Title: FEASTS: IGM cooling triggered by tidal interactions through the diffuse HI phase around NGC 4631 Show Chinese title

Title: FEASTS：由潮汐相互作用触发的NGC 4631周围弥散HI相中的IGM冷却

Abstract: We use the single-dish radio telescope FAST to map the HI in the tidally interacting NGC 4631 group with a resolution of 3.24$'$ (7 kpc), reaching a 5-$\sigma$ column density limit of $10^{17.9}$ cm$^{-2}$ assuming a line width of 20 km s$^{-1}$. Taking the existing interferometric HI image from the HALOGAS project of WSRT as reference, we are able to identify and characterize a significant excess of large-scale, low-density, and diffuse HI in the group. This diffuse HI extends for more than 120 kpc across, and accounts for more than one fourth of the total HI detected by FAST in and around the galaxy NGC 4631. In the region of the tidal tails, the diffuse HI has a typical column density above $10^{19.5}$ cm$^{-2}$, and is highly turbulent with a velocity dispersion around 50 km s$^{-1}$. It increases in column density with the dense HI, and tends to be associated with the kinematically ``hotter'' part of the dense HI. Through simple modeling, we find that the majority of the diffuse HI in the tail region is likely to induce cooling out of the hot IGM instead of evaporating or being radiatively ionized. Given these relations of gas in different phases, the diffuse HI may represent a condensing phase of the IGM. Active tidal interactions on-going and in the past may have produced the wide-spreading HI distribution, and triggered the gas accretion to NGC 4631 through the phase of the diffuse HI. Show Chinese abstract

Abstract: 我们使用单天线射电望远镜FAST对潮汐相互作用的NGC 4631星系群中的HI进行成图，分辨率为3.24$'$ (7 kpc)，达到5-$\sigma$柱密度极限为$10^{17.9}$ cm$^{-2}$，假设线宽为20 km s$^{-1}$。以WSRT的HALOGAS项目现有的干涉HI图像作为参考，我们能够识别和表征星系群中存在显著的大尺度、低密度和弥散HI过剩。这种弥散HI延伸超过120 kpc，并占FAST在NGC 4631星系及其周围检测到的总HI的四分之一以上。 在潮汐尾区域，弥散的HI具有典型的柱密度高于$10^{19.5}$ cm$^{-2}$，并且高度湍流，速度弥散约为50 km s$^{-1}$。 它随着致密HI的柱密度增加，并倾向于与致密HI的动力学“更热”部分相关联。 通过简单的建模，我们发现尾部区域大部分弥散HI可能倾向于从热IGM中冷却出来，而不是蒸发或被辐射电离。 鉴于不同相位气体之间的这些关系，弥散HI可能代表IGM的一个凝结相。 正在进行的和过去的活跃潮汐相互作用可能产生了广泛分布的HI，并通过弥散HI的阶段触发了气体向NGC 4631的吸积。