Astronomy and Astrophysics – Astronomy
Scientific paper
May 1987
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987nascp2466..263s&link_type=abstract
Star formation in galaxies, Proceedings of a conference at the California Institute of Technology, Pasadena, California, June 16
Astronomy and Astrophysics
Astronomy
101
Abundance, Dwarf Galaxies, Hydrogen, Irregular Galaxies, Local Group (Astronomy), Neutral Gases, Star Formation, Balmer Series, Density Distribution, H Alpha Line, H Ii Regions, Spatial Distribution
Scientific paper
The Very Large Array and WSTR H I synthesis observations of seven irregular galaxies are presented. The total H I images of four Local Group dwarf irregular galaxies and three larger more distant irregular galaxies are constructed at the identical resolution of 500 pc. When compared to H II region distributions derived from H alpha images, all galaxies studied show an excellent correlation between the H I surface density and the presence of H II regions. This correlation is most easily interpreted in terms of a requisite threshold H I surface density for massive star formation. This threshold is 1 x 10 to the 21st power H I atoms/sq cm for a resolution of 500 pc. Giant extragalactic H II regions are only found near H I surface densities of a factor of 3 to 5 times this threshold level. The observed threshold implies a Jeans length of 150 pc, which is the same as the size scale at which the structure in the H I complexes correlates well with the H II region distribution. This, combined with the fact that in none of the galaxies observed is there H I above the threshold level with concomitant H II regions, implies an exclusively gravitational origin for the star formation events. That is, there is no need to involve a trigger as in the SSPSF theory (Seiden 1983) or feedback as in Dopita (1985).
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