A New Method to Measure and Map the Gas Scale Height of Disk Galaxies

Abstract

We propose a new method to measure and map the gas scale height of nearby disk galaxies. This method is applied successfully to the Australia Telescope Compact Array interferometric HI survey of the Large Magellanic Cloud (LMC); it could also be applied to a significant number of nearby disk galaxies, thanks to the next generation of interferometric facilities, such as the extended VLA and CARMA. The method consists of computing the Spectral Correlation Function (SCF) for a spectral-line map of a face-on galaxy. The SCF quantifies the correlation between spectra at different map positions as a function of their separation, and is sensitive to the properties of both the gas mass distribution and the gas velocity field. It is likely that spatial correlation properties of the gas density and velocity fields in a galactic disk are sensitive to the value of the scale height of the gas disk. A scale-free turbulent cascade is unlikely to extend to scales much larger than the disk scale height, as the disk dynamics on those larger scales should be dominated by two dimensional motions. We find a clear feature in the SCF of the LMC HI disk, on the scale of approximately 180 pc, which we identify as the disk scale height. We are also tentatively able to map variations of the scale height over the disk.