Mid-UV Determination of Elliptical Galaxy Abundances and Ages

Abstract

We investigate the effects of abundance and age on the mid-UV spectra and Mg_{2} strengths of stellar populations using simple population synthesis models. These models are used to constrain the star formation history of four nearby elliptical galaxies and spiral bulges. The mid-UV (1800 - 3200 Å) light of evolved stellar populations (> 1 Gyr) is dominated by the main sequence turn-off, unlike the optical light which is dominated by the red giant branch. A detailed investigation of the mid-UV features of elliptical galaxies may help break the age-metallicity degeneracy that plagues optical techniques. Also, a better understanding of this wavelength region is useful for the studies of 0.5 $\leq$ z $\leq$ 1.5 galaxies for which the rest frame mid-UV is redshifted into the visible. We create simple, single age (3-20 Gyr), single metallicity (Z = 0.0004 - 0.05) spectral energy distributions (SEDs) extending into the UV using the Kurucz model stellar fluxes. Comparison to standard stars' mid-UV spectra reveals that the Kurucz model fluxes accurately model a blend feature of FeI and MgI at 2538 Å (Bl2538) and the slope of the continuum between 2600 and 3100 Å (S2850). We find that our simple single age, single metallicity SEDs agree well with these mid-UV features of globular clusters. However, the majority of the galaxies do not agree with the Bl2538, S2850, and Mg_{2} values given by these simple models. The mid-UV features appear to require both an old metal-rich and a small old metal-poor (Z $\leq$ 0.001) population. Despite being limited by the quality of the model stellar fluxes, our study has yielded two promising mid-UV spectral diagnostics (Bl2538 and S2850) and suggests unique and complex star formation histories for elliptical galaxies.