Scrutiny of a very young, metal-poor star-forming Lyα-emitter at z ~ 3.7

Abstract

The origin of the Lyman-$\alpha$ (Ly$\alpha$) emission in galaxies is a long-standing issue: despite several processes known to originate this line (e.g. AGN, star formation, cold accretion, shock heating), it is difficult to discriminate among these phenomena based on observations. Recent studies have suggested that the comparison of the ultraviolet (UV) and optical properties of these sources could solve the riddle. For this reason, we investigate the rest-frame UV and optical properties of A2895b, a strongly lensed Ly$\alpha$-emitter at redshift z ~ 3.7. From this study, we find that our target is a compact (r ~ 1.2 pkpc) star-forming (star formation rate ~ 11 M${}_{\odot }$/yr) galaxy having a young stellar population. Interestingly, we measure a high ratio of the H$\beta$ and the UV continuum monochromatic luminosities (L(H$\beta$)/L(UV) ~ 100). Based on tracks of theoretical stellar models (Starburst99, BPASS), we can only partially explain this result by assuming a recent (< 10 Myr), bursty episode of star-formation and considering models characterised by binary stars, a top-heavy initial-mass function (IMF) and sub-solar metallicities (Z < 0.01 Z${}_{\odot }$). These assumptions also explain the observed low (C/O) abundance of our target (~ 0.23(C/O)${}_{\odot }$). By comparing the UV and optical datasets, we find that the Ly$\alpha$ and UV continuum are more extended (x2) than the Balmer lines, and that the peak of the Ly$\alpha$ is offset (~ 0.6 pkpc). The multi-wavelength results of our analysis suggest that the observed Ly$\alpha$ emission originates from a recent star-formation burst, likely taking place in an off-centre clump.

18 pages (including Appendix), 5 figures. Paper accepted for publication on MNRAS. Accepted 2022 November 2. Received 2022 October 22; in original form 2022 August 3

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