衰落类星体的历史和环境：哈勃太空望远镜对Hanny's Voorwerp和IC 2497的观测
The History and Environment of a Faded Quasar: Hubble Space Telescope observations of Hanny's Voorwerp and IC 2497
摘要：
我们目前的哈勃太空望远镜成像和光谱的扩展高电离云被称为汉尼的Voorwerp，附近的螺旋星系IC 2497。WFC 3图像显示了IC 2497核附近复杂的尘埃吸收。STIS光谱显示了一个亮度相当低的2型Seyfert活动星系核。电离参数logU = -3.5与其弱X射线发射相一致。我们在核附近没有发现高电离气体，这进一步证明活动星系核目前处于低辐射输出状态（也许现在主要是动能）。在Hanny's Voorwerp对面的一侧，伴随着一个直径为500 pc的电离气体膨胀环，多普勒偏移量为300 km/s（运动学年龄< 7 x10^5年）。[O III]和H-alpha + [N II]图像显示了Hanny's Voorwerp的精细结构，边缘变亮的部分和H-alpha较强的小区域。我们确定这些地区电离最近的恒星形成，相反，AGN电离的整个云。这些候选的“正常”H II区域包含蓝色连续体天体，其颜色与年轻的恒星星族一致;它们仅出现在投影中IC 2497方向的2 kpc区域中。电离敏感的比率[O III]/H-α在电离气体中的突出“孔”附近没有可辨别的模式。电离和表面亮度的独立性表明，大量的空间结构仍然没有得到解决，在这样的程度上，表面亮度样本的数量更密集的灯丝，而不是在发射区的特征密度。这些结果与我们所描述的活动星系核的电离回波相吻合，活动星系核的电离亮度在过去的1-2 × 10^5年内下降了100倍以上（可能更多）;我们提出了一系列事件，并讨论了这种快速波动对活动星系核人口统计的影响。（节录）
英文摘要：
We present Hubble Space Telescope imaging and spectroscopy for the extended high-ionization cloud known as Hanny's Voorwerp, near the spiral galaxy IC 2497. WFC3 images show complex dust absorption near the nucleus of IC 2497. STIS spectra show a type 2 Seyfert AGN of rather low luminosity. The ionization parameter log U = -3.5 is in accord with its weak X-ray emission. We find no high-ionization gas near the nucleus, adding to evidence that the AGN is currently at low radiative output (perhaps now dominated by kinetic energy). The nucleus is accompanied by an expanding ring of ionized gas 500 pc in projected diameter on the side opposite Hanny's Voorwerp, with Doppler offset 300 km/s from the nucleus (kinematic age < 7 x10^5 years). [O III] and H-alpha + [N II] images show fine structure in Hanny's Voorwerp, with limb-brightened sections and small areas where H-alpha is strong. We identify these as regions ionized by recent star formation, in contrast to the AGN ionization of the entire cloud. These candidate "normal" H II regions contain blue continuum objects, whose colors are consistent with young stellar populations; they appear only in a 2-kpc region toward IC 2497 in projection. The ionization-sensitive ratio [O III]/H-alpha shows no discernible pattern near the prominent "hole" in the ionized gas. The independence of ionization and surface brightness suggests that substantial spatial structure remains unresolved, to such an extent that the surface brightness sample the number of denser filaments rather than the characteristic density in emission regions. These results fit with our picture of an ionization echo from an AGN whose ionizing luminosity has dropped by a factor > 100 (and possibly much more) within the last 1-2 x 10^5 years; we suggest a sequence of events and discuss implications of such rapid fluctuations for AGN demographics. (Abridged)
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