Evolution of the Lyman-α-emitting fraction and UV properties of lensed star-forming galaxies in the range 2.9 < z < 6.7

Abstract

Context. Faint galaxies are theorised to have played a major role, perhaps the dominant role, in reionising the Universe. Their properties, as well as the Lyman-α emitter (LAE) fraction, XLAE, could provide useful insights into this epoch. Aims. We used four clusters of galaxies from the Lensed Lyman-alpha MUSE Arcs Sample (LLAMAS) that also have deep HST photometry to select a population of intrinsically faint Lyman break galaxies (LBGs) and LAEs. We study the interrelation between these two populations, their properties, and the fraction of LBGs that display Lyman-α emission. Methods. The use of lensing clusters allows us to access an intrinsically faint population of galaxies, the largest such sample collected for this purpose: 263 LAEs and 972 LBGs with redshifts between 2.9 and 6.7, Lyman-α luminosities in the range 39.5 ≲ log(LLyα)(erg s−1)≲42, and absolute UV magnitudes in the range −22 ≲ M1500 ≲ −12. In addition to matching LAEs and LBGs, we define an LAE+continuum sample for the LAEs that match with a continuum object that is not selected as an LBG. Additionally, with the use of MUSE integral field spectroscopy, we detect a population of LAEs completely undetected in the continuum. Results. We find a redshift evolution of XLAE in line with literature results, with diminished values above z = 6. In line with past studies, we take this as signifying an increasingly neutral intervening intergalactic medium. When inspecting this redshift evolution with different limits on EWLyα and M1500, we find that the XLAE for the UV-brighter half of our sample is higher than the XLAE for the UV-fainter half, a difference that increases at higher redshifts. This is a surprising result and can be interpreted as the presence of a population of low Lyman-α equivalent width (EWLyα), UV-bright galaxies situated in reionised bubbles and overdensities. This result is especially interesting in the context of similar, UV-bright, low EWLyα objects recently detected during and around the epoch of reionisation. For intrinsically fainter objects, we confirm the previously observed trend of LAEs among LBGs as galaxies with high star formation rates and low dust content, as well as the trend of the strongest LAEs having, in general, fainter M1500 and steeper UV slopes.