Letter to the Editor

Discovery of a galaxy associated with the HI cloud FAST J0139+4328

¹ Astronomical Observatory Volgina 7 11060 Belgrade, Serbia
² Faculty of Mathematics, University of Belgrade Studentski trg 16 11158 Belgrade, Serbia
³ Special Astrophysical Observatory, Russian Academy of Sciences Nizhny Arkhyz 369167, Russia

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

Received: 3 December 2025
Accepted: 30 December 2025

Abstract

The search for “dark galaxies”, a key prediction of the lambda cold dark matter, has yielded few viable candidates. Recently, FAST J0139+4328 was reported as the first isolated dark galaxy in the nearby universe, based on a neutral hydrogen (HI) detection and a non-detection in the Pan-STARRS1 survey. To verify the nature of this candidate, we obtained deep optical imaging, using the 1.4 m Milanković and 0.6 m Nedeljković telescopes, and spectroscopic follow-up of the field. We report the unambiguous discovery of a low-surface-brightness (LSB) optical counterpart at the location of the HI cloud. Furthermore, the detection of Hα emission via the 6 m Big Telescope Alt-Azimuthal (BTA) confirms that the stellar system lies at a redshift consistent with the HI source, establishing their physical association. Through detailed photometry and employing color-dependent mass-to-light scaling relations, we derive a total stellar mass of M_★ = (7.2 ± 3.7)×10⁶ M_⊙, about an order of magnitude higher than the previously estimated upper limit. Using the literature HI mass, this implies a gas-to-stellar mass ratio of M_HI/M_★ = 11.5 ± 6.4. Our findings demonstrate that FAST J0139+4328 is not a dark galaxy but an extremely gas-rich LSB dwarf galaxy, whose stellar component was simply below the detection limit of the Pan-STARRS1 survey. This reclassification resolves the status of this prominent dark galaxy candidate and underscores the necessity of deep optical follow-up to classify faint HI-selected systems.