Astronomers from the Yonsei University in Seoul, South Korea and elsewhere have discovered a rare isolated group of five star-forming dwarf galaxies in the local universe. The finding was reported in a research paper published November 19 in The Astrophysical Journal Letters.

Dwarf galaxies are low-luminosity and low-mass stellar systems, usually containing a few billion stars. Observations suggest that groupings of these galaxies are rare as only fewer than 5% of dwarf galaxies have close companions. Moreover, there is less than a 0.004% chance that a dwarf galaxy would be part of a group containing at least four such objects.

However, now a team of astronomers led by Yonsei University’s Sanjaya Paudel reports the finding of such a rare group, which contains at least five dwarfs. The discovery is based on the analysis of data from various astronomical surveys, including the Sloan Digital Sky Survey (SDSS).

“We have identified a distinct group of dwarf galaxies, with all five members aligned along a straight line in the celestial plane and three sharing a common rotational direction,” the researchers wrote in the paper.

The group was detected in a near isolated environment, at a distance of about 117.3 million light years away. The five dwarf galaxies received designation from D1 to D5, and the images show that all of them are arranged nearly in a straight line, forming an exceptionally thin planar structure.

The observed spatial span of the member galaxies of the group was measured to be approximately 502,000 light years, and the maximum difference in line-of-sight velocity between them was found to be 75 km/s. All the five galaxies turned out to be star-forming, blue and gas-rich, and two of them (D3 and D4) show signs of mutual tidal interactions.

When it comes to the estimated mass of the group, the study found that its dynamical mass is about 60.2 billion solar masses. Dwarf galaxy D2 is the group’s most massive member with a stellar mass of 275 million solar masses, while D4 is the least massive—with a stellar mass of only 14.7 million solar masses. The median stellar mass of the group members was calculated to be 78.7 million solar masses.

The study also found that the member galaxies in the group have similar rotational directions. Moreover, three of them, namely D1, D2, and D5 have a common rotational direction in that the north parts of these galaxies are red-shifted, and the lower south parts are blue-shifted.

Summing up the results, the authors of the paper note that the discovery of such a unique group could be essential to improve our understanding of how dwarf galaxy groups form and evolve. The findings could also help us test the theory of large-scale structure formation.

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