Hunting for gas clouds illuminated by AGN

Some AGN emit so much ionizing radiation (in the ultraviolet and X-ray range) that they can ionize gas clouds tens of thousands of light-years away, in the galaxy's outskirts or even outside the galaxy altogether. The most (recently) famous of this second category is certainly Hanny's Voorwerp. These extensive gas clouds may be related to the AGN only by having it as a source of illumination, or they may have been ejected from the galaxy core by the AGN. In either case, their study gives us (literally) new views of the nucleus. Some galactic nuclei are surrounded by so much gas and dust that we do not have a direct view of the core; these clouds do, so that analyzing their spectra lets us find out about unseen objects. In addition, as in Hanny's Voorwerp, the extra time it has taken light to get from the core to the cloud and then on to us means that the cloud, as we see it, is affected by the AGN at an earlier time than our direct view, so we can find out something about the recent cosmic history of these objects.

Previous observations have revealed some AGN in the local Universe with these extended gas clouds. Further study of these has been important in our overall picture of AGN. It has become clear that Zoo participants can find such clouds from the SDSS images, because the gas emits strongly in only one or two filters. As a result, on the SDSS color images, the clouds have odd colors not usually found in galaxies. After a request in the Forum, participants have suggested numerous candidates, of which over 50 are very strong (and in some cases have already been confirmed with new data). These include the best examples known earlier in this part of the sky, adding to the confidence that the Zoo project can pick these out effectively from the existing images (and growing experience with how "ordinary" galaxies look). To build on this in a systematic way, with major input from Zookeeper Kevin and Zoo users laihro and waveney, we present the "Voorwerpje Hunt" (because these are like Hanny's Voorwerp but smaller, we use the Dutch diminutive form).

To begin with, we've selected galaxies classified as AGN from the SDSS spectra, and will present them for examination. You can select whether each galaxy definitely shows gas clouds, might show them, or definitely does not show them. For reference, here are some examples. First, these galaxies are good examples of the kinds of clouds lit up by AGN that we're looking for. The clouds can show up as blue, green, or violet in the SDSS images, depending on the galaxy redshift and which emission lines are strongest. These clouds can have almost any form and appear anywhere in the galaxy (or even off its edges).

Sample Images

587739845390761994	587736809916399664	587732483292266549
587739719750058064	587727221400862869	587735666377949228
587729158970736727	587739720846934175	587729386079059975

Possible Colours:

redshift	emission lines	colour
0.0-0.07	OIII	blue
0.0-0.07	NII+Hα	green to yellow
0.0-0.07	OIII+NII+Hα	turquoise
0.07-0.1	OIII	turquoise to green
0.07-0.1	NII+Hα	red
0.07-0.1	OIII+NII+Hα	pink to yellow

Imposters

Misregistered images Other objects in field may share color halos or offsets.
587736546312323142	587742013831971013	587734623787614268
587739630094647435
Spiral Arms
587739382061006898	587745544805351443	587728310178807862
587741727124553844	587727221948022819
Inner rings
588009370684555317	588017949888086075	587732702337368070

Bill, Laihro and waveney

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