The evolution of rest-frame UV properties, Lya EWs and the SFR-Stellar mass relation at z~2-6 for SC4K LAEs

Abstract

We explore deep rest-frame UV to FIR data in the COSMOS field to measure the individual spectral energy distributions (SED) of the ~4000 SC4K (Sobral et al. 2018) Lyman-alpha (Lya) emitters (LAEs) at z~2-6. We find typical stellar masses of 10${}^{9.3\pm 0.6}$ M${}_{\odot }$ and star formation rates (SFR) of SFR${}_{SED}=4.4_{-2.4}^{+10.5}$ M${}_{\odot }$/yr and SFR${}_{Lya}=5.9_{-2.6}^{+6.3}$ M${}_{\odot }$/yr, combined with very blue UV slopes of beta=-2.1${}_{-0.4}^{+0.5}$, but with significant variations within the population. M${}_{UV}$ and beta are correlated in a similar way to UV-selected sources, but LAEs are consistently bluer. This suggests that LAEs are the youngest and/or most dust-poor subset of the UV-selected population. We also study the Lya rest-frame equivalent width (EW${}_0$) and find 45 "extreme" LAEs with EW${}_0>240$ A (3 $\sigma$), implying a low number density of $(7\pm 1)\times 10^{-7}$ Mpc${}^{-3}$. Overall, we measure little to no evolution of the Lya EW${}_0$ and scale length parameter ($w_0$) which are consistently high (EW${}_0=140_{-70}^{+280}$ A, $w_0=129_{-11}^{+11}$ A) from z~6 to z~2 and below. However, $w_0$ is anti-correlated with M${}_{UV}$ and stellar mass. Our results imply that sources selected as LAEs have a high Lya escape fraction (f${}_{esc,Lya}$) irrespective of cosmic time, but f${}_{esc,Lya}$ is still higher for UV-fainter and lower mass LAEs. The least massive LAEs ($<10^{9.5}$ M${}_{\odot }$) are typically located above the star formation "Main Sequence" (MS), but the offset from the MS decreases towards z~6 and towards $10^{10}$ M${}_{\odot }$. Our results imply a lack of evolution in the properties of LAEs across time and reveals the increasing overlap in properties of LAEs and UV-continuum selected galaxies as typical star-forming galaxies at high redshift effectively become LAEs.

Accepted for publication in MNRAS. 23 pages, 11 figures, 3 tables + appendices. The full SC4K catalogue of LAEs with PSF photometry, stellar masses, SFRs and other properties for individual LAEs, is available at https://goo.gl/q9yfKo

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