Emission within a Damped Lyman Alpha Absorption Trough: the Complex Sight Line Towards Q2059-360

Abstract

We present new spectroscopic observations of the quasar Q2059-360, confirming the existence of an emission feature within the Damped Lyman Alpha (DLA) absorption trough. By observing also at slit positions offset from the quasar, we show that the emission is spatially extended by at least a few arcseconds, and hence confirm that the feature seen must be due to emission rather than unusual absorption characteristics. We find that the DLA trough is very close in redshift to the broad Lyman~$α$ emission line of the QSO, with the result that the DLA absorption removes much of the peak region of that line. Despite the similarity of the redshifts of the DLA and the QSO, the lack of high-ionization lines of the DLA system and the unresolved widths of the corresponding metal lines indicate that the DLA cloud is not an associated system. The emission feature has a large velocity offset of +490 km/s with respect to the DLA system, and is resolved in velocity, comprising two components with a separation of ~ 300 km/s. We consider three possibilities: (1) Both emission and absorption occur within an object similar to the high redshift Lyman-break galaxies; (2) The emission feature arises from an object distinct from both the DLA absorber and the QSO, perhaps a young star-forming galaxy or a proto-galactic clump. It could be associated with the DLA absorber and perhaps the QSO in a compact group or cluster; (3) The redshifts are such that the emission feature could be due to Narrow Line Region filaments of the QSO, if the DLA absorption covers a sufficiently small angular size to allow the filaments to be seen beyond the edge of the DLA cloud.