An international group of astronomers has reported the discovery of a new faint, extremely diffuse dwarf galaxy as part of a systematic, large-scale search for faint dwarf galaxies using the Dark Energy Survey (DES). The newly discovered object, called NGC 55-dw1, is a satellite of the NGC 55 galaxy. Details of the discovery are described in an article published September 8 on the preprocessing server. arXiv.

Ultradiffuse galaxies (UDGs) are galaxies with extremely low density. The largest UDGs are similar in size to the Milky Way, but have only 1% more stars than our galaxy. The mystery of the UDG still baffles scientists trying to explain why clusters of these faint but massive galaxies are not torn apart by the tidal field.

Recently, a team of astronomers led by Mitch McNanna of the University of Wisconsin-Madison discovered a new UDG that turned out to be a companion of the barred spiral galaxy NGC 55, located about 6.5 million light-years away in the constellation Sculptor. The discovery is based on six years of observations from the DES Large Survey (DES Y6).

“Using a simple matched filter search algorithm, we searched DES Y6 data for weak-field dwarf galaxies with heliocentric distances D = 0.3−2 Mpc. This algorithm identifies galaxies as angular minute overdensities of individually separated stars.” explained

The newly discovered UDG, named NGC 55-dw1, is separated from NGC 55 by only 47 arcmin. Therefore, if we assume that the galaxy and its satellite are approximately the same distance from each other, they are only 98,000 light-years apart.

NGC 55-dw1 has a V-band absolute magnitude of -8.0, a half-light radius of about 7,200 light-years, and a total stellar mass of about 142,000 solar masses. The age of the galaxy is estimated to be 6.5 billion years and its metallicity is estimated to be -1.8.

NGC 55-dw1’s large spatial extent relative to its luminosity makes it unusual among the known population of dwarf galaxies in the Local Volume (36 million light-years from Earth). NGC 55-dw1 has one of the lowest surface brightnesses (32.3 magnitudes/arcsecond)²) and with this brightness, it is the largest, most diffuse galaxy known.

The paper’s authors noted that NGC 55-dw1’s large, diffuse nature, high ellipticity (0.56), and proximity to its potential host galaxy suggest a tidal interaction with NGC 55. However, more studies are needed to confirm this.

“Tidal interaction is a possible explanation for its large size, high ellipticity and extremely low surface brightness. (…) However, due to the depth limitations of our ground-based images, confirmation of tidal separation will likely require further observations. Above,” concluded the scientists. Source