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We report the serendipitous detection of a 0.2 L$^*$, Lyman-$\alpha$ emittinggalaxy at redshift 2.5 at an impact parameter of 50 kpc from a brightbackground QSO sightline. A high-resolution spectrum of the QSO reveals apartial Lyman-limit absorption system ($N_\mathrm{HI}=10^{16.94\pm0.10}$cm$^{-2}$) with many associated metal absorption lines at the same redshift asthe foreground galaxy. Using photoionization models that carefully treatmeasurement errors and marginalise over uncertainties in the shape andnormalisation of the ionizing radiation spectrum, we derive the total hydrogencolumn density $N_\mathrm{H}=10^{19.4\pm0.3}$ cm$^{-2}$, and show that all theabsorbing clouds are metal enriched, with $Z=0.1$-$0.6 Z_\odot$. Thesemetallicities and the system's large velocity width ($436$ km$\,$s$^{-1}$)suggest the gas is produced by an outflowing wind. Using an expanding shellmodel we estimate a mass outflow rate of $\sim5 M_\odot\,$yr$^{-1}$. Ourphotoionization model yields extremely small sizes ($<$100-500 pc) for theabsorbing clouds, which we argue are typical of high column density absorbersin the circumgalactic medium (CGM). Given these small sizes and extremekinematics, it is unclear how the clumps survive in the CGM without beingdestroyed by hydrodynamic instabilities. The small cloud sizes imply that evenstate-of-the-art cosmological simulations require more than a $1000$-foldimprovement in mass resolution to resolve the hydrodynamics relevant for coolgas in the CGM.

Metal-enriched, sub-kiloparsec gas clumps in the circumgalactic medium of a faint z = 2.5 galaxy