Radiation pressure confinement – II. Application to the broad-line region in active galactic nuclei
Radiation pressure confinement – II. Application to the broad-line region in active galactic nuclei
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DOI：
10.1093/mnras/stt2230
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发表时间：
2013-09
影响因子：
4.8
通讯作者：
A. Baskin;A. Laor;J. Stern
中科院分区：
文献类型：
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作者：
A. Baskin;A. Laor;J. Stern
活动星系核（AGN）在所有光度下（$10^{39}$-$10^{47}$ erg s$^{-1}$）都具有相似的宽发射线特性。是什么在10^8 $的光度范围内产生了这种相似性？光致电离不可避免地与动量转移到光致电离气体有关。然而，宽线区（BLR）中的大多数光电离气体遵循开普勒轨道，这表明BLR起源于具有足够大的引力柱的气体。由于入射辐射力的作用，光致电离的气体表面层必然产生压力梯度。我们提出了解决方案的结构，这样的流体静力学光电离气体层中的BLR。气体是分层的，具有低密度高电离的表面层，向内的密度上升，以及均匀密度较冷的内部区域，其中气体压力达到入射辐射压力。这种光致电离层的辐射压力限制（RPC）导致在内部光致电离层中的通用电离参数$U\sim 0.1$，独立于亮度和距离。因此，RPC似乎可以解释活动星系核中BLR性质的普遍性。我们提出了预测的BLR发射每单位覆盖因子，从电离源的距离的函数，电离连续斜率和气体金属丰度的范围。大多数线（不包括H$\beta$）的预测平均强度，以及它们不同的平均发射半径，与现有的观测结果是一致的。
Active galactic nuclei (AGN) are characterized by similar broad emission lines properties at all luminosities ($10^{39}$-$10^{47}$ erg s$^{-1}$). What produces this similarity over a vast range of $10^8$ in luminosity? Photoionization is inevitably associated with momentum transfer to the photoionized gas. Yet, most of the photoionized gas in the Broad Line Region (BLR) follows Keplerian orbits, which suggests that the BLR originates from gas with a large enough column for gravity to dominate. The photoionized surface layer of the gas must develop a pressure gradient due to the incident radiation force. We present solutions for the structure of such a hydrostatic photoionized gas layer in the BLR. The gas is stratified, with a low-density highly-ionized surface layer, a density rise inwards, and a uniform-density cooler inner region, where the gas pressure reaches the incident radiation pressure. This radiation pressure confinement (RPC) of the photoionized layer leads to a universal ionization parameter $U\sim 0.1$ in the inner photoionized layer, independent of luminosity and distance. Thus, RPC appears to explain the universality of the BLR properties in AGN. We present predictions for the BLR emission per unit covering factor, as a function of distance from the ionizing source, for a range of ionizing continuum slopes and gas metallicity. The predicted mean strength of most lines (excluding H$\beta$), and their different average-emission radii, are consistent with the available observations.