In a recent report, an international team of astronomers has unveiled the discovery of a previously unnoticed, faint ultra-diffuse dwarf galaxy as part of their comprehensive exploration for dim dwarf galaxies utilizing the Dark Energy Survey (DES). This newfound celestial object, officially designated as NGC 55-dw1, orbits as a satellite around the larger galaxy known as NGC 55. The details of this groundbreaking discovery were disclosed in a paper published on September 8 and made available on the pre-print server arXiv.
Ultra-diffuse galaxies (UDGs) are characterized by their extraordinarily low density. Even though the largest UDGs possess dimensions akin to our Milky Way, they contain only a mere 1% of the stars found in our own galaxy. The enigmatic nature of UDGs continues to perplex scientists as they endeavor to elucidate why these faint, yet expansive galaxies manage to endure the gravitational forces exerted by their parent clusters.
Recently, an assemblage of astronomers, spearheaded by Mitch McNanna from the University of Wisconsin-Madison, identified a new UDG, ultimately revealing it to be a satellite of NGC 55—a barred spiral galaxy residing approximately 6.5 million light years away within the constellation Sculptor. This groundbreaking detection was rooted in a comprehensive examination of the full six-year dataset from the DES wide-area survey (DES Y6).
The research team elaborated on their methodology, stating, “We conducted a systematic search within the DES Y6 data for faint field dwarf galaxies within heliocentric distances D = 0.3−2 Mpc using a straightforward matched-filter search algorithm. This algorithm identifies galaxies as arcminute-scale overdensities of individually discernible stars.”
The newly discovered UDG, christened NGC 55-dw1, is situated a mere 47 arcminutes away from NGC 55. Consequently, assuming that both galaxies are approximately equidistant from Earth, their separation amounts to a mere 98,000 light years.
NGC 55-dw1 boasts an absolute V-band magnitude of -8.0 mag, a half-light radius spanning about 7,200 light years, and a cumulative stellar mass tallying approximately 142,000 times that of our Sun. It is estimated to have an age of 6.5 billion years, and its metallicity was measured at a level of -1.8.
The expansive spatial dimensions of NGC 55-dw1 in relation to its luminosity render it an anomaly within the known collection of dwarf galaxies located in the Local Volume—comprising celestial objects within a 36 million light-year radius of Earth. Notably, NGC 55-dw1 boasts one of the lowest surface brightness values recorded (32.3 mag/arcsec2) and stands as the largest, most diffuse galaxy yet discovered at this luminosity level.
The paper’s authors remarked that the extensive, diffused nature of NGC 55-dw1, coupled with its substantial ellipticity (0.56) and its proximity to its potential host galaxy, suggest possible tidal interactions with NGC 55. However, conclusive confirmation of these interactions may necessitate further in-depth investigations. The scientists concluded, “Tidal interactions present a plausible explanation for its substantial size, high ellipticity, and remarkably low surface brightness. Nonetheless, due to the constraints imposed by our ground-based imaging, verifying tidal stripping will likely mandate additional follow-up studies.”