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Old research pagesI'm keeping these old research pages here for the benefit of anyone who might arrive here from an external page. They are no longer updated. Please visit my new web site at http://sipapu.astro.illinois.edu/~ricker/. Eventually some content from these pages will migrate to the new site.
We find that in off-center collisions the cluster cores (the inner regions where the pressure exceeds the ram pressure) behave quite differently from the clusters' outer regions. A strong, roughly ellipsoidal shock front, similar to that noted in previous simulations of head-on collisions, enables the cores to become bound to each other by dissipating their kinetic energy as heat in the surrounding gas. These cores survive well into the collision, dissipating their orbital angular momentum via spiral bow shocks. After the ellipsoidal shock has passed well outside the interaction region, the material left in its wake falls back onto the merger remnant formed through the inspiral of the cluster cores, creating a roughly spherical accretion shock. For less than one-half of a sound crossing time after the cores first interact the total X-ray luminosity increases by a large factor; the magnitude of this increase depends sensitively on the size of the impact parameter. Observational evidence of the ongoing collision, in the form of bimodality and distortion in projected X-ray surface brightness and temperature maps, is present for 1--2 sound crossing times after the collision, but only for special viewing angles. The remnant actually requires at least five crossing times to reach virial equilibrium. Since the sound crossing time can be as large as 1--2 Gyr, the equilibration time can thus be a substantial fraction of the age of the universe. The final merger remnant is very similar for impact parameters of zero and five core radii. It possesses a roughly isothermal core, with central density and temperature twice the initial values for the colliding clusters. Outside the core the temperature drops as r-1, and the density roughly as r-3.8. The core radius shows a small increase due to shock heating during the merger. For an impact parameter of ten core radii the core of the remnant possesses a more flattened density profile, with a steeper dropoff outside the core. In both off-center cases the merger remnant rotates, but only for the ten-core-radius case does this appear to have an effect on the structure of the remnant.
The links below lead to images and movies of the gas density, temperature, and velocity fields in the xy plane (passing through the cluster centers). The units chosen were such that the initial radius, mass, temperature, and central sound speed of each cluster were all unity. Movies are in MPEG format.
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