The circumgalactic medium in Lyman-alpha: a new constraint on galactic
  outflow models
  Galactic outflows are critical to our understanding of galaxy formation and
evolution. However the details of the underlying feedback process remain
unclear. We compare Ly$\alpha$ observations of the circumgalactic medium (CGM)
of Lyman Break Galaxies (LBGs) with mock observations of their simulated CGM.
We use cosmological hydrodynamical `zoom-in' simulations of an LBG which
contains strong, momentum-driven galactic outflows. Simulation snapshots at
$z=2.2$ and $z=2.65$ are used, corresponding to the available observational
data. The simulation is post-processed with the radiative transfer code
\textsc{crash} to account for the impact of ionising photons on hydrogen gas
surrounding the simulated LBG. We generate mock absorption line maps for
comparison with data derived from observed close galaxy-galaxy pairs. We
perform calculations of Ly$\alpha$ photons scattering through the CGM with our
newly developed Monte-Carlo code \textsc{slaf}, and compare to observations of
diffuse Ly$\alpha$ halos around LBGs. Our fiducial galactic outflow model comes
closer to reproducing currently observed characteristics of the CGM in
Ly$\alpha$ than a reference inefficient feedback model used for comparison.
Nevertheless, our fiducial model still struggles to reproduce the observed data
of the inner CGM (at impact parameter $b<30$kpc). Our results suggest that
galactic outflows affect Ly$\alpha$ absorption and emission around galaxies
mostly at impact parameters $b<50$ kpc, while cold accretion flows dominate at
larger distances. We discuss the implications of this result, and underline the
potential constraining power of CGM observations - in emission and absorption -
on galactic outflow models.
