Title: Describing the Nonuniversal Galaxy Merger Timescales in IllustrisTNG: Effects of Host Halo Mass, Baryons, and Sample Selection Show Chinese title

Title: 描述IllustrisTNG中非普遍的星系并合时标：主晕质量、重子物质和样本选择的影响

Abstract: Galaxy merger timescales are crucial for understanding and modeling galaxy formation in our hierarchically structured Universe. However, previous studies have reported widely varying dependencies of merger timescales on initial orbital parameters and mass ratio at the first crossing of $r_{\rm vir}$. Using IllustrisTNG simulations, we find that these dependencies vary with host halo mass, suggesting that discrepancies in prior studies may arise from differences in the systems analyzed. Specifically, in low-mass halos, merger timescales show a stronger dependence on initial orbital parameters, while in high-mass halos, this dependence weakens. To account for these variations, we present a fitting formula that incorporates host mass dependence, achieving a logarithmic scatter smaller than 0.15 dex. Comparing dark matter-only and baryonic simulations, we observe similar merger timescales for circular orbits but notable differences for radial orbits. In halos with $M_{\rm{host}} < 10^{12.5} h^{-1} M_{\odot}$, mergers in dark matter-only runs take longer than in baryonic runs, whereas the trend reverses in more massive halos. We attribute these differences to the competing effects of tidal disruption by central galaxy disks and the resistance of baryonic satellites to tidal stripping. Finally, we extend our model to predict merger timescales from any starting radius within the halo. By fitting the extended model to the entire infall sample, we find that using only the merger sample can underestimate merger timescales, particularly for low mass ratios. Our model provides a valuable tool for improving semi-analytical and empirical models of galaxy formation. Show Chinese abstract

Abstract: 星系合并时间尺度对于理解并模拟我们分层结构的宇宙中的星系形成至关重要。然而，先前的研究报告称，合并时间尺度在首次穿过$r_{\rm vir}$时对初始轨道参数和质量比的依赖性存在广泛差异。使用 IllustrisTNG 模拟，我们发现这些依赖性随着主机晕的质量而变化，这表明之前研究中的差异可能源于所分析系统的不同。具体来说，在低质量晕中，合并时间尺度对初始轨道参数的依赖性更强，而在高质量晕中，这种依赖性则减弱。为了考虑这些变化，我们提出了一种包含主机质量依赖性的拟合公式，实现了小于 0.15 dex 的对数散射。比较暗物质仅模拟和重子模拟，我们观察到对于圆形轨道的合并时间尺度相似，但对于径向轨道则有显著差异。在具有$M_{\rm{host}} < 10^{12.5} h^{-1} M_{\odot}$的晕中，暗物质仅模拟中的合并时间比重子模拟更长，而在更高质量的晕中，趋势则相反。我们将这些差异归因于中心星系盘的潮汐破坏作用和重子卫星对潮汐剥离的阻力之间的竞争效应。最后，我们将模型扩展为从晕内的任何起始半径预测合并时间尺度。通过将扩展模型拟合到整个吸积样本，我们发现仅使用合并样本可能会低估合并时间尺度，尤其是在低质量比的情况下。我们的模型为改进星系形成的半解析和经验模型提供了一个有价值的工具。