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The Interstellar Medium and Star Formation in Edge-On Galaxies. I. NGC 891

We analyze images of BIMA 12CO (J = 1 --> 0), VLA HI, and Spitzer 3.6 and 24 \mum emission toward the edge-on galaxy NGC 891 and derive the radial and vertical distributions of gas and the radial distributions of stellar mass and recent star formation. We describe our method of deriving radial profiles for edge-on galaxies, assuming circular motion, and verify basic relationships between star formation rate and gas and stellar content, and between the molecular-to-atomic ratio and hydrostatic midplane pressure, that have been found in other galaxy samples. The Schmidt law index we find for the total gas (H2 + H I) is 0.85\pm0.55, but the Schmidt law provides a poor description of the SFR in comparison to a model that includes the influence of the stellar disk. Using our measurements of the thickness of the gas disk and the assumption of hydrostatic equilibrium, we estimate volume densities and pressures as a function of radius and height in order to test the importance of pressure in controlling the ρH2/ρHI ratio. The gas pressure in two dimensions P(r, z) using constant velocity dispersion does not seem to correlate with the ρH2/ρHI ratio, but the pressure using varying velocity dispersion appears to correlate with the ratio. We test the importance of gravitational instability in determining the sites of massive star formation, and find that the Q parameter using a radially varying gas velocity dispersion is consistent with self-regulation (Q - 1) over a large part of the disk.