PAH features within few hundred parsecs of active galactic nuclei
PAH features within few hundred parsecs of active galactic nuclei
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DOI：
10.1093/mnras/stx1447
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发表时间：
2017-06
影响因子：
4.8
通讯作者：
J. Jensen;J. Jensen;S. Hönig;S. Rakshit;S. Rakshit;A. Alonso-Herrero;D. Asmus;P. Gandhi;M. Kishimoto;A. Smette;K. Tristram
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文献类型：
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作者：
J. Jensen;J. Jensen;S. Hönig;S. Rakshit;S. Rakshit;A. Alonso-Herrero;D. Asmus;P. Gandhi;M. Kishimoto;A. Smette;K. Tristram
在中红外（mid-IR）范围内观察到的多环芳烃（PAH）分子的光谱特征通常用于推断活动星系核（AGN）周围千秒差距尺度上最近和正在进行的星星形成的数量，而传统方法则无法做到这一点。该方法假设观测到的PAH特征主要由星星形成激发。利用目前的地面望远镜和即将到来的詹姆斯韦伯太空望远镜，可以探测更小的空间尺度，我们的目标是测试这个假设是否仍然在几十到几百秒差距的范围内。为此，我们使用地基高角分辨率中红外光谱技术，将发射的11.3 μm多环芳烃表面通量空间映射为距28个附近活动星系核中心0.4-4角秒距离的函数。我们探测并提取了13个活动星系核中11.3 μm的PAH特征。每个孔径内的通量按比例缩放到一个亮度归一化的距离，从核，能够比较活动星系核辐射的内在空间尺度跨越约两个数量级的光度。为此，我们建立了吸收校正的X射线光度和升华半径在这些来源之间的经验关系。一旦标准化，发射的PAH表面通量的径向分布显示出相似的径向斜率，幂律指数约为-1.1，以及相似的绝对值，如内在尺度估计的不确定性所预期的那样，在几个因子内相互一致。我们解释这是一个共同的紧凑的中央物理过程，无论是活动星系核本身或环核星星的形成强度与活动星系核的权力的配置文件所造成的证据。活动星系核激发其环境中的密云的基于光电离的模型可以再现观测到的径向斜率，并证实活动星系核辐射场足够强，可以解释距核10-500秒差距内观测到的多环芳烃表面通量。我们的研究结果建议谨慎使用PAH排放作为星星形成示踪剂在一个kpc的AGN周围。
Spectral features from polycyclic aromatic hydrocarbon (PAH) molecules observed in the mid-infrared (mid-IR) range are typically used to infer the amount of recent and ongoing star formation on kiloparsec scales around active galactic nuclei (AGN) where more traditional methods fail. This method assumes that the observed PAH features are excited predominantly by star formation. With current ground-based telescopes and the upcoming James Webb Space Telescope, much smaller spatial scales can be probed and we aim at testing if this assumption still holds in the range of few tens to few hundreds of parsecs. For that, we spatially map the emitted 11.3 μm PAH surface flux as a function of distance from 0.4–4 arcsec from the centre in 28 nearby AGN using ground-based high-angular-resolution mid-IR spectroscopy. We detect and extract the 11.3 μm PAH feature in 13 AGN. The fluxes within each aperture are scaled to a luminosity-normalized distance from the nucleus to be able to compare intrinsic spatial scales of AGN radiation spanning about two orders of magnitude in luminosity. For this, we establish an empirical relation between the absorption-corrected X-ray luminosity and the sublimation radius in these sources. Once normalized, the radial profiles of the emitted PAH surface flux show similar radial slopes, with a power-law index of approximately −1.1, and similar absolute values, consistent within a factor of a few of each other as expected from the uncertainty in the intrinsic scale estimate. We interpret this as evidence that the profiles are caused by a common compact central physical process, either the AGN itself or circumnuclear star formation linked in strength to the AGN power. A photoionization-based model of an AGN exciting dense clouds in its environment can reproduce the observed radial slope and confirms that the AGN radiation field is strong enough to explain the observed PAH surface fluxes within ∼10–500 pc of the nucleus. Our results advice caution in the use of PAH emission as a star formation tracer within a kpc around AGN.