近日，北京大学的王凯翔及其研究小组与美国国家科学基金会NOIRLab的Eric W. Peng以及上海交通大学的刘成则等人合作并取得一项新进展。经过不懈努力，他们揭示从有核矮星系到星团的演化连续体。相关研究成果已于2023年11月8日在国际权威学术期刊《自然》上发表。

该研究团队发现，室女座星团中的106个星系展现出一种独特的形态，它们既不同于正常的星系，也有别于有核矮星系和单组分UCD。这一发现揭示了一个连续的形态转变过程，并将星团与星系之间的“大小差距”完全映射出来。这些星系的空间分布和较红的颜色与被剥离的卫星星系在环绕大质量星系的最初几个中心通道上的特征相一致。此外，这些星系周围的“超扩散”潮汐特征直观地展示了UCD如何通过潮汐剥离形成。这种演化路径或许还包含了有核超扩散星系的早期阶段。这些UCD被认为是星系团中古代矮星系留下的丰富可见化石残留物，未来或许还能发现更多的低质量残留物。

据悉，系统研究已经揭示了附近宇宙中数百个超致密矮星系(UCDs)。UCD的半光半径r_h约为10-100秒差距，恒星质量M_*≈10⁶-10⁸个太阳质量，是已知密度最大的恒星系统之一。尽管它们与大质量球状星团在外观上相似，但进一步的研究发现了扩展的恒星包层、复杂的恒星形成历史、高质量光比以及超大质量黑洞的存在。这些证据表明，部分UCD是潮汐剥离矮星系的残余核星团，甚至可能是古老的致密星系。然而，目前仅发现少数天体处于潮汐剥离的暂态阶段，而且这一假设的演化路径尚未被完全追踪。

附：英文原文

Title: An evolutionary continuum from nucleated dwarf galaxies to star clusters

Author: Wang, Kaixiang, Peng, Eric W., Liu, Chengze, Mihos, J. Christopher, Ct, Patrick, Ferrarese, Laura, Taylor, Matthew A., Blakeslee, John P., Cuillandre, Jean-Charles, Duc, Pierre-Alain, Guhathakurta, Puragra, Gwyn, Stephen, Ko, Youkyung, Lanon, Ariane, Lim, Sungsoon, MacArthur, Lauren A., Puzia, Thomas, Roediger, Joel, Sales, Laura V., Snchez-Janssen, Rubn, Spengler, Chelsea, Toloba, Elisa, Zhang, Hongxin, Zhu, Mingcheng

Issue&Volume: 2023-11-08

Abstract: Systematic studies have revealed hundreds of ultra-compact dwarf galaxies (UCDs) in the nearby Universe. With half-light radii r_h of approximately 10–100 parsecs and stellar masses M_* ≈ 10⁶–10⁸ solar masses, UCDs are among the densest known stellar systems. Although similar in appearance to massive globular clusters, the detection of extended stellar envelopes, complex star formation histories, elevated mass-to-light ratio and supermassive black holes suggest that some UCDs are remnant nuclear star clusters of tidally stripped dwarf galaxies, or even ancient compact galaxies. However, only a few objects have been found in the transient stage of tidal stripping, and this assumed evolutionary path has never been fully traced by observations. Here we show that 106 galaxies in the Virgo cluster have morphologies that are intermediate between normal, nucleated dwarf galaxies and single-component UCDs, revealing a continuum that fully maps this morphological transition and fills the ‘size gap’ between star clusters and galaxies. Their spatial distribution and redder colour are also consistent with stripped satellite galaxies on their first few pericentric passages around massive galaxies. The ‘ultra-diffuse’ tidal features around several of these galaxies directly show how UCDs are forming through tidal stripping and that this evolutionary path can include an early phase as a nucleated ultra-diffuse galaxy. These UCDs represent substantial visible fossil remnants of ancient dwarf galaxies in galaxy clusters, and more low-mass remnants probably remain to be found.

DOI: 10.1038/s41586-023-06650-z

Source: https://www.nature.com/articles/s41586-023-06650-z