are present within the virial radius, independent of galaxy age or activity.
- Standard simulations without mechanical AGN feedback fail to reproduce observed MgII properties.

## Abstract

We use the latest version of the metal line absorption catalogue of Zhu & M\'enard (2013) to study the clustering of MgII absorbers around massive galaxies (~10^11.5 M_sun), quasars and radio-loud AGN with redshifts between 0.4 and 0.75. Clustering is evaluated in two dimensions, by binning absorbers both in projected radius and in velocity separation. Excess MgII is detected around massive galaxies out to R_p=20 Mpc. At projected radii less than 800 kpc, the excess extends out to velocity separations of 10,000 km/s. The extent of the high velocity tail within this radius is independent of the mean stellar age of the galaxy and whether or not it harbours an active galactic nucleus. We interpret our results using the publicly available Illustris and Millennium simulations. Models where the MgII absorbers trace the dark matter particle or subhalo distributions do not fit the data. They overpredict the clustering on small scales and do not reproduce the excess high velocity separation MgII absorbers seen within the virial radius of the halo. The Illustris simulations which include thermal, but not mechanical feedback from AGN, also do not provide an adequate fit to the properties of the cool halo gas within the virial radius. We propose that the large velocity separation MgII absorbers trace gas that has been pushed out of the dark matter halos, possibly by multiple episodes of AGN-driven mechanical feedback acting over long timescales.