Accretion shocks and cold filaments in galaxy formation
Accretion shocks and cold filaments in galaxy formation
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DOI：
10.1111/j.1365-2966.2010.17641.x
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发表时间：
2011-02
影响因子：
4.8
通讯作者：
A. Benson;R. Bower
中科院分区：
文献类型：
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作者：
A. Benson;R. Bower
一般认为，由高质量晕吸积的气体会被冲击加热到晕的维里温度。然而，在低质量晕或高红移时，气体冷却速率足够快，吸积激波不太可能在维里半径附近形成。相反，吸积激波将在较小的半径处形成，也许靠近中央星系。半解析模型总是对吸积受热气体冷却时间和晕的动力学时间尺度限制的区域进行明确区分，使用从快速到缓慢冷却的过渡发生的质量尺度的简单估计。在这项工作中，我们重新审视这个问题，使用最新的理解和校准吸积激波的形成。从完善的Galform代码开始，我们调查的影响占的存在或否则的吸积冲击接近维里半径使用的冲击稳定性模型Birnboim和Dekel。正如预期的那样，当我们修改代码，使星系形成没有有效的反馈时，我们发现所谓的“冷模式”吸积是在高红移时将气体送入星系的主要通道，因此星系中90%的重子（平均所有星系）通过这个通道到达。质量尺度的快速冷却到缓慢的转变发生显着影响在高红移和吸积率成为主导的冷模吸积。然而，冷却通道的这种变化对星系性质的影响被星星形成和反馈周期的补偿效应所减轻。当有效的反馈，它重新加热气体从星系的维里温度，但它不允许气体从晕逃逸，包括在模型中，我们发现，“冷模式”是更不明显，因为存在的气体喷射从星系的光盘，虽然它仍然可以贡献近50%的净流入率时，平均在所有星系。因此，包括最新的校准吸积激波物理的基本结果，从早期的半解析模型，其中使用了一个简单的处理没有什么区别。我们的结论是，这种“冷模式”的物理已经充分占半解析模型和反馈代表一个更大的不确定性比任何这些影响。
A generic expectation for gas accreted by high-mass haloes is that it is shock-heated to the virial temperature of the halo. In low-mass haloes, or at high redshift, however, the gas cooling rate is sufficiently rapid that an accretion shock is unlikely to form close to the virial radius. Instead, the accretion shock will form at smaller radii, perhaps close to the central galaxy. Semi-analytic models have always made a clear distinction between the regimes in which accretion is limited by the cooling time of hot gas and by the dynamical time-scale of the halo, using simple estimates of the mass-scale at which the transition from rapid to slow cooling occurs. In this work, we revisit this issue using the latest understanding and calibration of accretion shock formation. Starting from the well-established Galform code, we investigate the effect of accounting for the presence or otherwise of an accretion shock close to the virial radius using the shock stability model of Birnboim & Dekel. As expected, when we modify the code so that there is no effective feedback from galaxy formation, we find that so-called ‘cold-mode’ accretion is the dominant channel for feeding gas into the galaxies at high redshifts, such that 90 per cent of baryons in galaxies (averaged over all galaxies) arrive via this channel. The mass-scale at which the rapid to slow cooling transition occurs is significantly affected at high redshifts and accretion rates become dominated by cold-mode accretion. However, the impact of this change in the cooling channel on galaxies properties is mitigated by compensating effects in the star formation and feedback cycle. When effective feedback, which reheats gas from galaxies to the virial temperature but which allows no gas to escape from a halo, is included in the model, we find that the ‘cold mode’ is even less apparent because of the presence of gas ejected from the galaxy's disc, although it can still contribute almost 50 per cent of the net inflow rate when averaged over all galaxies. Thus, the inclusion of the latest calibration of accretion shock physics makes little difference to basic results from earlier semi-analytic models, which used a simpler treatment. We conclude that this ‘cold-mode’ physics is already adequately accounted for in semi-analytic models and that feedback represents a much larger uncertainty than any of these effects.