2015MNRAS.446...18C

We report the serendipitous detection of a 0.2 L*, Lyα 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_H i=10^16.94±0.10/ cm2) with many associated metal absorption lines at the same redshift as the foreground galaxy. Using photoionization models that carefully treat measurement errors and marginalize over uncertainties in the shape and normalization of the ionizing radiation spectrum, we derive the total hydrogen column density N_H=10^19.4±0.3 cm^-2, and show that all the absorbing clouds are metal enriched, with Z = 0.1-0.6Z_☉. These metallicities and the system's large velocity width (436 kms- 1) suggest the gas is produced by an outflowing wind. Using an expanding shell model we estimate a mass outflow rate of ∼ 5M_☉/yr. Our photoionization model yields extremely small sizes (<100-500 pc) for the absorbing clouds, which we argue is 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.