Galaxy Zoo Paper on Dust in Spirals Submitted.
Before Kevin starts sending me friendly emails that I haven’t blogged about this yet, I want to announce the submission of the latest Galaxy Zoo paper (submitted to Monthly Notices on August 17th):
I’m delighted that I finally got this work submitted. Now I feel like I can properly call myself part of the Galaxy Zoo team. My first entry on the blog Blue Sky and Red Spirals was about this work, and you can also check out the scientific poster I made about it. I hinted several times over the past 8 months that we were very close to submission, so it’s great to be able to say it’s actually now in the referee process. I actually think this is one of the quickest papers I’ve ever written – only 10 months from when I started working on it, to submission of the paper. Fingers crossed for an equally smooth referee process.
Our main conclusions ended up being:
- Spiral galaxies are reddened as they become more inclined due to the presence of dust (this effect is explained in great detail in Blue Sky and Red Spirals)
- Spiral galaxies with large bulges are much redder than spiral galaxies with no/small bulges. This effect is larger than the dust reddening – face-on spirals with large bulges are redder than edge-on spirals with no bulge (on average).
- There is more dust reddening in spiral galaxies with small bulges than in those with large bulges.
- There is a peak in the dust content of spirals at moderate luminosities. Very luminous and very dim spirals both have less dust reddening. Very dim spirals are physically smaller, and make less dust than brighter ones. Very bright spirals usually don’t have a lot of recent star formation, and as dust is destroyed over time we may just be seeing that effect.
We compared the observed trends to a model published in 2004 (Tuffs etal. 2004: Attenuation of Stellar Light in Spiral Galaxies for the very keen!) and concluded that it works pretty well (especially considering how much you have to simplify a spiral galaxy to be able to model it), but there are some problems at the shortest wavelengths covered by SDSS – we see a lot less reddening there than the model predicts.
We finished by talking about the impact all these things have on galaxy surveys. It’s a fairly small effect, but because dust always dims galaxies that means that inclined spirals are often “left out” of samples which people use to study cosmology, or do galaxy evolution (just because you can’t see them, or they’re below a cut in brightness you needed to make). I don’t think I need to tell this crowd that spiral and elliptical galaxies are quite different objects, they also have different clustering properties. So if you preferentially leave out some of the spirals that could introduce some subtle biases, which when people are trying to use galaxy surveys to get percent level accuracy on cosmological parameters might actually start to matter!