Molecular Gas and Star Formation in the Central Regions of Virgo Spiral Galaxies

Citation

Canzian, Blaise J. (1990) Molecular Gas and Star Formation in the Central Regions of Virgo Spiral Galaxies. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/pcga-sm96. https://resolver.caltech.edu/CaltechETD:etd-06182007-104442

Abstract

The central regions of eight normal spiral galaxies in the Virgo Cluster spanning a wide range of morphological types have been observed in CO (J = 1 → 0) emission using the Owens Valley mm-wave interferometer. Broadband optical and Hα CCD photometry was obtained using the Palomar 60-inch telescope.

The H₂ masses in the central five kiloparsecs of the spirals are large, typically several 10⁸ solar masses, far exceeding the atomic hydrogen masses. H₂ densities in the cloud complexes are comparable to those of Galactic center complexes such as Sgr B2. Molecular gas comprises a substantial fraction of the dynamical mass of the central regions. The gas mass fraction, typically 5-15% at 2.5 kiloparsec radius, increases to 15-30% at the center. Molecular gas in the interferometer maps is concentrated in structures with scale lengths of only 400-750 parsecs. Such massive gas disks are, nonetheless, dynamically stable. The H₂ masses may be overestimated in some spirals because of cloud heating. The main source of heating is probably dissipation of turbulence, which is probably greater at the centers of spirals based on CO line widths.

Current central star formation rates are about a few solar masses per year. The star formation rate in the central regions of most of the spirals has been relatively constant over a Hubble time, although some galaxies are currently undergoing star formation bursts. Several mechanisms are suggested that could supply enough gas to maintain current star formation rates. The current rate of star formation is well correlated with the mean square molecular gas surface density computed from interferometer maps. Such a correlation is expected if cloud-cloud collisions or another stimulation mechanism acting between clouds is the principle means to produce high-mass stars where the concentration of clouds is greatest. The B luminosity is better correlated with the total gas mass surface density, which supports a stochastic or independent mode of forming intermediate-mass stars in the general cloud population.

Thesis Files