Title:How much gas and dust is in the $z=5.7$ Lyman Break Galaxy HZ10? An ALMA Band 10 to 4 and JWST/NIRSpec study of its interstellar medium

Abstract:A complete overview of the stellar, gas and dust contents of galaxies is key to understanding their assembly at early times. However, an estimation of molecular and atomic gas reservoirs at high redshift relies on various indirect tracers, while robust dust mass measurements require multi-band far-infrared continuum observations. We take census of the full baryonic content of the main-sequence star-forming galaxy HZ10 at $z=5.65$, a unique case study where all necessary tracers are available. We present new ALMA Band 10 ($\lambda_\mathrm{rest}=50\mu$m) and Band 4 ($300\mu$m) observations towards HZ10, which combined with previously taken ALMA Band 6 through 9 data ($70-200\mu$m) constrains its dust properties. We complete the baryonic picture using archival high-resolution [CII] observations that provide both a dynamical mass and molecular and atomic gas mass estimates, a JVLA CO(2-1)-based molecular gas mass, and JWST metallicity and stellar mass measurements. We detect continuum emission from HZ10 in Bands 10 and 4 at the $3.4-4.0\sigma$ level, and measure a dust temperature of $T_\mathrm{dust} = 37_{-5}^{+6}$K and dust mass $\log(M_\mathrm{dust}/M_\odot) = 8.0 \pm 0.1$. Leveraging the dynamical constraints, we infer its total gas budget, and find that commonly used [CII]-to-H$_2$ and [CII]-to-HI conversions overpredict the gas mass relative to the dynamical mass. For this reason, we derive a [CII]-to-total ISM mass (atomic + molecular) conversion factor, which for HZ10 corresponds to $\alpha_\mathrm{[CII]}^\mathrm{ISM} = 39^{+50}_{-25}M_\odot L_\odot^{-1}$. We also find that HZ10 falls below the local scaling relation between dust-to-gas ratio and metallicity, suggesting inefficient ISM dust growth. These results demonstrate a powerful synergy between ALMA and JWST in disentangling the baryonic components of early galaxies, paving the way for future studies of larger samples.