Our Galactic Neighbourhood: 1st December 2017

Owen Brazell (Webb Society) 

The Local Group is a small galaxy cluster containing approximately 54 galaxies, gravitationally bound within a radius of around 1.5 megaparsecs (Mpc). The Andromeda galaxy (M31) and our own Milky Way (MW) are the two largest members, but most are very small.

The existence of galaxies was first demonstrated by Edwin Hubble in 1926, and he also classified them by their shapes, elliptical or spiral, in an arrangement known as the “tuning-fork” diagram. Elliptical galaxies contain only old stars with little free gas or dust, and are classified from E0 (circular in appearance) to E7 (cigar-shaped). Spiral galaxies contain old and young stars, have gas and dust in their spiral arms, and most of them have star-forming regions. The classification ranges from class Sa with large nuclei and tightly-wound arms, through Sb to class Sc with small nuclei and open arms. Some spirals have a straight “bar” feature at the centre and are known as barred spirals. Lenticulars have the shape of spirals, but like ellipticals have no gas or dust. Galaxies that have no clearly-defined shape are referred to as irregulars.

The Local Group contains three spirals: the Milky Way (which is in fact a barred spiral), M31 and M33; four ellipticals including M32 and M110, the companions of M31; five irregulars and an unknown number of dwarf galaxies. Most of the small galaxies are orbiting either M31 or the MW, which are a gravitationally bound pair. The dwarf galaxies are hard to find: they typically have from 1000 to 10000 stars, and are detected in surveys by a slight increase in star density, or by differences in star velocities. The Sloan Digital Sky Survey discovered 24 dwarf galaxies. They are distinguished from star clusters by their greater mass, due to the presence of dark matter which affects the velocity distribution of the stars.

The MW has a pair of irregular companions: the Large and Small Magellanic Clouds. The LMC is an irregular that appears to be turning into a barred spiral. The LMCs are orbiting each other, and exchanging stars via the Magellanic Stream, but it is not certain that they are actually orbiting the MW. The LMC has several HII regions where star formation is occurring; the SMC has none.

M31 is approaching the MW, and the two galaxies will collide in around 4 billion years. The distance to M31 is currently 2¼ million light years: this was the first galactic distance measured by Hubble using Cepheid variables, although he made an error by using the wrong type of Cepheid. M31 has young stars in its spiral arms, and also many globular clusters, which can be seen with an 8-inch telescope. X-ray images show evidence for one, and possibly two, black holes in its core. M31 has two elliptical companions, M32 and M110, and 25 other known companions. All these companions orbit M31 in a plane that is inclined to the disc of the main galaxy: the reason for this is not known.

M33 is a spiral galaxy, smaller and lighter than the MW and M31. It has many HII regions resulting in a large amount of star formation. It is relatively faint, and is better seen in smaller telescopes at lower magnification.

Owen concluded his talk with a survey of some of the other galaxies and objects in the Local Group. These include Barnard’s galaxy (NGC6822) which is the next largest and, unusually for an irregular, shows a lot of star formation; IC10, in the plane of the MW, yet another irregular with unusual amounts of star formation, and Leo A which is a small irregular. Some objects have less certain classifications: Messier 54 appears at the centre of the Sagittarius Dwarf Elliptical Galaxy, and may actually be its core, rather than a separate globular cluster. Omega Centauri is generally classified as a globular cluster, but is elliptical rather than spherical, and has an unusual rotation pattern, so it may also be an old galactic nucleus.

Notes and summary by Chris Hooker.