Low-resolution, mid-infrared Spitzer/IRS spectral maps are presented for three nearby, low-metallicity dwarf galaxies (NGC 55, NGC 3109 and IC 5152) for the purpose of examining the spatial distribution and variation of polycyclic aromatic hydrocarbon (PAH) emission. The sample straddles a metallicity of 12+log(O/H)~8.0, a transition point below which PAH intensity empirically drops and the character of the interstellar medium changes. We derive quantitative radiances of PAH features and atomic lines on both global and spatially-resolved scales. The Spitzer spectra, combined with extensive ancillary data from the UV through the mid-infrared, allow us to examine changes in the physical environments and in PAH feature radiances down to a physical scale of 50 pc. We discuss correlations between various PAH emission feature and atomic line radiances. The (6.2 micron)/(11.3 micron), (7.7 micron)/(11.3 micron), (8.6 micron)/(11.3 micron), (7.7 micron)/(6.2 micron), and (8.6 micron)/(6.2 micron) PAH radiance ratios are found to be independent of position across all three galaxies, although the ratios do vary from galaxy to galaxy. As seen in other galaxies, we find no variation in the grain size distribution as a function of local radiation field strength. Absolute PAH feature intensities as measured by a ratio of PAH/(24 micron) radiances are seen to vary both positionally within a given galaxy, and from one galaxy to another when integrated over the full observed extent of each system. We examine direct comparisons of CC mode PAH ratios (7.7 micron)/(6.2 micron) and (8.6 micron)/(6.2 micron) to the mixed (CC/CH) mode PAH ratio (7.7 micron)/(11.3 micron). We find little variation in either mode, and no difference in trends between modes. While the local conditions change markedly over the observed regions of these galaxies, the properties of PAH emission show a remarkable degree of uniformity.
The Spitzer Spectroscopic Survey of the Small Magellanic Cloud (S4MC): Probing the Physical State of Polycyclic Aromatic Hydrocarbons in a Low-Metallicity Environment
The abundance and emission of PAHs from the local to early Universe
University of Florida（佛罗里达大学）；University of Copenhagen（哥本哈根大学）；Technical University of Denmark（丹麦技术大学）；University of Virginia（弗吉尼亚大学）；INAF, Osservatorio Astronomico di Trieste（意大利国家天体物理研究所的的里雅斯特天文台）；Institute for Fundamental Physics of the Universe（宇宙基础物理研究所）；Jet Propulsion Laboratory, California Institute of Technology（加州理工学院喷气推进实验室）；University of Texas at Austin（德克萨斯大学奥斯汀分校）；University of Massachusetts, Amherst（马萨诸塞大学阿默斯特分校）；European Southern Observatory（欧洲南方天文台）；Centro de Astrobiologia (CAB), CSIC-INTA（西班牙国家研究委员会-国家航空航天技术研究所天体生物学中心）
An In-Depth View of the Mid-Infrared Properties of Point Sources and the Diffuse ISM in the SMC Giant HII Region, N66
本文利用斯皮策空间望远镜的红外光谱仪研究了低金属丰度（~0.2太阳）的巨大HII区N66的中红外点源和弥漫星际介质（ISM）。我们研究了14个有针对性的红外点源，以及光谱的扩散ISM是代表的光解离区域（PDR）和HII区域。在点源光谱，我们光谱确认，最明亮的中红外点源是一个大规模的嵌入式年轻恒星的对象，我们检测到硅酸盐的发射与两个年轻的星团，我们观察到光谱特征的一个已知的B[e]超巨星，更常见的是与赫比格Be星。在漫ISM中，我们提供了额外的证据表明，非常小的颗粒人口被光破坏的硬辐射场。11.3 μ m PAH复合物发射在点源和ISM光谱中都表现出无法解释的质心偏移，应在更高的信噪比和分辨率下进行研究。与其他地区的研究不同，6.2微米和7.7微米波段的通量是解耦的;数据点覆盖了I6.2/I11.3比值的窄带内的I7.7/I11.3 PAH比值的大范围。此外，在PAH电离中存在扩散，在辐射场相对较软的密集PDR中更中性，但在扩散ISM/PDR中电离。相比之下，PAH的尺寸分布似乎是独立的本地电离状态。对于河外低金属丰度恒星形成区尚未解决的研究很重要，我们发现来自红外亮点源的发射仅占整个区域PAH发射的20-35%。这些结果与其他具有类似硬辐射场和强辐射场的恒星形成区域进行了比较。
