Paper  astro-ph/0703232

The speed of the `bullet' in the merging galaxy cluster 1E0657-56

Deep Chandra exposures of the hot galaxy cluster 1E0657-56 have revealed a highly dynamical state of the system due to an ongoing merger with a massive subcluster. The system is observed shortly after the first core-passage of the infalling subcluster, which moves approximately in the plane of the sky and is preceded by a prominent bow shock with Mach number M~3. The inferred shock velocity of ~4700 km/s has been commonly interpreted as the velocity of the 'bullet' subcluster itself. This velocity is unexpectedly high in the LCDM cosmology, which may require non-trivial modifications in the dark sector such as additional long-range scalar forces if taken at face value. Here we present explicit hydrodynamical toy models of galaxy cluster mergers which very well reproduce the observed dynamical state of 1E0657--56 and the mass models inferred from gravitational lensing observations. However, despite a shock speed of 4500 km/s, the subcluster's mass centroid is moving only with ~2600 km/s in the rest frame of the system. The difference arises in part due to a gravitationally induced inflow velocity of the gas ahead of the shock towards the bullet, which amounts to ~1100 km/s for our assumed 10:1 mass ratio of the merger. A second effect is that the shock front moves faster than the subcluster itself, enlarging the distance between the subcluster and the bow shock with time. We also discuss the expected location of the lensing mass peak relative to the hydrodynamical features of the flow, and show that their spatial separation depends sensitively on the relative concentrations and gas fractions of the merging clusters, in addition to being highly time dependent. A generic LCDM collision model, where a bullet subcluster with concentration c=7.2 merges with a parent cluster with concentration c=3 on a zero-energy orbit, reproduces all the main observational features seen in 1E0657-56 with good accuracy, suggesting that 1E0657-56 is well in line with expectations from standard cosmological models. In theories with an additional "fifth" force in the dark sector, the bullet subcluster can be accelerated beyond the velocity reached in LCDM, and the spatial offset between the X-ray peak and the mass centroid of the subcluster can be significantly enlarged. Our results stress the need for explicit hydrodynamical models for the proper interpretation of actively merging systems such as 1E0657-56.

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This movie shows the time evolution of a simulation model for the bullet cluster. The four panels give the projected X-ray emissivity (top left), the gas temperature (top right), the dissipation rate in hdyrodynamical shock waves (bottom left), and the mean mass-weighted gas velocity in the x-direction (bottom right). The merger has a 10:1 mass ratio, with a mass of 1.5x1015 Msun for the parent cluster (with concentration c=2.0), and 1.5x1014 Msun for the subcluster (with concentration c=3.0), and occurs on a zero-energy collision orbit. Credit: Springel & Farrar (2007)		bullet.mp4 [mp4, 8.6 MB, 1024x768] bullet.avi [divx5, 6.5 MB, 1024x768] The video data is compressed using divx5 or standard mpeg4, respectively, and has fairly high resolution. To play the movie, you can use the 'mplayer' program under Linux. On a Mac, 'quicktime' should work directly for the mp4-file. Alternatively, you can install the divx-codec to play the avi-file, available free of charge here. Likewise for `windows mediaplayer'.