Probing IGM accretion onto faint Ly{\alpha} emitters at z~2.8
  (abridged) Observing the signature of accretion from the intergalactic medium
(IGM) onto galaxies at z~3 requires the detection of faint (L<<L*) galaxies
embedded in a filamentary matrix of low-density, metal-poor gas coherent over
hundreds of kpc. We study the gaseous environment of three Lyman$\alpha$
emitters (LAEs) at z=2.7-2.8, found to be aligned in projection with a
background QSO over ~250 kpc along the slit of a long-slit spectrum. The lack
of detection of the LAEs in deep continuum images and the low inferred
Ly$\alpha$ luminosities show the LAEs to be intrinsically faint, low-mass
galaxies (L<0.1 L*, M_star< 0.1 M*). An echelle spectrum of the QSO reveals
strong Ly-alpha absorption within $\pm200$ km/s from the LAEs. Our absorption
line analysis leads to HI column densities in the range of log N(HI) =16-18.
Associated absorption from ionic metal species CIV and SiIV constrains the gas
metallicities to ~0.01 solar if the gas is optically thin, and possibly as low
as ~0.001 solar if the gas is optically thick, assuming photoionization
equilibrium. While the inferred metallicities are at least a factor of ten
lower than expected metallicities in the interstellar medium (ISM) of these
LAEs, they are consistent with the observed chemical enrichment level in the
IGM at the same epoch. Total metal abundances and kinematic arguments suggest
that these faint galaxies have not been able to affect the properties of their
surrounding gas. The projected spatial alignment of the LAEs, together with the
kinematic quiescence and correspondence between the LAEs and absorbing gas in
velocity space suggests that these observations probe a possible filamentary
structure. Taken together with the blue-dominant Ly$\alpha$ emission line
profile of one of the objects, the evidence suggests that the absorbing gas is
part of an accretion stream of low-metallicity gas in the IGM.
