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

Abstract

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 ({$Sigma$}$_{dyn}$). By subtracting our observed atomic and inferred molecular gas mass surface densities from {$Sigma$}$_{dyn}$, we derived the stellar mass surface densities ({$Sigma$}$_{∗}$), 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 ({Upsilon}$_{∗}$ = {$Sigma$}$_{∗}$/I$_K$) and adopted the radial mean (overline${$mls$}$) for each galaxy to extrapolate {$Sigma$}$_{∗}$ beyond the outermost kinematic measurement. The derived overline${$mls$}$ of individual galaxies are consistent with all galaxies in the sample having equal {Upsilon}$_{∗}$. We find a sample average and scatter of mlab overline${$mls$}$mrab = 0.31 {plusmn} 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$$^{2.2hR}$) are less than 0.75. We find this ratio to be nearly constant between 1-6h$_R$ within individual galaxies. We find a sample average and scatter of mlab F$_b$$^{2.2hR}$mrab = 0.57 {plusmn} 0.07, with trends of larger F$_b$$^{2.2hR}$ for more luminous and higher-surface-brightness galaxies. To enforce these being maximal, we need to scale {Upsilon}$_{∗}$ 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-{Upsilon}$_{∗}$ (submaximal) case, we find that the derived NFW-halo parameters have values consistent with {$Lambda$}CDM 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-{Upsilon}$_{∗}$ decompositions yield halo-concentration parameters that are too low compared to the {$Lambda$}CDM simulations. Appendix is available in electronic form at http://www.aanda.org