New stellar streams in Andromeda galaxy show galaxy formation through mergers

Washington, January 28 (ANI): A team of astronomers has discovered new stellar streams in a vast region surrounding the disk of Andromeda, in its so-called stellar halo, which shows galaxy formation through mergers.

These stellar or tidal streams, which are localized in space and move as a coherent group through the parent galaxy, intensify the density of stars and are remnants of past mergers of relatively small (dwarf) galaxies.

The data from the team's observations using both Subaru's Suprime Cam for photometry and Keck II's Deep Extragalactic Imaging Multi-Object Spectrograph (DEIMOS) for spectroscopy provided detailed spatial and velocity distributions of the stellar streams and led to this discovery.

According to the current theory of galaxy formation, we expect a halo to preserve evidence of past galaxy mergers and/or tidal dissolution in the course of halo formation.

Since the merging and dissolution of a dwarf galaxy typically last for a couple of billion years, these events are occasionally seen in a large galaxy.

Given the assumption that past merging events are recorded as stellar streams, identification of these stellar substructures in a halo plays a key role in studying the past history of galaxies.

The Andromeda Galaxy is an excellent test case for this purpose: it is the nearest, large spiral galaxy similar to our own Milky Way Galaxy and is close enough for individual stars to be studied in great detail.

Motivated by the scientific significance of examining Andromeda's halo, an international research team led by Mikito Tanaka (Tohoku University) carried out photometric observations of Andromeda's halo fields with V and I bands of Suprime Cam, a wide field imager mounted on the Subaru Telescope.

Rather than spending an enormous number of observation nights mapping its entire halo, the team looked at specific portions of Andromeda's minor axis fields, including the hitherto uncharted north side as well as some fields at the major axis.

This survey led to the discovery of two stellar streams to the northwest at projected distances of 200,000 and 300,000 light years from Andromeda's center.

The study also confirmed a few previously known streams, including the little-studied diffuse stream to the southwest, which lies at a projected distance of 200,000 to 300,000 light years from Andromeda's center. (ANI)