The impact of cluster mergers on arc statistics

Abstract

We study the impact of merger events on the strong lensing properties of galaxy clusters. Previous lensing simulations were not able to resolve dynamical time-scales of cluster lenses, which arise on time-scales that are of the order of a Gyr. In this case study, we first describe qualitatively with an analytic model how some of the lensing properties of clusters are expected to change during merging events. We then analyse a numerically-simulated lens model for the variation in its efficiency for producing both tangential and radial arcs while a massive substructure falls on to the main cluster body. We find that: <listgroup type="i" id="l1"> <li> during the merger, the shape of the critical lines and caustics changes substantially;</li> <li> the lensing cross-sections for long and thin arcs can grow by one order of magnitude and reach their maxima when the extent of the critical curves is largest;</li> <li> the cross-section for radial arcs also grows, but the cluster can efficiently produce these kind of arcs only while the merging substructure crosses the main cluster centre, and</li> <li> while the arc cross-sections pass through their maxima as the merger proceeds, the X-ray emission of the cluster increases by a factor of ~5.</li></listgroup> Thus, we conclude that accounting for these dynamical processes is very important for arc statistics studies. In particular, they may provide a possible explanation for the arc statistics problem

Strong lensing in the MARENOSTRUM UNIVERSE

“…This seems to suggest that some cluster property rather than the mass plays an important role for boosting the lensing cross sections of these small lenses, which also influences their X-ray luminosity. As discussed in Torri et al (2004) and in Fedeli & Bartelmann (2007), mergers are likely to explain the effect we observe here.…”

“…As discussed in Torri et al (2004), the shear produced by the substructures enhance the ability of the clusters to produce strong lensing, because it makes critical even those lenses where the convergence is not enough to ensure it (κ ≥ 1 at some point).…”

“…The gas physics, in particular cooling, could also affect the strong lensing properties of clusters (Puchwein et al 2005;Wambsganss et al 2008;Mead et al 2010). Finally, Torri et al (2004) show that the cluster ability to produce gravitational arcs can also be enhanced by the dynamical activity in the lens. By studying with high time resolution how the lensing cross section changes during an edge-on collision between the main cluster clump and a substructure, these authors find that the strong lensing efficiency is boosted by a factor of 10 during the merging phase.…”

The strongest gravitational lenses

“…5.2, for summaries), and references therein). In particular, Torri et al (2004) analysed numerically simulated mergers of galaxy clusters. These authors found that mergers substantially change the shape of the critical curves and can boost a cluster's efficiency to produce giant arcs by an order of magnitude.…”

Constraints on Dark Energy from Strong Gravitational Lensing by Galaxy Clusters

“…In fact, the curves exhibit strong peaks which are connected to merger events arising in single clusters. It has been recently shown that during such events, which occur on timescales of some Gyr, the cluster efficiency for strong lensing is strongly enhanced (Torri et al, 2004), due to the combined effect of the increasing shear and convergence. It is interesting to note that, by comparing the cross sections as function of redshift for different cosmological models, there is a correspondence between the number of the peaks in the curves.…”