New constraints on the molecular gas content of a $z\sim 8$ galaxy from JVLA CO(J=2-1) observations

As the primary fuel for star formation, molecular gas plays a key role in galaxy evolution. A number of techniques have been used for deriving the mass of molecular reservoirs in the early Universe (e.g., [CII]158$\mu$m, [CI], dust continuum), but the standard approach of CO-based estimates has been limited to a small number of galaxies due to the intrinsic faintness of the line. We present Jansky Very Large Array (JVLA) observations of the $z\sim 8.31$ galaxy MACS0416_Y1, targeting CO(2-1) and rest-frame radio continuum emission, which result in upper limits on both quantities. Adding our continuum limit to the published far-infrared (FIR) spectral energy distribution (SED), we find a small non-thermal contribution to the FIR emission, a low dust mass (${\log }_{10}({\mathrm{M}}_{\mathrm{D}}\mathrm{/}{\mathrm{M}}_{\odot })\sim 5$), and an abnormally high dust temperature (${\mathrm{T}}_{\mathrm{D}}\gtrsim 90\mathrm{K}$) that may indicate a recent starburst. Assuming a low metallicity ($Z\mathrm{/}{Z}_{\odot }\sim 0.25$), we find evidence for $M_{{\mathrm{H}}_2,\mathrm{C}\mathrm{O}}\lesssim {10}^{10}$M${}_{\odot }$, in agreement with previous [CII] investigations ($M_{{\mathrm{H}}_2,[\mathrm{C}\mathrm{I}\mathrm{I}]}\sim {10}^{9.6}$M${}_{\odot }$). Upcoming JWST observations of this source will result in a precise determination of $Z$, enabling better constraints and an unprecedented view of the gaseous reservoir in this primordial starburst galaxy.