Radial distributions of surface mass density and mass-to-luminosity ratio in spiral galaxies

Abstract

Abstract We present radial profiles of the surface mass density (SMD) in spiral galaxies directly calculated using rotation curves of two approximations of flat-disk (SMD-F) and spherical mass distribution (SMD-S). The SMDs are combined with surface brightness using photometric data to derive radial variations of the mass-to-luminosity ratio (ML). It is found that the ML generally has a central peak or a plateau, and decreases to a local minimum at R ∼ 0.1–0.2 h, where R is the radius and h is the scale radius of optical disk. The ML, then, increases rapidly until ∼0.5 h, and is followed by gradual rise till ∼2 h, remaining at around ∼2 [$M_{\odot}\,L^{-1}_{\odot}$] in the w1 band (infrared λ3.4 μm) and $\sim 10\ [M_\odot L_\odot ^{-1}]$ in the r band (λ6200–7500 Å). Beyond this radius, the ML increases steeply with approaching the observed edges at R ∼ 5 h, attaining to as high values as ∼20 in w1 and $\sim 10^2\ [M_\odot L_\odot ^{-1}]$ in the r band, which are indicative of dominant dark matter. The general properties of the ML distributions will be useful for constraining cosmological formation models of spiral galaxies.