远距离恒星形成星系中IRAC过量：3.3 $μ$ m多环芳烃特征的初步证据？
IRAC Excess in Distant Star-Forming Galaxies: Tentative Evidence for the 3.3$μ$m Polycyclic Aromatic Hydrocarbon Feature ?
摘要：
我们提出的证据存在的IRAC过剩的光谱能量分布（SED）的5个星系在0.6<z<0.9和1个星系在z=1.7。这6个星系，位于大天文台起源深巡天（GOODS-N），是恒星形成，因为他们目前强大的6.2，7.7和11.3 μ m多环芳烃（PAH）线在他们的斯皮策IRS中红外光谱。我们使用一个图书馆的模板计算PEGASE.2适合他们的多波长测光，并推导出他们的恒星连续。减去恒星的连续，使我们能够检测到在5个星系的IRAC带通的3.3微米PAH是预期的显着过剩。然后，我们评估，如果IRAC过剩的物理起源是由于一个模糊的活动星系核（AGN）或温暖的尘埃排放。其中一个星系的活动星系核被遮蔽的证据被发现，而其余四个星系没有表现出任何显着的活动星系核活动。讨论了在银河漫发射中发现的来源不明的暖尘埃连续体可能造成的污染。这样一个连续体的性质必须与本地宇宙不同，才能解释测量到的IRAC过剩，但在了解其起源之前，我们不能明确排除这种可能性。假设IRAC过剩是由3.3微米PAH功能为主，我们发现与观察到的11.3微米PAH线通量产生相同的C-H弯曲和拉伸模式，与模型的预期一致。最后，IRAC过剩似乎与星系中的恒星形成率相关。因此，一旦詹姆斯·韦伯太空望远镜的中红外光谱能力可用，它就可以为测量z>3星系中尘埃恒星的形成提供强有力的诊断。
英文摘要：
We present evidence for the existence of an IRAC excess in the spectral energy distribution (SED) of 5 galaxies at 0.6<z<0.9 and 1 galaxy at z=1.7. These 6 galaxies, located in the Great Observatories Origins Deep Survey field (GOODS-N), are star forming since they present strong 6.2, 7.7, and 11.3 um polycyclic aromatic hydrocarbon (PAH) lines in their Spitzer IRS mid-infrared spectra. We use a library of templates computed with PEGASE.2 to fit their multiwavelength photometry and derive their stellar continuum. Subtraction of the stellar continuum enables us to detect in 5 galaxies a significant excess in the IRAC band pass where the 3.3 um PAH is expected. We then assess if the physical origin of the IRAC excess is due to an obscured active galactic nucleus (AGN) or warm dust emission. For one galaxy evidence of an obscured AGN is found, while the remaining four do not exhibit any significant AGN activity. Possible contamination by warm dust continuum of unknown origin as found in the Galactic diffuse emission is discussed. The properties of such a continuum would have to be different from the local Universe to explain the measured IRAC excess, but we cannot definitively rule out this possibility until its origin is understood. Assuming that the IRAC excess is dominated by the 3.3 um PAH feature, we find good agreement with the observed 11.3 um PAH line flux arising from the same C-H bending and stretching modes, consistent with model expectations. Finally, the IRAC excess appears to be correlated with the star-formation rate in the galaxies. Hence it could provide a powerful diagnostic for measuring dusty star formation in z>3 galaxies once the mid-infrared spectroscopic capabilities of the James Webb Space Telescope become available.
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