Timing the last major merger of galaxy clusters with large halo sparsity

Abstract

Numerical simulations have shown that massive dark matter haloes, which today host galaxy clusters, assemble their mass over time alternating periods of quiescent accretion and phases of rapid growth associated with major merger episodes. Observations of such events in clusters can provide insights on the astrophysical processes that characterise the properties of the intra-cluster medium, as well as the gravitational processes that contribute to their assembly. It is therefore of prime interest to devise a fast and reliable way of detecting such perturbed systems. We present a novel approach to identifying and timing major mergers in clusters characterised by large values of halo sparsity. Using halo catalogues from the MultiDark-Planck2 simulation, we show that major merger events disrupt the radial mass distribution of haloes, thus leaving a distinct universal imprint on the evolution of halo sparsity over a period not exceeding two dynamical times. We exploit this feature using numerically calibrated distributions to test whether an observed galaxy cluster with given sparsity measurements has undergone a recent major merger and to eventually estimate when such an event occurred. We implement these statistical tools in a specifically developed public python library lammas, which we apply to the analysis of Abell 383 and Abell 2345 as test cases. Finding that, for example, Abell 2345 had a major merger about 2.1 ± 0.2 Gyr ago. This work opens the way to detecting and timing major mergers in galaxy clusters solely through measurements of their mass at different radii.

From inflation to dark matter halo profiles: the impact of primordial non-Gaussianities on the central density cusp

“…To have a deeper understanding of our halo sample, we do not only focus on the median of the halo but plot the full probability density function of the relative ratios of the sparsities as in figure 1 of ref. [79]. At z = 0, there is no obvious deviation from NFW compared to the Gaussian case, but the tails of the NG+ simulation indicate a possible deviation at high redshift.…”

“…However, it had nothing to say on whether the NFW profile is indeed a correct description of the halo density profiles in the non-Gaussian cases under consideration, which is a more profound question than the one relating to the mass-concentration relation. The halo sparsity, namely the ratio between cumulative masses enclosing two different overdensities, provides a non-parametric estimate of the halo mass distribution that encodes information beyond the simple NFW description [74][75][76][77][78][79][80]. It is defined as:…”

“…This means that the sparsities give an image of the inner structure of the halo without relying on any parametric density function. It has been shown to contain a wealth of cosmological (e.g., [75]) and astrophysical (e.g., [79,80]) information. Regarding observational prospects, it is also simpler to measure than a full profile (see ref.…”

On the choice of the most suitable indicator for the assembly state of dark matter haloes through cosmic time

“…Many works (e.g., Neto et al 2007;Wang et al 2020) have pointed out the relation between the time spanned since the last major merger and halo concentration, 𝑐 vir = 𝑅 vir /𝑅 s , being 𝑅 s the scale radius of the Navarro et al (1997) profile (or the radius where the logarithmic slope of the DM density profile equals −2). More recently, sparsity has been suggested as a non-parametric alternative to concentration, which reduces the scatter with halo mass (Balmès et al 2014;Corasaniti et al 2018), and has also been found to correlate with the timing since the last relevant merger (Richardson & Corasaniti 2022). While sparsity is generally defined as the quotient between the masses at different spherical overdensities, we find that the one maximising the correlation with merging activity is…”