DISSECTING THE RED SEQUENCE—I. STAR FORMATION HISTORIES OF QUIESCENT GALAXIES: THE
COLOR-MAGNITUDE VS. THE COLOR-SIGMA RELATION

Genevieve J. Graves, S. M. Faber, & Ricardo P. Schiavon

From DR4 SDSS data with the NYU-VAGC, the authors select quiescent (i.e. emission line-free) galaxies. These lie on the red sequence. They measure luminosities, colors, velocity dispersion and a few element abundances (mainly Fe abundance and alpha-enrichment). In a plot that I don't show here, it shows that at fixed velocity dispersion there is no relation between luminosity and color (luminosities vary more than color, and contours of number density are largely horizontal). The red sequence is inclined, because higher sigma galaxies, have higher luminosities and are redder. Adding up different sigma bins results in the red sequence as we know it.

To investigate this further, the authors bin in L-sigma-color space and stack all spectra in a bin together to measure abundances and age. Here I will concentrate on age. In the plot you see six panels in the luminosity color plane, for six bins in velocity dispersion (these show no L-color relation!). The color coding is luminosity weighted mean age.

As you can see, low-sigma galaxies are younger and have a bigger spread in ages than luminous, high-sigma galaxies. Also, the variation in age is perpendicular to the red sequence, i.e. the width is set by the age distribution of the galaxies.

Quiescent galaxies are a multi-parameter family. Age (and Fe/H and alpha/Fe) all increase with sigma, and vary at fixed sigma depending on L. Age also varies as a function of color, at fixed sigma: brighter galaxies have lower age at fixed sigma and are bluer.