Dark Matter in Spiral Galaxies

This thesis presents a detailed dynamic analysis of five high surface brightness, late type spiral galaxies NGC 3810, NGC 3893, NGC 4254, NGC 5676 and NGC 6643, which were studied with the aim to quantify the luminous-to-dark matter ratio inside their optical radii. In order to meet this goal, the galaxies' morphology and gas kinematics have been observed and compared to hydrodynamic gas simulations which were performed to predict the gas dynamics arising in response to realistic two dimensional gravitational potentials, combined from stellar disk and dark halo contributions. The gravitational potential of the stellar disk was derived from color-corrected NIR photometry and for the dark halo, the mass density distribution of an axisymmetric isothermal sphere with a core was chosen. The rather good agreement between the simulated and observed gas kinematics permitted to conclude that the most massive galaxies tend to possess also very massive stellar disks, dominating the gas dynamics within the optical radius. In less massive galaxies with a maximal rotation velocity of <200 km/s, the mass of the dark halo already equals the stellar mass. The maximal disk stellar mass-to-light ratio in the K-band was found to lie at about M/L_K ~ 0.6. Furthermore, the gas dynamic simulations provide a powerful tool to accurately determine the spiral pattern speed for galaxies, independent of a specific density wave theory. It was found that the location of the corotation resonance falls into a narrow range of around three exponential disk scale lengths for all galaxies from the sample. The corotation resonance encloses the strong part of the stellar spiral in all cases. Based on the experience gained from this project, the use of a color-correction to account for local stellar population differences is strongly encouraged when properties of galactic disks are studied which rely on their stellar mass distributions

Probing for Dark Matter within Spiral Galaxy Disks

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.Comment: 16 pages including 9 figures, accepted for publication in the Ap

Measuring Stellar and Dark Mass Fractions in Spiral Galaxies

We explore the relative importance of the stellar mass density as compared to the inner dark halo, for the observed gas kinematics thoughout the disks of spiral galaxies. We perform hydrodynamical simulations of the gas flow in a sequence of potentials with varying the stellar contribution to the total potential. The stellar portion of the potential was derived empirically from K-band photometry. The output of the simulations - namely the gas density and the gas velocity field - are then compared to the observed spiral arm morphology and the H-alpha gas kinematics. We solve for the best matching spiral pattern speed and draw conclusions on how massive the stellar disk can be at most. For the case of the galaxy NGC 4254 (Messier 99) we demonstrate that the prominent spiral arms of the stellar component would overpredict the non-circular gas motions unless an axisymmetric dark halo component adds significantly in the radial range R_exp < R < 3*R_exp.Comment: 5 pages, 3 figures, to appear in "Dark 2000: Third International Conference on Dark Matter in Astro and Particle Physics", H.V. Klapdor-Kleingrothaus, B. Majorovits (eds.

Dark matter within high surface brightness spiral galaxies

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.Comment: 28 pages, 7 figures; to appear in the Astrophysical Journal, Vol. 586, March 200

Exploring spiral galaxy potentials with hydrodynamical simulations

Received...; accepted... We study how well the complex gas velocity fields induced by massive spiral arms are modelled by the hydrodynamical simulations we used to constrain the dark matter fraction in nearby spiral galaxies (Kranz et al. 2001, 2003). More specifically, we explore the dependence of the positions and amplitudes of features in the gas flow on the temperature of the interstellar medium (assumed to behave as a one-component isothermal fluid), the non-axisymmetric disk contribution to the galactic potential, the pattern speed, Ωp and finally the numerical resolution of the simulation. We argue that, after constraining the pattern speed reasonably well by matching the simulations to the observed spiral arm morphology, the amplitude of the non-axisymmetric perturbation (the disk fraction) is left as the primary parameter determining the gas dynamics. However, due to the sensitivity of the positions of the shocks to modeling parameters, one has to be cautious when quantitatively comparing the simulations to observations. In particular, we show that a global least squares analysis is not the optimal method fo

Chronic kidney disease aggravates arteriovenous fistula damage in rats

Neointimal hyperplasia (NIH) and impaired dilatation are important contributors to arteriovenous fistula (AVF) failure. It is unclear whether chronic kidney disease (CKD) itself causes adverse remodeling in arterialized veins. Here we determined if CKD specifically triggers adverse effects on vascular remodeling and assessed whether these changes affect the function of AVFs. For this purpose, we used rats on a normal diet or on an adenine-rich diet to induce CKD and created a fistula between the right femoral artery and vein. Fistula maturation was followed noninvasively by high-resolution ultrasound (US), and groups of rats were killed on 42 and 84 days after surgery for histological and immunohistochemical analyses of the AVFs and contralateral femoral vessels. In vivo US and ex vivo morphometric analyses confirmed a significant increase in NIH in the AVFs of both groups with CKD compared to those receiving a normal diet. Furthermore, we found using histological evaluation of the fistula veins in the rats with CKD that the media shrank and their calcification increased significantly. Afferent artery dilatation was significantly impaired in CKD and the downstream fistula vein had delayed dilation after surgery. These changes were accompanied by significantly increased peak systolic velocity at the site of the anastomosis, implying stenosis. Thus, CKD triggers adverse effects on vascular remodeling in AVFs, all of which contribute to anatomical and/or functional stenosis