Centaurus A (Cen A) is one of the most famous galaxies hosting an active galactic nucleus (AGN), where the interaction between AGN activities and surrounding interstellar and intergalactic media has been investigated. Recent studies reported detections of the H{\alpha} emission from clouds in the galactic halo toward the northeast and southwest of the nucleus of Cen A, suggesting that AGN jets may have triggered star formation there. We performed near-infrared line mapping of Cen A with the IRSF 1.4-m telescope, using the narrow-band filter tuned for Pa{\beta}, from which we find that the Pa{\beta} emission is not detected significantly from either northeast or southwest regions. The upper limit of the Pa{\beta}/H{\alpha} ratio in the northeast region is compatible with that expected for a typical HII region, in line with the scenario that AGNs have triggered star formation there. On the other hand, the upper limit of Pa{\beta}/H{\alpha} in the southwest region is significantly lower than that expected for a typical HII region. A possibility to explain the low Pa{\beta}/H{\alpha} ratio in the southwest region is the scattering of H{\alpha} and Pa{\beta} photons from the center of Cen A by dust grains in the halo clouds. From the upper limit of Pa{\beta}/H{\alpha} in the southwest region, we obtain constraints on the dust size distribution, which is found to be compatible with those seen in the interstellar medium of our Galaxy.

中文翻译：

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半人马座 A 与 IRSF/SIRIUS 的氢复合近红外谱线图

半人马座 A (Cen A) 是最著名的拥有活动星系核 (AGN) 的星系之一，研究了 AGN 活动与周围星际和星系间介质之间的相互作用。最近的研究报告了从星系晕中的云向 Cen A 核的东北和西南方向检测到 H{\alpha} 发射，这表明 AGN 喷流可能触发了那里的恒星形成。我们使用 IRSF 1.4 米望远镜对 Cen A 进行近红外线测绘，使用针对 Pa{\beta} 调谐的窄带滤波器，从中我们发现 Pa{\beta} 发射未从任何一个显着检测到东北或西南地区。东北地区 Pa{\beta}/H{\alpha} 比率的上限与典型 HII 地区的预期一致，与 AGN 在那里引发恒星形成的情景一致。另一方面，西南地区 Pa{\beta}/H{\alpha} 的上限明显低于典型 HII 地区的预期。解释西南地区低 Pa{\beta}/H{\alpha} 比率的一种可能性是光晕中的尘埃颗粒从 Cen A 中心散射 H{\alpha} 和 Pa{\beta} 光子云。从西南地区的 Pa{\beta}/H{\alpha} 的上限，我们获得了尘埃尺寸分布的约束，发现这与我们银河系星际介质中看到的那些兼容。解释西南地区低 Pa{\beta}/H{\alpha} 比率的一种可能性是晕中的尘埃颗粒从 Cen A 中心散射 H{\alpha} 和 Pa{\beta} 光子云。从西南地区的 Pa{\beta}/H{\alpha} 的上限，我们获得了尘埃尺寸分布的约束，发现这与我们银河系星际介质中看到的那些兼容。解释西南地区低 Pa{\beta}/H{\alpha} 比率的一种可能性是光晕中的尘埃颗粒从 Cen A 中心散射 H{\alpha} 和 Pa{\beta} 光子云。从西南地区的 Pa{\beta}/H{\alpha} 的上限，我们获得了尘埃尺寸分布的约束，发现这与我们银河系星际介质中看到的那些兼容。