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

¹ ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD, Dwingeloo, The Netherlands
² Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV, Groningen, The Netherlands
³ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
⁴ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China
⁵ Guizhou Radio Astronomical Observatory, Guizhou University, Guiyang 550000, PR China
⁶ CAS Key Laboratory of FAST, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 18 August 2025
Accepted: 16 January 2026

Abstract

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 × 10⁶ 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.