NGC 4449 (Caldwell 21) – An Irregular Magellanic Starburst Dwarf Galaxy with a Faint Tidal Tail

Before we discuss NGC 4449, let’s unpack that complicated title, starting with what it means for a galaxy to be irregular. An irregular galaxy is one that has an odd shape, often just a blob, rather than a spiral or globular form. A Magellanic galaxy is a galaxy (usually a dwarf galaxy) with an irregular structure, a single spiral arm, and a bar—similar to the nearby Large Magellanic Cloud and Small Magellanic Cloud. A starburst galaxy is one currently engaging (and usually briefly on a cosmological timescale) in an unusually large volume of star formation. A dwarf galaxy is one that comprises no more than a few billion stars (unlike our Milky Way galaxy, which contains as many as 400 billion stars!). A tidal tail is a stream of stars pulled out of a galaxy by gravitational interaction with another galaxy.

So just with the basic description of NGC 4449, we can learn a lot about what it is and why it is the way it is. It’s small, which means it hasn’t absorbed a large number of smaller galaxies—nor has it been absorbed by a larger galaxy. Both its prolific rate of star formation and its tidal stream, however, are strong evidence that it has collided with a smaller galaxy. Deeper studies indicate that the starburst activity in NGC 4449 has been going on for about 10 million years, so it may soon begin to diminish.