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

• Published in: arXiv.org 2021-01
• Main Authors: Pelliccia, Debora, Strait, Victoria, Lemaux, Brian C, Bradac, Marusa, Coe, Dan, Bolan, Patricia, Bradley, Larry D, Frye, Brenda, Gandhi, Pratik J, Mainali, Ramesh, Mason, Charlotte, Ouchi, Masami, Keren, Sharon, Trenti, Michele, Zitrin, Adi
• Format: Article
• Language: English
• Subjects: Active galactic nuclei; Cosmic dust; Emitters; Galactic clusters; Galaxies; Ionization; James Webb Space Telescope; Luminosity; Occultation; Population III stars; Space telescopes; Stellar populations
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2011.08857

We report the discovery of a spectroscopically-confirmed strong Lyman-\(\alpha\) emitter at \(z=7.0281\pm0.0003\), observed as part of the Reionization Cluster Lensing Survey (RELICS). This galaxy, dubbed "Dichromatic Primeval Galaxy" at \(z\sim7\) (DP7), shows two distinct components. While fairly unremarkable in terms of its ultraviolet (UV) luminosity (\(\sim0.3L^{\ast}_{UV}\), where \(L^{\ast}_{UV}\) is the characteristic luminosity), DP7 has one of the highest observed Lyman-\(\alpha\) equivalent widths (EWs) among Lyman-\(\alpha\) emitters at \(z>6\) (\(>200\) Angstrom in the rest frame). The strong Lyman-\(\alpha\) 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, \(-1.13\pm 0.84\), 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 Lyman-\(\alpha\) is spatially extended and likely larger than the galaxy size, hinting to the possible existence of a Lyman-\(\alpha\) 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 Lyman-\(\alpha\) 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.