Voorwerpjes – results now ready for prime time!
Yesterday marked a milestone in the Galaxy Zoo study of AGN-ionized gas clouds (“voorwerpjes”), when we received notice that the paper reporting the GZ survey and our spectroscopic study of the most interesting galaxies
has been accepted for publication in the Monthly Notices of the Royal Astronomical Society. We’ve now posted the preprint online – at http://arxiv.org/abs/1110.6921 on the preprint server, or, until publication, I have a PDF with full-resolution graphics. Here’s the front matter:
The Galaxy Zoo survey for giant AGN-ionized clouds: past and present black-hole accretion events
William C. Keel, S. Drew Chojnowski, Vardha N. Bennert, Kevin Schawinski, Chris J. Lintott, Stuart Lynn, Anna Pancoast, Chelsea Harris, A.M. Nierenberg, Alessandro Sonnenfeld, & Richard Proctor
Abstract: Some active galactic nuclei (AGN) are surrounded by extended emission-line regions (EELRs), which trace both the illumination pattern of escaping radiation and its history over the light-travel time from the AGN to the gas. From a new set of such EELRs, we present evidence that the AGN in many Seyfert galaxies undergo luminous episodes 20,000-200,000 years in duration. Motivated by the discovery of the spectacular nebula known as Hanny’s Voorwerp, ionized by a powerful AGN which has apparently faded dramatically within 100,000 years, Galaxy Zoo volunteers have carried out both targeted and serendipitous searches for similar emission-line clouds around low-redshift galaxies. We present the resulting list of candidates and describe spectroscopy identifying 19 galaxies with AGN-ionized regions at projected radii > 10 kpc. This search recovered known EELRs (such as Mkn 78, Mkn 266, and NGC 5252) and identified additional previously unknown cases, one with detected emission to r = 37
kpc. One new Sy 2 was identified. At least 14/19 are in interacting or merging systems, suggesting that tidal tails are a prime source of distant gas out of the galaxy plane to be ionized by an AGN. We see a mix of one- and two-sided structures, with observed cone angles from 23-112 degrees. We consider the energy balance in the ionized clouds, with lower and upper bounds on ionizing luminosity from recombination and ionization-parameter arguments, and estimate the luminosity of the core from the far-infrared data. The implied ratio of ionizing radiation seen by the clouds to that emitted by the nucleus, on the assumption of a nonvariable nuclear source, ranges from 0.02 to > 12; 7/19 exceed unity. Small values fit well with a heavily obscured AGN in which only a small fraction of the ionizing output escapes to be traced by surrounding gas. However, large values may require that the AGN has faded over tens of thousands of years, giving us several examples of systems in which such dramatic long-period variation
has occurred; this is the only current technique for addressing these timescales in AGN history. The relative numbers of faded and non-faded objects we infer, and the projected extents of the ionized regions, give our estimate for the length of individual bright phases.
Our main conclusions are:
Another advance, perhaps sociological rather than scientific: the journal editor was OK with our thanking each participant in the AGN Hunt project individually in the acknowledgements section (I think that was 185 names or IDs), and listing the first poster of each candidate posted on the forum in the large object table.
The broad project doesn’t stop here. We are now a week away from the first scheduled Hubble observations of 7 of the Voorwerpjes which show the best evidence for a faded AGN. The European Space Agency’s XMM-Newton orbiting observatory has several of these scheduled for X-ray observations. Meanwhile, colleagues (amateur and professional alike) are sending us new candidates from the sky region newly covered by SDSS DR8, or the GALEX ultraviolet sky survey. Just today, I had email from a prominent theorist of gaseous nebulae, noting that their software has the capability of tracking what happens when the ionizing source changes more rapidly than the response time of low-density gas, and this is the first context where they might be able to apply this kind of calculation. UA grad student Erin Darnell is now finishing her master’s thesis project, a search using [O III] narrowband images for new and potentially very faint Voorwerpes around Seyfert galaxies known to have extended gaseous tails from H I 21-cm observations. And alert Zooites continue to post candidates on the forum, including some from Hubble Zoo (which is especially challenging since we often have only 2 filters, and which emission lines show up where depends on the galaxies’ redshift even more than in the low-redshift SDSS sample).
Once again, thanks to everyone who’s participated so far! One of these days, if you should see a car with an Alabama license plate saying ZOOROOLZ, that just might be me.