Abstract
We map the lensing-inferred substructure in the first three clusters observed by the Hubble Space Telescope Frontier Fields Initiative (HSTFF): Abell 2744 (z = 0.308), MACSJ0416, (z = 0.396) and MACSJ1149 (z = 0.543). Statistically resolving dark-matter subhaloes down to ~10^{9.5} solar masses, we compare the derived subhalo mass functions (SHMFs) to theoretical predictions from analytical models and with numerical simulations in a Lambda Cold Dark Matter (LCDM) cosmology. Mimicking our observational cluster member selection criteria in the HSTFF, we report excellent agreement in both amplitude and shape of the SHMF over four decades in subhalo mass (10^{9-13} solar masses). Projection effects do not appear to introduce significant errors in the determination of SHMFs from simulations. We do not find evidence for a substructure crisis, analogous to the missing satellite problem in the Local Group, on cluster scales, but rather excellent agreement of the count-matched HSTFF SHMF down to M_{sub halo}/M_{halo} ~ 10^{-5}. However, we do find discrepancies in the radial distribution of sub haloes inferred from HSTFF cluster lenses compared to determinations from simulated clusters. This suggests that although the selected simulated clusters match the HSTFF sample in mass, they do not adequately capture the dynamical properties and complex merging morphologies of these observed cluster lenses. Therefore, HSTFF clusters are likely observed in a transient evolutionary stage that is presently insufficiently sampled in cosmological simulations. The abundance and mass function of dark matter substructure in cluster lenses continues to offer an important test of the LCDM paradigm, and at present we find no tension between model predictions and observations.
Highlights
While the bulk of the matter content of our Universe is inventoried to be dark matter – cold, collisionless particles that drive the formation of all observed structure – its nature remains elusive
The abundance and mass function of dark matter substructure in cluster lenses continues to offer an important test of the Lambda Cold Dark Matter (LCDM) paradigm, and at present we find no tension between model predictions and observations
In Natarajan et al (2007), we presented high resolution mass models for five Hubble Space Telescope (HST) cluster lenses, and performed a detailed comparison of the subhalo mass functions (SHMFs), the velocity dispersion and aperture radii function with an ensemble of cluster–sized haloes selected from the Millennium Simulation including an implementation of a semianalytic model for the galaxy formation detailed in De Lucia et al (2006)
Summary
While the bulk of the matter content of our Universe is inventoried to be dark matter – cold, collisionless particles that drive the formation of all observed structure – its nature remains elusive. We study the detailed distribution of substructure derived directly from mass models constrained by more than a hundred lensed images each gleaned from the HSTFF imaging data for Abell 2744, MACSJ 0416.1–2403 (hereafter MACSJ 0416; Mann & Ebeling 2012) and 65 images for MACSJ 1149.5+2223 (hereafter MACSJ 1149; Ebeling et al 2010) These three clusters, spanning a redshift range 0.308-0.554, represent various stages of cluster mass assembly. The inferred substructure - referred to as the subhalo mass function (SHMF thereafter) - is compared with mimicked ”measurements” from simulated clusters in the Illustris cosmological boxes (Vogelsberger et al 2014a, details are available at http://www.illustris-project.org/); as well as analytic estimates that take halo-to-halo scatter into account This exercise offers a concrete and powerful test of the standard LCDM model, its implementation in cosmological simulations and our analytic calculational framework.
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