Discovery of a Ly-alpha galaxy near a damped Ly-alpha absorber at Z = 2.3

Abstract

We report the detection of a galaxy associated with the damped Lyα absorbing cloud.seen at z = 2.309 toward the QSO PHL 957 (Z_em_= 2.681). The galaxy was discovered in deep Fabry-Perot narrow-band CCD frames and was subsequently imaged spectroscopically. In addition to a Strong but narrow Lyα emission line (F_Lyα_= 5.6 X 10^-16^ ergs s^-1^ cm^-2^, FWHM~700 km s^-1^) and weaker C IV and He II lines, the object shows continuum at V~24, with a slope (in F_v_) rising slightly toward the red, similar to what is seen in high-redshift radio galaxies; however, the galaxy does not correspond to any known radio source in the literature. The detected emission lines and continuum are most easily interpreted as light from hot, recently formed stars, implying not only a sizable star formation rate (SFR >= 5 M_sun_ yr^-1^) but also a scarcity of dust, which readily quenches Lyα photons. The emission region appears to be marginally resolved spatially and is located 48" to the NW of the QSO, corresponding to a projected distance of 270h^-1^ kpc (for q_0_= 0.1); the velocity difference with respect to the damped Lyα cloud is ~350 km s^-1^. The spatial correlation of the absorbing cloud and the companion galaxy supports the interpretation of damped Lyα clouds as objects fundamentally different from the lower column density Lyα forest clouds, which show weak or no clustering. The absorption trough itself shows no Lyα emission, extended or unresolved, in either the Fabry-Perot frames or in deep, moderate resolution (~3 A FWHM), two-dimensional spectra, down to a limiting flux (3 σ) for an unresolved line of 2 x 10^-17^ ergs s^-1^ cm^-2^,~30 times fainter than the Lyα flux detected from the companion galaxy. The lack of strong Lyα emission from the absorbing cloud can be interpreted as evidence either for a low SFR or for heavy dust obscuration.