Insights into the reionization epoch from cosmic-noon-Civ emitters in the VANDELS survey

Abstract

Recently, intense emission from nebular CaIII] and CIV emission lines have been observed in galaxies in the epoch of reionization (z > 6) and have been proposed as the prime way of measuring their redshift and studying their stellar populations. These galaxies might represent the best examples of cosmic reionizers, as suggested by recent low-z observations of Lyman continuum emitting galaxies, but it is hard to directly study the production and escape of ionizing photons at such high redshifts. The ESO spectroscopic public survey VANDELS offers the unique opportunity to find rare examples of such galaxies at cosmic noon (z∼ 3), thanks to the ultra deep observations available. We have selected a sample of 39 galaxies showing CIV emission, whose origin (after a careful comparison to photoionization models) can be ascribed to star formation and not to active galactic nuclei. By using a multiwavelength approach, we determined their physical properties including metallicity and the ionization parameter and compared them to the properties of the parent population to understand what the ingredients are that could characterize the analogs of the cosmic reionizers. We find that CIV emitters are galaxies with high photon production efficiency and there are strong indications that they might also have a large escape fraction: given the visibility of CIV in the epoch of reionization, this could become the best tool to pinpoint the cosmic reioinzers.