RELICS-DP7: Spectroscopic confirmation of a dichromatic primeval galaxy at z ~ 7

We report the discovery of a spectroscopically-confirmed strong Lyman-α emitter at z = 7.0281 ± 0.0003, observed as part of the Reionization Cluster Lensing Survey (RELICS). This galaxy, dubbed “Dichromatic Primeval Galaxy” at z ∼ 7 (DP7), shows two distinct components. While fairly unremarkable in terms of its ultraviolet (UV) luminosity (∼ 0.3L^∗_UV , where L^∗_UV is the characteristic luminosity), DP7 has one of the highest observed Lyman-α equivalent widths (EWs) among Lyman-α emitters at z > 6 (> 200Å in the rest frame). The strong Lyman-α emission generally suggests a young metal-poor, low-dust galaxy; however, we find that the UV slope β of the galaxy as a whole is quite red, −1.13 ± 0.84, likely indicating, on average, a considerable amount of dust obscuration, or an older stellar population. When we measure β for the two components separately, however, we find evidence of differing UV colors, suggesting two separate stellar populations. Also, we find that Lyman-α is spatially extended and likely larger than the galaxy size, hinting to the possible existence of a Lyman-α 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 Lyman-α 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.