In the News this month: first results from a panoramic survey of the Andromeda galaxy
The Andromeda galaxy, M31 CREDIT: John Lanoue
Edwin Hubble's original classification of galaxies into various types based on their visible shapes and structures has been a feature of extra-galactic astronomy since the 1920s. The scheme, originally thought to depict an evolutionary sequence, has two major groups: spiral galaxies with a small central bulge, spiral arms and possibly a central bar, and elliptical galaxies that are more spherical in structure with no spiral arms or disk. There are however, many galaxies which do not fit into this scheme, being neither spherical or disk-like, and these are usually lumped together into a class called the irregulars. These disturbed galaxies are surprisingly common, and many are the result of collisions or close encounters between galaxies. Such interactions happened frequently throughout the history of the universe, but it is also going on right now in our own galactic neighbourhood.
The nearest major galaxy to our own is the Andromeda Galaxy, otherwise known as M31, slightly larger than the Milky Way and located 2.5 million light years away. It is heading towards the Milky Way at some 300 km/s and, in a few billion years, the two galaxies will eventually collide. In some cosmological models, galaxies grow over time by disrupting and absorbing smaller galaxies in such collisions. In such violent processes, a significant number of stars should be tossed out of the galaxies involved, forming a diffuse halo which can provide clues to the merger history of a galaxy, if they are bright enough to be detected. In research reported in the journal Nature on the 3rd of September, a team of astronomers led by Alan McConnachie at the Herzberg Institute of Astrophysics in Canada, report a panoramic survey of Andromeda and its nearby neighbour, the Triangulum galaxy, M33, which shows clear evidence of the remnants of galactic mergers.
Detecting this evidence is difficult as these stellar populations are extremely faint and distributed over a huge area, so the astronomers are using the MegaCam camera on the 3.6-metre Canada-France-Hawaii telescope to build up a sensitive wide-field survey of the Andromeda galaxy and it's companions. The so-called Pan-Andromeda Archaeological Survey will cover more than 300 square degrees when completed in 2011, but has already produced results showing the vast extent of M31's stellar halo, covering an area of nearly 100 times the classical optical disk of the galaxy.
These early results from the survey lend support to the idea that large galaxies build up through the accretion of smaller galaxies. The halo stars discovered away from the disk of M31 are unlikely to have been formed at their present positions because there is not enough gas there for star formation to occur. The most likely explanation is that they have been thrown out in a tidal interaction. Another piece of evidence that they are relics from previous galactic mergers is that the stars in this faint population are often located in huge arcs, loops and other diffuse structures which are characteristic of the gravitational disruption of dwarf galaxies undergoing a merger with a larger galaxy.
As well as lending support to the hierarchical model of galaxy formation, the team's results also show a new diffuse stellar structure around M33, M31's largest companion galaxy. This newly discovered feature matches up with a distortion in the disk of M33, as well as a mild warp seen in the outer disk of M31, adding to the evidence of a past tidal interaction between the two galaxies.
McConnachie, A., Irwin, M., Ibata, R., Dubinski, J., Widrow, L., Martin, N., Côté, P., Dotter, A., Navarro, J., Ferguson, A., Puzia, T., Lewis, G., Babul, A., Barmby, P., Bienaymé, O., Chapman, S., Cockcroft, R., Collins, M., Fardal, M., Harris, W., Huxor, A., Mackey, A., Peñarrubia, J., Rich, R., Richer, H., Siebert, A., Tanvir, N., Valls-Gabaud, D., & Venn, K. (2009). The remnants of galaxy formation from a panoramic survey of the region around M31 Nature, 461 (7260), 66-69 DOI: 10.1038/nature08327