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

TL;DR

This study tests whether FAST J0139+4328 is a true dark galaxy by performing deep optical imaging and spectroscopy to search for a stellar counterpart and confirm association with an HI cloud. The authors detect a faint optical counterpart and, via Hα spectroscopy, establish a redshift consistent with the HI source, confirming their physical connection. They derive a stellar mass of $M_\star = (7.2 \pm 3.7) \times 10^{6}\,M_\odot$ and a gas-to-stellar mass ratio of $M_{\mathrm{HI}} / M_\star = 11.5 \pm 6.4$, indicating that FAST J0139+4328 is an extremely gas-rich LSB dwarf rather than a starless dark halo. This reclassification highlights that deep optical follow-up is essential for HI-selected candidates, as many may be faint LSB systems missed by shallower surveys.

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{Milanković} and $0.6\,\mathrm{m}$ \textit{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 H$α$ 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.

Figures (3)

• Figure 1: LBVI multiband image with magnified galaxy (inset, upper left corner). The position of the HI cloud is marked with the red plus sign. The circles mark the foreground stars identified using Gaia, corresponding to the same sources highlighted in Xu+2023ApJ...944L..40X.
• Figure 2: Surface brightness profile of the FAST J0139+4328 galaxy in the $BVI$ bands in the Vega system, corrected for Galactic extinction. In the lower panel, the $B-V$ color is shown with a horizontal gray line indicating its mean value (see Sect. \ref{['subsection:stellarmass']}).
• Figure 3: Fragment of the 2D spectrum obtained with BTA/SCORPIO-2 (bottom) and integrated spectrum (101 along the slit) with Gaussian fitting of the H$\alpha$ emission line (top). The blue dotted lines indicate $\pm 1 \sigma$ around the stellar continuum level.