Sunday, July 26, 2015

Ultra-Diffuse Galaxies in the Virgo Cluster

Recently, a large number of low surface brightness (LSB) galaxies were discovered in the Coma Cluster - a large cluster of galaxies located over 300 million light years away. Galaxies do not possess well defined boundaries. They simply get fainter and fainter towards their outer regions. As a result, the size of a galaxy is defined by its effective radius, also known as the half-light radius. The effective radius is the radius within which half of the galaxy’s luminosity is emitted.

Many of the LSB galaxies in the Coma Cluster are large, with effective radii between 2 to 5 kpc. One kpc (kiloparsec) is equivalent to 3,260 light years, and for comparison, the effective radius of the Milky Way galaxy is estimated to be ~3.6 kpc. They are also extremely diffuse galaxies with central surface brightnesses between 24 to 26 mag/arcsec². LSB galaxies are vulnerable to tidal perturbations as they move through the cluster and interact with other galaxies. Tidal disruption of a LSB galaxy can strip the galaxy down to only its dense nucleus, leading to the formation of an ultra-compact dwarf (UCD) galaxy.

Mihos et al. (2015) present the discovery of three LSB galaxies in the Virgo Cluster - a much nearer cluster of galaxies located ~50 million light years away. The three LSB galaxies are dubbed VLSB-A, VLSB-B and VLSB-C. They are extremely diffuse galaxies with central surface brightnesses around 27 mag/arcsec² and effective radii between 3 to 10 kpc. All three LSB galaxies appear quite diverse in their physical properties.

VLSB-A appears as a nucleated LSB galaxy with a tidal stream extending off it. This indicates that VLSB-A is presently experiencing tidal perturbations and its diffuse component is currently being stripped away. Since the nucleus of VLSB-A shares the same structural properties as UCD galaxies, VLSB-A will most likely become a new UCD galaxy after its diffuse component is stripped away.

The properties of VLSB-B and VLSB-C are not as clear compared to VLSB-A. However, both VLSB-B and VLSB-C do show a lack of obvious tidal distortion. This suggests they may lie in the outskirts of the Virgo Cluster or may be “falling” into the Virgo Cluster for the first time. Alternatively, they may be highly dominated by dark matter, making them less susceptible to tidal stripping. Interestingly, VLSB-B is appears to host a small population of globular clusters. These globular clusters may indicate the presence of a massive halo of dark matter around the galaxy which means that the galaxy has stronger self-gravity and is therefore more protected against tidal stripping.

Surface brightness of the three LSB galaxies in the Virgo Cluster. Mihos et al. (2015).

Structural properties of the three LSB galaxies in the Virgo Cluster compared with other stellar systems, including early type galaxies in the Virgo Cluster and Fornax Cluster, and in the Local Group, as well as globular clusters and UCD galaxies in the Virgo Cluster, and the LSB galaxies found in the Coma Cluster. The dashed orange lines show the globular cluster selection box, while lines of constant surface brightness are shown in green. Mihos et al. (2015).

Mihos et al. (2015), “Galaxies at the extremes: Ultra-diffuse galaxies in the Virgo Cluster”, arXiv:1507.02270 [astro-ph.GA]