Diffuse Lyman Alpha Haloes around Lyman Alpha Emitters at z=3: Do Dark
  Matter Distributions Determine the Lyman Alpha Spatial Extents?
  Using stacks of Ly-a images of 2128 Ly-a emitters (LAEs) and 24 protocluster
UV-selected galaxies (LBGs) at z=3.1, we examine the surface brightness
profiles of Ly-a haloes around high-z galaxies as a function of environment and
UV luminosity. We find that the slopes of the Ly-a radial profiles become
flatter as the Mpc-scale LAE surface densities increase, but they are almost
independent of the central UV luminosities. The characteristic exponential
scale lengths of the Ly-a haloes appear to be proportional to the square of the
LAE surface densities (r(Lya) \propto Sigma(LAE)^2). Including the diffuse,
extended Ly-a haloes, the rest-frame Ly-a equivalent width of the LAEs in the
densest regions approaches EW_0(Lya) ~ 200 A, the maximum value expected for
young (< 10^7 yr) galaxies. This suggests that Ly-a photons formed via shock
compression by gas outflows or cooling radiation by gravitational gas inflows
may partly contribute to illuminate the Ly-a haloes; however, most of their
Ly-a luminosity can be explained by photo-ionisation by ionising photons or
scattering of Ly-a photons produced in HII regions in and around the central
galaxies. Regardless of the source of Ly-a photons, if the Ly-a haloes trace
the overall gaseous structure following the dark matter distributions, it is
not surprising that the Ly-a spatial extents depend more strongly on the
surrounding Mpc-scale environment than on the activities of the central
galaxies.
