Abstract
Abridged With VLT/X-shooter, we obtain optical and NIR spectra of six
Ly-alpha blobs at z~2.3. Using three measures --- the velocity offset between
the Lya line and the non-resonant OIII or H-alpha line (Dv_Lya), the offset
of stacked interstellar metal absorption lines, and the spectrally-resolved
OIII line profile --- we study the kinematics of gas along the line of sight
to galaxies within each blob center. These three indicators generally agree in
velocity and direction, and are consistent with a simple picture in which the
gas is stationary or slowly outflowing at a few hundred km/s from the embedded
galaxies. The absence of stronger outflows is not a projection effect: the
covering fraction for our sample is limited to <1/8 (13%). The outflow
velocities exclude models in which star formation or AGN produce "super" or
"hyper" winds of up to ~1000km/s. The Dv_Lya offsets here are smaller than
typical of LBGs, but similar to those of compact LAEs. The latter suggests that
outflow speed cannot be a dominant factor in driving extended Lya emission. For
one Lya blob (CDFS-LAB14), whose Lya profile and metal absorption line offsets
suggest no significant bulk motion, we use a simple radiative transfer model to
make the first column density measurement of gas in an embedded galaxy, finding
it consistent with a DLA system. Overall, the absence of clear inflow
signatures suggests that the channeling of gravitational cooling radiation into
Lya is not significant over the radii probed here. However, one peculiar system
(CDFS-LAB10) has a blueshifted Lya component that is not obviously associated
with any galaxy, suggesting either displaced gas arising from tidal
interactions among blob galaxies or gas flowing into the blob center. The
former is expected in these overdense regions, and the latter might signify the
predicted but elusive cold gas accretion along filaments.
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