Astronomers explore globular cluster system of ultra-diffuse galaxy NGC5846_UDG1

Astronomers from Swinburne University in Australia and elsewhere have employed the Keck II telescope to investigate the globular cluster system of an ultra-diffuse galaxy designated NGC5846_UDG1. Results of the observations, published April 4 on the arXiv pre-print server , could help us better understand the nature of this galaxy.

Globular clusters (GCs) are collections of tightly bound stars orbiting galaxies. Astronomers perceive them as natural laboratories enabling studies on the evolution of stars and galaxies. In particular, GCs could help researchers to better understand the formation, history and evolution of early-type galaxies, as the origin of GCs seems to be closely linked to periods of intense star formation.

Ultra-diffuse galaxies (UDGs) are extremely-low-density galaxies. The largest UDGs have sizes similar to the Milky Way, but have only about 1% as many stars as our home galaxy. The mystery of UDGs is still baffling scientists as they try to explain why these faint but large galaxies are not ripped apart by the tidal field of their host clusters.

NGC5846_UDG1 is an ultra-diffuse galaxy in the NGC 5846 group of galaxies located some 81.5 million light years away from the Earth. The galaxy has an effective radius of about 6,200 light years and stellar mass of some 120 million solar masses.

Previous observations of NGC5846_UDG1 have found that it hosts a rich globular cluster system with an estimated number of around 50 GCs and only one confirmed member GC.

Searching for new GCs of this system, a team of astronomers led by Swinburne University's Lydia Haacke decided to perform spectroscopic observations of NGC5846_UDG1 with the Keck Cosmic Web Imager (KCWI) at the Keck II telescope.

The observations made by Haacke's team confirmed 19 GCs as members of NGC5846_UDG1. Therefore, a total of 20 GCs are now spectroscopically confirmed to be members of NGC5846_UDG1 and approximately 9% of the stellar light of this UDG is contained in these confirmed clusters.

The mean velocity of the confirmed GC was measured to be 2,153.9 km/s, while the velocity dispersion was found to be approximately 29.8 km/s. It turned out that the velocity dispersion increases with increasing magnitudes of the clusters.

Based on the new data, the researchers obtained a dynamical mass of NGC5846_UDG1, which was found to be at a level of 2.09 billion solar masses, enclosed within the galaxy's de-projected half-light radius. The total halo mass of NGC5846_UDG1 was estimated to be around 270 billion solar masses.

Summing up the results, the authors of the paper conclude that the rich GC system, an overly massive halo, and a high GC luminosity fraction suggest that NGC5846_UDG1 is a failed galaxy that may have formed in a short, intense burst of star formation, which was largely confined to its GCs.

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