星暴星系M82超风细丝的分光偏振测量：尘埃流出的运动学
Spectropolarimetry of the superwind filaments of the starburst galaxy M82: kinematics of dust outflow
摘要：
本文给出了星暴星系M82的光谱偏振测量结果。在M82核爆发区的能量外流（“超级风”）中，暗条的光发射线基本上是偏振的。通过我们的研究测量的H-α偏振度和角度与以前的窄带成像偏振测量数据是一致的。偏振发射线相对于星系总光和系统运动中的发射线发生红移。偏振光中的发射线强度比[NII]/H-α和[SII]/H-α与核恒星形成区相似。此外，从偏振光的[SII]6731/6717线比得到的电子密度N_e在距离原子核1 kpc以上的地方为600 ~ 1000 cm^-3，而从总光得到的N_e则小于300 cm^-3。这些事实有力地表明，来自M82核爆发的辐射是被超级风携带和向外输送的尘埃颗粒散射的。一个简单的空心双锥外流模型表明，尘埃颗粒的外流速度v_d在核附近为100 ~ 200 km/s，随离核距离的增加而单调减小，在1 kpc附近达到10 km/s左右。在距离M82核相同的距离上，尘埃的运动速度比电离气体（~600 km/s）和分子气体（~200 km/s）慢得多。这表明，在大半径的超级风中的尘埃颗粒是运动学解耦的气体成分。由于尘埃的速度v_d远小于M82的逃逸速度（在距离星系核1.5千秒差距时约为170 km/s），因此大部分被超级风携带的尘埃无法逃逸到星系际空间，而可能在没有任何额外加速机制（如辐射压）的情况下落回星系盘。
英文摘要：
Spectropolarimetry results for the starburst galaxy M82 are presented. The optical emission lines of the filaments in the energetic outflow ("superwind") from the nuclear starburst region of M82 are substantially polarized. The H-alpha polarization degrees and angles measured by our study are consistent with previous narrowband imaging polarimetry data. The polarized emission lines are redshifted with respect to the emission lines in the total light and systemic motion of the galaxy. The emission line intensity ratios [NII]/H-alpha and [SII]/H-alpha in the polarized light are similar to those of the nuclear star-forming region. In addition, the electron density N_e derived from the[SII]6731/6717 line ratio of the polarized light is 600 - 1000 cm^-3 at a distance of more than 1 kpc from the nucleus, whereas the N_e derived from the total light are less than 300 cm^-3. These facts strongly suggest that the emission from the nuclear starburst of M82 is scattered by dust grains entrained and transported outward by the superwind. A simple hollow biconical outflow model shows that the velocity of the outflowing dust grains, v_d, ranges from 100 to 200 km/s near the nucleus, decreases monotonically with the distance from the nucleus, and reaches about 10 km/s at around 1 kpc. The motion of the dust is substantially slower than that of both ionized gas (~600 km/s) and molecular gas (~200 km/s) at the same distance from the nucleus of M82. This indicates that dust grains in the superwind are kinematically decoupled from both gas components at large radii. Since the dust velocity v_d is much less than the escape velocity of M82 (~170 km/s at 1.5 kpc from the nucleus), most of the dust entrained by the superwind cannot escape to intergalactic space, and may fall back into the galaxy disk without any additional acceleration mechanisms (such as radiation pressure).
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