RELICS-DP7: Spectroscopic Confirmation of a Dichromatic Primeval Galaxy at z ∼ 7

We report the discovery of a spectroscopically confirmed strong Ly alpha emitter at z = 7.0281 0.0003, observed as part of the Reionization Lensing Cluster Survey (RELICS). This galaxy, dubbed Dichromatic Primeval Galaxy at z similar to 7 (DP7), shows two distinct components. While fairly unremarkable in terms of its ultraviolet (UV) luminosity (similar to 0.3L(UV)*, where L-UV* is the characteristic luminosity), DP7 has one of the highest observed Ly alpha equivalent widths (EWs) among Ly alpha emitters at z > 6 (>200 A in the rest frame). The strong Ly alpha emission generally suggests a young metal-poor, low-dust galaxy; however, we find that the UV slope beta of the galaxy as a whole is redder than typical star-forming galaxies at these redshifts, -1.13 0.84, likely indicating, on average, a considerable amount of dust obscuration, or an older stellar population. When we measure beta for the two components separately, however, we find evidence of differing UV colors, suggesting two separate stellar populations. Also, we find that Ly alpha is spatially extended and likely larger than the galaxy size, hinting to the possible existence of a Ly alpha halo. Rejuvenation or merging events could explain these results. Either scenario requires an extreme stellar population, possibly including a component of Population III stars, or an obscured active galactic nucleus. DP7, with its low UV luminosity and high Ly alpha EW, represents the typical galaxies that are thought to be the major contribution to the reionization of the universe, and for this reason DP7 is an excellent target for follow-up with the James Webb Space Telescope.

Automated summary

DP7 is a strong Lyα emitter at redshift z ≈ 7, with high Lyα equivalent width and low UV luminosity. The redder UV slope of the galaxy suggests dust obscuration or an older stellar population, while measuring beta separately for two components indicates the presence of different stellar populations.