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

Dark galaxies – systems rich in neutral hydrogen (H I ) 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 H I 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 J0139+4328, 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, J0139+4328 has a stellar mass of 3 × 10 6 M ⊙ and a relatively high gas richness of M HI / M * = 18. Despite its high ratio, the galaxy is consistent, within the scatter, with the stellar-to-H I mass relation of H I -selected 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 H I surveys for detecting low surface brightness and gas-rich galaxies, but underscores the need for careful interpretation of low-resolution H I data, with potentially large centroid errors, and for sufficiently deep optical imaging to ensure robust identification.