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FAST detects new ultra-faint dwarf galaxy

Using the Five-hundred-meter Aperture Spherical radio Telescope (FAST), Chinese astronomers have detected a new ultra-faint dwarf galaxy, which turned out to be gas-rich. The finding was reported in a research paper March 12 on the preprint server arXiv.

The so-called ultra-faint dwarf galaxies (UFDs) are the least luminous, most dark matter–dominated, and least chemically evolved galaxies known. Therefore, they are perceived by astronomers as the best candidate fossils from the universe at its early stages.

A team of astronomers led by Jin-Long Xu of the Chinese Academy of Sciences (CAS) is carrying out a FAST extragalactic H I (neutral atomic hydrogen) survey (FASHI). One of the objectives of this survey is to search for dark and weak galaxies. Now, they report the finding of a new UFD as part of this project.

"In this paper, based on the H I survey using the FAST, we have discovered a gas-rich UFD galaxy in the Local Group or its outskirts," the researchers wrote.

Xu's team searched for UFDs by identifying compact H I gas clouds with system velocities below 250 km/s. From the detected clouds, they found an isolated cloud with a heliocentric system velocity of about 127.0 km/s. An optical counterpart, which is a galaxy designated KK153 (LEDA 41920), was found at the center of this cloud.

According to the study, the newfound ultra-faint dwarf KK153 was found at a distance of about 6.5 million light years and has an optical effective radius of 682 light years. The observations show that the galaxy's neutral atomic hydrogen gas showcases a typical disk-galaxy structure.

The stellar mass of KK153 was derived to be 410,000 solar masses, while the neutral atomic hydrogen gas mass was obtained to be some 520,000 solar masses. The neutral gas fraction for the galaxy was calculated to be 0.63.

Therefore, these results indicate that KK153 is one of the smallest gas-rich disk galaxies discovered so far. Moreover, the dynamical mass of KK153 was estimated to be at a level of 69 million solar masses, which is at least one order of magnitude lower than the threshold value.

Furthermore, the study found that KK153 shows a cool (with a temperature of about 200 K) and warm (7,400 K) two-phase neutral medium. Such a two-phased medium is typical for gas-rich faint dwarf irregular galaxies.

The collected data also indicate that KK153 is a blue galaxy that has recently formed stars and that the new ones are mostly forming in its inner disk. The astronomers noted that this galaxy not only contains a large number of blue-colored stars but also red-colored stars.