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arXiv 1403.3587astro-ph.GAastro-ph.CO
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边缘星系NGC891的近红外偏振测量

Near-Infrared Polarimetry of the Edge-On Galaxy NGC891

Jordan D. Montgomery, Dan P. Clemens

摘要:

用1.8米帕金斯望远镜上的Mimir仪器在H波段(1.6微米)用近红外成像偏振测量法探测了边缘星系NGC 891。在表面亮度为18.8 mag arcsec^-2时,探测到的偏振信号的信噪比大于3。整个椎间盘的偏振百分比(P)的未加权平均值和离散度分别为0.7%和0.3%,偏振位置角(P.A.)分别为12度和19度。至少有一个偏振零点,P下降到接近零,被检测到在NE盘,但不是SW盘。其他几个不对称的P之间的北部和南部的磁盘被发现,可能与螺旋结构。P和P.A.的分布沿着NGC 891的短轴的方向的变化表明,从磁(B)场跟踪盘中平面附近的二向色偏振到偏离盘中平面的散射主导偏振的转变。近红外光谱分析法(NIR P.A.)和射电(3.6cm)同步加速器偏振P.A.数值显示,在东北部的中心区域,B场的方向相似,这表明热等离子体和冷的恒星形成星际介质可能共享一个共同的B场。以前在可见光波段看到的垂直偏振很可能是由明亮星系中心的散射光引起的,不太可能追踪外盘中的极向B场。

英文摘要:

The edge-on galaxy NGC 891 was probed using near-infrared (NIR) imaging polarimetry in the H-band (1.6 um) with the Mimir instrument on the 1.8 m Perkins Telescope. Polarization was detected with signal-to-noise ratio greater than three out to a surface brightness of 18.8 mag arcsec^-2. The unweighted average and dispersion in polarization percentage (P) across the full disk were 0.7% and 0.3%, respectively, and the same quantities for polarization position angle (P.A.) were 12 deg and 19 deg, respectively. At least one polarization null point, where P falls nearly to zero, was detected in the NE disk but not the SW disk. Several other asymmetries in P between the northern and southern disk were found and may be related to spiral structure. Profiles of P and P.A. along the minor axis of NGC 891 suggest a transition from magnetic (B) field tracing dichroic polarization near the disk mid-plane to scattering dominated polarization off the disk mid-plane. A comparison between NIR P.A. and radio (3.6 cm) synchrotron polarization P.A. values revealed similar B-field orientations in the central-northeast region, which suggests that the hot plasma and cold, star-forming interstellar medium may share a common B-field. Disk-perpendicular polarizations previously seen at optical wavelengths are likely caused by scattered light from the bright galaxy center and are unlikely to be tracing poloidal B-fields in the outer disk.

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