Metal-enriched, sub-kiloparsec gas clumps in the circumgalactic medium
  of a faint z = 2.5 galaxy
  We report the serendipitous detection of a 0.2 L$^*$, Lyman-$\alpha$ emitting
galaxy at redshift 2.5 at an impact parameter of 50 kpc from a bright
background QSO sightline. A high-resolution spectrum of the QSO reveals a
partial Lyman-limit absorption system ($N_\mathrm{HI}=10^{16.94\pm0.10}$
cm$^{-2}$) with many associated metal absorption lines at the same redshift as
the foreground galaxy. Using photoionization models that carefully treat
measurement errors and marginalise over uncertainties in the shape and
normalisation of the ionizing radiation spectrum, we derive the total hydrogen
column density $N_\mathrm{H}=10^{19.4\pm0.3}$ cm$^{-2}$, and show that all the
absorbing clouds are metal enriched, with $Z=0.1$-$0.6 Z_\odot$. These
metallicities and the system's large velocity width ($436$ km$\,$s$^{-1}$)
suggest the gas is produced by an outflowing wind. Using an expanding shell
model we estimate a mass outflow rate of $\sim5 M_\odot\,$yr$^{-1}$. Our
photoionization model yields extremely small sizes ($<$100-500 pc) for the
absorbing clouds, which we argue are typical of high column density absorbers
in the circumgalactic medium (CGM). Given these small sizes and extreme
kinematics, it is unclear how the clumps survive in the CGM without being
destroyed by hydrodynamic instabilities. The small cloud sizes imply that even
state-of-the-art cosmological simulations require more than a $1000$-fold
improvement in mass resolution to resolve the hydrodynamics relevant for cool
gas in the CGM.
