Galaxies in the Young Universe

For a long time, well before the invention, creation, and deployment of the Hubble Space Telescope, classifying galaxies was easy for the closest ones and quite a bit tougher for the more distant ones. When I say “classify” I mean determining a galaxy’s size and shape and structure — its morphology.  There are several main shapes: spiral, elliptical, lenticular, irregular — and within those classifications you get subclasses like barred spirals and so forth.  There was also a class of galaxies called “peculiar” because they looked — well, peculiar.

Making out the shape of a distant, dim blob of light is difficult unless you have a really good, strong telescope able to look out through the guck. So, until we had such observatories as Hubble and Gemini and Spitzer and European Southern Observatory and others, it was easier to determine the morphology of galaxies that were easier to see. That also meant astronomers classified galaxies that were closer to us. Edwin Hubble, for whom the HST is named, developed a classification scheme that astronomers came to call the “tuning fork” diagram because it … looks like a tuning fork. Nearby galaxies were observed and fit into this scheme and that helped astronomers talk definitively about the structure and characteristics of, oh say, a barred spiral galaxy and compare it to the characteristics of an elliptical galaxy.

But, the burning question always has remained — what are more distant galaxies like? Do they follow the same morphological classification scheme? And if so, what does that tell us about the time in which they formed and in which we observe them? Remember: when we look at distant galaxies, we are seeing them as they appeared very long ago. Looking at a galaxy that is six billion light-years away only tells us what it looked like roughly six billion years ago. Did galaxies conform to the same kinds of shapes they do in the closer and more recent universe? That was a question that modern telescopes were designed to answer.  Scientists used data from both Hubble Space Telescope and the ground-based Sloan Digital Sky Survey to create a tuning fork for the more distant universe.  The image above shows the tuning fork for the local galaxies in the top section. The bottom section is a classification of more distant galaxies that lie some six billion light-years away.

Even just a quick look at the image will show that the nearby galaxies — also known as the “local universe” is mostly (about 72 percent) spiral galaxies.  Another 15 percent are lenticular (labeled S0), and only three percent are ellipticals (indicated by the letter E).

Now, look at the bottom half of the image and you see that among the galaxies as they existed half the age of the universe ago, there are way more peculiar galaxies (more than half, actually). Ellipticals are just about four percent, 31 percent are spirals, and 13 percent are lenticular.

Astronomers think that many of the pecular galaxies will evolve to become spirals through galaxy mergers.  That process of interaction and merger is how the Andromeda Galaxy was formed, and indeed, our own galaxy is in process of gobbling up smaller, dwarf companions.

So, what’s the take-away message from this work?  There are a couple: Classification is more than just make-work.   Classification helps us trace the history of galaxy formation. Ultimately, it brings us back to the present-day universe and helps us understand why it looks the way it does.  If you want to read more about the observations behind this result, check out the Hubble Europe web page story.