The luminous and dark matter content of disk galaxies

We have compiled a sample of disk galaxies with available photometry in the B and K bands, velocity line-widths and HI integral fluxes. Several parameters that trace the luminous, baryonic and dark matter contents were inferred. We investigated how these parameters vary with different galaxy properties, and compared the results with predictions of galaxy evolutionary models in the context of the Λ Cold Dark Matter (ΛCDM) cosmogony. The ratio of disk-to-total maximum circular velocity, (V_d,m/V_t,m), depends mainly on the central disk surface density Σ_d,0 (or surface brightness, SB), increasing roughly as Σ_d,0^0.15. While a fraction of high SB galaxies have a (V_d,m/V_t,m) 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 (V_d,m/V_t,m) ratios than observations, in particular at the highest SBs and when small baryon fractions are used. The scatter in the (V_d,m/V_t,m)- Σ_d,0 plot is large. An analysis of residuals shows that (V_d,m/V_t,m) 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 dynamical-to-baryon mass and dynamical mass-to-light (B and K) ratios at a given radius were also estimated. All these ratios, for observations and models, decrease with Σ_d,0; (or SB) and do not correlate significantly 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 of solving the controversy of whether the dark and luminous contents in disk galaxies depend on SB or luminosity. The broad agreement between the models and observations presented here regarding the trends of the dynamical-to-baryon matter and mass-to-light ratios with several galaxy properties favors the ΛCDM scenario. However, the excess of dark matter inside the optical region of disk galaxies remains the main difficulty.

Appendices A and B are only available in electronic form at http://www.edpsciences.org. Table 1 is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/412/633