The Warm Ionized Medium: A High Resolution Northern Hemisphere [SII] Survey and an Emission Measure Atlas

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AST 0098487SimonettiThis research probes the structure and physical properties of the warm ionized medium (WIM), a phase of the interstellar medium in which gas is in mostly ionized form at approximate temperatures of 6000 to 10,000 K. The investigators use a sensitive charge-coupled device (CCD) camera system in conjunction with narrowband interference filters to image the distribution of the WIM over the entire northern sky with approximately arcminute resolution. Their previous work included a survey of the WIM in the H-alpha emission line ( 6563A ). In this research they extend previous imaging in the [S II] forbidden-line doublet to cover the northern hemisphere. Hydrogen is excited by photoionization, whereas [S II] is collisionally excited. Thus, the [S II] survey is an ideal complement to their H-alpha survey, making it possible to study the physical properties of the gas. Most importantly, this research will globally address fundamental questions concerning the sources of heat in the WIM by providing the [S II]/H-alpha emission ratio is a large variety of structures. These surveys are presented to the astronomical community (consisting of both researchers and educators ) at the Virginia Tech Spectral-line Survey (VTSS) website (http://www.phys.vt.edu/vtss). The researchers will also produce an H-alpha atlas which combines their VTBB Ha-alpha survey with the WHAM H-alpha survey of Reynolds and collaborators at the University of Wisconsin. These two surveys are ideally complementary. The surveys produced by this research will enable astronomers to study the relationship of the WIM to other phases of the interstellar medium. Through comparison with maps of H I, CO, infrared, and X-ray emission, astronomers will be able to discern how the WIM is distributed in space relative to neutral gas, cold gas, dust, and gas heated to very high temperatures by supernovae and stellar outflows. The angular resolution of the VTSS surveys is well matched to major surveys such as IRAS and ROSAT. This work will involve Virginia Tech undergraduate and graduate students. The researchers are building on previous successes with students trained in conjunction with this research. The camera system is located at a dark site within easy reach of the Virginia Tech campus -- an arrangement that fits the scientific needs of the project, minimizes cost, and enables student involvement. In addition to research uses, the survey images are readily accessible for a wide range of educational uses at the high school and college levels. With this in mind, a simple to use web-based image viewing and processing program is provided for displaying, manipulating, and analyzing the FITS-format images.***