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arXiv 0704.1765astro-ph
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z~2 II红外发光星系的Spitzer中红外光谱诊断

Spitzer Mid-Infrared Spectroscopy of Infrared Luminous Galaxies at z~2 II: Diagnostics

Anna Sajina, Lin Yan, Lee Armus, Phil Choi, Dario Fadda, George Helou, Henrik Spoon

摘要:

我们给出了48个Spitzer选择的ULIRGs样本的中红外光谱分解,这些ULIRGs的跨度为z~1-3,可能为L_IR~10^12-10^13Lsun。我们的研究旨在量化这些来源中的恒星形成和活动星系核过程,最近的结果表明,这些来源在观测时代和今天之间已经发生了强烈的演变。为此,我们研究了PAH发射,连续和消光的中红外贡献。大约3/4的样本是连续体(即活动星系核)主导的源,但其中约60%显示PAH发射,这表明存在恒星形成活动。这些源具有比典型的光学选择类星体更红的中红外颜色。大约25%的样品有很强的PAH发射,但没有一个可能是纯粹的星爆,反映在他们相对较高的5微米热尘埃连续体。然而,它们陡峭的30微米至14微米的斜率表明恒星形成可能主导了总的红外光度。其中6个z~2源的EW>~0.3um,L_14um>~10^12Lsun(意味着L_IR>~10^13Lsun)。在这样的光度下,如此高的EW 6.2 ULIRGs在本地宇宙中不存在。我们发现在9.7um处的中值光学深度<tau_9.7>=1.4。这与局部IRAS选择的ULIRG一致,但与SCUBA选择的z~2 ULIRG的早期结果不同。与局部ULIRG相似,我们的样品中约25%显示出极端的遮蔽(tau_9.7&gt;~3),表明埋藏的核。总的来说,我们发现我们的源与本地ULIRGs相似,但亮度要高出一个数量级。目前尚不清楚我们的z~2 ULIRG是局部ULIRG的简单放大版本,还是受到根本不同的物理过程的影响。

英文摘要:

We present mid-IR spectral decomposition of a sample of 48 Spitzer-selected ULIRGs spanning z~1-3 and likely L_IR~10^12-10^13Lsun. Our study aims at quantifying the star-formation and AGN processes in these sources which recent results suggest have evolved strongly between the observed epoch and today. To do this, we study the mid-IR contribution of PAH emission, continuum, and extinction. About 3/4 of our sample are continuum- (i.e. AGN) dominated sources, but ~60% of these show PAH emission, suggesting the presence of star-formation activity. These sources have redder mid-IR colors than typical optically-selected quasars. About 25% of our sample have strong PAH emission, but none are likely to be pure starbursts as reflected in their relatively high 5um hot dust continua. However, their steep 30um-to-14um slopes suggest that star-formation might dominate the total infrared luminosity. Six of our z~2 sources have EW6.2>~0.3um and L_14um>~10^12Lsun (implying L_IR>~10^13Lsun). At these luminosities, such high EW6.2 ULIRGs do not exist in the local Universe. We find a median optical depth at 9.7um of <tau_9.7>=1.4. This is consistent with local IRAS-selected ULIRGs, but differs from early results on SCUBA-selected z~2 ULIRGs. Similar to local ULIRGs about 25% of our sample show extreme obscuration (tau_9.7>~3) suggesting buried nuclei. In general, we find that our sources are similar to local ULIRGs, but are an order of magnitude more luminous. It is not clear whether our z~2 ULIRGs are simply scaled-up versions of local ULIRGs, or subject to fundamentally different physical processes.

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