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[116.07] Measuring Molecular Hydrogen in M 51

N.R. Hinkel (Oberlin College), E.D. Miller, J.N. Bregman (University of Michigan)

The molecular mass as derived from CO emission depends upon an assumed conversion factor, which for M 51, is suggested to be significantly different from the Milky Way value. Another and more direct method of determining the molecular gas mass is from the UV molecular hydrogen absorption lines. Here we use the stellar continuum of the disk, against which we detect the H₂ absorption lines in the far UV with FUSE. From this, we estimate a column density and compare it to the CO line intensity to find the value of the conversion factor (X) for M 51.

We have obtained FUSE spectra toward five lines of sight in M 51, including the center, spiral arms, and interarm regions. Absorption due to H₂ and low- and high-ionization atomic species from both M 51 and the Milky Way are seen. Using models of the background stellar continuum, we estimate the H₂ column density to range from 10¹⁹--10²⁰ cm^-2, with a covering fraction between 20--50%. We discuss the implications of these results on the CO conversion factor and the ISM within M 51.

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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.