The Red and the Blue

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Science  10 Aug 2007:
Vol. 317, Issue 5839, pp. 725
DOI: 10.1126/science.317.5839.725a

All galaxies can be built up from two major components: a central spherical bulge and a surrounding flattened disk of stars. The relative size of each has been widely used to classify galaxies, so that a variety of types can be identified with disks of varying size, and when there is no disk, the galaxy is classified as elliptical. Complementing this picture, galaxies tend to favor one of two colors, either red or blue. Ellipticals are mostly red and spirals blue, leading to suggestions that this color preference is simply due to the relative contributions of stars in the bulge and disk to the galaxy's overall light.

Drory and Fisher have found that this simple view is inadequate, however. Decomposing the shapes of tens of galaxies selected from the Sloan Digital Sky Survey using images taken by the Hubble Space Telescope, they find that some particular galaxies do not follow the expected relationship between color and brightness expected from their bulge-to-disk ratio. The galaxies in question instead have so-called “pseudobulges,” or central puffed-up concentrations of stars whose properties are more similar to those in the disks than in ellipticals or normal bulges. Such differences imply that it is the type of central bulge that is important, not its size, in characterizing a galaxy. Also, the characteristics ultimately depend on the environment in which the galaxy first formed, so that pseudobulges are indicative of later, less violent formation in comparison with the conditions that give rise to normal bulges. — JB

Astrophys. J. 664, 640 (2007).

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