Molecular gas in HII regions of M101

Abstract

The molecular component of three giant H 11 regions (NGC 5461, NGC 5462, NGC 5471) and one normal H II region (H660) in the galaxy M101 are investigated with new observations from single dish telescopes (JCMT and NRAO 12-meter) and from the Owens Valley millimeter array. The data are analyzed using three independent methods to calculate the mass of the molecular gas in the H II regions.

The molecular mass for he association of clouds in NGC 5461 is approximately 10^8 M. and is accompanied by a few times more atomic mass. The observed CO emission in NGC 5461 was stronger than in NGC 5462 while it was not possible to detect molecular gas in NGC 5471 with the JCMT. The fairly large ratio of atomic to molecular gas in NGC 5471 might be attributed to inefficient conversion of the atomic gas to giant molecular clouds.

The masses of the indiviudal giant molecular clouds in NGC 5461, which are gravitationally bound, cover a range of (10 - 40) x 10^5 M. - an order of magnitude larger than Galactic giant molecular clouds. Massive clouds might be a prerequisite for the formation of the large number of stars whose radiation produces the giant H 11 regions. Furthermore, the giant molecular clouds in NGC 5461 are larger in size than expected given their small velocity dispersions. These small velocity dispersions indicate that collisions of smaller clouds are important for the formation of the large giant molecular clouds in NGC 5461.