A KINEMATIC STUDY OF THE FORNAX DWARF SPHEROIDAL GALAXY

We have obtained precise radial velocities of 44 stars and four globular clusters located in two fields of the Fornax dwarf spheroidal (dSph) galaxy. The fields are located in the center of Fornax, and about two core radii SW of the galaxy center along the major axis. Our results are based on photon-counting Echelle spectroscopy with a resolution of approximately 14 km s-1. We also present BV CCD photometry of the giant branch of Fornax in both fields. Using a variety of kinematic and photometric criteria, we have identified 10-12 probable nonmembers in our sample of spectroscopically observed stars. Based on the most probable members, the mean heliocentric systemic velocity of Fornax is 53.0 +/- 1.8 km s-1, with no evidence for any significant rotation about the minor axis. The intrinsic velocity dispersion of the stars in the central field of Fornax is 9.9 +/- 1.7 km s-1 (12.1 +/- 1.9 km s-1), while for the outer field the velocity dispersion is 12.0 +/- 2.8 km s-1 (13.6 +/- 3.0 km s-1). The first values refer to dispersions estimated using only the most probable members; the second values show the effect of including a marginal nonmember in each sample. The true central velocity dispersion is less-than-or-equal-to 1.6 km s-1 larger than the observed central dispersions for a number of reasonable models. The central mass-to-light ratio (M/L) of Fornax is 12.3 +/- 4.5, or 11.4 +/- 4.2 based on global galaxy parameters or core fitting, respectively; the full range of acceptable M/L ratios is between 5.3 and 26. The uncertainties in the published structural parameters of Fornax are the dominant source of this large range. The observed velocity dispersion "profile" suggests that any dark matter (DM) halo in Fornax is more extended than the visible material. A model-independent lower limit to the central mass density of Fornax also implies that the galaxy contains a significant DM component. If the velocity dispersion in the inner parts of Fornax is assumed to be isotropic, then the central mass density of DM is 0.07 +/- 0.03 M . pc-3. This is comparable to the central DM densities observed in gas-rich dwarf irregular galaxies, but is lower than in other dSph systems. Whether this latter effect is related to galaxy luminosity or Galactic tidal effects is unclear.