Probing IGM accretion on to faint Lyα emitters at <i>z</i> ∼ 2.8

Observing the signature of accretion from the intergalactic medium (IGM) on to galaxies at z similar to 3 requires the detection of faint (L << L*) galaxies embedded in a filamentary matrix of low-density (rho < 100 <(rho)over bar>), metal-poor gas (Z similar to 10(-2.5) Z(circle dot)) coherent over hundreds of kpc. We study the gaseous environment of three Ly alpha emitters (LAEs) at z = 2.7 - 2.8, found to be aligned in projection with a background QSO over similar to 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 less than or similar to 0.1L*, M-star less than or similar to 0.1M*). An echelle spectrum of the QSO reveals strong Ly alpha absorption within +/- 200 km s(-1) from the LAEs. Our absorption line analysis leads to HI column densities in the range of logN(H I)/cm(-2) = 16-18. Associated absorption from ionic metal species CIV and Si IV constrains the gas metallicities to similar to 0.01 solar if the gas is optically thin, and possibly as low as similar to 0.001 solar if the gas is optically thick, assuming photoionization equilibrium. While the inferred metallicities are at least a factor of 10 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.