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arXiv 0809.3397astro-ph
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AGN中的星爆和环面演化

Starbursts and torus evolution in AGN

B. Vollmer, T. Beckert, R. Davies

摘要:

最近VLT SINFONI对邻近活动星系核的封闭环境(~ 30 pc)的观测表明,经常发现年龄在10 ~ 1.5亿年之间的厚气体环和星暴,将这些观测结果应用于先前建立的星团吸积盘的分析模型,我们提出了星暴和活动星系核相的演化序列。观测到的气体性质告诉我们环的当前状态,而星暴特征则提供了环历史的信息。在所提出的演化中,一个环面会经历3个不同的阶段,这3个阶段是由外部质量吸积速率所决定的。由初始的、短暂的、大量的气体流入开始,一个湍流的、由恒星风驱动的Q~1盘形成,星暴在其中进行。一旦超新星爆炸,云间介质被移除,留下一个巨大的、几何厚度的碰撞盘,其吸积率下降,但仍然很高。当质量吸积率显著降低时,碰撞环变得薄而透明,就像银河系中心的环核盘一样。当有第二次短暂而大量的气体流入时,这种情况可能会发生变化,在这种情况下,环面可能会切换回恒星爆发模式,或者当没有初始的短暂大量气体流入时。到目前为止,所有观察到的环面都是碰撞的和厚的。观测表明,这一阶段可以持续超过1亿年。在这一阶段,环内的质量吸积速率下降缓慢(在1.5亿年内是4倍)。碰撞环面也形成恒星,但与湍流盘相比,效率约为10%。

英文摘要:

Recent VLT SINFONI observations of the close environments (~30pc) of nearby AGNs have shown that thick gas tori and starbursts with ages between 10 and 150Myr are frequently found. By applying these observations to a previously established analytical model of clumpy accretion disks, we suggest an evolutionary sequence for starburst and AGN phases. Whereas the observed properties of the gas tell us about the current state of the torus, the starburst characteristics provide information on the history of the torus. In the suggested evolution, a torus passes through 3 different phases predetermined by an external mass accretion rate. Started by an initial, short, and massive gas infall, a turbulent and stellar wind-driven Q~1 disk is formed in which the starburst proceeds. Once the supernovae explode the intercloud medium is removed, leaving a massive, geometrically thick, collisional disk with a decreasing, but still high-mass accretion rate. When the mass accretion rate has significantly decreased, the collisional torus becomes thin and transparent as the circumnuclear disk in the Galactic center of the Milky Way. Variations on this scenario are possible either when there is a second short and massive gas infall, in which case the torus may switch back into the starburst mode, or when there is no initial short massive gas infall. All observed tori up to now have been collisional and thick. The observations show that this phase can last more than 100Myr. During this phase the decrease in the mass accretion rate within the torus is slow (a factor of 4 within 150Myr). The collisional tori also form stars, but with an efficiency of about 10% when compared to a turbulent disk.

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