Beecroft Institute for Particle Astrophysics and Cosmology

Abstract:

Using mean relative peculiar velocity measurements for pairs of galaxies, we estimate the cosmological density parameter $\Omega_m$ and the amplitude of density fluctuations $\sigma_8$. Our results suggest that our statistic is a robust and reproducible measure of the mean pairwise velocity and thereby the $\Omega_m$ parameter. We get $\Omega_m = 0.30^{+0.17}_{-0.07}$ and $\sigma_8 = 1.13^{+0.22}_{-0.23}$. These estimates do not depend on prior assumptions on the adiabaticity of the initial density fluctuations, the ionization history, or the values of other cosmological parameters.

Abstract:

We have used composite spectra generated from more than 22000 QSOs observed in the course of the 2dF and 6dF QSO Redshift Surveys to investigate the relationship between the velocity width of emission lines and QSO luminosity. We find that the velocity width of the broad emission lines Hbeta, Hgamma, MgII, CIII] and CIV are correlated with the continuum luminosity, with a significance of more than 99 per cent. Of the major narrow emission lines ([OIII] 5007, [OII] 3727, NeIII 3870 and NeV 3426) only [OIII] exhibits a significant correlation between line width and luminosity. Assuming that the gas is moving in Keplerian orbits and that the radius of the broad line region is related to the QSO continuum luminosity, we use the velocity widths of the broad lines to derive average black hole masses for the QSOs contributing to the composite spectra. The resultant QSO mass-luminosity relationship is consistent with M ~ L^0.97+-0.16. We find that the correlation between line width and redshift, if present, must be weak, and only CIV shows significant evidence of evolution. This enables us to constrain the redshift evolution of the black hole mass-luminosity ratio to be ~(1+z)^beta with beta ~< 1, much less than the ~(1+z)^3 evolution seen in QSO luminosity evolution. Assuming that the motion of the broad line region gas is Keplerian and that its radius depends on the QSO luminosity, our models indicate that the observed weak redshift dependence is too small for the observed QSO luminosity function to be due to the evolution of a single long-lived population of sources.

Abstract:

We present results from a detailed dynamical 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 of quantifying 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 that predict the gasdynamics 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 yield a constant stellar mass-to-light ratio (M/L); for the dark halo, the mass density distribution of an axisymmetric isothermal sphere with a core was chosen. 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 modeled gas distribution and kinematics. The agreement between the simulated and observed gas kinematics permitted us to conclude that the galaxies with the highest rotation velocities tend to possess very massive stellar disks that dominate the gasdynamics within the optical radius. In less massive galaxies, with a maximal rotation velocity of less than 200 km s-1, the mass of the dark halo at least equals the stellar mass within 2-3 disk scale lengths. The maximal disk stellar mass-to-light ratio in the K band was found to lie at about M/LK ≈ 0.6. Furthermore, the gasdynamic simulations provide a powerful tool for accurately determining the dominant 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 that rely on their stellar mass distributions.