The diversity of the circumgalactic medium around z = 0 Milky Way-mass
  galaxies from the Auriga simulations
  Galaxies are surrounded by massive gas reservoirs (i.e. the circumgalactic
medium; CGM) which play a key role in their evolution. The properties of the
CGM, which are dependent on a variety of internal and environmental factors,
are often inferred from absorption line surveys which rely on a limited number
of single lines-of-sight. In this work we present an analysis of 28 galaxy
haloes selected from the Auriga project, a cosmological magneto-hydrodynamical
zoom-in simulation suite of isolated Milky Way-mass galaxies, to understand the
impact of CGM diversity on observational studies. Although the Auriga haloes
are selected to populate a narrow range in halo mass, our work demonstrates
that the CGM of L* galaxies is extremely diverse: column densities of commonly
observed species span ~3-4 dex and their covering fractions range from ~5 to 90
per cent. Despite this diversity, we identify the following correlations: 1)
the covering fractions (CF) of hydrogen and metals of the Auriga haloes
positively correlate with stellar mass, 2) the CF of H I, C IV, and Si II
anticorrelate with active galactic nucleus luminosity due to ionization
effects, and 3) the CF of H I, C IV, and Si II positively correlate with galaxy
disc fraction due to outflows populating the CGM with cool and dense gas. The
Auriga sample demonstrates striking diversity within the CGM of L* galaxies,
which poses a challenge for observations reconstructing CGM characteristics
from limited samples, and also indicates that long-term merger assembly history
and recent star formation are not the dominant sculptors of the CGM.
