The X-SHOOTER Lyman α survey at z = 2 (XLS-z2) I: What makes a galaxy a Lyman α emitter? Matthee, Jorryt J ; https://orcid.org/0000-0003-2871-127X Sobral, David Hayes, Matthew Pezzulli, Gabriele Gronke, Max Schaerer, Daniel Naidu, Rohan P Röttgering, Huub Calhau, João Paulino-Afonso, Ana Santos, Sérgio Amorín, Ricardo Space and Planetary Science Astronomy and Astrophysics galaxies: formation galaxies: ISM galaxies: starburst dark ages reionization first stars 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) utilizing ≈90 h of exposure time with Very Large Telescope/X-SHOOTER and covers rest-frame Ly α to H α 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 characterized by an interstellar medium with little dust, a low metallicity, and a high ionization state. The ionizing sources are young hot stars that power strong emission lines in the optical and high-ionization lines in the ultraviolet (UV). The LAEs exhibit clumpy UV morphologies and have outflowing kinematics with blueshifted Si II absorption, a broad [O III] component, and a red-skewed Ly α line. Typically, 30 per cent of the Ly α photons escape, of which one quarter on the blue side of the systemic velocity. A fraction of Ly α photons escape directly at the systemic suggesting clear channels enabling an ≈10 per cent escape of ionizing photons, consistent with an inference based on Mg II. A combination of a low effective H I column density, a low dust content, and young starburst determines whether a star-forming galaxy is observed as an 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⊙. Oxford University Press 2021 info:eu-repo/semantics/article doc-type:article text http://purl.org/coar/resource_type/c_2df8fbb1 https://research-explorer.ista.ac.at/record/11523 Matthee JJ, Sobral D, Hayes M, et al. The X-SHOOTER Lyman α survey at z = 2 (XLS-z2) I: What makes a galaxy a Lyman α emitter? <i>Monthly Notices of the Royal Astronomical Society</i>. 2021;505(1):1382-1412. doi:<a href="https://doi.org/10.1093/mnras/stab1304">10.1093/mnras/stab1304</a> eng info:eu-repo/semantics/altIdentifier/doi/10.1093/mnras/stab1304 info:eu-repo/semantics/altIdentifier/issn/0035-8711 info:eu-repo/semantics/altIdentifier/e-issn/1365-2966 info:eu-repo/semantics/altIdentifier/arxiv/2102.07779 info:eu-repo/semantics/openAccess