Adrianne Slyz

Abstract:

We present results from a detailed dynamical analysis of five high surface brightness, late type spirals, studied with the aim to quantify the luminous-to-dark matter ratio inside their optical radii. The galaxies' stellar light distribution and gas kinematics have been observed and compared to hydrodynamic gas simulations, which predict the 2D gas dynamics arising in response to empirical gravitational potentials, which are combinations of differing stellar disk and dark halo contributions. The gravitational potential of the stellar disk was derived from near-infrared photometry, color-corrected to constant (M/L); the dark halo was modelled by an isothermal sphere with a core. Hydrodynamic gas simulations were performed for each galaxy for a sequence of five different mass fractions of the stellar disk and for a wide range of spiral pattern speeds. These two parameters mainly determine the modelled gas distribution and kinematics. The agreement between the non-axisymmetric part of the simulated and observed gas kinematics permitted us to conclude that the galaxies with the highest rotation velocities tend to possess near-maximal stellar disks. In less massive galaxies, with v_max<200 km/s, the mass of the dark halo at least equals the stellar mass within 2-3 R_disk. The simulated gas morphology provides a powerful tool to determine the dominant spiral pattern speed. The corotation radius for all galaxies was found to be constant at R_corotation ~ 3 R_disk and encloses the strong part of the stellar spiral in all cases.

Abstract:

Although K-band images provide quite a reliable estimate of the stellar mass distribution within the disks of spiral galaxies, we discuss how performing a color correction to reduce local stellar mass-to-light ratio variations leads to shorter disk scale lengths and has a notable effect on the morphology. We use the mass maps prepared in this way to derive an accurate potential for the stellar mass and perform 2D hydrodynamical gas simulations to model the gas flow in a combined disk-halo potential. By comparison with the galaxy's measured kinematics, the simulated velocity fields allow us to constrain the contribution of a smooth dark matter component to the total mass in the inner few exponential disk scale lengths in spiral galaxies. We show for a sample of 3 galaxies that the simulations enable us to explore stellar disk and dark halo mass fractions.

Abstract:

We explore the relative importance of the stellar mass density as compared to the inner dark halo, using the observed gas kinematics throughout the disk of the spiral galaxy NGC 4254 (Messier 99). We perform hydrodynamical simulations of the gas flow for a sequence of gravitational potentials in which we vary the stellar disk contribution to the total potential. This stellar portion of the potential was derived empirically from color corrected K-band photometry reflecting the spiral arms in the stellar mass, while the halo was modelled as an isothermal sphere. The simulated gas density and the gas velocity field are then compared to the observed stellar spiral arm morphology and to the H-alpha gas kinematics. We find that this method is a powerful tool to determine the corotation radius of the spiral pattern and that it can be used to place an upper limit on the mass of the stellar disk. For the case of the galaxy NGC 4254 we find R_cr = 7.5 +/- 1.1 kpc, or R_cr = 2.1 R_exp(K'). We also demonstrate that for a maximal disk the prominent spiral arms of the stellar component over-predict the non-circular gas motions unless an axisymmetric dark halo component contributes significantly (>~ 1/3) to the total potential inside 2.2 K-band exponential disk scale lengths.