NGC 2915 is a blue compact dwarf galaxy with a very extended H I disk. This disk shows a short central bar and extended spiral arms, both reaching far beyond the optical component. We use Tremaine & Weinberg's method to measure the pattern speed of the bar and spiral arms from H I radio synthesis data. Our measurements yield a pattern speed Ωp = 0.21 ± 0.06 km s-1 arcsec-1 (8.0 ± 2.4 km s-1 kpc-1 for D = 5.3 kpc), in disagreement with the general view that corotation in barred disks lies just outside the end of the bar, but consistent with recent models of barred galaxies with dense dark matter halos. Our adopted bar semilength, rb ≈ 180'', puts corotation at more than 1.7rb. The existence of the pattern is also problematic. Because NGC 2915 is isolated, gravitational interactions cannot account for the structure observed in the H I disk. We also demonstrate that the low surface density observed in the disk and the location of the pseudorings make it unlikely that swing amplification or bar-driven spiral arms could explain the bar and spiral pattern. Based on the similarity of the dark matter and H I surface density profiles, we discuss the possibility of dark matter distributed in a disk and following closely the H I distribution. This disk then becomes gravitationally unstable and can naturally form a bar and spiral pattern. However, this explanation is difficult to reconcile with some properties of NGC 2915. We also consider the effect of a massive and extended triaxial dark matter halo with a rotating figure. The existence of such halos is supported by cold dark matter simulations showing strongly triaxial dark halos with slow figure rotation. The observed structure of the H I disk can then arise through forcing by the rotating triaxial figure. We associate the measured pattern speed in NGC 2915 with the figure rotation of its dark halo.
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机译：NGC 2915是一个蓝色紧凑的矮星系，具有非常扩展的H I盘。该光盘显示出一个短的中心杆和延伸的螺旋臂，两者都远远超出了光学组件。我们使用Tremaine＆Weinberg的方法从H I无线电合成数据中测量棒和螺旋臂的图案速度。我们的测量结果得出图案速度Ωp= 0.21±0.06 km s-1 arcsec-1（D = 5.3 kpc时为8.0±2.4 km s-1 kpc-1），与通常的观点不同，即禁止盘中的同向旋转位于外部柱状图的尽头，但与最近的具有密集暗物质光环的条纹星系模型一致。我们采用的钢筋半长rb≈180''，使同心度超过1.7rb。模式的存在也是有问题的。由于NGC 2915是孤立的，因此重力相互作用无法解释在H I盘中观察到的结构。我们还证明，在磁盘中观察到的低表面密度和伪环的位置使得摆幅放大或杆驱动的螺旋臂不可能解释杆和螺旋图案。基于暗物质和H I表面密度分布的相似性，我们讨论了暗物质在磁盘中分布并紧随H I分布的可能性。然后，该圆盘在重力上变得不稳定，并且可以自然地形成条形和螺旋形图案。但是，这种解释很难与NGC 2915的某些特性相吻合。我们还考虑了具有旋转图形的庞大且扩展的三轴暗物质晕的影响。这种晕圈的存在得到冷暗物质模拟的支持，冷暗物质模拟显示了具有缓慢图形旋转的强三轴暗晕。 H I盘的观察结构然后可以通过旋转三轴图形的强迫而产生。我们将NGC 2915中测得的图案速度与其暗晕的图形旋转关联起来。
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