For understanding the relationship between star formation and the interstellar medium properties, measuring the disk thickness of a galaxy is very important since it allows us to derive the volume density, a quantity that may be better correlated with star formation rate (SFR) than surface density. In this talk, I present a study of the vertical structure of the gaseous and stellar disks in a sample of edge-on galaxies (NGC 891, 4157, 4565, and 5907) using BIMA/CARMA CO (J = 1-0), VLA HI, and Spitzer 3.6 micron data.
In order to take into account projection effects when measuring the disk thickness as a function of radius, I obtain the inclination by modeling the radio data. Using the measurement of the disk thicknesses and the derived radial profiles of gas and stars, I estimate the corresponding volume densities and vertical velocity dispersions. Both stellar and gas disks have smoothly varying scale heights and velocity dispersions, contrary to assumptions of previous studies. Using these scale heights and velocity dispersions, the gravitational instability parameter Q follows a fairly uniform profile with radius and is greater than or similar to 1 across the star forming disk. The star formation law has a slope that is significantly different from those found in more face-on galaxy studies, both in deprojected and pixel-by-pixel plots and this is likely due to IR opacity. Midplane density appears to be a better predictor of molecular-to-atomic ratio than midplane gas pressure.