The DiskMass Survey. VII. The distribution of luminous and dark matter in spiral galaxies

We present dynamically-determined rotation-curve mass decompositions of 30 spiral galaxies, which were carried out to test the maximum-disk hypothesis and to quantify properties of their dark-matter halos. We used measured vertical velocity dispersions of the disk stars to calculate dynamical mass surface densities (Σ_dyn). By subtracting our observed atomic and inferred molecular gas mass surface densities from Σ_dyn, we derived the stellar mass surface densities (Σ_∗), and thus have absolute measurements of all dominant baryonic components of the galaxies. Using K-band surface brightness profiles (I_K), we calculated the K-band mass-to-light ratio of the stellar disks (Υ_∗ = Σ_∗/I_K) and adopted the radial mean (overline{mls}) for each galaxy to extrapolate Σ_∗ beyond the outermost kinematic measurement. The derived overline{mls} of individual galaxies are consistent with all galaxies in the sample having equal Υ_∗. We find a sample average and scatter of mlab overline{mls}mrab = 0.31 ± 0.07. Rotation curves of the baryonic components were calculated from their deprojected mass surface densities. These were used with circular-speed measurements to derive the structural parameters of the dark-matter halos, modeled as either a pseudo-isothermal sphere (pISO) or a Navarro-Frenk-White (NFW) halo. In addition to our dynamically determined mass decompositions, we also performed alternative rotation-curve decompositions by adopting the traditional maximum-disk hypothesis. However, the galaxies in our sample are submaximal, such that at 2.2 disk scale lengths (h_R) the ratios between the baryonic and total rotation curves (F_b^2.2h_R) are less than 0.75. We find this ratio to be nearly constant between 1-6h_R within individual galaxies. We find a sample average and scatter of mlab F_b^2.2h_Rmrab = 0.57 ± 0.07, with trends of larger F_b^2.2h_R for more luminous and higher-surface-brightness galaxies. To enforce these being maximal, we need to scale Υ_∗ by a factor 3.6 on average. In general, the dark-matter rotation curves are marginally better fit by a pISO than by an NFW halo. For the nominal-Υ_∗ (submaximal) case, we find that the derived NFW-halo parameters have values consistent with ΛCDM N-body simulations, suggesting that the baryonic matter in our sample of galaxies has only had a minor effect on the dark-matter distribution. In contrast, maximum-Υ_∗ decompositions yield halo-concentration parameters that are too low compared to the ΛCDM simulations.

Appendix is available in electronic form at http://www.aanda.org