On the contribution of fluorescence to Ly$\alpha$ halos (LAHs) around
  star forming galaxies
  We quantify the contribution of Ly$\alpha$ fluorescence to observed spatially
extended Ly$\alpha$ halos around Ly$\alpha$ emitters (LAE) at redshift ${\rm
z=3.1}$. The key physical quantities that describe the fluorescent signal
include (${\it i}$) the distribution of cold gas in the circum-galactic medium
(CGM); we explore simple analytical models and fitting functions to recent
hydrodynamical simulations; (${\it ii}$) local variations in the ionizing
background due to ionizing sources that cluster around the central galaxy. We
account for clustering by boosting the observationally inferred volumetric
production rate of ionizing photons, $\epsilon_{\rm LyC}$, by a factor of
$1+\xi_{\rm LyC}(r)$, in which $\xi_{\rm LyC}(r)$ quantifies the clustering of
ionizing sources around the central galaxy. We compute $\xi_{\rm LyC}(r)$ by
assigning an 'effective' bias parameter to the ionizing sources. This novel
approach allows us to quantify our ignorance of the population of ionizing
sources in a simple parametrized form. We find a maximum enhancement in the
local ionizing background in the range $50-200$ at $r \sim 10$ physical kpc.
For spatially uncorrelated ionizing sources and fluorescing clouds we find that
fluorescence can contribute up to $\sim 50-60\%$ of the observed spatially
extended Ly$\alpha$ emission. We briefly discuss how future observations can
shed light on the nature of Ly$\alpha$ halos around star forming galaxies.
