A z=3.045 Lyα emitting halo hosting a QSO and a possible candidate for AGN-triggered star formation

In this third paper in a series on the nature of extended, asymmetric Ly alpha emitters at z similar to 3 we report the discovery, in an ultra-deep, blind, spectroscopic long-slit survey, of a Ly alpha emitting halo around a QSO at redshift 3.045. The QSO is a previously known, obscured active galactic nucleus (AGN). The Ly alpha emitting halo appears extended along the direction of the slit and exhibits two faint patches separated by 17 proper kpc in projection from the QSO. Comparison of the two-dimensional spectrum with archival Hubble Space Telescope ACS images shows that these patches coincide spatially with emission from a peculiar, dumbbell-shaped, faint galaxy. The assumptions that the Ly alpha emission patches are originating in the galaxy and that the galaxy is physically related to the QSO are at variance with photometric estimates of the galaxy redshift. We show, however, that a population of very young stars at the redshift of the QSO may fit the existing rest-frame broad-band UV photometry of the galaxy. If this scenario is correct, then the symmetry of the galaxy in continuum and Lya emission, the extension of the QSO's Ly alpha emission in its direction, and the likely presence of a young stellar population in close proximity to a (short-lived) AGN suggest that this may be an example of AGN feedback triggering external star formation in high-redshift galaxies.