Age Determination of Elliptical Galaxies using H γ Profile

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This research aimed to reveal the formation epoch of elliptical galaxies by measuring their spectroscopic ages. We have taken spectra of 27 elliptical galaxies (13 in the field and group of galaxies and 14 in the Virgo cluster of galaxies) of extremely high quality with Subaru (NAOJ), WTT(Spain, UK) and NTT(ESO) telescopes and have measured their ages and chemical abundances by using the Hg profile method, which we have developed by ourselves. The Hg profile method is a quite efficient approach to disentangle age and metallicity effects in galaxy spectra and has been appreciated a lot since it was proposed by Vazdekis & Arimoto (1999). This study is the first one that applies the Hg profile method to a significantly large number of galaxy samples. In this study, we have made it clear how the ages and metallicities of elliptical galaxies depend on galaxy mass, luminosity, and the environment. The spatial distribution of age and metallicity within a galaxy has also been studied. Our resu … More lts show that 1)massive elliptical galaxies in cluster environment are uniformly old (<10 Gyrs), while those in the field show a scatter in ages spanning from 6 Gyrs to 10 Gyrs old. This indicates that some of filed ellipticals had experienced long lasting star formation until rather recently. 2)Dwarf elliptical galaxies show wide scatter in their ages, from 3 Gyrs to 10 Gyrs old, independent of where they locate. This suggests a rather complicated history of star formation in contrast to what has been thought previously. 3)Mg/Fe ratio of elliptical galaxies shows strong correlation with galaxy mass,; ie., moremassive galaxies tend to have smaller Mg/Fe ratio, suggesting shorter star formation period in more massive elliptical galaxies. 4) In a dwarf elliptical M32 (a companion galaxy of M31 spiral galaxy), the stellar age decreases towards galaxy centre and Mg/Fe ratio too. This is a strong evidence for a long lasting star formation at the galaxy center due to perhaps dissipative gas infall.In summary we could say that although it is true that formation of elliptical galaxies shows wide variety (in particular among dwarf ellipticals) more massive galaxies formed earlier in the dense cluster environment. Less