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 { \cal M } \sim 3 .
The inferred shock velocity of \sim 4700 { km s ^ { -1 } } has been commonly interpreted as the velocity of the ‘ bullet ’ subcluster itself .
This velocity is unexpectedly high in the \Lambda CDM 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 ^ { -1 } } , the subcluster ’ s mass centroid is moving only with \sim 2600 { km s ^ { -1 } } 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 \sim 1100 { km s ^ { -1 } } 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 \Lambda CDM 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 \Lambda CDM , 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 .