We love looking at pictures of big, gorgeous spiral galaxies. In fact, that's probably what pops into your head when you think of the word "galaxy." But by the numbers, most galaxies are tiny little things like the one you see above, newly discovered around our closest spiral galaxy, neighbor, the Andromeda Galaxy.
Our galaxy lives in a pretty typical, boring region of of the cosmic web of galaxies in what we call "The Local Group." (See how creative astronomers are?) It is dominated by Andromeda and the Milky Way, but has a whole host of smaller galaxies associated with these two giants.
Most of these galaxies are dwarf galaxies, and this shouldn't be too surprising. From planetary bodies to stars to galaxies, the universe tends to make a lot of "little" things and very few spectacularly large ones.
There are something like a hundred galaxies in the Local Group when you add up all the dwarf galaxies. The exact number is hard to determine, as there is some debate over whether some objects are "really" galaxies or just a chance grouping of stars.
A model of some of the galactic members of the Local Group.
These two new dwarf galaxies are named Andromeda XXVIII and Andromeda XXIX, because, again, astronomers are so clever at naming things, and these are the 28th and 29th dwarf galaxies to be found associated with the Andromeda Galaxy. Both are actually a bit far removed from the large spiral, and thus may clue us in to how dwarf galaxies look before they get too close to a big galaxy.
What happens when a little galaxy gets too close? Well, just ask the famous Sagittarius dwarf galaxy, if you can. It's been gravitationally ripped to shreds, leaving a stream of stars looping around the Milky Way at least twice. The "galactic cannibalism" is how large galaxies grow larger over time, consuming the myriad of smaller galaxies.
Model of the Milky Way and what is left of the Sagittarius dwarf galaxy.
Since these galaxies are so faint, we can really only study the ones that live in our Local Group and extrapolate to other galaxy groups. However, being in the middle of our galaxy's disk means that our view isn't necessarily unobstructed.
Finding and classifying new dwarf galaxies is painstaking work. Will there ever be an end? Will there be something too small to classify as a galaxy and a Pluto-like debate over its name? Probably, but finding out how the universe works is way more interesting than agonizing over a name.
Images: Top – Andromeda XXIX, outlined in yellow for clarity. Credit - Gemini Observatory/AURA/Eric Bell; Middle – Model of the Local Group. Credit - Richard Powell; Bottom – It's not that obvious in the sky, but the Sagittarius dwarf galaxy has left a stream of stars around the Milky Way. Credit – David Law, UCLA.
This research has been published in Astrophysical Journal Letters, and the preprints can be found at arxiv.org. Thanks again to Rachael Beaton and Gail Zasowski for helping me navigate the Local Group madness!