Extended Lyman-Alpha Emission around Star-forming Galaxies
  Lyman-alpha (Lya) photons that escape the interstellar medium of star-forming
galaxies may be resonantly scattered by neutral hydrogen atoms in the
circumgalactic and intergalactic media, thereby increasing the angular extent
of the galaxy's Lya emission. We present predictions of this extended, low
surface brightness Lya emission based on radiative transfer modeling in a
cosmological reionization simulation. The extended emission can be detected
from stacked narrowband images of Lya emitters (LAEs) or of Lyman break
galaxies (LBGs). Its average surface brightness profile has a central cusp,
then flattens to an approximate plateau beginning at an inner characteristic
scale below ~0.2 Mpc (comoving), then steepens again beyond an outer
characteristic scale of ~1 Mpc. The inner scale marks the transition from
scattered light of the central source to emission from clustered sources, while
the outer scale marks the spatial extent of scattered emission from these
clustered sources. Both scales tend to increase with halo mass, UV luminosity,
and observed Lya luminosity. The extended emission predicted by our simulation
is already within reach of deep narrowband photometry using large ground-based
telescopes. Such observations would test radiative transfer models of emission
from LAEs and LBGs, and they would open a new window on the circumgalactic
environment of high-redshift star-forming galaxies.
