Line-of-sight velocity dispersions and a mass-distribution model of the Sa galaxy NGC 4594

Abstract

In this paper, we develop an algorithm allowing to calculate line-of-sight velocity dispersions in an axisymmetric galaxy outside the galactic plane. When constructing a self-consistent model, we take into account the galactic surface brightness distribution, stellar rotation curve and velocity dispersions. We assume that the velocity dispersion ellipsoid is triaxial and lies under a certain angle with respect to the galactic plane. This algorithm is applied to an Sa galaxy NGC 4594 = M 104, for which there exist velocity dispersion measurements outside the galactic major-axis. The mass-distribution model is constructed in two stages. In the first stage, we construct a luminosity-distribution model, where only galactic surface brightness distribution is taken into account. Here, we assume the galaxy to consist of the nucleus, the bulge, the disc and the stellar metal-poor halo and determine structural parameters of these components. Thereafter, in the second stage, we develop, based on the Jeans equations, a detailed massdistribution model and calculate line-of-sight velocity dispersions and the stellar rotation curve. Here, a dark matter (DM) halo is added to visible components. Calculated dispersions are compared with observations along different slit positions perpendicular and parallel to the galactic major-axis. In the best-fitting model, velocity dispersion ellipsoids are radially elongated with σ θ/σR ≃ 0.9-0.4, σ z/σR ≃ 0.7-0.4, and lie under the angles less than or equal to 30° with respect to the galactic equatorial plane. Outside the galactic plane, velocity dispersion behaviour is more sensitive to the DM density distribution and allows to estimate dark halo parameters. For visible matter, the total M/LB = 4.5 ± 1.2, M/LR = 3.1 ± 0.7. The central density of the DM halo is ρDM(0) = 0.033 M⊙ pc-3. © 2006 RAS.