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

TL;DR

Deep optical and spectroscopic observations revealed that the HI cloud FAST J0139+4328 is an extremely gas-rich low-surface-brightness dwarf galaxy, not a dark galaxy as previously thought.

Contribution

This study provides the first confirmed optical counterpart and redshift for FAST J0139+4328, reclassifying it from a dark galaxy candidate to a gas-rich dwarf galaxy.

Findings

Discovered a low-surface-brightness optical counterpart at the HI cloud location.

Confirmed the physical association through Hα emission and redshift measurement.

Derived a stellar mass of approximately 7.2 million solar masses, with a high gas-to-stellar mass ratio.

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 $1.4\,\mathrm{m}$ \textit{Milankovi\'c} and $0.6\,\mathrm{m}$ \textit{Nedeljkovi\'c} 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 H$\alpha$ emission via the $6\,\mathrm{m}$ 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 $M_\star = (7.2 \pm 3.7) \times 10^6\, M_{\odot}$, 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 $M_{\mathrm{HI}} / M_{\star} = 11.5 \pm 6.4$. 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.