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基于~（12）CO（1-0）、（2-1）和（3-2）谱线的两个邻近活动星系物理条件的高分辨率成图

High-resolution mapping of the physical conditions in two nearby active galaxies based on 12CO(1-0), (2-1) and (3-2) lines

F. Boone, S. Garcia-Burillo, F. Combes, J. Lim, P. Ho, A. J. Baker, S. Matsushita, M. Krips, Dinh-van-Trung, E. Schinnerer

备注：
accepted for publication in A&A
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摘要：

我们提出了一个详细的分析，在两个附近的活动星系，NGC 4569和NGC 4826的三个最低CO跃迁的高分辨率观测。CO（1-0）和（2-1）线用Plateau de Bure干涉仪观测，CO（3-2）线用亚毫米波列阵观测。结合这些数据，我们可以比较三条谱线中的发射，并绘制谱线比率，R21= I_{CO(2-1)} / I_{CO(1-0)}，R32= I_{CO(3-2)} / I_{CO(1-0)}，分辨率约为2”，即，NGC 4569的线性分辨率为160 pc，NGC 4826的线性分辨率为40 pc。在这两个星系中，三条谱线的发射在空间和速度上的分布相似，CO的激发程度（R32<0.6）低于银河系中心或迄今为止研究的其他活动星系中心。根据伪LTE模型，NGC 4569中的分子气体是冷的，并且主要在CO（1-0）和（2-1）线中是光学厚的;在CO（3-2）线中少于50%的气体是光学薄的。LVG模型表明存在一个细长的冷而致密的气体环，与恒星棒的ILR相吻合。在NGC 4826的环核盘中，有较多的激发态气体被解析。根据我们的伪LTE模型，这对应于较暖的气体，其中约50%的CO（3-2）发射是光学薄的。LVG模型表明，存在一个半圆形的密集和冷气体的动力中心和半径约70 pc为中心。气体温度增加，其密度向中心减小。在CO、R32和Pa-α图中明显的近侧/远侧不对称表明不透明度效应起作用。检查附近活动星系的CO地图，我们发现类似的不对称性，这可能是活动星系中的一种常见现象。这些主要是定性的结果打开了新的视角，未来的阿塔卡玛大型毫米/亚毫米阵列的活动星系的研究。

英文摘要：

We present a detailed analysis of high resolution observations of the three lowest CO transitions in two nearby active galaxies, NGC4569 and NGC4826. The CO(1-0) and (2-1) lines were observed with the Plateau de Bure Interferometer and the CO(3-2) line with the Submillimeter Array. Combining these data allows us to compare the emission in the three lines and to map the line ratios, R21=I_{CO(2-1)}/I_{CO(1-0)} and R32=I_{CO(3-2)}/I_{CO(1-0)} at a resolution of ~2", i.e., a linear resolution of 160 pc for NGC4569 and 40 pc for NGC4826. In both galaxies the emission in the three lines is similarly distributed spatially and in velocity, and CO is less excited (R32<0.6) than in the Galactic Center or the centers of other active galaxies studied so far. According to a pseudo-LTE model the molecular gas in NGC4569 is cold and mainly optically thick in the CO(1-0) and (2-1) lines; less than 50% of the gas is optically thin in the CO(3-2) line. LVG modeling suggests the presence of an elongated ring of cold and dense gas coinciding with the ILR of the stellar bar. More excited gas is resolved in the circumnuclear disk of NGC4826. According to our pseudo-LTE model this corresponds to warmer gas with a ~50% of the CO(3-2) emission being optically thin. LVG modeling indicates the presence of a semicircular arc of dense and cold gas centered on the dynamical center and ~70 pc in radius. The gas temperature increases and its density decreases toward the center. A near side/far side asymmetry noticeable in the CO, R32 and Pa-alpha maps suggests that opacity effects play a role. Examining published CO maps of nearby active galaxies we find similar asymmetries suggesting that this could be a common phenomenon in active galaxies. These mainly qualitative results open new perspectives for the study of active galaxies with the future Atacama Large Millimeter/submillimeter Array.

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