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arXiv 0812.2594astro-ph
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局部S0星系中恒星形成的证据:SAURON样品的斯皮策观测

Evidence of Star Formation in Local S0 Galaxies: Spitzer Observations of the SAURON Sample

Pasquale Temi, Fabrizio Brighenti, William G. Mathews, ;

摘要:

本文讨论了SAURON样品中早型星系在24、60和170微米波段的红外斯皮策观测。当与2 MASS Ks波段光度比较时,透镜状(S 0)星系比椭圆(E)星系表现出更宽的中红外到远红外光度范围。E星系的中红外和远红外辐射是来自本地质量损失的红巨星的星周或星际辐射、从星系核心被强制输送到遥远的热星际气体中的尘埃和从环境中吸积的尘埃的组合。在S 0星系的中红外和远红外发射源是完全不同的,是一致的低水平的恒星形成,0.02 - 0.2 Msol/年,在寒冷的,充满灰尘的气体盘。对于已知分子盘的星系(大多数是S 0),红外线24微米至70微米的颜色系统性地较低。我们的观测结果支持了这样的猜想,即在一些S 0星系中,冷尘埃气体是由恒星质量损失产生的,其速度与恒星形成消耗的速度大致相同,因此恒星形成对这些盘的质量消耗将是缓慢的。与E星系不同的是,S 0星系的红外光度与分子气体质量和恒星的Hbeta光谱指数都相关,并且都与最近的恒星形成率有关。然而,恒星形成率估计从β发射线光度L_{Hbeta}在SAURON S 0星系一般要小得多。由于L_{Hbeta}与年轻恒星加热尘埃产生的24微米辐射没有关联,因此光辐射线似乎不能很好地指示SAURON S 0星系的恒星形成率。没有β辐射可能是由于初始质量函数中相对没有OB星,或者是由于尘埃吸收了β辐射线。

英文摘要:

We discuss infrared Spitzer observations of early type galaxies in the SAURON sample at 24, 60 and 170 microns. When compared with 2MASS Ks band luminosities, lenticular (S0) galaxies exhibit a much wider range of mid to far-infrared luminosities then elliptical (E) galaxies. Mid and far-infrared emission from E galaxies is a combination of circumstellar or interstellar emission from local mass-losing red giant stars, dust buoyantly transported from the galactic cores into distant hot interstellar gas and dust accreted from the environment. The source of mid and far-IR emission in S0 galaxies is quite different and is consistent with low levels of star formation, 0.02 - 0.2 Msol/yr, in cold, dusty gaseous disks. The infrared 24micron-70micron color is systematically lower for (mostly S0) galaxies with known molecular disks. Our observations support the conjecture that cold dusty gas in some S0 galaxies is created by stellar mass loss at approximately the same rate that it is consumed by star formation, so the mass depletion of these disks by star formation will be slow. Unlike E galaxies, the infrared luminosities of S0 galaxies correlate with both the mass of molecular gas and the stellar Hbeta spectral index, and all are related to the recent star formation rate. However, star formation rates estimated from the Hbeta emission line luminosities L_{Hbeta} in SAURON S0 galaxies are generally much smaller. Since L_{Hbeta} does not correlate with 24 microns emission from dust heated by young stars, optical emission lines appear to be a poor indicator of star formation rates in SAURON S0 galaxies. The absence of Hbeta emission may be due to a relative absence of OB stars in the initial mass function or to dust absorption of Hbeta emission lines.

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