Not so-dark: High resolution HI imaging of J0139+4328 and identification of an optical counterpart

Dark galaxies -- systems rich in neutral hydrogen ( gas but with no stars -- are a common prediction of numerous theoretical models and cosmological simulations. However, the unequivocal identification of such sources in current surveys has proven challenging. In this work, we present interferometric follow-up observations with the Karl G. Jansky Very Large Array (VLA) of a former dark galaxy candidate originally detected with the single-dish FAST telescope. The improved spatial resolution of the VLA data allows us to identify a faint optical counterpart and characterize the galaxy. Located at a distance of about 31 Mpc, has a stellar mass of 3 10^6 and a relatively high gas richness of = 18. Despite its high ratio, the galaxy is consistent, within the scatter, with the stellar-to- mass relation of samples in the literature and with the baryonic Tully–Fisher relation (BTFR), although its kinematic measurement is subject to large uncertainties. This case highlights the potential of modern high-sensitivity surveys for detecting low surface brightness and gas-rich galaxies, but underscores the need for careful interpretation of low-resolution data, with potentially large centroid errors, and for sufficiently deep optical imaging to ensure robust identification.