We’re In Your Universe, Classifying Your Galaxies

As every astronomy enthusiast knows — and as many people are now learning by looking at great images from Hubble Space Telescope and other observatories — there are countless galaxies out there. They seem to exist as far as we can detect — as far back as the earliest epochs of cosmic history after the Big Bang. Last week, I posted about an image that HST took that showed galaxies as they looked at least 13 billion years ago. The big job for astronomers now is to understand the long line of evolution that begins with the first shreds of galactic matter (stars, gas, dust) that clumped together early in a galaxy’s existence — and continues through galaxy mergers and acquisitions to make the galactic objects we see today. At the same time, astronomers need to classify each galaxy they see by shape, mass, distance, color, and size.

The way to do that is take data about galaxies that we already know about, from surveys and observations by such telescopes as HST and the Two Micron All-sky Survey (2MASS, a ground-based observatory) and dump that data into computer models that slice and dice the data. The output?  An image that looks remarkable like actual survey images of galaxy fields.  That’s what two astronomers — Dr. Andrew Benson of the California Institute of Technology (Caltech) and Dr. Nick Devereux from Embry-Riddle University — did. Their work was just announced and is written up in the Monthly Notices of the Royal Astronomical Society.

Their results explain the diversity of galaxy shapes we see.   Their image (to the left) contains images of data from HST and 2MASS, and looks very much like that Hubble Deep Field image we saw at the AAS meeting last week.

Now, galaxy shapes are an interesting substory in astronomy history because, up until less than a hundred years ago, astronomers didn’t know what galaxies were, much less how to classify them. We didn’t even know WE lived in a galaxy until the early years of the 20th century.  Some of the most flashy arguments of that time were about just what the “spiral nebulae” were that astronomers were observing. There was, in fact, a well-known public debate in 1920 called the Shapley-Curtis Debate, and it really was about the scale of the universe and these spiral nebulae that seemed to be scattered around in nearby space.

It really took the advent of larger telescopes, reliable photographic instruments, and the science of astrophysics before astronomers fully understood the nature of galaxies. And, once they began looking out at the cosmos with improved instruments, they saw galaxies everywhere!  As far as we can see, we detect galaxies. And, they come in an amazing array of shapes. Classifying galaxies became a growth industry.

Astronomers began with the basic shapes:  elliptical, spiral, and irregular. But, as astronomers examined more galaxies, the simple three classes began to fragment into subclasses. Edwin Hubble (for whom the HST is named) came up with a classification scheme that is named after him, and is the basis for the more extensive classifications astronomers use today.

If you look at this scheme and then look at the image above — or at any HST image of galaxies — you’ll see these basic shapes. Astronomers see these shapes back almost to the earliest epochs of galaxy formation — although the very earliest galaxies often looked more like shreds of material, as opposed to the more fully formed objects we see today.

So, what does galaxy classification do for us? Certainly it helps astronomers understand how many different galactic shapes there are out there. And, how many of certain types of galaxies lie at huge distances from us (at presumably earlier times) versus how many are relatively nearby and more recent.

But, look at this another way — at some level and for many galaxies, we’re seeing an evolutionary history as we look at the shapes. For example — astronomers can see places where spiral galaxies are colliding. In the fullness of time, those collisions will produce NOT new spirals, but elliptical galaxies with old stars with very little starbirth activity, but sporting supermassive black holes (and possibly high-speed jets).  In the process, the colliding galaxies will tear out huge tidal streams of gas and dust, where starbirth activity will go off like firecrackers.

Spirals, on the other hand, are hotbeds of star formation. How do they form?  Like other galaxies, they are created as smaller galaxies merge and coalesce and the complex gravitational interactions shape the spiral arms.  Our Milky Way galaxy is a barred spiral, and its shape implies that it has experienced a pretty complex history, with only a few minor collisions and at least one episode where the inner disk collapsed to form the large central bar.

And, there’s a lot of evidence that black holes play a part in galaxy formation and evolution as well. And, just to make things interesting, galaxies are very likely all embedded in haloes of dark matter — invisible “stuff” that seems to affect the evolution of galaxies as they ride along the expansion of the universe and traverse the cosmos.

As it turns out,  some irregular galaxies are also the sites of intense star formation.  In at least one case, which we saw at the American Astronomical Society meeting last week, the Small Magellanic Cloud (which is an irregular galaxy in the Milky Way’s neighborhood), is glowing with star-forming activity.

Galaxies are fascinating objects — just when we think we understand them, we find more of them to study– and classify. There’s more than enough work to do, and the observatories of the future (like James Webb Space Telescope and others) will give us deeper looks at more distant galaxies and their structures.