What is a Galaxy? …the return

Abell S740 / HST

The first time I gave a public talk, I spent an hour describing why galaxy classification is fundamentally important to the study of the Universe, the origins of Galaxy Zoo, the amazing response of the volunteers and the diverse results from their collective classifications of a million galaxies near and far. I showed many gorgeous galaxy images, a few dark matter simulations and even a preview of the Hubble image of Hanny’s Voorwerp.

As I finished my talk and the Q&A began, I braced myself for the inevitably interesting and challenging questions (I seem to get a lot of questions about black holes and spacetime).

A brief pause, and then the first question echoed from somewhere in the darkened auditorium: …”What’s a galaxy?

Oops. Apparently I’d forgotten that little detail at the start of the talk. So I described a typical galaxy (if there is such a thing): a collection of stars, gas, dust, dark matter, all gravitationally bound together. Then I made a joke about scientists forgetting to define their terms, and we moved on to the next raised hand.

Turns out, though, it’s not such an easy question. Even though my casual definition works fine for most galaxies, it’s not at all an agreed-upon standard. We’ve discussed this on the blog before, and even in the short time (astronomically speaking) since Karen wrote that very nice post, more work has been done to find galaxies that push the boundaries and force us to re-think what it really means to be a galaxy.

Segue 1

The circled stars (plus a lot of dark matter you can’t see) are Segue 1, one of the smallest galaxies we know about. To read more on this, click the image.

So, spurred by a very broad interpretation of a question left for us in the comments on the post announcing this hangout, we decided to re-visit the discussion, covering the various properties a galaxy must have, should have, could have, and can’t have. We discussed the smallest galaxies, found by counting and measuring each of their individual stars. We discussed the biggest, brightest galaxies in the universe, living in rich environments and grown fat by eating other galaxies. And everything in between.

(podcast version here)

Note: when we talk about Segue 1 and 2, I say that these galaxies are unique because they have low mass-to-light ratios. Despite the pause that indicated I was trying to keep from inverting numerator and denominator… that’s exactly what I did. The galaxies have very few stars compared to the amount of dark matter in them, so their mass is high and their light is low, so their mass-to-light ratios are high. Oops (again)!

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