Variations in Spectral-Energy Distributions and Absorption-Line Strengths among Elliptical Galaxies

Abstract

Thirty-one elliptical galaxies and 10 globular clusters have been observed on a 10-color intermediate-band photometric system designed for the study of old stellar populations. To within the accuracy of the present data, the integrated colors of elliptical galaxies can be specified with only two independent parameters which in turn may be utilized to determine both the intrinsic colors and the reddening between the galaxy and the observer. The intrinsic colors and line strengths are closely correlated with absolute magnitude at all luminosities. Most of the galaxies observed are members of pairs or small groups. No variations in galaxy colors were found that correlated with group properties, morphological types of companions, or the presence of optical line or radio emission. In short, these integrated colors of elliptical galaxies are independent of all physical properties studied other than luminosity. The strengths of the absorption features CN and Mg "b" plus MgH, together with blanketing by metallic lines near 3800 A, increase monotonically with increasing luminosity. A reddening-free line absorption index monitoring CN and Mg can be used to determine the absolute magnitude of an elliptical galaxy with an accuracy of 10.56 mag. Estimates of the integrated color and absorption-line changes expected solely from variations in metal abundance have been calculated for an old stellar population with a main-sequence turnoff near 1 . The difference in color and line strength between M32 and the most luminous elliptical galaxies is consistent with an increase in [FeiH] from the solar value in M32 to twice the solar value in luminous ellipticals. The uncertainties in this result are discussed. Subject headings: galaxies, photometry of - globular clusters