M31中的灰尘
The Dust in M31
摘要：
本文用PPMAP程序分析了M31的Herschel观测。PPMAP图像的分辨率是足够的（31 pc的M31），我们可以分析远红外尘埃排放的巨型分子云的规模。通过比较PPMAP估计的远红外发射光学深度在300微米（tau_300），和近红外消光光学深度在1.1微米（tau_1.1）从红的RGB恒星，我们表明，比率R_OBS.tau = tau_1.1/tau_300落在范围500至1500。如此低的数值与许多常用的理论尘埃模型不相容，这些模型预测的R_MODEL.kappa = kappa_1.1/kappa_300（其中kappa是尘埃不透明度系数）的值在2500到4000的范围内。也就是说，除非在300微米处发射的尘埃中有很大一部分（至少60%）是在如此紧凑的来源中，以至于它们不太可能拦截像RGB星这样的分布式群体的视线。这不是一个新的结果：使用不同的观测和/或不同的波长获得的变体已经被其他研究报道过。我们提出了两个分析参数，为什么它是不可能的，至少60%的排放尘埃是在足够紧凑的来源。因此，可能有必要探讨R_OBS.tau的观测值与R_MODEL.kappa的理论值之间的差异是由于现有尘埃模型的局限性造成的可能性。PPMAP还允许我们推导出发射率指数的光学深度加权平均值，β = - dln（kappa_lambda）/dln（lambda），以及尘埃温度T，表示为betabar和Tbar。我们发现，在M31中，R_OBS.tau与β_（10）呈反相关，R_OBS.tau = 2042（+/-24）-557（+/-10）β_（10）。如果得到证实，这对M31中星际尘埃的性质提供了一个具有挑战性的限制。
英文摘要：
We have analysed Herschel observations of M31, using the PPMAP procedure. The resolution of PPMAP images is sufficient (31 pc on M31) that we can analyse far-IR dust emission on the scale of Giant Molecular Clouds. By comparing PPMAP estimates of the far-IR emission optical depth at 300 microns (tau_300), and the near-IR extinction optical depth at 1.1 microns (tau_1.1) obtained from the reddening of RGB stars, we show that the ratio R_OBS.tau = tau_1.1/tau_300 falls in the range 500 to 1500. Such low values are incompatible with many commonly used theoretical dust models, which predict values of R_MODEL.kappa = kappa_1.1/kappa_300 (where kappa is the dust opacity coefficient) in the range 2500 to 4000. That is, unless a large fraction, at least 60%, of the dust emitting at 300 microns is in such compact sources that they are unlikely to intercept the lines of sight to a distributed population like RGB stars. This is not a new result: variants obtained using different observations and/or different wavelengths have already been reported by other studies. We present two analytic arguments for why it is unlikely that at least 60% of the emitting dust is in sufficiently compact sources. Therefore it may be necessary to explore the possibility that the discrepancy between observed values of R_OBS.tau and theoretical values of R_MODEL.kappa is due to limitations in existing dust models. PPMAP also allows us to derive optical-depth weighted mean values for the emissivity index, beta = - dln(kappa_lambda)/dln(lambda), and the dust temperature, T, denoted betabar and Tbar. We show that, in M31, R_OBS.tau is anti-correlated with betabar according to R_OBS.tau = 2042(+/-24)-557(+/-10)betabar. If confirmed, this provides a challenging constraint on the nature of interstellar dust in M31.
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