We report the serendipitous detection of a 0.2 L ^ { * } , Ly \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 ( { \it N } _ { \mathrm { HI } } = 10 ^ { 16.94 \pm 0.10 } 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 { \it N } _ { \mathrm { H } } = 10 ^ { 19.4 \pm 0.3 } ~ { } 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 \sim 5 { \it M } _ { \odot } { yr } ^ { -1 } .
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 .