2018ApJ...864L..22S

The most distant galaxies known are at z ∼ 10-11, observed 400-500 Myr after the Big Bang. The few z ∼ 10-11 candidates discovered to date have been exceptionally small, barely resolved, if at all, by the Hubble Space Telescope. Here we present the discovery of SPT0615-JD1, a fortuitous z ∼ 10 (z_phot = 9.9_–0.6^+0.8) galaxy candidate stretched into an arc over ∼2.''5 by the effects of strong gravitational lensing. Discovered in the Reionization Lensing Cluster Survey (RELICS) Hubble Treasury program and companion S-RELICS Spitzer program, this candidate has a lensed H-band magnitude of 24.6 ± 0.1 AB mag. With a magnification of µ ∼ 4-7 estimated from our lens models, the delensed intrinsic magnitude is 26.7 ± 0.1 AB mag, and the half-light radius is r_e < 0.8 kpc, both consistent with other z > 9 candidates. The inferred stellar mass (log[M_*/M_☉]=8.3_–0.2^+0.3) and star formation rate (log[SFR/M_☉yr^–1]=1.1_–0.4^+0.5) indicate that this candidate is a typical star-forming galaxy on the z > 6 SFR-M_* relation. We note that three independent lens models predict two counter images, at least one of which should be of a similar magnitude to the arc, but these counter images are not yet detected. Counter images would not be expected if the arc were at lower redshift. The relatively large physical size could be due to a merger or accretion event, while the unprecedented lensed size of this z ∼ 10 candidate offers the potential for ALMA and the James Webb Space Telescope to study the geometric and kinematic properties of a galaxy observed 500 Myr after the Big Bang.