Astron. Astrophys. 328, 471-482 (1997)

4. Conclusions

1. IRAS far-infrared surface brightnesses and HI column densities are used to indepently estimate column densities towards CO clouds observed in the LMC and SMC. Generally, in these clouds mass surface densities exceed those of HI by a factor of about 1.5 on average. This is in contrast to the global to HI mass ratios which are of the order of 20 10 .
2. By combining the newly derived column densities with published CO intensities, the CO to conversion factors X are determined to be = 1.3 0.2 10²¹ molecules cm^-2 ()^-1 and = 12 2 10²¹ molecules cm^-2 ()^-1. The global mass of (warm) molecular hydrogen is estimated to be = 1.0 0.3 10⁸ for both LMC and SMC.
3. On average somewhat higher molecular to atomic hydrogen mass surface densities are found in the irregular dwarf galaxies NGC 55, NGC 1569, NGC 4214, NGC 4449 and NGC 6822, as well as in the extragalactic HII region complexes NGC 604, NGC 595, both in M 33, and NGC 5461 in M 101. The X- values derived for the HII regions and NGC 4449 are comparable to that of the LMC, while the X -values derived for NGC 55, NGC 4214 and NGC 6822 are typically two to four times higher; NGC 1569 has a very high value comparable to that of the SMC.
4. Analysis suggests that the CO to conversion factor X is linearly dependent on the strength of the ambient radiation field per nucleon, and inversely dependent on a steep function of metallicity [O]/[H]: log X = 0.9 0.1 log - 3.5 0.2 log + 34.6 2.2. If the dependence on radiation field is neglected, the relation log X = -2.7 0.3 log + 11.6 1.0 also fits the data. Similarly derived Milky Way values fit these same relations. They are interpreted as the result of selective photodissociation of CO under conditions of high radiation field energy densities and poor shielding and selfshielding in low-metallicity environments. Thus, over the parameter range studied, the CO content of galaxies varies strongly as a function of conditions.
5. Estimates of the global (warm) to HI mass ratios and the (warm) gas fractions yield very similar results for all galaxies. On average, = 0.20 M(HI), and = 0.12 . These ratios are very close to the global Milky Way ratios: the global warm fraction in irregular dwarf galaxies appears to be very similar to that of our Galaxy, notwithstanding the large differences in total mass, luminosity, metallicity and observed CO luminosity.

© European Southern Observatory (ESO) 1997

Online publication: March 26, 1998

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