The Ages of Elliptical Galaxies from Mid-Infrared Emission
The Ages of Elliptical Galaxies from Mid-Infrared Emission
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DOI：
10.1086/427867
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发表时间：
2004-12
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影响因子：
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通讯作者：
P. Temi;W. Mathews;F. Brighenti
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作者：
P. Temi;W. Mathews;F. Brighenti
椭圆星系的中红外（10-20μm）光度主要由红巨星星周尘埃的综合发射决定。随着单个恒星族的演化，红巨星的尘埃质量损失率随着时间的推移而降低，因此中红外光度也应该随着恒星年龄的增长而降低。为了寻求这种相关性，我们使用档案红外空间天文台 (ISO) 观测来确定 17 个早期型星系的表面亮度分布和 15 μm 处的中心通量，这些早期型星系的恒星年龄是根据光谱指数确定的。正如预期的那样，15 μm 处的径向表面亮度分布通常遵循恒星 de Vaucoulurs 轮廓。我们发现，年龄≥5 Gyr 的椭圆星系和有近期合并证据的星系中，表面亮度比μ15 μm/μI 波段总体较高。在我们观察的准确性范围内，μ15 μm/μI 带显示年龄≥5 Gyr 时不存在年龄依赖性。缩放到有效半径 (Re/8) 的孔径内相应的通量比 F15 μm/FI 波段与较大星系半径处的 μ15 μm/μI 波段比成正比，这表明在我们的一些样本星系的光学图像中可见的中心尘埃云中没有检测到 15 μm 发射。 NGC 1316 是一个椭圆星系，被认为最近发生了合并，在由尘埃和冷气体组成的非中心大质量云中观察到了 15 μm 的发射。最近的斯皮策太空望远镜数据还表明，8 μm 处存在多环芳烃 (PAH) 排放。一些椭圆机具有扩展的 15 μm 发射区域，而在其他频率下没有明显的对应区域。
The mid-infrared (10-20 μm) luminosity of elliptical galaxies is dominated by the integrated emission from circumstellar dust in red giant stars. As a single stellar population evolves, the rate of dusty mass loss from red giant stars decreases with time, so the mid-infrared luminosity should also decline with stellar age. To seek such a correlation, we have used archival Infrared Space Observatory (ISO) observations to determine surface brightness profiles and central fluxes at 15 μm in 17 early-type galaxies for which stellar ages have been determined from optical spectral indices. The radial surface brightness distributions at 15 μm generally follow the stellar de Vaucouleurs profile, as expected. We find that the surface brightness ratio μ15 μm/μI band is systematically higher in elliptical galaxies with ages ≲5 Gyr and in galaxies that exhibit evidence of recent mergers. Within the accuracy of our observations, μ15 μm/μI band shows no age dependence for ages ≳5 Gyr. The corresponding flux ratios F15 μm/FI band within apertures scaled to the effective radius (Re/8) are proportional to the μ15 μm/μI band ratios at larger galactic radii, indicating that no 15 μm emission is detected from central dust clouds visible in optical images in some of our sample galaxies. Emission at 15 μm is observed in noncentral massive clouds of dust and cold gas in NGC 1316, an elliptical galaxy that is thought to have had a recent merger. Recent Spitzer Space Telescope data also indicate the presence of polycyclic aromatic hydrocarbon (PAH) emission at 8 μm. Several ellipticals have extended regions of 15 μm emission that have no obvious counterparts at other frequencies.