Observational evidence of density wave motion from the study of star formation and evolution in M 33

Details Observational evidence of density wave motion from the study of star formation and evolution in M 33 Observational evidence of density wave motion from the study of star formation and evolution in M 33

Apr 1977

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1977a%26a....56..293d&link_type=abstract

Astronomy and Astrophysics, vol. 56, no. 1-2, Apr. 1977, p. 293-301.

Astronomy and Astrophysics

Astrophysics

19

Astrophysics, Density Wave Model, Spiral Galaxies, Star Formation, Astronomical Photography, Galactic Rotation, Galactic Structure, Gas Expansion, H Ii Regions, Hydrogen Ions, Shock Fronts, Star Distribution

Scientific paper

H-alpha photographs and H I observations are used to localize the site of a density wave in the southern spiral arm of M 33. It is shown that the usual spiral tracers (dust, H II regions, and O stars) are concentrated along a chain front in the southern arm that is 4 kpc long and well represented by a logarithmic spiral. A test of Robert's (1969) kinematic conditions for the existence of a shock front reveals that a shock wave is present at the inner edge of the southern arm but not in the northern arm. It is found that stellar associations at the outer edge of the southern arm are older than those at the inner edge, indicating the propagation of a density wave with a pattern velocity less than the rotational velocity of the gas. A study of the regular dimensions of H II regions and the number of exciting stars in inner-edge associations demonstrates that star formation occurs rather uniformly along the 4-kpc length of the chain front in the southern arm but is random and less efficient elsewhere. It is concluded that sudden compression due to a density wave is the only plausible mechanism for inducing cloud fragmentation and star formation in the southern arm of M 33.

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