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arXiv 0712.0022astro-ph
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M51中的热分子气体:H_2激发温度和质量的Spitzer红外光谱测绘

Warm Molecular Gas in M51: Mapping the Excitation Temperature and Mass of H_2 with the Spitzer Infrared Spectrograph

G. Brunner, K. Sheth, L. Armus, M. Wolfire, S. Vogel, E. Schinnerer, G. Helou, R. Dufour, J. Smith, D. Dale

摘要:

利用Spitzer空间望远镜上的红外光谱仪(IRS),在M51(NGC 5194)的核盘径向带上绘制了H_2S(0)-H_2S(5)纯转动中红外辐射线示踪的暖分子气体图。六条H_2谱线具有明显不同的发射分布。我们采用两温模型:由低J(S(0)- S(2))线跟踪的暖相(T = 100 - 300 K)和由高J(S(2)- S(5))线跟踪的热相(T = 400 - 1000 K),得到了H_2的温度和表面密度分布。最低的分子气体温度被发现在螺旋臂内(T ~ 155 K),而最高的温度被发现在臂间区域(T > 700 K)。暖气体面密度在西北旋臂处达到最大值11 Msun/pc ^2,而热气体面密度在核处达到最大值0.24 Msun/pc ^2。暖相和热相峰值的空间偏移以及H_2谱线发射分布的差异表明,暖相主要由恒星形成区的紫外光子产生,而热相主要由与核活动有关的激波或X射线产生。温暖的H_2位于M51的尘埃带中,在空间上偏离最亮的HII区。热H_2与CO(J = 1 - 0)辐射所追踪的冷分子气体在空间上基本一致,这与稠密光解区(PDR)中热相的激发相一致。相反,热H_2在核区最为突出。在M51核周围0.5kpc半径范围内,热H_2与[O IV](25.89 μ m)和X射线辐射相一致,表明激波和/或X射线是激发这一相的原因。

英文摘要:

We have mapped the warm molecular gas traced by the H_2 S(0) - H_2 S(5) pure rotational mid-infrared emission lines over a radial strip across the nucleus and disk of M51 (NGC 5194) using the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. The six H_2 lines have markedly different emission distributions. We obtained the H_2 temperature and surface density distributions by assuming a two temperature model: a warm (T = 100 - 300 K) phase traced by the low J (S(0) - S(2)) lines and a hot phase (T = 400 - 1000 K) traced by the high J (S(2) - S(5)) lines. The lowest molecular gas temperatures are found within the spiral arms (T ~ 155 K), while the highest temperatures are found in the inter-arm regions (T > 700 K). The warm gas surface density reaches a maximum of 11 M_sun/pc^2 in the northwestern spiral arm, whereas the hot gas surface density peaks at 0.24 M_sun/pc^2 at the nucleus. The spatial offset between the peaks in the warm and hot phases and the differences in the distributions of the H_2 line emission suggest that the warm phase is mostly produced by UV photons in star forming regions while the hot phase is mostly produced by shocks or X-rays associated with nuclear activity. The warm H_2 is found in the dust lanes of M51, spatially offset from the brightest HII regions. The warm H_2 is generally spatially coincident with the cold molecular gas traced by CO (J = 1 - 0) emission, consistent with excitation of the warm phase in dense photodissociation regions (PDRs). In contrast, the hot H_2 is most prominent in the nuclear region. Here, over a 0.5 kpc radius around the nucleus of M51, the hot H_2 coincides with [O IV](25.89 micron) and X-ray emission indicating that shocks and/or X-rays are responsible for exciting this phase.

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