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外文期刊>The Astrophysical journal
>SPITZER SPECTROSCOPY OF INFRARED-LUMINOUS GALAXIES: DIAGNOSTICS OF ACTIVE GALACTIC NUCLEI AND STAR FORMATION AND CONTRIBUTION TO TOTAL INFRARED LUMINOSITY
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SPITZER SPECTROSCOPY OF INFRARED-LUMINOUS GALAXIES: DIAGNOSTICS OF ACTIVE GALACTIC NUCLEI AND STAR FORMATION AND CONTRIBUTION TO TOTAL INFRARED LUMINOSITY
We use mid-infrared (MIR) spectroscopy from the Spitzer Infrared Spectrograph to study the nature of star-formation and supermassive black hole accretion for a sample of 65 IR-luminous galaxies at 0.02 z 0.6 with F(24 μm) 1.2 mJy. The MIR spectra cover wavelengths 5-38?μm, spanning the polycyclic aromatic hydrocarbon (PAH) features and important atomic diagnostic lines. Our sample of galaxies corresponds to a range of total IR luminosity, L IR?= L(8-1000?μm)?= 1010-1012 L ☉ (median L IR of 3.0 × 1011 L ☉). We divide our sample into a subsample of galaxies with Spitzer Infrared Array Camera 3.6-8.0?μm colors indicative of warm dust heated by an active galactic nucleus (AGN; IRAGN) and those galaxies whose colors indicate star-formation processes (non-IRAGN). Compared to the non-IRAGN, the IRAGN show smaller PAH emission equivalent widths, which we attribute to an increase in mid-IR continuum from the AGN. We find that in both the IRAGN and star-forming samples, the luminosity in the PAH features correlates strongly with [Ne II] λ12.8?μm emission line, from which we conclude that the PAH luminosity directly traces the instantaneous star-formation rate (SFR) in both the IRAGN and star-forming galaxies. We compare the ratio of PAH luminosity to the total IR luminosity, and we show that for most IRAGN star-formation accounts for 10%-50% of the total IR luminosity. We also find no measurable difference between the PAH luminosity ratios of L 11.3/L 7.7 and L 6.2/L 7.7 for the IRAGN and non-IRAGN, suggesting that AGN do not significantly excite or destroy PAH molecules on galaxy-wide scales. Interestingly, a small subset of galaxies (8 of 65 galaxies) show a strong excess of [O IV] λ25.9?μm emission compared to their PAH emission, which indicates the presence of heavily-obscured AGN, including 3 galaxies that are not otherwise selected as IRAGN. The low PAH emission and low [Ne II] emission of the IRAGN and [O IV]-excess objects imply the IR luminosity of these objects is dominated by processes associated with the AGN. Because these galaxies lie in the "green valley" of the optical color-magnitude relation and have low implied SFRs, we argue their hosts have declining SFRs and these objects will transition to the red sequence unless some process restarts their star-formation.
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机译：我们使用Spitzer红外光谱仪的中红外（MIR）光谱研究了0.02 1.2的65个IR发光星系样品的恒星形成和超大质量黑洞积聚的性质嗯MIR光谱涵盖5-38？μm的波长，跨多环芳烃（PAH）特征和重要的原子诊断线。我们的星系样本对应于总IR发光度的范围，LIR≥L（8-1000μm）≥1010-1012L☉（中值L IR为3.0×1011 L☉）。我们使用Spitzer红外阵列摄像机3.6-8.0？μm颜色将样本分为星系子样本，这些颜色表示由活跃银河核（AGN; IRAGN）加热的暖尘，以及那些颜色指示恒星形成过程的星系（非IRAGN） 。与非IRAGN相比，IRAGN的PAH发射当量宽度更小，这归因于AGN的中红外连续谱的增加。我们发现，在IRAGN和恒星形成样品中，PAH特征的光度都与[Ne II]λ12.8？μm发射谱线密切相关，由此我们得出结论，PAH光度直接追踪了瞬时恒星形成率（IRAGN）和恒星系中的（SFR）。我们比较了PAH发光度与总IR发光度的比率，并且我们发现大多数IRAGN恒星形成占总IR发光度的10％-50％。对于IRAGN和非IRAGN，我们还没有发现L 11.3 / L 7.7和L 6.2 / L 7.7的PAH光度比之间没有可测量的差异，这表明AGN不会在整个银河系尺度上显着激发或破坏PAH分子。有趣的是，与PAH发射相比，一小部分星系（65个星系中的8个）显示出[O IV]λ25.9？m的发射量大大过量，这表明存在严重遮盖的AGN，包括3个不存在的星系否则选择为IRAGN。 IRAGN和[O IV]过量物体的低PAH发射和低[Ne II]发射意味着这些物体的IR光度受与AGN相关的过程的控制。由于这些星系位于光学色度关系的“绿色山谷”中，并且隐含的SFR较低，因此我们认为它们的宿主的SFR不断下降，除非某些过程重新开始其恒星形成，否则这些天体将转变为红色序列。
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