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arXiv 1708.09396astro-ph.GA
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星系合并在宇宙时间内驱动恒星质量增长中的有限作用

The limited role of galaxy mergers in driving stellar mass growth over cosmic time

G. Martin, S. Kaviraj, J. E. G. Devriendt, Y. Dubois, C. Laigle, C. Pichon

摘要:

一个尚未解决的关键问题是星系合并在推动恒星质量在宇宙时间内增长中所起的作用。最近的观测工作暗示,“主要”合并(质量比$\gtrsim$ 1:4)对宇宙恒星质量增长的总体贡献可能很小,因为它们在高红移时增加了相对较小的恒星形成率,而今天的大部分恒星质量都是在高红移时聚集的。然而,今天的数据集的异质性和相对较小的尺寸,再加上难以识别真正的合并,使得$\textit{empirically}$量化合并对恒星质量增长的贡献具有挑战性。在这里,我们使用地平线活动星系核,宇宙流体动力学模拟,全面量化的贡献合并的恒星形成预算在宇宙的寿命。我们证明:(1)大合并和小合并都使恒星形成的数量增加到相似的程度,(2)在所有红移下,直接归因于合并的恒星形成比例都很小(例如,在z $\sim$ 3和z $\sim$ 1处,分别为$\sim$ 35和$\sim$ 20%),(3)在宇宙时间内,只有$\sim$ 25%的今天恒星质量直接归因于星系合并。我们的研究结果表明,平滑的吸积,而不是合并,是宇宙一生中恒星质量增长的主要驱动力。

英文摘要:

A key unresolved question is the role that galaxy mergers play in driving stellar mass growth over cosmic time. Recent observational work hints at the possibility that the overall contribution of `major' mergers (mass ratios $\gtrsim$1:4) to cosmic stellar mass growth may be small, because they enhance star formation rates by relatively small amounts at high redshift, when much of today's stellar mass was assembled. However, the heterogeneity and relatively small size of today's datasets, coupled with the difficulty in identifying genuine mergers, makes it challenging to $\textit{empirically}$ quantify the merger contribution to stellar mass growth. Here, we use Horizon-AGN, a cosmological hydrodynamical simulation, to comprehensively quantify the contribution of mergers to the star formation budget over the lifetime of the Universe. We show that: (1) both major and minor mergers enhance star formation to similar amounts, (2) the fraction of star formation directly attributable to merging is small at all redshifts (e.g. $\sim$35 and $\sim$20 per cent at z$\sim$3 and z$\sim$1 respectively) and (3) only $\sim$25 per cent of today's stellar mass is directly attributable to galaxy mergers over cosmic time. Our results suggest that smooth accretion, not merging, is the dominant driver of stellar mass growth over the lifetime of the Universe.

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