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arXiv 1506.02663astro-ph.GA
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大规模遗迹星系挑战SMBH及其宿主星系的共同演化

Massive relic galaxies challenge the co-evolution of SMBHs and their host galaxies

Anna Ferré-Mateu, Mar Mezcua, Ignacio Trujillo, Marc Balcells, Remco C. E. van den Bosch

摘要:

我们研究了在M $_{\bullet}$ -M $_\mathrm{bulge}$局部标度关系中属于极端异常值(3-5 $σ$)的8个大质量星系的样本。其中两个星系被证实拥有极大的超大质量黑洞(SMBH),而其他六个星系的维里质量估计也与异常大的SMBH一致。通过对它们的恒星形成历史和结构特征的分析,我们发现所有这些极端异常星系都可以被认为是早期(z $\sim$ 2)宇宙的遗迹星系:即它们是致密的(R $_{\mathrm{e}}$ $<$ 2 kpc),并且具有纯粹的古老恒星星族(t $\gtrsim$ 10 Gyr)。为了解释这种偏离局部关系的性质,我们提出了一种假设,在这种假设中,这些超大质量SMBH的宿主是遵循不同演化路径的星系,而不是为大质量星系假设的两阶段生长通道。一旦SMBH和星系的核心在z $\sim$ 2处形成,星系就跳过了第二阶段,保持结构不变,质量和大小也没有进一步增加。我们表明,如果离群值遵循正常的进化路径,通过合并活动的规模增长,预期的(温和的)质量增长将使它们更接近观察到的局部关系。我们的研究结果表明,大多数大质量星系的SMBH生长时期在$\sim$ 10 Gyr前停止。

英文摘要:

We study a sample of eight massive galaxies that are extreme outliers (3-5$σ$) in the M$_{\bullet}$-M$_\mathrm{bulge}$ local scaling relation. Two of these galaxies are confirmed to host extremely large super massive black holes (SMBHs), whereas the virial mass estimates for the other six are also consistent with having abnormally large SMBHs. From the analysis of their star formation histories and their structural properties we find that all these extreme outliers can be considered as relic galaxies from the early (z$\sim$2) Universe: i.e. they are compact (R$_{\mathrm{e}}$$<$2 kpc) and have purely old stellar populations (t$\gtrsim$10 Gyr). In order to explain the nature of such deviations from the local relations, we propose a scenario in which the hosts of these über-massive SMBHs are galaxies that have followed a different evolutionary path than the two-phase growth channel assumed for massive galaxies. Once the SMBH and the core of the galaxy are formed at z$\sim$2, the galaxy skips the second phase, remaining structurally untouched and without further mass and size increase. We show that if the outliers had followed the normal evolutionary path by growing in size via merger activity, the expected (mild) growth in mass would place them closer to the observed local relations. Our results suggest that the SMBH growth epoch for the most massive galaxies stopped $\sim$10Gyr ago.

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