English: What is this strange, enormous green thing next to this distant pair of galaxies? The leading hypothesis seems to be that gas from the larger, centered galaxy, strewn about by cosmically recent interaction with another galaxy, was set alight by a very energetic event originating from a quasar hosted at the center of the pictured galaxy. And it's green because some of the gas is oxygen, and that's apparently what oxygen atoms do when supplied with enough energy. You can also find oxygen glowing green around many planetary nebulas, though it is often depicted as blue. See some more voorwerpjes in this APOD.

The following is a verbose description about image processing. It may be of limited interest.

Combining the data for this image is no easy task. It took me about a week to get this right even though it is not an especially vast image. The trouble is that there is very good data for near-infrared and narrowband OIII filters in the Hubble archive, but nothing at all for any shorter wavelengths. Almost any wide filter from 600nm or below would make composing this image much easier, but there is simply nothing available from Hubble.

To solve this problem, I had to look elsewhere. Where better to look than hannysvoorwerp.com? Yes, it has its own website created by Hanny herself. Along with information about the voorwerp is also her blog and other interesting bits of astronomy. From this convenient gathering of images, I decided it would likely be worth combining Keel and Manning's WIYN data with Hubble's offering. I hoped to find it in the NOAO Science Archive, but it wasn't there. Luckily, I know Bill Keel is a nice guy, and he Dropboxed the data to me after a quick Twitter exchange. I was sad to be informed that the NOAO archive doesn't get a lot of funding, so it doesn't necessarily contain much beyond large surveys.

Combining ground-based data and Hubble data can often result in jarringly blurry areas that can contrast quite harshly against the impeccable crispness of the space telescope. This time, I found it complemented it quite nicely, like they were meant to be together. Bill says it may be the sharpest ground-based image he ever took, so that's probably why. This is dryly noted in the footnotes of his own voorwerp page as (V image has 0.45" FWHM). Just an amusing comparison of ways to say the same thing.

Hubble data from Proposal 11620: A Quasar Light Echo in the Local Universe?

Red: WFC3/IR F160W + WFC3/UVIS F814W Green: ACS/WFC FR505N#5240 + WFC3/UVIS F814W + WIYN V band Blue: WIYN B band + WFC3/UVIS F225W

Note: The WFC3/IR F160W data was also used on top of the RGB image to help brighten the dimmest parts of the image. It is because of this that many galaxies which are mainly visible in this near-infrared data appear gray rather than reddish.

North is up.