Abstract
We report the detection of diffuse Ly$\alpha$ emission, or Lyman alpha halos
(LAHs), around star-forming galaxies at $z\approx3.78$ and $2.66$ in the NOAO
Deep Wide-Field Survey Boötes field. Our samples consist of a total of
$\approx$1,400 galaxies, within two separate regions containing
spectroscopically confirmed galaxy overdensities. They provide a unique
opportunity to investigate how the LAH characteristics vary with host galaxy
large-scale environment and physical properties. We stack Ly$\alpha$ images of
different samples defined by these properties and measure their median LAH
sizes by decomposing the stacked Ly$\alpha$ radial profile into a compact
galaxy-like and an extended halo-like component. We find that the exponential
scalelength of LAHs depends on UV continuum and Ly$\alpha$ luminosities, but
not on Ly$\alpha$ equivalent widths or galaxy overdensity parameters. The full
samples, which are dominated by low UV-continuum luminosity Ly$\alpha$ emitters
($M_UV -21$), exhibit LAH sizes of 5$\,-\,6\,$kpc. However, the
most UV luminous galaxies have more extended halos with scalelengths of
8$\,-\,9\,$kpc. The stacked Ly$\alpha$ radial profiles of our samples decline
more steeply than recent theoretical predictions that include the contributions
from gravitational cooling of infalling gas and from low-level star formation
in satellites. However, the LAH extent and surface brightness profile matches
what one would expect for photons produced in the galaxy and then resonantly
scattered by gas in an outflowing envelope. The observed trends of LAH sizes
with host galaxy properties suggest that the physical conditions of the CGM
(covering fraction, HI column density, and outflow velocity) change with halo
mass and/or star formation rates.
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