After consultation with the Zookeepers, it seems sensible to open a thread for this. I hereby advertise for posting of galaxy pairs which overlap from our point of biew, but are distinctly not merging. Such pairs are pretty rare (at least they were in our corner of the Universe before the SDSS harvest got going) and can tell us important things about dust in galaxies. So, the details -
What? We are looking for pairs of galaxies which appear overlapping from our point of view, but are not actually interacting or merging. In fact, the farther apart they are in space, the better. We already know that the Sloan survey is a rich source of such pairs, and are seeking the best collection of galaxies for followup study.
Why? In a word, dust. Dust grains play a crucial role in the evolution of galaxies and in how we see them. Dust intermingled with interstellar gas helps it cool to form stars, and in turn these grains are produced in the atmospheres of cool stars and in the expanding envelopes of nova and supernova explosions. Dust reddens and dims starlight passing through it, to an extent that increases for bluer and ultraviolet light (just as particles in our atmosphere do for sunrise and sunset). We need to correct for this effect if we are to understand the star-forming history of galaxies, one of the most important ingredients to cosmic history. One of the most straightforward approaches to this problem is use of silhouetted galaxies, in which we see one galaxy partially backlit by another. The ideal case is a spiral half projected against the smooth light of an elliptical or lenticular galaxy. However, we need to look at spiral/spiral overlaps as well, since looking at the high-redshift Universe and comparing with deep Hubble observations means that we need to look into the ultraviolet(which has been shifted into visible light by cosmic expansion for such distant galaxies). Ellipticals and lenticulars are quite red, and too dim in the UV to be effective backlights. With a list in hand of the best local galaxy pairs for this project, we can initially extend these measurements into the UV using the ongoing survey by the GALEX satellite, which is in many ways a good match for the Sloan data. Eventually it is our hope to extend the project to include followup with such other facilities as Hubble. This approach dovetails with measurements of the dust in the far-infrared, but we are still some way from having the sensitivity to do such measurements wholesale in the early Universe at a level sensitive enough to match what the absorption technique does. Furthermore, to say something about the whole population, we need to start with enough pairs for a wide range of galaxy type and luminosity to be represented. Just the thing for GalaxyZoo.
The attached montages show some of the kinds of pairs we are looking for. First, in greatest detail, Hubble images from the 20 or so useful pairs we identified from earlier galaxy catalogs. Next, some of the ones we knew about from catalog searches or simple luck before the SDSS catalogs. And in the third montage, some of the best of the ones newly identified simply from being posted on galaxyzooforum.org. Just from looking over each day's postings on galaxyzooforum.org, the current list has increased tenfold from the pre-SDSS collection, with last weekend's harvest pushing the galaxyzoo collection past 200 candidates..
Since this is not a category which participants were looking for, and indeed these would be rejected as mergers, we hope to get an even more complete census in this way.
The ideal pairs for this project consist of galaxies which
- include at least one spiral or lenticular (the SDSS data may or may not be enough to say which galaxy is in front)
- are symmetric and undisturbed. This way we learn about normal galaxies, without the complication of working out where material pulled out of its original location during an interaction may have started.
- are of roughly comparable angular size
- overlap half their diameters, so we have a good measurement of their appearance "alone", and certainly overlap at bright enough levels to make out structure in this region.
- appear large enough for the SDSS images to show details such as dust lanes (there's no gain to worrying about galaxies we can't see well enough to tell).
Maybe we'll get enough pairs that I won't look at any more images of galaxies and mutter "could you move about 30,0000 light-years to the side, please?".