射电泡Seyfert星系NGC 6764的秒差距尺度成像
Parsec-scale Imaging of the Radio-bubble Seyfert galaxy NGC 6764
摘要：
我们用甚长基线天线阵在1.6和4.9 GHz对复合活动星系核-星暴星系NGC 6764进行了观测。这些观测发现了一个“核心喷流”结构和一个可能的弱反喷流分量在1.6 GHz。核心和喷流（1.6-4.9 GHz）光谱指数的上限分别为0.6和0.3。再加上核心区域约10^7 K的高亮度温度，射电辐射似乎来自同步加速器喷流。在25度角的位置上，秒差距尺度的喷流似乎正紧密地指向NGC 6764中南部kpc尺度气泡的西部边缘。秒差距和次千秒差距尺度的辐射之间的真正联系不仅表明存在弯曲喷流，而且还表明活动星系核喷流和NGC 6764中的射电泡之间存在密切联系。我们表明，旋进射流模型可以解释从秒差距到次千秒差距尺度的无线电形态，射流速度和方向的模型最佳拟合参数是完全符合所观察到的喷流对喷流表面亮度比。然而，喷流似乎在数百秒差距的尺度上被破坏，可能是由于与星际介质气体的相互作用和夹带，随后导致气泡的形成。NGC 6764中的喷流能量学表明，膨胀（南部）气泡需要1200万~ 2100万年。这个时间尺度大致对应于大约1500 - 5000万年前在NGC 6764中发生的星暴事件，并且可能表明喷流形成和该星系中的星暴活动之间存在密切联系。
英文摘要：
We have observed the composite active galactic nucleus (AGN)-starburst galaxy NGC 6764 with the Very Long Baseline Array at 1.6 and 4.9 GHz. These observations have detected a "core-jet" structure and a possible weak counterjet component at 1.6 GHz. The upper limits to the core and jet (1.6-4.9 GHz) spectral index are 0.6 and 0.3, respectively. Taken together with the high brightness temperature of ~10^7 K for the core region, the radio emission appears to be coming from a synchrotron jet. At a position angle of 25 degrees, the parsec-scale jet seems to be pointing closely toward the western edge of the southern kpc-scale bubble in NGC 6764. A real connection between the parsec- and sub-kpc-scale emission would not only suggest the presence of a curved jet, but also a close link between the AGN jet and the radio bubbles in NGC 6764. We demonstrate that a precessing jet model can explain the radio morphology from parsec- to sub-kpc scales, and the model best-fit parameters of jet speed and orientation are fully consistent with the observed jet-to-counterjet surface brightness ratio. The jet however appears to be disrupted on scales of hundreds of parsecs, possibly due to interaction with and entrainment of the interstellar medium gas, which subsequently leads to the formation of bubbles. The jet energetics in NGC 6764 suggest that it would take 12-21 Myr to inflate the (southern) bubble. This timescale corresponds roughly to the starburst episode that took place in NGC 6764 about 15-50 Myr ago, and could be indicative of a close connection between jet formation and the starburst activity in this galaxy.
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