Chapter 15: Galaxies

Current estimates indicate that roughly 3% of all galaxies defy classification into one of the three primary types defined by Hubble: elliptical, lenticular, or spiral. These systems vary in size from small systems that look rather like bright blue bug splatter on a windshield, to large systems of unusual morphology. Classified by Edwin Hubble as Irregular galaxies (Irr), these systems have few unifying characteristics. The exact cause for all these different ill-defined shapes is poorly understood. Other than having stars (and most likely a dormant central supermassive black hole), the only unifying characteristic of these systems is their ill-defined shape. Many, but not all, are undergoing star formation. Many, but not all, show signs of recent gravitational interactions with another galaxy, or of a recent merger event. Most, but not all, irregular systems are found in dense environments or at least near a few other luminous galaxies. Systematic studies of all the different shapes, colors, and environments of these systems are ongoing, making for a complex puzzle for computer simulations to try and replicate.

Seen from the southern skies, the Large and Small Magellanic Clouds (the LMC and SMC, respectively) are bright patches in the sky. 

The nearest two examples of irregular galaxies are the Large and Small Magellanic Clouds (LMC and SMC). Thanks to these systems, we have studied irregulars up close and understand how at least these two operate. They are part of the Local Group and are orbiting the Milky Way. Easily visible to the naked eye in the southern hemisphere, they look like wads of cotton wool in the sky; the Large Magellanic Cloud has an angular size of 11° × 9° and the Small Magellanic Cloud has an angular size of 5° × 3°. They have been known to South Pacific voyagers and Asian astronomers for thousands of years and their naming after the Spanish explorer Ferdinand Magellan is relatively recent. The LMC is 4300 pc across and 50 kpc distant (160,000 light years), while the SMC is 2300 pc across and 61 kpc distant (200,000 light-years). Observations and theoretical arguments indicate that the Magellanic Clouds have been greatly distorted by the gravity of the Milky Way as they pass close to it. Diffuse streams of cold hydrogen gas connect them to the Milky Way and to each other. They are gas-rich and their stars have heavy element abundances less than the stars in the disk of the Milky Way. In 1987, the Large Magellanic Cloud was host to SN 1987A, the brightest supernova to be observed in four centuries.

SN1987A in the Large Magellanic Cloud.

The Magellanic Clouds are nearly examples of gas-rich dwarf galaxies. Small and low mass galaxies are the most abundant stellar systems in the universe. A typical dwarf galaxy has a few billion stars and is a few thousand light-years across. The Milky Way has about fifty dwarf galaxies that orbit as its satellites. The distribution of galaxies of all sizes and masses shows that there are many dwarf galaxies for every normal-sized galaxy like the Milky Way. So most galaxies in the universe are dwarf galaxies. Most dwarf galaxies are gas-poor ellipticals rather than gas-rich irregulars; the nearest example of a dwarf elliptical is M32, the satellite companion to Andromeda (M31). Dwarf galaxies play an important role in cosmology and our understanding of galaxy formation and evolution. They formed first in the history of the universe and merged and combined over billions of years to form the large galaxies we see in the present universe. They also contain a large amount of dark matter and are good "laboratories" for studying the properties of this enigmatic component of the universe.