HST Imaging of Giant Ionized Clouds Around Fading AGN
Abstract
We present HST images in [O III], Hα, and adjacent continuum bands, for 7 AGN surrounding by photoionized clouds extending 10-35 kpc from the nuclei, whose energy budgets suggest substantial fading over timescales 30,000-100,000 years. These targets were found as a result of the efforts of nearly 200 Galaxy Zoo volunteers in both targeted and serendipitous searches, aimed at finding objects similar to Hanny's Voorwerp near IC 2497. The images reveal a fascinating variety of emission-line structures. Sub-kpc loops near the nuclei are frequent. A larger 5-kpc ring of emission appears near one AGN, not obviously connected to the gas near the nucleus. In NGC 5972, two systems of gas and dust appear, in a pattern suggesting that they result from a merger; there is a gap in line emission around the nucleus which would fit with a rapid cutoff in ionizing luminosity. Some of these clouds have their emission concentrated in very narrow filaments kiloparsecs in length, which might suggest a role for extensive magnetic fields. As in Hanny's Voorwerp, we see little spatial correlation between ionization parameter and Hα surface brightness, indicating that there is substantial density structure on yet-unresolved scales. The two AGN with large double radio sources show extended line emission perpendicular to the radio axis, violating a common pattern, as if the AGN axis had recently and rapidly precessed by ~90 degrees. This, plus the common signs of recent interaction in the hosts, informs our speculations on what might drive a rapid switch in the mode of energy output from accretion into the black holes in these objects, from radiative to kinetic, including the possibility that a special epoch near the merger of a binary black hole might be involved. This work has been supported by NASA through STSCI grant GO-12525.01.
- Publication:
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American Astronomical Society Meeting Abstracts #221
- Pub Date:
- January 2013
- Bibcode:
- 2013AAS...22133947K