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近邻宇宙大星系潮汐结构的搜寻研究

The Search for Tidal Structures around Nearby Large Galaxies

  • 摘要: 在星系形成和演化的等级成团模型中, 星系之间的相互作用和并合过程是星系质量增长的重要机制, 会在星系周围产生一些低面亮度的潮汐结构. 观测上, 需要长时间曝光并结合特定的图像处理技术以获得更深的图像, 从而能够探测和研究这些较暗的结构. 为寻找近邻星系周围的潮汐结构, 从前人给出的源表中筛选出76个大星系样本, 并利用ZTF (Zwicky Transient Facility)时域巡天的g和r两个波段的数据进行图像叠加, 得到的深场图像的平均深度(面亮度极限)为 \mu_\mathrmg(3\sigma,10^''\times10^'')=\left(28.86\pm0.27\right)~\mathrmmag/(^'')^2 , \mu_\mathrmr(3\sigma,10^''\times 10^'')=\left(28.65\pm0.27\right)~\mathrmmag/(^'')^2 (其中\sigma为天光背景噪声标准差), 与国际上多数同类深场巡天深度相当. 在所有样本星系中, 22个星系周围已知存在潮汐结构, 成功探测到其中20个星系附近的结构, 并在一个星系周围首次发现了潮汐结构. 对其中的星系NGC 5907周围的星流结构分析了形态并与前人文章进行了比较, 验证了单一环状星流的结论. 其次, 分析了在星系NGC 4725周围新发现的3个暗弱潮汐结构, 其中位于东北侧的结构颜色较蓝(g、r波段星等差g-r约0.4−0.6), 与东北方向的伴星系NGC 4747颜色相似, 可能源于两个星系之间的相互作用; 位于东侧和东南侧的结构在位置和形态上对应于一个整体的环状结构, 其颜色均较红(g-r约0.7−0.9)且与NGC 4725东侧外晕颜色一致, 可能来源于较年老的矮椭球星系的潮汐瓦解, 或者是NGC 4725在与NGC 4747相互作用的过程中被撕扯出去的盘成分. 后续还需利用更高分辨率、更深的WFST (Wide Field Survey Telescope)及其他波段数据对发现的潮汐结构进一步验证, 并通过数值模拟帮助理解星系相互作用过程. 最后, 在部分样本星系图像中发现延展的前景银河系卷云污染, 其面亮度较低(暗于\mu_\mathrmg = 26~\mathrmmag/(^\prime\prime)^2)且颜色(g-r约0.41)蓝于河外星系等天体成分. 结果显示, 这类卷云反射光会影响星系周围暗弱潮汐结构的探测分析, 但是容易被辨认出来.

     

    Abstract: In the hierarchical model of galaxy formation and evolution, interactions and mergers between galaxies play a pivotal role in their mass growth. These processes lead to the formation of low surface brightness tidal structures around galaxies, which are inherently faint. To capture these structures effectively, a combination of long telescope exposure time and specific image processing techniques is necessary. To search for tidal structures around nearby galaxies, the authors selected 76 large galaxy samples from catalogs in the literature and performed image coaddition using g-band and r-band data from ZTF (the Zwicky Transient Facility) time-domain survey. They achieved an average depth (surface brightness limit) of \mu_\mathrmg(3\sigma,10^''\times10^'')=\left(28.86\pm0.27\right)~\mathrmmag/(^'')^2 , \mu_\mathrmr(3\sigma,10^''\times10^'')= \left(28.65\pm0.27\right)~\mathrmmag/(^'')^2 (\sigma is the standard deviation of sky background noise) for the deep images, comparable to the depth of most international deep imaging surveys. Among the 76 sample galaxies, tidal structures were known to exist around 22 galaxies, and the authors successfully detected the structures around 20 galaxies using their image processing methods. What's more, they first discovered tidal structures around one of the sample galaxies. They specifically analysed the morphology of the tidal stream around NGC 5907. The comparison with previous studies well proved the existence of a single circular stream structure. Secondly, they analysed the three newly discovered faint tidal structures around NGC 4725. The structure in the northeast exhibits a blue color (the magnitude difference g-r\sim 0.4−0.6), similar to the companion galaxy NGC 4747 in the same direction, likely resulting from their interaction. The two structures on the east and southeast sides correspond to an overall ring structure both in location and morphology, displaying a reddish color (g-r \sim 0.7−0.9) akin to the eastern outer halo of NGC 4725. These structures may originate from the tidal disruption of an old dwarf spheroid galaxy or the torn-off disk components of NGC 4725 during interaction with NGC 4747. Future work will necessitate further verification of the tidal structures using higher-resolution and deeper WFST (Wide Field Survey Telescope) images, along with data in other wavebands. Besides, numerical simulations will help gain a better understanding of galaxy interaction processes. Finally, in some sample galaxy images, they identified diffuse foreground Galactic cirrus contamination, characterized by rather low surface brightness (fainter than \mu_\mathrmg = 26~\rm mag/(^\prime\prime)^2) and a bluer color (g-r \sim 0.41) compared to extragalactic sources, facilitating their differentiation. The results show that the light reflected by these cirrus affects the detection and analysis of faint tidal structures around galaxies, but is easy to identify.

     

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