November 1, 2011  

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 identifi ed 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:

  • We found 19 galaxies with AGN-ionized clouds extending more than 10 kiloparsecs from the nuclei. That cutoff emphasizes gas outside the main body of the galaxy, and lets us sample the history of the AGN luminosity on times 30,000 years and up (in some cases beyond 100,00 years). The largest clouds, in UGC 7342, are slightly over half as far from the core as we see in Hanny’s Voorwerp and IC 2497.
  • Our special interest in these clouds lies in the way they can show us whether the central AGN has changed much in brightness over the time it takes light to go from the core to the cloud (plus a geometric factor depending on exactly where the gas lies with respect to the nucleus, on which the referee corrected my trigonometry). We’re particularly sensitive to AGN which have faded. A powerful way to distinguish this from an AGN which is strongly hidden by dust from our direction, but not toward the cloud, is considering its energy balance between various parts of the spectrum. If the nucleus is absorbed along most directions, most of its energy output will be soaked up by those dust grains and given off again in the far-infrared. Thus, the ratio of energy needed to ionize the gas we observe to the total-infrared output tells us how plausible this kind of obscuration is. This ratio varies widely – in some of these galaxies, it is clear that we see gas ionized by radiation escaping only through rare clear patches in a dusty cocoon. But in others, only very specific (and unlikely) arrangements of dust could hide the AGN from us but leave it clearly exposed to the gas. In these, we have the best cases for strong variations in timescales of tens of millennia, as in Hanny’s Voorwerp. Long by human timescales, but surprisingly fast for the accretion disk around a supermassive black hole.
  • Most of these ionized clouds are found in interacting or indeed merging pairs. This makes sense if tidal tails of gas provide the most likely source of gas far from the nucleus and far from the plane of the galaxy, to be easily ionized by radiation emerging from the core. An excellent example is UGC 7342, where we see a distorted pair of galaxies and gas whose distribution is very different from the stars:

    UGC 7342 from SDSS

  • The relative numbers of hidden and faded AGN give us a first hint at the duration of “on” phases for Seyferts – 20,000 to 200,000 years.

    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.

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    5 responses to “Voorwerpjes – results now ready for prime time!”

    1. veggy2 says :

      Way to go Bill and the zoo! And I get a little mention in a scientific paper, almost 40 years after spectacularly flunking a great chance to be a professional astronomer. That is so cool and makes me so happy. I know the main goal is to further scientific knowledge, but for a shallow guy like me seeing my name in the credits is a great incentive for doing more than the little I do already.

      Reply
    2. Kyle says :

      Congratulations on the paper, and thanks for a great summary of the content, Bill. Looking forward to the HST and XMM observations!

      Reply
    3. mitch says :

      Fantastic job, Bill and co-authors – I also am looking forward to all the exciting stuff yet to come.

      Reply
    4. SUMO_2011 says :

      Congratulations, then unquestionable success is and stimulus for volunteers

      Reply

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