Dark matter measurements combining stellar and HI kinematics: 30% 1−σ outliers with low dark matter content at 5Re
Résumé
We construct the Schwarzschild dynamical models for 11 early-type galaxies with the SAURON and Mitchell stellar IFUs out to 2−4Re, and construct dynamical models with combined stellar and HI kinematics for a subsample of 4 galaxies with HI velocity fields out to 10Re obtained from the Westerbork Synthesis Radio Telescope, thus robustly obtaining the dark matter content out to large radii for these galaxies. Adopting a generalised-NFW dark matter profile, we measure an NFW-like density cusp in the dark matter inner slopes for all sample galaxies, with a mean value of 1.00±0.04 (rms scatter 0.15). The mean dark matter fraction for the sample is 0.2 within 1Re, and increases to 0.4 at 2Re, and 0.6 at 5Re. The dark matter fractions within 1Re of these galaxies are systematically lower than the predictions of both the TNG-100 and EAGLE simulations. For the dark matter fractions within 2Re and 5Re, 40% and 70% galaxies are 1−σ consistent with either the TNG-100 or the EAGLE predictions, while the remaining 60% and 30% galaxies lie below the 1−σ region. Combined with 36 galaxies with dark matter fractions measured out to 5Re in the literature, about 10% of these 47 galaxies lie below the 3−σ region of the TNG-100 or EAGLE predictions.