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We quantify the contribution of Ly$\alpha$ fluorescence to observed spatiallyextended Ly$\alpha$ halos around Ly$\alpha$ emitters (LAE) at redshift ${\rmz=3.1}$. The key physical quantities that describe the fluorescent signalinclude (${\it i}$) the distribution of cold gas in the circum-galactic medium(CGM); we explore simple analytical models and fitting functions to recenthydrodynamical simulations; (${\it ii}$) local variations in the ionizingbackground due to ionizing sources that cluster around the central galaxy. Weaccount for clustering by boosting the observationally inferred volumetricproduction 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 ofionizing sources around the central galaxy. We compute $\xi_{\rm LyC}(r)$ byassigning an 'effective' bias parameter to the ionizing sources. This novelapproach allows us to quantify our ignorance of the population of ionizingsources in a simple parametrized form. We find a maximum enhancement in thelocal ionizing background in the range $50-200$ at $r \sim 10$ physical kpc.For spatially uncorrelated ionizing sources and fluorescing clouds we find thatfluorescence can contribute up to $\sim 50-60\%$ of the observed spatiallyextended Ly$\alpha$ emission. We briefly discuss how future observations canshed light on the nature of Ly$\alpha$ halos around star forming galaxies.

On the contribution of fluorescence to Ly$\alpha$ halos (LAHs) around star forming galaxies