Age Determination of Elliptical Galaxies using H γ Profile

这项研究旨在通过测量椭圆星系的光谱年龄来揭示它们的形成时代。我们利用斯巴鲁（NAOJ）、WTT（西班牙、英国）和NTT（ESO）望远镜对27个椭圆星系（13个在星系场和星系群中，14个在室女星系团中）进行了高质量的光谱采集，并利用我们自行开发的Hg剖面法测量了它们的年龄和化学丰度。Hg剖面法是一种非常有效的方法来解开星系光谱中的年龄和金属丰度效应，自Vazdekis和Arimoto（1999）提出以来，一直受到广泛的重视。这项研究是首次将汞剖面方法应用于大量星系样本。在这项研究中，我们已经弄清楚椭圆星系的年龄和金属丰度是如何依赖于星系质量、光度和环境的。星系中年龄和金属丰度的空间分布也被研究过。结果表明：1)星系团环境中的大质量椭圆星系年龄分布均匀（<10 Gyrs），而野外的大质量椭圆星系年龄分布在6 ~ 10 Gyrs之间；这表明一些椭圆场直到最近才经历了持续很长时间的恒星形成。2)矮椭圆星系的年龄分布很广，在3 ~ 10 Gyrs之间，与它们的位置无关。这表明恒星形成的历史相当复杂，这与之前的想法不同。3)椭圆星系的Mg/Fe比值与星系质量有较强的相关性；ie。，更大质量的星系往往具有更小的Mg/Fe比率，这表明更大质量的椭圆星系的恒星形成周期更短。4)在矮椭圆星系M32 （M31螺旋星系的伴星系）中，恒星年龄也向星系中心和Mg/Fe比值方向减小。这是一个强有力的证据，证明在星系中心可能由于耗散气体的下降而长期形成恒星。总之，我们可以说，尽管椭圆星系的形成确实表现出多种多样（特别是在矮椭圆星系中），但更大质量的星系在密集的星团环境中形成得更早。少

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