Blue ellipticals – lots of them!

Hey all, as some of you know, I’m working on the blue ellipticals in Galaxy Zoo. I’ve been working on the formation and evolution of elliptical galaxies ever since I started my PhD. In many ways, they’re the most interesting galaxy type out there because they never really want to come out “right” in simulations. They are rather enigmatic objects and we’re not sure how they form.

They *appear* to be completely quiescent – i.e. not forming any stars* – but more recent work by our group has shown that that’s not entirely true. We used the GALEX ultraviolet space telescope to look for small amounts of hot, blue young stars in elliptical galaxies and to our surprise found them to be very common!

It turns out that it’s just too hard to really disentangle such small populations of young stars against the background of really old stars. Unfortunately, this idea that elliptical galaxies are all old and have no young stars in them led some people to specifically *exclude* any galaxies that might have young stars in them from the elliptical class. So our discovery from the ultraviolet data led us to search for more of these elliptical galaxies with young stars in them.

We knew that the only way to find them was my making our own catalogue of ellipticals where we would *not* throw out things that look like ellipticals but had the blue colours or spectra indicating young stars.

I did classify 50 000 galaxies from the SDSS by eye in a week, dividing galaxies into ellipticals vs. everything else; once I was done, I checked to see what was left. And lo and behold, there was a small but significant population of *very* blue elliptical galaxies! This project of course led to Galaxy Zoo, since 50 000 may sound a lot, but it’s only a tiny fraction of the 1 million in SDSS. So now with all the classifications from all you guys (thanks so much!), I’ve been able to study blue ellipticals in much more detail.

Here’s what I did: I selected a redshift (distance) range and a limit in absolute magnitude (luminosity – how bright the galaxies actually are) to create a “volume-limited” sample. That’s a sample where I know that I’ve got all galaxies down to a certain luminosity limit in a certain volume. That’s important when you want to compare numbers (e.g. blue vs. red ellipticals), because blue and red galaxies can have different luminosities, and we must compare apples to apples.

I already knew from earlier tests that your classifications are absolutely awesome, but when I pulled up the images of those galaxies that you classified as “elliptical” that also had very blue colours, I was amazed. Here they were! Blue ellipticals, lots of them!

So with this incredible sample in my hands, I started work on a paper. In Chris’s words, it’s a “classic astronomy paper” because we’re doing nothing fancy, but simply report what we find. The most important finding (I think) is that blue ellipticals exist (i.e. they aren’t misclassified spirals) and that they aren’t super-rare, but make up ~5% of the elliptical population.

We’ve also measured their star formation rates in a variety of ways and the measured rates make them by far the highest ever reported for ellipticals. We have some ellipticals with star formation rates of over 50 solar masses per year. To compare, our own Milky Way only manages about 3 per year!

Here are some example images (click for a larger view):

What’s next? I am still polishing the text and we’re doing some comparisons to a simulation. After that, I will circulate the draft with the other team members again for a final round of comments and then it’s probably good to submit to a journal.