Wednesday, 23 January 2008

The Intrinsic Properties of SDSS Galaxies

This is Figure 11 from Maller et al. (arXiv:0801.3286), which looks at the impact of inclination corrections on SDSS galaxies.

One of the things they do is show that Sersic index is not a good enough discriminator between disks and ellipticals (see also Van der Wel from last week); the axis ratio provides an important second piece of information. They also argue that the axis ratio is good enough to derive (wavelength dependent) inclination corrections.

This plot shows how the color--magnitude diagram changes with these inclination corrections: the left panel is uncorrected, the right panel is inclination corrected. In each panel, the yellow points are edge on (b/a < 0.35) galaxies; the purple points are face on (b/a > 0.85) galaxies.

Lots of the edge on galaxies are disks, and so heavily extincted -- so they can be even redder than the red sequence. Correcting for inclination (effectively assigning them the (g-r) colors of b/a > 0.85 -- ie, face on -- galaxies with the same K magnitude and Sersic index), most of these galaxies fall back into the blue cloud.

Things i think are cool: 1. the correction is not so important for the brightest galaxies; 2. there are precious few edge on systems on the red sequence after inclination correction: if this is right, then the quenching mechanism really is associated with the morphological transformation from disk to spheroidal.

There is also some discussion on how the inclination correction affects mass estimates, which comes up with some surprising (and i think fishy) results.

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