The Spectrally Resolved Lyman-alpha Emission of Three Lyman-alpha
  Selected Field Galaxies at z~2.4 from the HETDEX Pilot Survey
  We present new results on the spectrally resolved Lyman-alpha (LyA) emission
of three LyA emitting field galaxies at z~2.4 with high LyA equivalent width
(>100 Angstroms) and LyA luminosity (~10^43 erg/s). At 120 km/s (FWHM) spectral
resolution, the prominent double-peaked LyA profile straddles the systemic
velocity, where the velocity zero-point is determined from spectroscopy of the
galaxies' rest-frame optical nebular emission lines. The average velocity
offset from systemic of the stronger redshifted emission component for our
sample is 176 km/s while the average total separation between the redshifted
and main blueshifted emission components is 380 km/s. These measurements are a
factor of ~2 smaller than for UV continuum-selected galaxies that show LyA in
emission with lower LyA equivalent width. We compare our LyA spectra to the
predicted line profiles of a spherical "expanding shell" LyA radiative transfer
grid that models large-scale galaxy outflows. Specifically blueward of the
systemic velocity where two galaxies show a weak, highly blueshifted (by ~1000
km/s) tertiary emission peak, the model line profiles are a relatively poor
representation of the observed spectra. Since the neutral gas column density
has a dominant influence over the shape of the LyA line profile, we caution
against equating the observed LyA velocity offset with a physical outflow
velocity, especially at lower spectral resolution where the unresolved LyA
velocity offset is a convoluted function of several degenerate parameters.
Referring to rest-frame ultraviolet and optical Hubble Space Telescope imaging,
we find that galaxy-galaxy interactions may play an important role in inducing
a starburst that results in copious LyA emission, as well as perturbing the gas
distribution and velocity field which have strong influence over the LyA
emission line profile.
