正常边对半导体NGC 4565和NGC 5907的热分子氢发射
Warm Molecular Hydrogen Emission in Normal Edge-On Galaxies NGC 4565 and NGC 5907
摘要：
我们用斯皮策高分辨率红外光谱仪观测了两个邻近的边缘盘星系NGC 4565和NGC 5907中的热分子氢。0-0 S（0）28.2微米和0-0 S（1）17.0微米的纯旋转线在长轴两侧距离每个星系中心10 kpc处被探测到，而在NGC 4565中，S（0）线在一侧r = 15 kpc处被探测到。这个位置位于圆盘中宇宙射线的无线电连续辐射的急剧下降之外。尽管有迹象表明恒星形成活动随着半径的增加而减少，但H2激发温度和H2线与远红外光度表面密度之比Sigma_L（H2）/Sigma_L（TIR）作为半径的函数变化很小，甚至在NGC 4565的盘的扩散外部区域也是如此。这表明H2的激发源在很大的半径范围内运作，并且与年轻恒星紫外线辐射的强度和相对位置无关。虽然在光解离区域的激发是最常见的解释广泛的H2排放，宇宙射线加热或冲击不能排除。在这两个侧边星系中，推测的热分子氢的质量面密度有很大的不同，在r = 10 kpc时，NGC 4565和NGC 5907的质量面密度分别为4（-60）M_solar/pc ^2和3（-50）M_solar/pc ^2。较高的值代表非常不可能的点源上限。点源的情况下，不支持所观察到的光谱狭缝中的发射分布。这些质量表面密度不能支持观测到的超过200 km/s的自转速度。因此，温暖的氢分子不能解释这些盘状星系中的暗物质，这与之前ISO对附近边缘星系NGC 891的研究所暗示的相反。
英文摘要：
We have observed warm molecular hydrogen in two nearby edge-on disk galaxies, NGC 4565 and NGC 5907, using the Spitzer high-resolution infrared spectrograph. The 0-0 S(0) 28.2 micron and 0-0 S(1) 17.0 micron pure rotational lines were detected out to 10 kpc from the center of each galaxy on both sides of the major axis, and in NGC 4565 the S(0) line was detected at r = 15 kpc on one side. This location lies beyond a steep drop in the radio continuum emission from cosmic rays in the disk. Despite indications that star formation activity decreases with radius, the H2 excitation temperature and the ratio of the H2 line and the far-IR luminosity surface densities, Sigma_L(H2}/Sigma_L(TIR}, change very little as a function of radius, even into the diffuse outer region of the disk of NGC 4565. This suggests that the source of excitation of the H2 operates over a large range of radii, and is broadly independent of the strength and relative location of UV emission from young stars. Although excitation in photodissociation regions is the most common explanation for the widespread H2 emission, cosmic ray heating or shocks cannot be ruled out. The inferred mass surface densities of warm molecular hydrogen in both edge-on galaxies differ substantially, being 4(-60) M_solar/pc^2 and 3(-50) M_solar/pc^2 at r = 10 kpc for NGC 4565 and NGC 5907, respectively. The higher values represent very unlikely point-source upper limits. The point source case is not supported by the observed emission distribution in the spectral slits. These mass surface densities cannot support the observed rotation velocities in excess of 200 km/s. Therefore, warm molecular hydrogen cannot account for dark matter in these disk galaxies, contrary to what was implied by a previous ISO study of the nearby edge-on galaxy NGC 891.
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