Galaxy Zoo goes observing

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.


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7 responses to “Galaxy Zoo goes observing”

  1. Waveney says :

    Other than obtaining pretty pictures, what are you hoping to learn about the dust?

  2. NGC3314 says :

    In the example above, we don’t just have the pretty picture; for each pixel in the overlap region (and in each filter we observe with), we have a quantitative estimate of the dust absorption. (And being proper scientists, we also have estimates of the error on those, based on the un-overlapped parts of the galaxy). This gives us a corresponding estimate of the dust mass in each region. As we get more galaxies, not only do we have the dust maps, we can start to see how they differ among galaxies, for example in central obscuration, relative scales of the distribution of stars and dust, and contrast between spiral arms and the interarm regions as seen in dust. We used to think we knew the dust/stars comparison, but a dramatic counterexample recently turned up in the corner of a Hubble image taken for another purpose. Maybe we should try posting some of these maps as we get them produced at the telescope.

  3. Waveney says :

    Would taking area spectographs of the unoverlapped portions of the two galaxies and the overlap region also yield useful data?

  4. NGC3314 says :

    There’s an additional technique you can use with spectra – the strengths of absorption lines with Doppler shifts matching the two galaxies tells you what fraction of the light comes from foreground and background systems. You’re still vulnerable to asymmetry in the galaxies in the interpretation, but not as much as with imaging alone for spiral/spiral pairs. We’ve done that for a few back in the 1990s, finding that the approach works, and with area spectrographs coming into wider use, we may be revisiting this idea for the new sample.

  5. Joseph K. H. Cheng says :

    Very detailed, interesting and exciting preview of the upcoming real time observations from Kitt Peak, Arizona. Thank you so much for the narration. I have been keeping records of some overlapping galaxy pairs from ” MY GALAXIES ” database of over 670,000 galaxy analysis. I will spend tomorrow comparing my references with your PDF file paying particular attention to the high Z value samples. Will touch base with you asap on my results. I am elated as well as very excited to participate in the preparations leading to the upcoming important observations at Kitt Peak.

  6. Daniel Cornwall says :

    From a purely aesthetic perspective, I am very happy you shared your target book with us!

  7. Half65 says :

    Thanks my overlapping guru to share your album with us. Great work.

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