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