The luminous and dark matter content of disk galaxies

Abstract

For a compiled sample of disk galaxies with available photometry (B and K bands), velocity line-widths and HI integral fluxes, several parameters which trace the luminous, baryonic and dark matter contents were inferred. We investigated how these parameters do vary with different galaxy properties, and confronted the results with predictions of galaxy evolutionary models in the context of the LCDM cosmogony. The ratio of disk-to-total maximum circular velocity, vd/vt, depends mainly on the central disk surface density Sig_d (or surface brightness, SB), increasing roughly as Sig_d^0.15. While a fraction of high SB galaxies have a vd/vt ratio corresponding to the maximum disk solution, the low SB are completely dark matter dominated. The trend is similar for the models, although they have slightly smaller vd/vt ratios than observations. An analysis of residuals of the vd/vt-Sig_d relation shows that vd/vt tends to decrease as the galaxy is redder, more luminous (massive), and of earlier type. The models allow us to explain the physics of these results, which imply a connexion between halo structure and luminous properties. The estimated dynamical-to-baryon mass and dynamical mass-to-light ratios at a given radius, for observations and models, decrease with the SB and do not correlate with the galaxy scale, contrary to what has been reported in previous works, based on the analysis of rotation curve shapes. We discuss this difference and state the importance to solve the controversy on whether the dark and luminous contents in disk galaxies depend on SB or luminosity. The broad agreement between models and observations favors the LCDM scenario. However, the excess of dark matter inside the optical region of disk galaxies remains as the main difficulty. (abridged)