The small covering factor of cold accretion streams
The small covering factor of cold accretion streams
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DOI：
10.1111/j.1745-3933.2011.01018.x
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发表时间：
2010-11
影响因子：
--
通讯作者：
C. Faucher-Giguère;D. Keres̆
中科院分区：
文献类型：
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作者：
C. Faucher-Giguère;D. Keres̆
星系形成的理论模型预测，星系通过冷模吸积获得大部分重子。对高红移星系的观测，虽然显示了无处不在的外流，但到目前为止，还没有发现令人信服的预测冷流的痕迹，这被解释为对当前模型的挑战。通过对莱曼破裂星系晕的高分辨率、放大光滑粒子流体力学模拟，结合电离辐射传输，我们量化了z=2-4时冷流的覆盖因子。我们特别关注莱曼极限系统(LLSS)和阻尼型Lyα吸收体(DLA)，它们可以通过使用背景星系或类星体视线的吸收光谱来探测，它们与低电离金属吸收体密切相关。结果表明，这些系统的覆盖因子相对较小，且随时间的延长而减小。在z=2时，模拟星系维里半径内的DLA的覆盖系数为∼3%(在这个投影距离的两倍内∼为1%)，并且主要来自星系本身。LLS的相应价值分别为∼10%和4%。由于冷流的覆盖系数较小，与银河系风预期的秩序单位覆盖比例相比，冷流自然以堆叠光谱中的外流为主。我们得出的结论是，现有的观测结果与冷模吸积的预测是一致的，并勾勒出有希望的运动学和化学诊断来区分星系吸积和反馈的信号。
Theoretical models of galaxy formation predict that galaxies acquire most of their baryons via cold mode accretion. Observations of high-redshift galaxies, while showing ubiquitous outflows, have so far not revealed convincing traces of the predicted cold streams, which has been interpreted as a challenge for the current models. Using high-resolution, zoom-in smooth particle hydrodynamics simulations of Lyman break galaxy (LBG) haloes combined with ionizing radiative transfer, we quantify the covering factor of the cold streams at z= 2–4. We focus specifically on Lyman limit systems (LLSs) and damped Lyα absorbers (DLAs), which can be probed by absorption spectroscopy using a background galaxy or quasar sightline, and which are closely related to low-ionization metal absorbers. We show that the covering factor of these systems is relatively small and decreases with time. At z= 2, the covering factor of DLAs within the virial radius of the simulated galaxies is ∼3 per cent (∼1 per cent within twice this projected distance), and arises principally from the galaxy itself. The corresponding values for LLSs are ∼10 and 4 per cent. Because of their small covering factor compared to the order unity covering fraction expected for galactic winds, the cold streams are naturally dominated by outflows in stacked spectra. We conclude that the existing observations are consistent with the predictions of cold mode accretion, and outline promising kinematic and chemical diagnostics to separate out the signatures of galactic accretion and feedback.