Early types E - Early types E/S0 have red colors, as...

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Early types E/S0 have red colors, as expected for systems with very low SFR. Later types have bluer colors, indicating a larger relative rate of recent star formation. This test alone does not tell whether this is due only to bulge/disk variations, chemical abundance effects or a real difference in the disk histories (I recall a very probing conversation with Sandy Faber about this, as a lowly grad student). We may regard this as a very low-spectral-resolution kind of spectral synthesis. The color-type relation is shown in this figure from Roberts and Haynes (1994 ARA&A 32, 115, reproduced from the ADS). That review also summarizes the evidence for changes in chemical abundance, dynamical mass, and H I content along the Hubble sequence. It has long been known that the colors of E/S0 galaxies form a very well-defined sequence (the red sequence ), reddening for brighter galaxies due to metallicity. It took the large uniform data set from the SDSS, augmented by GALEX, to show that star-forming galaxies have a set of
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Early types E - Early types E/S0 have red colors, as...

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