Overlapping galaxies in color and in detail
We’re making progress in working on the Kitt Peak images of overlapping galaxies (encouraged by recently learning that we have 3 more nights this November, so keep those candidates coming in!). Now that various meetings and proposal deadlines are over, there’s some time to show off color composite images and point out some of the things we’re looking at. For most of these galaxies, we took images in two filters – the B band (blue), which lies between the SDSS u and g filters, and the I filter, which used to stand for infrared but nowadays (with the proliferation of genuine infrared imagers at much longer wavelengths) we think of as a red just a bit too deep for our eyes to see. Making a color image takes three filters, not just two. Fortunately, the colors of most galaxies are so well-behaved that we can synthesize the middle filter (green) from the two we have; this is something done pretty often with Hubble images as well. I then pasted the three colors together using a brightness mapping that is logarithmic starting slightly below the sky-brightness level and consistent across the various galaxies – this ends up much like the familiar SDSS color display. But enough of the details – let’s get to the galaxies.
One of our highest-priority targets was NGC 3861, a gorgeous partial silhouette of two ringed spirals. It lies in a part of the sky with a deep ultraviolet exposure as part of the GALEX sky survey, so we can further leverage our images to interpret the UV data and measure the dust at wavelengths seldom possible. The basic approach to measuring absorption by dust is to compare the brightness in a region where we see both galaxies with what we expect for the sum of both, based on symmetry and the non-overlapping regions. Here we certainly have enough light to do that in detail in the small overlap region; our limits will come from how clumpy the star-forming regions are in both galaxies. For comparison, this is SDSS object 587742572687917067. Here’s another beauty – the face-on spirals UGC 5769 and 5770 (you might recognize them better by the SDSS number 587742013810343948 ). The bluer spiral is in front of the slightly yellower ringed galaxy, with the outer ring backlighting some of its distant spiral arms. Here’s another spectacular pair, the spectacle increased even as our work is complicated by the bright star almost in front. This is Arp 198 ( SDSS 587742865816944738).The edge-on galaxy is in the background, showing clearly where the dust in the arms of the foreground spiral blocks its light as they cross in front. You’ll notice more speckles caused by cosmic-ray events in the detector than in the previous ones. Normally we take three exposures in each filter so they can e combined to reject any values that are wild from a single image. but sometimes the atmosphere was so much steadier in one of these that we get a better product by just using that one (and either putting up with the cosmic rays or interpolating across them if in an important part of the image). In Arp 198, the atmosphere was unusally steady during one of the blue images, so we went with that one by itself. I’m getting a little wordy, so maybe this is a good time to leave off. As dessert, here are a few more. “Few” out of a lot – at the rate Zoo participants continue to post new candidates even on the temporary forum, within the next week or so the total number of pairs in our working sample should pass 1000!