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arXiv 1306.0268astro-ph.CO
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经典QSO宿主星系中的中等年龄恒星星族

Intermediate-Age Stellar Populations in Classical QSO Host Galaxies

Gabriela Canalizo, Alan Stockton

摘要:

虽然在讨论类星体活动时,经常会在星系演化的背景下提到合并和星爆,但有几项研究质疑它们的重要性,甚至质疑它们在类星体宿主星系中的存在。因此,我们正在进行一项研究的z~0.2类星体宿主星系以前被归类为被动演变的椭圆星系。我们提出了深凯克LRIS光谱的15个主机的样本和恒星吸收光谱模型使用恒星合成模型。我们的光谱的高S/N使我们能够打破各种简并性,这些简并性来自不同的模型组合、不同的金属丰度和类星体光的污染。我们发现,没有一个主机的光谱可以模拟纯粹的老恒星星族和大多数的主机(14/15)有相当大的贡献,年龄范围从0.7至2.4 Gyr的中间年龄的人口。一个平均的寄主谱与一个老种群和一个2.1(+0.5,-0.7)Gyr种群的组合非常吻合。宿主星系的形态表明,这些老化的星爆是在合并的早期阶段引起的,导致了我们观察到的椭圆形星系。目前的活动星系核活动可能对应于数值模拟预测的后期吸积事件,发生在合并结束时,而早期的事件可能更难观察到,由于遮蔽。我们的离轴观测使我们无法探测到任何当前的恒星形成或年轻的恒星星族,可能存在于中心的几千秒差距。

英文摘要:

Although mergers and starbursts are often invoked in the discussion of QSO activity in the context of galaxy evolution, several studies have questioned their importance or even their presence in QSO host galaxies. Accordingly, we are conducting a study of z~0.2 QSO host galaxies previously classified as passively evolving elliptical galaxies. We present deep Keck LRIS spectroscopy of a sample of 15 hosts and model their stellar absorption spectra using stellar synthesis models. The high S/N of our spectra allow us to break various degeneracies that arise from different combinations of models, varying metallicities, and contamination from QSO light. We find that none of the host spectra can be modeled by purely old stellar populations and that the majority of the hosts (14/15) have a substantial contribution from intermediate-age populations with ages ranging from 0.7 to 2.4 Gyr. An average host spectrum is strikingly well fit by a combination of an old population and a 2.1 (+0.5, -0.7) Gyr population. The morphologies of the host galaxies suggest that these aging starbursts were induced during the early stages of the mergers that resulted in the elliptical-shaped galaxies that we observe. The current AGN activity likely corresponds to the late episodes of accretion predicted by numerical simulations, which occur near the end of the mergers, whereas earlier episodes may be more difficult to observe due to obscuration. Our off-axis observations prevent us from detecting any current star formation or young stellar populations that may be present in the central few kiloparsecs.

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