The Carina dwarf spheroidal galaxy: How dark is it?

We have obtained precise (〈σ〉=3.1 km s^-1) radial velocities of 23 photometrically-selected normal giant stars located near the center of the Carina dwarf spheroidal (dSph) galaxy. Of this sample, 17 stars are Carina members based on their heliocentric radial velocities in excess of 200 km s^-1. From this subsample, we derive a mean systemic velocity for Carina of 223.1±1.8 km s^-1, and a central velocity dispersion of 6.8 km s^-1. Using the luminosity function of the brightest stars in Carina, we have estimated the central surface brightness of the galaxy to be Σ_0,V=25.2±0.3. Assuming the mass distribution follows that of the visible material in Carina, we derived the V band mass-to-light ratio to be (script M sign/L)_V=39±23, based on published core parameters of the galaxy and using estimates of the integrated mass and luminosity. We have also fit the kinematic and structural data for Carina to a variety of one- and two-component Michie-King models. Those models in which the velocity dispersion of the visible matter is extremely anisotropic are ruled out by the surface brightness profile; acceptable fits correspond to models with global V-band script M sign/L ratios in excess of 35. These fits do not allow us to place any constraints on the distribution of dark matter in Carina except that the central mass density must be ≳0.02 script M sign_⊙pc^-3. We consider and reject the possibility that atmospheric or binary motions among the program stars may have artificially inflated a small intrinsic velocity dispersion to the observed value. The inferred dark matter content of Carina is consistent with the overall trend of increasing script M sign/L ratio with decreasing total luminosity exhibited by the best-studied dSph galaxies. We conclude that an assumed highly radially anisotropic velocity dispersion tensor in low-luminosity dSph galaxies is unlikely to be able to reduce the script M sign/L ratios within the visible volumes of these systems significantly. Similarly, we conclude that tidal forces probably do not play a significant role in generating the large range of script M sign/L ratios inferred for dSph systems. The current data for dSph galaxies are consistent with the conjecture that these systems all have similar dark matter halos with total masses in the range 1-5×10⁷ script M sign_⊙.