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