Using WISE data to study the stellar populations of giant and dwarf ellipticals

This work investigates the Color-Magnitude (C-M) relation for early-type galaxies. The recent WISE and Spitzer infrared surveys have opened up the possibility to study the C-M relation for galaxies in the nearand mid-Infrared. For the C-M relation four different samples from different sources are used, ranging from a Virgo Cluster sample of dwarf ellipticals to a large sample of bright early-types from the Sloan Sky Survey. The WISE survey is used to determine the [3.4]-[4.6] and [4.6]-[12] colors of all the samples, corresponding with the observational bands of the WISE mission. These infrared colors give us information about the possible presence of Asymptotic Giant Branch (AGB) stars in the early-type dwarfs, and the age and metallicity gradient of the galaxies. Although the results for the fainter galaxies are noisy, we find a clear C-M relation indicative of a metallicity gradient. The outlier galaxies, in both the [3.4]-[4.6] and [4.6]-[12] colors, are then age deviations from the main C-M relation, which is strong evidence that AGB stars are present in these galaxies. For the larger early-types we find a nearly flat slope, meaning that there is no real metallicity dependency for those galaxies. In this sample we also find outliers, but many of these can also be attributed to Active Galactic Nuclei (AGN’s) which are more commonly present in larger elliptical galaxies. The sample from the Sloan Sky Survey is harder to fit into the C-M relation, mainly due to redshift effects, further amplified by the reddening effects of the CO absorption band. However, selecting a subsample with lower redshift does flatten the slope, causing it to closer agree with the other sample of similar type galaxies.