Weekly Science Update

Wednesday, June 25, 2008
Objects Both Hot and Cold
The sun's photosphere is hot, about 6000 kelvin, and so the sun emits about 70% of its light in the visible portion of the electromagnetic spectrum, and about one-quarter in the infrared. The sun has a very hot outer corona, however, whose temperatures reach over one million kelvin, and it contributes X-ray emission at a level of about one-millionth of the total solar radiative energy output. Most cosmic sources that we know about are like the sun, radiating significantly across a wide range of wavelengths because they harbor multiple processes, each process with its own characteristic temperature. Astronomers trying to figure out what happens in the nuclei of galaxies, as well as in the environments of stars, piece together information from many wavelengths as they sort out the various physical processes at work.

SAO's astronomers have expertise in the phenomena and wavelengths that span this wide range of cosmic energetics and sources, and this month two groups published separate articles on different topics, with each combining X-ray and infrared observations. A team of eight SAO astronomers led by Steve Murray, together with eleven colleagues, compared the X-ray emission of galaxies in a portion of the sky studied carefully by both the Chandra X-ray Observatory and the Spitzer Space Telescope. They find that nearly every one of the 3086 X-ray emitting sources in the field has detectable infrared emission that helps to categorize the source: nearly all are types of galaxies with black holes at their nuclei, although a few are stars. Their results, the largest such study that has ever been done, also help to identify the nature of the sources that contribute overall to the cosmic infrared background because a substantial fraction of it comes from the ensemble of distant galactic nuclei.

SAO astronomer Howard Smith and nine of his colleagues published the second paper, the largest study ever made of the mid-infrared emission from the X-ray sources seen in the direction of our own galactic center. Some objects more exotic than the sun can emit predominantly in X-rays, for example, massive stars with strong winds, or binary stars in which one member is a black hole or neutron star. The team compared 2357 Chandra X-ray sources with about 20,000 Spitzer infrared sources in order to determine if the conditions in the center of our galaxy are conducive to producing such extreme objects. They report that, at least to within the ability of the images to distinguish separate sources (the direction of the galactic center is densely packed with stars), almost none of the X-ray objects has an infrared counterpart. The paper helps to confirm that the galactic center region, despite its harboring a massive black hole and being unusual in many other respects, does not have an unusual population of stars. Although the two papers reach opposite statistical conclusions (one finds near-perfect matches, the other near-perfect mismatches), each paper combines both infrared and X-ray observations to improve our understanding of the nature of cosmic sources.

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