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Astro: WISE All Sky Survey

Posted 4/2/2012 12:04 AM by Ka Chun Yu | Comments

All sky surveys provide a sort of fundamental baseline to astronomical observational science.  Such surveys are done using the same telescope and/or instrument, or at least instruments that are similar enough that the differences between them are well understood.  (The latter occurs in the case of surveys from the ground, where different telescopes located at observatories in the northern and southern hemispheres are necessary to provide coverage over the entire sky.)  The observations from such a survey provide a consistent set of data probing the same wavelengths in the electromagnetic spectrum, and to similar sensitivities -- that is, detecting objects down to the same level of faintness.

By uniformly observing the sky without concentrating on one part more than another, the resulting data is not biased towards just the regions of the universe that happen to be the most popular with astronomers.  Serendipitous discoveries are possible.  Interesting objects discovered in the survey can be probed in the future with other telescopes.

WISE has instruments covering four wavebands.

The WISE four wavelength bands showing the responses for each detector. A relative response of 1.0 means 100% of the photons reaching the instrument are detected. The plot shows that there is a broad range of wavelengths that each detector is sensitive to, and the response varies quite a bit even within each band.

These are some of the reasons why the just released all-sky from the Wide-field Infrared Survey Explorer is important. Its coverage of the mid-infrared between 3.4 and 23 microns helps plug a big gap in wavelength coverage. The objects discovered in the WISE survey will be targets for future James Webb Space Telescope observations.

The two shortest wavelength bands at 3.4 and 4.6 microns, are just beyond the "near-infrared," a part of the electromagnetic spectrum accessible from ground-based telescopes.  Stars emit enough light at these wavelengths that they will still show up in the WISE images.  The longer bands at 12 and 22 microns reveal light that is dominated by emission from cool dust and gas in the interstellar medium.  We can see this in the following four images, taken in each of the four wavebands, from shortest to longest:

In the two pictures on the left, the field is dominated by countless stars.  However for the two images on the right, the stellar point sources drop from the thousands to just a few dozen.  The "nebulous" emission is also more prominent in the longer wavelength images on the right, with the frilly feature in the bottom center showing up only in the longest 22 micron image.  When combined together, so that the 3.4 and 4.6 micron data is colored blue, the 12 micron image green, and the 22 micron data red, we get the following composite:

The arcing wisps showing up only in red is the Puppis A supernova remnant.  This was a massive star that exploded and became visible in our night skies 3,700 years ago (although there is no historical record of it being observed).  The expanding shockwave from the original explosion is heating up the interstellar dust in the general vicinity, as well as gas that was expelled by the progenitor star before its final death throes.  This has resulted in the rosy rosette visible here.

There are lots more images of objects within our galaxy that WISE has observed in the process of making its all-sky that are highlighted here at the mission website.  If you want to explore the entire all-sky dataset, I'd recommend playing with World Wide Telescope, the free astronomical data visualization platform from Microsoft Research (available for install on Windows PCs, but accessible via the web client for both PCs and Macs).


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