Thursday, December 3, 2009
Is the Green Valley a mythical land?
Here's a paper I read a month ago, but somehow have procrastinated writing about... because I don't really believe the outcome. Though, honestly, there's nothing obviously wrong with the analysis. And... I suppose, it acts as a cautionary tale. In this paper, Brammer et al. argue that the so-called green valley (where galaxies are thought to be transitioning from the blue cloud to the red sequence) is really just a dusty figment of our imaginations. Using MIPS infrared data to study a large (25000 galaxies between z=0-2.5) sample, they find that the green valley galaxies are dusty (S24µm > 20 µJy) galaxies. Plotting the S24µm < 20 µJy cases rather separates the color-magnitude diagram into a clean red and blue sequence (okay, there are a few green valley galaxies, but most vanish). In fact, they quote that only 14% of the RED SEQUENCE remains truly red after dust corrections. The conclusion: to truly pick green valley/transitioning or "red and dead" galaxies, you have to be very careful to do proper reddening correction accounting for dust properly. Or: Science is hard.
Subscribe to:
Post Comments (Atom)
ok, putting this in the right place:
ReplyDeletethere's definitely alot of dusty galaxies in the green valley - i saw this in my 2007 paper, and it has been known for awhile. It's easy to see if you use spectroscopic indices as measures of sSFR, since these are less sensitive to dust. However, there still *are* green valley galaxies after dereddening, and these are interesting. Note also that the location and width of the green valley moves alot.
Also, they are getting their attenuations from SED fits with broadband data, and have to be careful there that the model library they are fitting to actually *has* 'true' green valley models, or the fits will always require dust to get green colors.
And if you are fitting SEDs, it makes more sense to talk about the green valley in terms of sSFR than color anyway.....
nice. Thanks, Ben.
ReplyDelete