The speed of the ‘bullet’ in the merging galaxy cluster 1E0657−56

By Volker Springel, G. Farrar. 2007

198 citations recorded by the source corpus.

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 numberM ∼ 3. The inferred shock velocity of ∼ 4700 km s -1 has been commonly interpreted as the velocity of the 'bullet' subclu ster itself. This velocity is unexpectedly high in theCDM 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 ∼ 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 ∼ 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 dist ance 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 se nsitively on the relative concentrations and gas fractions of the merging clusters, in addition to being high ly time dependent. A genericCDM 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 secto r, the bullet subcluster can be accelerated beyond the velocity reached inCDM, and the spatial offset between the X-ray peak and the mass centroid of the subcluster can be significantly enlarged. Our results stres s the need for explicit hydrodynamical models for the proper interpretation of actively merging systems such as 1E0657-56. Subject headings:clusters: structure - clusters: x-ray observations - metho ds: numerical

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/ee29bf0cbecbb25bad49f8a1cc0a4f7e04e16fc7

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Citations in the corpus

Colliding clusters and dark matter self-interactions (2013, background): “We assume a collision velocity of 4500 km s−1, even though it has been argued that the velocity of the Bullet Cluster may be significantly smaller (Springel & Farrar 2007).”
X-ray spectroscopy of galaxy clusters: studying astrophysical processes in the largest celestial laboratories (2009, background): “that try to reconstruct the merger configuration of 1E0657-56 and recover such high velocities which are partly boosted by the accretion inflow of ICM at the position of the bullet (Springel and Farrar 2007; Mastropietro and Burkert 2008).”
Hydrodynamical N-body simulations of coupled dark energy cosmologies (2008, background): “…have been discussed, for example, in Macciò et al. (2004), Nusser, Gubser & Peebles (2005), Stabenau & Jain (2006), Kesden & Kamionkowski (2006), Springel & Farrar (2007), Laszlo & Bean (2008), Sutter & Ricker (2008), Oyaizu (2008), Keselman, Nusser & Peebles (2009), but to our knowledge…”
Shock waves in Eulerian cosmological simulations: main properties and acceleration of cosmic rays (2008, background): “…to the fluctuation of the gravitational potential may cause infall of the pre-shocked gas on to the shock discontinuity increasing the Mach number (Springel & Farrar 2007), while other kind of strong shocks are reverse shocks propagating trough the innermost regions of accreting and cold…”
Constraints on the Self-Interaction Cross Section of Dark Matter from Numerical Simulations of the Merging Galaxy Cluster 1E 0657–56 (2007, background): “…bandwas not included in the simulations (doing so would greatly increase the computing time and the complexity involved with matching the observations in detail; for simulations of this system that include a gas component with noninteracting DM, see, e.g., Takizawa 2006; Springel & Farrar 2007).”
STIRRING UP THE POT: CAN COOLING FLOWS IN GALAXY CLUSTERS BE QUENCHED BY GAS SLOSHING? (2009, methodology): “Such simulations have been employed in many previous works, typically in order either to explore a parameter space of mergers (e.g., Ricker & Sarazin 2001; Poole et al. 2006, 2007) or to simulate specific cluster merger scenarios (e.g., Springel & Farrar 2007; ZuHone et al. 2009).”
Moving mesh cosmology: the hydrodynamics of galaxy formation (2011, background): “…works which analysed collisions of two galaxy clusters in isolation (see e.g. Ricker & Sarazin 2001; Ritchie & Thomas 2002; McCarthy et al. 2007; Springel & Farrar 2007; Mitchell et al. 2009; ZuHone 2011), but here we devise a cleaner set-up in order to minimize additional possible numerical…”
Creation of cosmic structure in the complex galaxy cluster merger Abell 2744 (2011, background): “Improvements are continuing in numerical simulations (Milosavljević et al. 2007; Springel & Farrar 2007; Mastropietro & Burkert 2008; Lee & Komatsu 2010).”
Masses of Galaxy Clusters from Gravitational Lensing (2013, methodology): “The interpretation, also corroborated by follow-up studies (e.g. Springel & Farrar 2007), is that while the ICM of the two clusters experienced deceleration due to the collision (as indicated best by the shock visible with Chandra), the dark matter halos passed through each other without…”
Revealing the Properties of Dark Matter in the Merging Cluster MACS J0025.4–1222 (2008, background): “…dispersion of 1132+117 −106 km/s (from 52 early-type galaxy spectra) and Markevitch et al. (2006) estimate a merger velocity for 1E0657−56 (based on a 500ks Chandra observation) of ∼ 4500 km/s (although this value is thought to be extreme; e.g. Hayashi & White 2006; Springel & Farrar 2007).”