Spectroscopic Ages of Elliptical Galaxies – Subaru Observation

Abstract

Breaking age/metallicity degeneracy ([19], [1]) is crucial for understanding stellar populations in elliptical galaxies. Spectroscopic properties, such as line strength, can partially break age/metallicity degeneracy. Reference [19] found that Hβ index, defined in [4] (Lick indices), is relatively sensitive to luminosity-weighted age of galaxies. But emission line makes Hβ shallower, therefore correction of emission line effect is necessary. Hγ and Hδ indices, such as Hγ _A , Hδ _A , Hγ _F and Hδ _F , which are less affected by emission than Hβ, are defined in [8] and [20]. However, those indices are still affected by age/metallicity degeneracy, since they are measured on low resolution spectra (9–11 Å). Reference [8] defined Hγ _HR to find it strongly sensitive to age. But Hγ _HR is much affected by velocity dispersion (σ) of galaxies, since the bandpasses of feature and pseudocontinua are very narrow.