扰动的羽毛：EAGLE模拟中并合驱动的星系尺寸增长与结构转变
Ruffled Feathers: Merger-driven galaxy size growth and structural transformation in EAGLE
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中文总结 AI 辅助
本研究以EAGLE模拟的约4500次并合为样本，检验能量守恒估计法预测并合后星系尺寸的效果，量化并合驱动的形态转变，发现小质量并合无法确定为大质量星系快速尺寸增长的驱动因素。
中文摘要 AI 辅助
星系并合既驱动星系尺寸增长，也促使星系从盘状结构向椭球星系转变，但半解析模型中用于模拟这些过程的 prescription（模型预设）尚未结合宇宙学流体动力学模拟中的真实并合群体进行检验。利用EAGLE模拟中识别出的约4500次并合事件，我们检验了一种用于预测并合后星系尺寸的能量守恒估计方法，并量化了并合驱动的形态学转变。预测的残余恒星半质量半径与模拟的后代星系尺寸的离散度约为0.12-0.15 dex，且对前身星系属性无显著系统依赖性；而在宇宙学环境下，对富气并合应用常用的耗散修正会低估并合后尺寸达约0.4 dex，并增加整体离散度。每次并合的尺寸增长随并合对的恒星质量比单调递增，从并合质量比小于0.03 dex的 minor mergers（并合）到质量相当的并合约0.10 dex。从能量守恒估计方法中，我们解析推导出单位吸积恒星质量的尺寸增长效率η≡dlog₁₀r⋆/dlog₁₀M⋆，并表明仅在无轨道能量的无碰撞小质量并合的理想极限下，η才达到约2；由于η对并合时的轨道能量高度敏感，在未更好约束该量的情况下，无法将小质量并合通道确定为大质量星系快速尺寸增长的驱动因素。除尺寸增长外，合并系统会按质量比降低旋转支撑并增加三轴度，但即使是质量最接近相等的并合也并非总能完全破坏盘状结构，这与一些半解析模型中假设的盘状结构完全破坏存在矛盾。
英文摘要
Galaxy mergers drive both the size growth and the transformation from discs to spheroids, yet the prescriptions used to model these processes in semi-analytic frameworks have not been tested against the realistic merger population in cosmological hydrodynamical simulations. Using $\approx 4{,}500$ mergers identified in the EAGLE simulation, we test an energy-conservation estimator for post-merger galaxy sizes and quantify merger-driven morphological transformation. The predicted remnant half-stellar-mass radius matches the simulated descendant size with a scatter of $\approx 0.12$-$0.15$ dex and no significant systematic dependence on progenitor properties, while a commonly used dissipation correction applied to gas-rich mergers under-predicts the post-merger size by up to $\approx 0.4$ dex in a cosmological context and increases the overall scatter. The per-merger size growth increases monotonically with the stellar mass ratio of the merging pair, from $\lesssim 0.03$ dex for minor mergers to $\approx 0.10$ dex for equal-mass mergers. From the energy-conservation estimator, we analytically derive the size growth efficiency per unit accreted stellar mass, $η\equiv \mathrm{d}\log_{10} r_{\star}/\mathrm{d}\log_{10} M_{\star}$, and show that $η$ reaches $\approx 2$ only in the idealised limit of collisionless minor mergers with zero orbital energy; as $η$ is highly sensitive to the orbital energy at the time of merging, the minor merger channel cannot be established as the driver of the rapid size growth of massive galaxies without better constraints on this quantity. Beyond the size growth, mergers systematically reduce rotational support and increase triaxiality in proportion to mass ratio, but even the most nearly equal-mass mergers do not always fully destroy the disc, in tension with the complete disc destruction assumed in several semi-analytic models.
