The X-SHOOTER Lyman-α survey at z=2 (XLS-z2) I: What makes a galaxy a Lyman-α emitter?
Abstract
We present the first results from the X-SHOOTER Lyman-α survey at z=2 (XLS-z2). XLS-z2 is a deep spectroscopic survey of 35 Lyman-α emitters (LAEs) utilising ≈90 hours of exposure time with VLT/X-SHOOTER and covers rest-frame Lyα to Hα emission with R≈4000. We present the sample selection, the observations and the data reduction. Systemic redshifts are measured from rest-frame optical lines for 33/35 sources. In the stacked spectrum, our LAEs are characterised by an interstellar medium with little dust, a low metallicity and a high ionisation state. The ionising sources are young hot stars that power strong emission-lines in the optical and high ionisation lines in the UV. The LAEs exhibit clumpy UV morphologies and have outflowing kinematics with blue-shifted SiII absorption, a broad [OIII] component and a red-skewed Lyα line. Typically 30 % of the Lyα photons escape, of which one quarter on the blue side of the systemic velocity. A fraction of Lyα photons escapes directly at the systemic suggesting clear channels enabling a ≈10 % escape of ionising photons, consistent with an inference based on MgII. A combination of a low effective HI column density, a low dust content and young star-burst determine whether a star forming galaxy is observed as a LAE. The first is possibly related to outflows and/or a fortunate viewing angle, while we find that the latter two in LAEs are typical for their stellar mass of 109 M.