Prevalence of Tidal Interactions among Local Seyfert Galaxies
Cheng-Yu Kuo, Jeremy Lim, Ya-Wen Tang, Paul T. P. Ho
备注：
64 pages, 44 figures, accepted for publication in ApJ. Changes: Nothing is changed except that the paper was compiled differently to reduce the number of pages. To be friendly to the Earth forest, you could also choose to only print the text part of the paper and view images on line. (see http://www.astro.virginia.edu/~ck2v/paper/)
No mechanisms have hitherto been conclusively demonstrated to be responsible for initiating optically-luminous nuclear (Seyfert) activity in local disk galaxies. Only a small minority of such galaxies are visibly disturbed in optical starlight, with the observed disturbances being at best marginally stronger than those found in matched samples of inactive galaxies. Here, we report the first systematic study of an optically-selected sample of twenty-three active galaxies in atomic hydrogen (HI) gas, which is the most sensitive and enduring tracer known of tidal interactions. Eighteen of these galaxies are (generally) classified as Seyferts, with over half (and perhaps all) having [OIII] luminosities within two orders of magnitude of Quasi-Stellar Objects. Only ~28% of these Seyfert galaxies are visibly disturbed in optical DSS2 images. By contrast, ~94% of the same galaxies are disturbed in HI, in nearly all cases not just spatially but also kinematically on galactic (>~20 kpc) scales. In at least ~67% and perhaps up to ~94% of cases, the observed HI disturbances can be traced to tidal interactions with neighboring galaxies detected also in HI. The majority of these neighboring galaxies have projected separations of <~ 100 kpc and differ in radial velocities by <~100 km/s from their respective Seyfert galaxies, and many have optical luminosities ranging from the Small to Large Magellanic Clouds. In a companion paper, we show that only ~15% of a matched control sample of inactive galaxies display comparable HI disturbances. Our results suggest that: i) most Seyfert galaxies (with high nuclear luminosities) have experienced tidal interactions in the recent past; ii) in most cases, these tidal interactions are responsible for initiating events that lead to their nuclear activity.
The giant X-ray outburst in NGC 5905 - a tidal disruption event ?
NGC 5905是极少数经历过巨大X射线爆发的星系之一，其ROSAT光子计数率的变化约为100倍。爆发光谱既柔和又明亮。我们对NGC 5905的高分辨率后续光谱学没有发现任何塞弗特活动的迹象。目前，这使得NGC 5905成为X射线爆发星系中唯一不活跃的星系。我们讨论了几种方案来解释NGC 5905的特殊性质，包括致密介质中的超新星，吸积盘不稳定性，极端引力透镜事件，以及GRB的X射线余辉来解释X射线爆发。我们发现，最有可能解释观测结果的模型似乎是恒星被中央SMBH的潮汐破坏，这是Rees（1988）提出的一种场景，作为附近非活动星系中SMBH的示踪剂。这个HII星系中的X射线爆发进一步支持了所有星系都经历了一个活跃阶段的说法，在它们的中心留下了休眠的SMBH。
