Galaxy Zoo Voorwerpjes – now with Hubble data!
Some of us continue to exult over the approval of our Hubble proposal to look at some of the “voorwerpjes” found through the GZ forum. These are clouds of gas ionized by an active galactic nucleus, similar to Hanny’s Voorwerp except for being smaller and dimmer (hence the Dutch diminutive form of the word). This has been a very fruitful project, going back to the first few possibilities posted for discussion in different contexts by Zooites. From these, I realized that such clouds could be spotted based on their unusual colors from the SDSS images, and with contributions from Waveney and laihro setting up the web interface and one of our source lists, the hunt was on! The results staggered us – within 6 weeks each of the 18,000 candidate galaxies had been examined by at least 10 Zooites. Seven of you looked at them all! Then we could examine the highest-scoring galaxies, in three sessions from Kitt Peak and Lick observatories, measuring spectra to see which ones really show gas ionized by an active nucleus. Once again, Drew, alias sdrew123, has done a lot of the data reduction and Python action in this part of the project. Our sample of giant AGN clouds now includes 19, each showing gas more than 10 kiloparsecs (32,000 light-years) from the galaxy core, so we get information on its history over at least that many years.
Why do we want to find these? From what we’ve learned about Hanny’s Voorwerp, we have the possibility of tracing the history of active galactic nuclei – how fast they can fade, how long they stay on at once, and maybe how they influence their surroundings. It’s hard to generalize from a single instance (though astronomers are notorious for trying), so we want enough of a sample for statistically defensible conclusions.
Which of our objects should we ask to look at, balancing the demand for Hubble time against the fact that very often more data are better? From our current point of view, investigating whether active nuclei shut down very often within time spans roughly 100,000 years, we want to concentrate on the ones that show evidence for a deficit of energy from the core compared to that needed to light up the gas we see. This gives us a set of 7 (plus NGC 5252, which has been known for many years and already has archival Hubble images). They are:
SDSS J143029.88+133912.0, the Teacup (Kevin started calling it that in honor of the handle-like loop of gas extending more than 15,000 light-years to the side). This is the most distant galaxy in the sample, at z=0.085.
UGC 7342 (also known in some Zoo threads as the Crab galaxy), with its enormous filaments of gas stretching more than 100,000 light-years on each side (fully half as large as Hanny’s Voorwerp, and at about the same distance).
Another new SDSS AGN, and new Zoo find for its gas clouds, is
SDSS J220141.64+115124.4 (which I tend to abbreviate to SDSS 2201+11 for my own sanity).
SDSS J151004.01+074037.1, with symmetric clouds around a type 2 Seyfert nucleus, is another SDSS//GZ discovery.
NGC 5972 is pretty close to us at z=0.0297. This galaxy attracted some brief notice in a 1995 paper by Mira Veron-Cetty and Phillippe Veron, who established that what look like spiral arms are purely ionized gas features, and noted that this AGN is surrounded by a double radio source.
UGC 11185 is part of a very disturbed interacting pair, with a bright spray of ionized gas seen to the east of the AGN. Note to self: we need to take care in where we center the Hubble images to avoid problems with scattered light from the annoyingly bright star.
Mkn 1498, as the name suggests, has a long-known AGN, but its ionized gas shows such strong radiation
reaching it that this one may have faded even to its observed brightness.
We based our observation request on what we learned from working on the Hanny’s Voorwerp data. The most valuable results for these will probably come from images in Hα and [O III] for these, the clouds sometimes extend into the galaxy, so we need red and green starlight images to distinguish stars and gas. Our filter selection depends on the galaxy redshift – the brand-new Wide-Field Camera 3 (WFC) has cleaner performance but only a limited set of narrow filters, while the decade-old Advanced Camera for Surveys (ACS) has ramp filters which can tune to any desired optical wavelength, but its CCD detectors are really showing the ill effects of damage from years in the space environment. So we use WFC where we can and ACS otherwise, emboldened by the release of Jay Anderson’s software for partially undoing the effects of that radiation damage. We end up using 3 orbits per galaxy – one for each emission line and the other for broad-band filters to map the structure and color of each galaxy’s starlight.
We know some of the things to look for in the data – regions of star formation, gas outflow, maybe shadowing effects from material near the galaxy nuclei. But the best is often in what we don’t know to look for beforehand…
Next up – detailed observation planning, at which point we get our first hints as to when each galaxy could be observed.