Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000aas...196.6304h&link_type=abstract
American Astronomical Society, 196th AAS Meeting, #63.04; Bulletin of the American Astronomical Society, Vol. 32, p.1285
Astronomy and Astrophysics
Astronomy
Scientific paper
The approximately flat rotation curve of the Milky Way implies the existence of a substantial amount of (unobserved) mass in the outer regions of the Galaxy which may comprise cold molecular hydrogen. Although this hypothesis is best tested by observing far-IR transitions from space, these ground-state lines may be exploitable at visible wavelengths in pure rotational Raman spectra excited by H-alpha radiation. This approach has several advantages. The Raman intensity is directly proportional to the specie number density permitting direct estimates of the molecular hydrogen mass and ortho-to-para ratio within the sampled region. Furthermore, the distribution of population among the lowest rotational lines can be determined from a single spectrum obtained with a single detector, minimizing the selective effects of extinction on the results. The cloud and radiation source do not need to lie along the same line-of-sight as they would for absorption-line studies, and the spatial structure of the emitting cloud can be probed at much finer detail in the deep-red than at 28 \micron. To estimate the viability of this concept, the Stokes flux density, AB magnitude, and on-orbit signal-limited signal-to-noise ratio (SNR) in a 5 Angstroms band have been computed as a function of the hydrogen volume density in spherically symmetric, homogeneous clouds. The molecular cloud was placed at 1/4, 1/2, and 3/4 of the source distance which was varied between 1-10 kpc. The computed SNR initially rises to a peak value as the hydrogen volume density increases, followed by a gradual decline as the dust optical depth becomes dominant. With an on-orbit aperture of 2.5 m, 50 pc diameter sources as distant as 3 kpc produce detectable Raman emission with exposures of 10,000 sec. Although the SNR exceeds 5 only over a very limited range of hydrogen densities with this aperture, it exceeds 10 for clouds with hydrogen densities between about 1 and 100 per cubic centimeter when the aperture is increased to 8-m. If these results can be confirmed in background-limited situations when multiple clouds exist along the line of sight, this technique may permit NGST to achieve its ground-state Galactic hydrogen science goals simply by extending its short wavelength limit to 0.5 \micron.
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