The current cosmological model implies the existence of “dark galaxies” composed almost entirely of hypothetical dark matter. They have no stellar population, but may contain some gas. One of the few “dark galaxy” candidates is the object FAST J0139+4328, discovered in 2023 using the 500-meter Chinese FAST radio telescope. Based on the rotation of a neutral hydrogen (HI) cloud, the mass of the dark halo was measured at 80 million solar masses, with no optical emission from stars detected. Importantly, since the cloud turned out to be highly isolated, it could not be considered the result of galaxy interactions (a tidal structure), as was the case with a number of other candidates.

    A team of astronomers from the Astronomical Observatory and University of Belgrade (Serbia) and SAO RAS studied FAST J0139+4328 to determine whether it can indeed be classified as a “dark galaxy”. Using exposures with a total duration of 28 hours on the 1.4-meter and 0.6-meter telescopes of the Astronomical Station Vidojevica, very deep images of this region of the sky were obtained. A faint stellar system—a galaxy that had not been detected in previous sky surveys was found almost at the center of the HI cloud.

Fig. 1. Left: an image of the galaxy FAST J0139+4328, obtained using the AS Vidojevica telescopes. The cross marks the center of the HI cloud; the inset shows a magnified image of the discovered dwarf galaxy. Right: the spectrum of the galaxy in the H-alpha line region, obtained with the 6-meter telescope of SAO RAS.

    The optical spectrum of FAST J0139+4328 was studied using the SCORPIO-2 instrument on the 6-meter telescope of SAO RAS. It revealed an ionized hydrogen H-alpha emission line, associated with the effect of stellar radiation on the surrounding gas. The coincidence of the velocities of the galaxy’s emission lines and those of the neutral hydrogen cloud indicates that they are located at the same distance, i.e., are physically connected. The stellar mass of the galaxy is approximately 7 million solar masses. Thus, FAST J0139+4328 cannot be classified as a “dark galaxy”, but belongs rather to a known type of low surface brightness gas-rich dwarf galaxies. The distance to the galaxy is 29 megaparsecs (94 million light years).

Published:
Mitrašinović Ana, Grozdanović Marko, Lalović Ana, Jovanović Milena, Bílek Michal, Pavlov Nataša, Moiseev Alexei, Oparin Dmitry “Discovery of a galaxy associated with the HI cloud FAST J0139+4328”, 2026, Astronomy & Astrophysics, Vol. 705, id. L9, 5 pp.

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