Star formation rate vs. color in galaxy groups

Toady’s guest blog is from Andrew Wetzel, a postoc at Yale University. We asked Andrew to write this blog since he and his collaborators had used the public Galaxy Zoo 1 data in their own work (that is, they weren’t part of the team). Without any further ado, here’s Andrew’s experience with the Zoo data:

Recently, Jeremy Tinker, Charlie Conroy, and I posted a paper to the arXiv (click the link to access the paper) in which we sought to understand why galaxies located in groups and clusters have significantly lower star formation rates, and hence significantly redder colors, than galaxies in the field. Among the interesting things we found is that the likelihood of a galaxy to have its star formation quenched increases with group mass and increases towards the center of the group. Furthermore, galaxies are more likely to be quenched even if they are in groups as low in mass as 3 x 10^{11} Msol (for comparison, the `group’ comprised of the Milky Way and its satellites has a mass of about 10^{12} Msol). All together, these results place strong constraints on what quenches star formation in group galaxies. However, many of the above results disagree with what some other authors have found recently, and here is where Galaxy Zoo has been useful for us.

Because galaxies that are actively forming stars have a significant population of young, massive, blue stars, while galaxies that have very little star formation retain just long-lived, low-mass, red stars, astronomers often differentiate between star-forming and quenched galaxies based on their observed color. But using observed color can be dangerous, because if a galaxy contains a significant amount of gas and dust, it can appear red even if it is actively star-forming (analogous to how the sun appears redder on the horizon as the light passes through more of Earth’s atmosphere). To get a more robust measurement of a galaxy’s star formation, we used star formation rates derived from their spectra, because spectroscopic features are fairly immune to dust attenuation. But, we wanted to check how these spectroscopically-derived star formation rates compare with the color-based selection that many previous authors have used. What we found was striking: in lower mass galaxies, over 1/3 of those that appear red and dead actually have high star formation rates!

What is going on? Here is where Galaxy Zoo provided us with insight. We examined the Galaxy Zoo morphologies of these red-but-star-forming galaxies, and the result was telling: 70% of these galaxies are spirals (which have particularly high gas/dust content) and furthermore, 50% are edge-on-spirals (for which the dust attenuation is particularly strong). The image shows a good example of a galaxy which has a high star formation rate but appears red. You can even see the dust lane.

So, Galaxy Zoo helped to confirm our suspicion that many spiral galaxies that appear red are in fact actively forming stars, but their colors are reddened via dust (Karen Masters has done a lot of work in this direction as well). This gave us further confidence in our spectroscopic star formation rates and insight into why previous authors, using observed color, came to such different conclusions. Thanks to the Galaxy Zoo team and all the volunteers.