Five dwarf galaxies lined up: cosmic parade or accident?

A team of astronomers from Yonse University in Seoul, South Korea, together with colleagues, has discovered an isolated group of five star-forming dwarf galaxies in the local Universe. This rare phenomenon was described in a study published on 19 November in The Astrophysical Journal Letters.

Dwarf galaxies are stellar systems with low luminosity and low mass, typically containing a few billion stars (for comparison, our Milky Way galaxy contains about 200 billion stars). According to observations, such galaxies rarely have close neighbours: less than 5% of them are in groups. The probability of a dwarf galaxy being in the company of four or more similar objects is less than 0.004 per cent.

Led by Sanjaya Paudel, the team discovered this rare group by analysing data from various astronomical surveys, including the Sloan Digital Sky Survey (SDSS). All the five galaxies have been labelled D1 to D5. They are located in an isolated environment about 117.3 million light-years away and are lined up in an almost perfect straight line, forming an exceptionally thin planar structure.

Characteristics of the group

• Spatial extent: The galaxies of the group are about 502,000 light years apart.
• Velocities: The maximum difference in radial velocities between them is only 75 km/s.
• Properties: All galaxies are blue, rich in gas and actively forming stars.
• Interactions: Two of them, D3 and D4, show signs of mutual tidal interactions.

Masses of the members

• Total mass: The dynamic mass of the group is about 60.2 billion solar masses.
• Most massive: D2 with a stellar mass of 275 million solar masses.
• Lightest: D4 with a mass of only 14.7 million solar masses.
• Median mass: About 78.7 million solar masses.

Interestingly, the galaxies in the group have similar rotational directions. Three of them - D1, D2 and D5 - show a common rotation: their northern parts are shifted to the red region of the spectrum, while their southern parts are shifted to the blue. This may indicate a common formation or interaction history.

Supplementary information: Sanjaya Paudel et al, Discovery of a Rare Group of Dwarf Galaxies in the Local Universe, The Astrophysical Journal Letters(2024). DOI: 10.3847/2041-8213/ad8f3c

Journal information: Astrophysical Journal Letters