THE STELLAR VELOCITY DISPERSION OF THE EDGE-ON SPIRAL GALAXY NGC-891

At three positions, along a line perpendicular to the major axis, absorption line spectra have been obtained of the edge-on spiral galaxy NGC 891. These have been used to determine the stellar kinematics at three distances from the centre. For a distance of 52" radial velocities and velocity dispersions are presented as a function of height above the plane. At that position the average dispersion value amounted to 82 +/- 15 km s-1, while at distances of 5" and 132" average velocity dispersions of 112 +/- 20 km s-1 and 54 +/- 10 km s-1 were determined. The observations were translated into dispersion values as a function of radius inside the galaxy. To that aim a galaxy model was constructed including a disk, bulge and absorbing layer. When the radial velocity dispersion decreases exponentially as a function of radius with a scalelength twice the photometric scalelength, the central dispersion amounts to 120 +/- 20 km s-1. This is comparable to the value of 100 +/- 10 km s-1 for our Galaxy. From the dispersion a M/L ratio (in the B-band) between 6 and 10 was determined by assuming a constant Toomre Q value of 1.7 and a (M/L)B of 5.3 +/- 2.5 by assuming the old disk stars make up an isothermal disk. These mass-to-light ratios compare well with (M/L)B = 7 given by measuring the thickness of the hydrogen layer. Finally, it is concluded that the dispersion is not large enough to allow a so called "maximum disk" fit to the rotation curve.