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

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. active galactic nucleus, 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 Abell 2895b, a strongly lensed Ly alpha emitter at redshift z similar to 3.7. From this study, we find that our target is a compact (r(n) similar to 1.2 pkpc) star-forming (star formation rate similar or equal to 11 M-circle dot yr(-1)) 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) similar or equal to 100). Based on tracks of theoretical stellar models (starburst99 and bpass), we can only partially explain this result by assuming a recent (less than or similar to 10 Myr), bursty episode of star formation and considering models characterized by binary stars, a top-heavy initial mass function and subsolar metallicities (Z less than or similar to 0.01 Z(circle dot)). These assumptions also explain the observed low (C/O) abundance of our target (similar or equal to 0.23(C/O)(circle dot)). By comparing the UV and optical data sets, 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 (similar or equal to 0.6 pkpc). The multiwavelength 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.