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Session 67 - ISM in Galaxies.
Display session, Wednesday, January 15
Metropolitan Ballroom,
The spiral galaxy M101 has many spectacular giant HII regions, of which the most impressive is NGC 5461. There is no theory that explains why giant HII regions form in certain spiral galaxies and not in others (such as our own). We are testing whether giant HII regions are associated with gas characterized by different properties than those of molecular clouds near more typical HII regions. We provide some observational data for the molecular gas associated with a giant and a normal HII region in M101.
We have used the James Clerk Maxwell Telescope to observe two rotational transitions of CO (J=2\to1 and J=3\to2) and ^13CO J=2\to1 toward NGC 5461. The line ratio of ^12CO/^13CO is between 6.7 and 8.1, which agrees well with the ratios found in M33.
Using the Owens Valley Millimeter Array, we obtained higher resolution (3''-4'') CO J=1\to0 observations in NGC 5461 and resolved the peak seen in the J=2\to1 map to several smaller peaks. These giant molecular associations have diameters up to 200 pc and masses of 3\times 10^5 \cal M_ødot.
In addition, we have observed a normal HII region in M101 and found that the ^12CO 3\to2/^12CO 2\to1 ratio is slightly lower than that observed for NGC 5461 (0.51 \pm 0.1 and 0.70 \pm 0.1 respectively). We use these ratios in a large velocity gradient code and find that the molecular gas around NGC 5461 has an excitation temperature of 90 K whereas the gas around the normal HII region has a temperature of 10 K.
