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 coors 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!

If you’ve arrived via the Galaxy Zoo newsletter, you’re probably wondering where the posts about our observing trip are. They’re here:  

Bill started things off by writing about our quarry for this Zoo-inspired hunt.

I then got excited about where we were, and about our first images. And about the telescope.

Later in the run we were seeing more and more perfect pairs, all of which will help us track down the galaxies’ dust.

Despite a few problems we pressed on, and started to produce colour images for the first time.

Since we got back, work has continued - and I’ll leave it to Bill to give you the latest.

 Not that I’m about to change my avatar or anything, but I think this is one of the best pairs we observed from Kitt Peak. First pointed out in the Galaxy Zoo forum by GwydionM, this features a face-on spiral almost exactly in front of an edge-on spiral. Like NGC 3314, it gives a rare chance to see the dust content of a galaxy almost all the way from outer regions to the nucleus (limited there by the accuracy with which we can extrapolate the profile of the background galaxy inward). This also drives home a point which isn’t always obvious from pictures, and is especially insidious when looking at books where the pictures tend to be all about the same size. Similar-looking galaxies can span a wide range of sizes, even among spirals which don’t come as small and faint as spheroidal or irregular galaxies. We don’t yet have good redshifts for both; the Sloan data give the single value z=0.067 for what must be the blended light of both, probably meaning that they are at similar redshifts so the size comparison in this picture is pretty close to reality.  This image come from the first fruits of the next stage in processing our data, one which leaves them ready to analyze. To sample red light, we used an I filter rather similar to the SDSS i band. For the particular CCD we used, the skyglow that it sees causes a pattern of interference fringes from light reflecting within the chip. This can be calibrated and subtracted only using data on the night sky itself. We combined images where the target galaxies were at different places on the detector while rejecting objects that were at a certain pixel value only once (that is, things on the sky) to leave, ideally, only the interference pattern. I’m still tweaking until we decide that we’re close enough to that ideal…                                If you look closely, you can spot the heavily reddened core of the background galaxy behind the spiral arm to the lower right of the foreground galaxy core, and note the darker absorption next  to those spiral arms. (I’ll be watching to see how this image shows up - this is the first time the regular Zookeepers let me have the keys to the blog, and I’m feeling my way around. It looked sort of odd in preview). 

We’ll find a Kitt Peak related prize for whoever makes us laugh the most… 

We’re only a couple of weeks from the observing session to look at some of the overlapping galaxies pointed out by the good people of Galaxy Zoo, so it’s high time for us to think about how to do this most effectively. We are scheduled for the nights of April 25-29, using the 3.5-meter WIYN telescope at Kitt Peak, Arizona. The timing is just right for the SDSS sample, since the northern galactic hemisphere that it covered most completely is up all night this time of year and the moon is waning and moving out of the way. Here are some outside and inside views of the telescope:

You can even see the current weather at Kitt Peak (as long as it’s daylight there) using this webcam view - WIYN is at upper right, on the mountain horizon directly above the nearby University of Arizona telescopes.We’ll be using a fairly new CCD camera called OPTIC, which is visiting from the University of Hawaii. This camera has chips with a special architecture allowing the accumulating image to be moved around on the chip in any direction by purely electronic means, so it can keep up with atmospheric motions as long as there is a bright enough guide star in the right field of view. The advantage of doing this is that electrons can be moved around much faster than the whole telescope, and without introducing any mechanical vibrations. This device will help us work out how far out in redshift we can pursue galaxies for this project using ground-based telescopes.

Now - which galaxy pairs do we start with? You all have furnished a magnificent sample, something like 875 overlapping pairs that look useful for dust measurements by the time I fold in the last few weeks’ harvest. I keep my target book for this project as a set of PDF files accessible from anywhere - for those who really want to see, the list is broken into four parts starting at right ascension zero, 10 hours = 150 degrees, 13 hours = 195 degrees, and 15 hours = 225 degrees. (These are really cool to flip through rapidly page by page). This huge sample gives us the luxury of being able to select our targets carefully, making sure to span ranges of foreground galaxy type and luminosity. An obvious starting point is the nearest galaxies, where we can see the most detail in the dust. We want to include pairs with both elliptical and spiral background galaxies; the dust detail is clearest when the background galaxy is smooth, but we also want to correlate with ultraviolet data from the GALEX satellite, and only spirals are bright enough in the UV to do this sensibly.If we think about choosing galaxies based on a score, we give them points for being bright and nearby, points for having two known and quite different redshifts, and points allocated according to how long the GALEX UV sky survey looked at that piece of the sky.

But we also want to pick a few higher-redshift galaxies, at z=0.1 or greater, to show just how well we can measure their dust from the ground. (Recall that we do have pending Hubble proposals to do some of these, but there are something like 960 competing proposals for next year’s observations and the oversubscription will be fierce).Our goal is to look at something like 50 pairs for an hour each over the 5 nights, and naturally we want to start with the most interesting ones in case the weather goes downhill. Chris will be blogging during the observing run, and certainly we’ll be getting action pictures. There are also plans for a writer to watch the action for a night and report the scene from a non-astronomer’s perspective. We hope to be able to do some analysis near real-time and get dust maps to show how well the data match our needs; if so you’ll be seeing some. I’m attaching a couple of images showing this on NGC 5544/5, which has been posted a couple of times in different threads (but from older data here). First the blue-light image shows the overall geometry. Then we have subtracted a symmetric model for the light from the foreground galaxy and divided by a symmetric model for the light of the background galaxy (both models being based on the data in the non-overlapping regions), giving a map of the absorption by dust in the overlap area. We hope to get a lot more of these.