The most distant galaxies known are at z \sim 10 - 11 , observed 400 - 500 Myr after the Big Bang .
The few z \sim 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-JD , a fortuitous z \sim 10 ( z _ { phot } = 9.9 \pm 0.6 ) galaxy candidate stretched into an arc over \sim 2.5 \arcsec 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 25.7 \pm 0.1 AB mag .
With a magnification of \mu \sim 4 - 7 estimated from our lens models , the de-lensed intrinsic magnitude is 27.6 \pm 0.3 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 _ { \star } / M _ { \sun } ] = 9.7 ^ { +0.7 } _ { -0.5 } ) and star formation rate ( \log [ SFR / { M } _ { \sun } { yr } ^ { -1 } ] = 1.3 ^ { +0.2 } _ { -0.3 } ) indicate that this candidate is a typical star-forming galaxy on the z > 6 SFR– M _ { \star } relation .
We note that three independent lens models predict two counterimages , at least one of which should be of a similar magnitude to the arc , but these counterimages are not yet detected .
Counterimages would not be expected if the arc were at lower redshift .
However , the only spectral energy distributions capable of fitting the Hubble and Spitzer photometry well at lower redshifts require unphysical combinations of z \sim 2 galaxy properties .
The unprecedented lensed size of this z \sim 10 candidate offers the potential for the James Webb Space Telescope to study the geometric and kinematic properties of a galaxy observed 500 Myr after the Big Bang .