[论文解读] Fossil group origins IX. Probing the formation of fossil galaxy groups with stellar population gradients of their central galaxies

E. M. Corsini, L. Morelli+8 more|arXiv (Cornell University)|Sep 4, 2018

Galaxies: Formation, Evolution, Phenomena参考文献 104被引用 10

一句话总结

本研究通过分析两个邻近星系化石群（NGC 6482 和 NGC 7556）中心星系（BCG）的空间分辨恒星族群梯度，探讨了化石星系群的形成机制。利用长缝光谱和单星族模型，研究发现金属丰度梯度较浅且呈负值，而α元素丰度梯度可忽略不计，表明这些大质量星系是通过湿式主要并合形成，而非从未拥有明亮卫星的失败星系群。结果支持与宇宙学模拟一致的并合驱动形成情景。

ABSTRACT

Fossil groups (FGs) are galaxy aggregates with an extended and luminous X-ray halo, which are dominated by a very massive early-type galaxy and lack of L* objects. FGs are indeed characterized by a large magnitude gap between their central and surrounding galaxies. This is explained by either speculating that FGs are failed groups that formed without bright satellite galaxies and did not suffer any major merger, or by suggesting that FGs are very old systems that had enough time to exhaust their bright satellite galaxies through multiple major mergers. Since major mergers leave signatures in the stellar populations of the resulting galaxy, we study the stellar population parameters of the brightest central galaxies (BCGs) of FGs as a benchmark against which the formation and evolution scenarios of FGs can be compared. We present long-slit spectroscopic observations along different axes of NGC 6482 and NGC 7556, which are the BCGs of two nearby FGs. The measurements include spatially resolved stellar kinematics and radial profiles of line-strength indices, which we converted into stellar population parameters using single stellar-population models. NGC 6482 and NGC 7556 are very massive and large galaxies and host a centrally concentrated stellar population, which is significantly younger and more metal rich than the rest of the galaxy. The age gradients of both galaxies are somewhat larger than those of the other FG BCGs studied so far, whereas their metallicity gradients are similarly negative and shallow. They have negligible gradients of alpha-element abundance ratio. The measured metallicity gradients are less steep than those predicted for massive galaxies that formed monolithically and evolved without experiencing any major merger. We conclude that the observed FGs formed through major mergers rather than being failed groups that lacked bright satellite galaxies from the beginning.

研究动机与目标

• 检验化石星系群中心星系（BCG）是否通过主要并合形成，或是否为从一开始便缺乏明亮卫星的失败星系群。
• 确定化石星系群BCG中恒星族群梯度是否反映了分层并合与动力学演化的特征。
• 将观测到的年龄、金属丰度和α元素丰度梯度与单体坍缩和并合驱动形成模型的预测进行比较。
• 评估化石星系群BCG的恒星族群特性是否与其他大质量椭圆星系和S0星系不同，从而挑战其为一类独立系统的观点。

提出的方法

• 利用大天区望远镜（Gran Telescopio Canarias）沿NGC 6482和NGC 7556的主轴、副轴及对角线方向获取长缝光谱观测。
• 从光谱中测量空间分辨的恒星动力学特性及谱线深度指数（如H$δ$、Fe5270、Mgb）的径向分布。
• 使用Thomas等（2003）的单星族模型，将谱线深度指数转换为恒星族群参数——年龄、金属丰度和[$α$/Fe]。
• 分析BCG中年龄、金属丰度和α元素丰度的径向梯度，以推断其形成历史。
• 将观测到的梯度与单体坍缩和湿式并合模拟的理论预测进行比较。
• 利用NED、RC3和SDSS的测光与红移数据，推导出恒星质量（$M_{\ast} \simeq 10^{11.5}$ M$_{\odot}$）、距离和结构参数。

实验结果

研究问题

• RQ1 化石星系群BCG中的恒星族群梯度是否支持主要并合形成情景，还是表明其为从未拥有明亮卫星的失败星系群？
• RQ2 NGC 6482和NGC 7556中观测到的年龄、金属丰度和[$\alpha$/Fe]梯度与大质量星系单体坍缩模型的预测相比如何？
• RQ3 化石星系群BCG中是否存在金属丰度梯度与恒星质量的相关性？其与早期型星系总体样本的对比如何？
• RQ4 化石星系群BCG中观测到的梯度是否与其它大质量椭圆星系和S0星系存在显著差异，暗示其具有独特的形成路径？
• RQ5 化石星系群BCG的恒星族群特性在多大程度上与宇宙学流体动力学模拟中星系群形成过程的预测一致？

主要发现

• NGC 6482和NGC 7556是极为巨大的BCG，恒星质量约为$M_{\ast} \simeq 10^{11.5}$ M$_{\odot}$，有效直径约为$D_{25} \simeq 50$ kpc。
• 两星系均拥有中心集中分布的恒星族群，其年龄明显较年轻且金属丰度明显高于外区，表明近期存在恒星形成爆发。
• 两星系的年龄梯度均大于其他化石星系群BCG，表明其近期或持续的恒星形成活动更显著。
• 金属丰度梯度为负但较浅，显著低于大质量星系单体坍缩模型的预测值。
• [$\alpha$/Fe]丰度比表现出可忽略的径向梯度，表明缺乏强烈且短暂的恒星形成爆发，支持其起源于长期或湿式并合。
• 化石星系群BCG中观测到的恒星族群梯度与一般大质量早期型星系的梯度一致，最合理的解释为湿式主要并合，而非失败星系群形成。