利用ALFALFA捕捉处于较晚演化阶段的潮汐矮星系
Catching Tidal Dwarf Galaxies at a Later Evolutionary Stage with ALFALFA
AI总结：
本文利用ALFALFA巡天数据研究四个近暗星系，结合深度光学成像和HI综合观测，发现其中两个具有潮汐矮星系特征且处于更晚演化阶段，揭示了盲HI巡天对完善低质量星系普查和潮汐矮星系演化研究的重要价值。
摘要：
我们展示了对ALFALFA巡天中四个被归类为“近暗”星系的天体的深度光学成像和测光结果，这些天体因富含气体且在现有星表中光学辐射极弱或缺失而被如此归类。它们的HI质量为$10^7$-$10^9$ $M_{\odot}$，距离约为9-100 Mpc。利用WIYN 3.5米望远镜和One Degree Imager进行的观测揭示了微弱的恒星成分，其在g波段的中心面亮度约为24-25 $\mathrm{mag}\\,\mathrm{arcsec}^{-2}$。我们还展示了利用Westerbork Synthesis Radio Telescope进行的HI综合观测结果。这些近暗星系因其靠近一个或多个大质量星系而被认定为可能的潮汐矮星系（TDGs）。我们证明AGC 229398和AGC 333576很可能具有潮汐矮星系所代表的低暗物质含量和较大的有效半径。它们距离其前身星系的位置比先前研究的潮汐矮星系远得多，表明它们更古老且演化程度更高。AGC 219369很可能以暗物质为主导，而AGC 123216的暗物质含量对于潮汐矮星系而言异常高，但对于普通矮星系而言则偏低。我们考虑了形成这些潮汐矮星系候选体的可能机制，例如传统的主并合情景以及高速飞掠导致的气体抛射。像ALFALFA这样的盲HI巡天能够探测到富含气体、光学上暗淡的潮汐矮星系，这些天体在其他巡天中可能被忽略，从而提供对低质量星系族群的更完整普查，并为研究处于生命周期更晚阶段的潮汐矮星系提供了机会。
英文摘要：
We present deep optical imaging and photometry of four objects classified as "Almost-Dark" galaxies in the ALFALFA survey because of their gas-rich nature and extremely faint or missing optical emission in existing catalogs. They have HI masses of $10^7$-$10^9$ $M_{\odot}$ and distances of $\sim$9-100 Mpc. Observations with the WIYN 3.5m telescope and One Degree Imager reveal faint stellar components with central surface brightnesses of $\sim$24-25 $\mathrm{mag}\,\mathrm{arcsec}^{-2}$ in the g-band. We also present the results of HI synthesis observations with the Westerbork Synthesis Radio Telescope. These Almost-Dark galaxies have been identified as possible tidal dwarf galaxies (TDGs) based on their proximity to one or more massive galaxies. We demonstrate that AGC 229398 and AGC 333576 likely have the low dark matter content and large effective radii representative of TDGs. They are located much farther from their progenitors than previously studied TDGs, suggesting they are older and more evolved. AGC 219369 is likely dark matter dominated, while AGC 123216 has a dark matter content that is unusually high for a TDG, but low for a normal dwarf galaxy. We consider possible mechanisms for the formation of the TDG candidates such as a traditional major merger scenario and gas ejection from a high velocity fly-by. Blind HI surveys like ALFALFA enable the detection of gas-rich, optically faint TDGs that can be overlooked in other surveys, thereby providing a more complete census of the low-mass galaxy population and an opportunity to study TDGs at a more advanced stage of their life cycle.
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