  Through the Backyard Worlds: Planet 9 citizen science project, we have
identified a wide-separation ($\sim$10', $\sim$9900 au projected) substellar
companion to the nearby ($\sim$17.5 pc), mid-M dwarf Ross 19. We have developed
a new formalism for determining chance alignment probabilities based on the
BANYAN $\Sigma$ tool, and find a 100% probability that this is a physically
associated pair. Through a detailed examination of Ross 19A, we find that the
system is metal-poor ([Fe/H]=$-$0.40$\pm$0.12) with an age of
7.2$^{+3.8}_{-3.6}$ Gyr. Combining new and existing photometry and astrometry,
we find that Ross 19B is one of the coldest known wide-separation companions,
with a spectral type on the T/Y boundary, an effective temperature of
500$^{+115}_{-100}$ K, and a mass in the range 15-40 $M_{\rm Jup}$. This new,
extremely cold benchmark companion is a compelling target for detailed
characterization with future spectroscopic observations using facilities such
as the Hubble Space Telescope or James Webb Space Telescope.
