arXiv subjects

Hans-Walter Rix

Publications and source records attributed to Hans-Walter Rix.

489 records · Page 28Linked to original sources

arXiv1994

Galaxy Mergers and Gravitational Lens Statistics

We investigate the impact of hierarchical galaxy merging on the statistics of gravitational lensing of distant sources. Since no definite theoretical predictions for the merging history of luminous galaxies exist, we adopt a parametrized prescription, which allows us to adjust the expected number of pieces comprising a typical present galaxy at z=0.65. The existence of global parameter relations for elliptical galaxies and constraints on the evolution of the phase space density in dissipationless mergers, allow us to limit the possible evolution of galaxy lens properties under merging. We draw two lessons from implementing this lens evolution into statistical lens calculations: (1) The total optical depth to multiple imaging (e.g. of quasars) is quite insensitive to merging. (2) Merging leads to a smaller mean separation of observed multiple images. Because merging does not reduce drastically the expected lensing frequency it cannot make $λ$-dominated cosmologies compatible with the existing lensing observations. A comparison with the data from the HST Snapshot Survey shows that models with little or no evolution of the lens population are statistically favored over strong merging scenarios. The specific merging scenario proposed by Toomre (1977) can be rejected (95\% level) by such a comparison. Some versions of the scenario proposed by Broadhurst, Ellis \& Glazebrook (1992), are statistically acceptable.

astro-ph↗

arXiv1993

Can the Dark Matter be $10^6$ Solar Mass Objects?

If the dark matter in galactic halos is made up of compact, macroscopic objects (MO), such as black holes with $M_\MO >>M_{stars}$, gravitational scattering will lead to kinematic heating of the stars. Observational constraints on the amount of heating in the disk of the Milky Way put upper limits on $M_\MO \ltorder 10^{6.3}\msun $. We find limits that are three orders of magnitude more stringent by examining the heating limits in low mass stellar systems, where higher densities of dark matter and lower relative velocities disperse the stars in less than a billion years. Specifically, the structure and kinematics of the Local Group member GR8 yield a limit of $M_\MO \ltorder 6\times 10^{3}\msun$. If the properties of the dark matter are universal these results preclude the dominance of dark matter constituents in the cosmologically interesting mass range $\sim 10^6\msun$ and limit them to $M_\MO \ltorder 10^{3.7}\msun$. These results also rule out massive compact halo objects as significant contributors to the kinematic heating of the Galactic disk.

astro-ph↗

arXiv1993

M51 Stripped To Its Bones

We present optical and IR surface photometry of M51 (NGC~5194) at B~V~R~I~J~K and CO$(2.3μ)$. These data are used to establish whether K-band ($2.2μ$) images of spiral galaxies provide reliable maps of stellar surface mass density features such as massive spiral arms or bars. The main distorting agents in the mapping at shorter wavelengths are dust extinction and luminous young stars. From modeling the color changes across the main dust lanes in M51, we find that the K-band flux is attenuated by only $\ltorder 10\%$ even in the dust lanes. From monitoring the gravity-sensitive CO$(2.3μ)$ index across the spiral arms we find that young, red supergiants do not distort significantly the K-band image except in one small patch. On this basis, {\it we conclude that K band images do trace the massive disk star population}. In M51 we find the surface mass density contrast (arm/inter-arm) to range from 1.8 to 3, comparable to results from N-body simulations of the galaxy's tidal encounter with NGC~5195. The spiral arm amplitude in M51 also shows smooth, strong radial variations, with a maximum at $\sim 130\asec$ and minima at $45\asec$ and $170\asec$.

astro-ph↗