The mass function dependence on the dynamical state of dark matter haloes

R Seppi,F Prada,A Klypin,E Bulbul,J Ider Chitham,P Predehl,J Comparat,K Nandra,A Merloni

doi:10.1051/0004-6361/202039123

Abstract

Context. Galaxy clusters are luminous tracers of the most massive dark matter haloes in the Universe. To use them as a cosmological probe, a detailed description of the properties of dark matter haloes is required. Aims. We characterize how the dynamical state of haloes impacts the dark matter halo mass function at the high-mass end (i.e., for haloes hosting clusters of galaxies). Methods. We used the dark matter-only MultiDark suite of simulations and the high-mass objects M > 2.7 × 1013 M⊙ h−1 therein. We measured the mean relations of concentration, offset, and spin as a function of dark matter halo mass and redshift. We investigated the distributions around the mean relations. We measured the dark matter halo mass function as a function of offset, spin, and redshift. We formulated a generalized mass function framework that accounts for the dynamical state of the dark matter haloes. Results. We confirm the recent discovery of the concentration upturn at high masses and provide a model that predicts the concentration for different values of mass and redshift with one single equation. We model the distributions around the mean values of concentration, offset, and spin with modified Schechter functions. We find that the concentration of low-mass haloes shows a faster redshift evolution compared to high-mass haloes, especially in the high-concentration regime. We find that the offset parameter is systematically smaller at low redshift, in agreement with the relaxation of structures at recent times. The peak of its distribution shifts by a factor of ∼1.5 from z = 1.4 to z = 0. The individual models are combined into a comprehensive mass function model, which predicts the mass function as a function of spin and offset. Our model recovers the fiducial mass function with ∼3% accuracy at redshift 0 and accounts for redshift evolution up to z ∼ 1.5. Results. This new approach accounts for the dynamical state of the halo when measuring the halo mass function. It offers a connection with dynamical selection effects in galaxy cluster observations. This is key toward precision cosmology using cluster counts as a probe.

Highlights

Galaxy clusters are the most massive virialized, gravitationally bound structures in the Universe
We investigate the variations of the dark matter halo mass function as a function of the dynamical state of the constituting haloes
Where X, Y, Z are permutations of the variables σ, Xoff, λ. With this method we recover the multiplicity function f (σ), which in this notation is fXoff,λ(σ). This allows us to study the behavior of the dark matter halo mass function according to different variables, making sure that in the end our analysis provides an accurate multiplicity function

Summary

Introduction

Galaxy clusters are the most massive virialized, gravitationally bound structures in the Universe They grow hierarchically, starting from matter perturbations in the initial density field. An early theoretical description of the mass function was given by Press & Schechter (1974; PS) based on the assumption that Gaussian density perturbations overcoming a fixed density contrast collapse into haloes This formally accounts for only half of the total halo mass in the Universe. An alternative approach, employing the excursion set theory, solved these shortcomings by considering the probability of crossing a given barrier with random walks (Bond et al 1991) This provides a good prediction for high-mass haloes, but it predicts too many low-mass objects. The introduction of ellipsoidal collapse corrected these differences between simulations and theory (Sheth & Tormen 1999, 2002)

Results

Discussion

Conclusion