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arXiv 1410.5437astro-ph.GA
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早型星系中原子和分子气体含量的起源。II。错位气体吸积

The origin of the atomic and molecular gas contents of early-type galaxies. II. Misaligned gas accretion

Claudia del P. Lagos, N. D. Padilla, T. A. Davis, C. G. Lacey, C. M. Baugh, V. Gonzalez-Perez, M. Zwaan, S. Contreras

摘要:

本文研究了早型星系(ETG)中恒星角动量与气体成分$α_{\rm G,S}$之间夹角宽分布的成因。我们使用GALFORM模型的星系形成,设置在$Λ$冷暗物质的框架,并将其与蒙特卡罗模拟跟随角动量翻转驱动的物质吸积到晕和星系。如果$α_{\rm G,S}>30$ ~度,我们认为气体盘相对于恒星体是不对准的。通过假设星系中错位的唯一来源是星系合并,我们将ETG中气体/恒星成分错位的比例下限设为$2-5$ %。这些低分数与ATLAS $^{\rm 3D}$中观察到的$\approx 42\pm 6$ %值不一致。在更一般的情况下,除了星系合并外,平滑的气体吸积也会导致错位,我们的计算预测,$\approx 46$ %的ETG具有$α_{\rm G,S}>30$ ~度。在计算中,我们发现了$α_{\rm G,S}$与恒星质量、冷气体分数和恒星形成率之间的相关性,使得质量高、冷气体分数低和恒星形成率低的ETG更有可能显示出冷气体和恒星成分的排列。我们首次使用ATLAS $^{\rm 3D}$数据观测证实了这些趋势。我们认为,在ETG中观察到的错位气体盘的高比例主要是由于平滑的气体吸积(例如,从星系的热晕冷却),这发生在星系的大部分恒星质量到位后,相对于恒星组件错位。在大部分恒星质量到位之前,已经吸积了大部分冷气的星系团显示出对齐的成分。

英文摘要:

We study the origin of the wide distribution of angles between the angular momenta of the stellar and gas components, $α_{\rm G,S}$, in early-type galaxies (ETGs). We use the GALFORM model of galaxy formation, set in the $Λ$ cold dark matter framework, and coupled it with a Monte-Carlo simulation to follow the angular momenta flips driven by matter accretion onto haloes and galaxies. We consider a gas disk to be misaligned with respect to the stellar body if $α_{\rm G,S}>30$~degrees. By assuming that the only sources of misaligments in galaxies are galaxy mergers, we place a lower limit of $2-5$ per cent on the fraction of ETGs with misaligned gas/stellar components. These low fractions are inconsistent with the observed value of $\approx 42\pm 6$ per cent in ATLAS$^{\rm 3D}$. In the more general case, in which smooth gas accretion in addition to galaxy mergers can drive misalignments, our calculation predicts that $\approx 46$ per cent of ETGs have $α_{\rm G,S}>30$~degrees. In this calculation, we find correlations between $α_{\rm G,S}$ and stellar mass, cold gas fraction and star formation rate, such that ETGs with high masses, low cold gas fractions and low star formation rates are more likely to display aligned cold gas and stellar components. We confirm these trends observationally for the first time using ATLAS$^{\rm 3D}$ data. We argue that the high fraction of misaligned gas discs observed in ETGs is mostly due to smooth gas accretion (e.g. cooling from the hot halo of galaxies) which takes place after most of the stellar mass of the galaxy is in place and comes misaligned with respect to the stellar component. Galaxies that have accreted most of their cold gas content prior to the time where most of the stellar mass was in place show aligned components.

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