Mapping substructure in the HST Frontier Fields cluster lenses and in cosmological simulations (1702.04348v2)

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.

• The paper shows that observed subhalo mass functions from HST Frontier Fields align closely with LCDM simulation predictions across four orders of magnitude in mass.
• It identifies radial distribution discrepancies in subhalo placements, likely due to transient merger dynamics not fully captured in current simulations.
• Utilizing both Illustris and analytic methods, the study underscores the need for refined baryonic physics and larger simulation volumes in modeling cluster structures.

Mapping Substructure in HST Frontier Fields Cluster Lenses and Cosmological Simulations

The paper by Natarajan et al. systematically explores the substructure observed in massive galaxy cluster lenses from the Hubble Space Telescope Frontier Fields Initiative (HSTFF) and compares these observations with theoretical predictions from cosmological simulations executed under the Lambda Cold Dark Matter (LCDM) paradigm. Using meticulous gravitational lensing techniques, the paper reconstructs the subhalo mass functions (SHMFs) for three clusters: Abell 2744, MACSJ\,0416, and MACSJ\,1149, largely aspiring to bridge empirical data with simulations that incorporate both dark matter and baryonic physics.

A major focus is on robust lensing-inferred SHMFs spanning four orders of magnitude in subhalo mass, reaching as low as $\sim 10^{9.5} \, \text{M}_\odot$. The empirical mass functions are meticulously derived by correlating the positions, brightnesses, and multiplicity of lensed images with mass models. Key among the findings is the striking alignment between these observed subhalo distributions and the dynamically simulated ones, especially over the high-mass range, suggesting no substructure crisis akin to the missing satellite problem previously encountered on galaxy scales.

Highlighted are several essential results and propositions:

• Subhalo Mass Functions: For both Abell 2744 and MACSJ\,0416, there is excellent agreement with simulations on the abundance and distribution of subhalos up to $M_{\text{sub halo}}/M_{\text{halo}}{\sim}10^{-5}$. This match supports the LCDM model's prediction of abundant substructure over the four decades in mass.
• Discrepancies in Radial Distributions: While the amplitude and shape of SHMFs concur with simulation predictions, the radial positioning of subhalos inferred from HSTFF clusters reveal inconsistencies with the simulations. It is suggested that transient dynamical states of cluster mergers witnessed in observations might not be captured in simulations, attributing increased mass segregation and concentration differences.
• Simulation and Analytic Comparisons: The paper utilizes both the Illustris simulation suite and analytic estimates to test LCDM predictions. Despite accounting for cosmic variance, variances in subhalo abundance for MACSJ\,1149 remain, possibly correlated with its complex mass distribution and comparable rarity in simulated clusters.
• Implications and Future Prospects: By substantiating the SHMF results against different methodologies, including scaling laws for mass-to-light ratios, future explorations are proposed to focus on improving the fidelity of baryonic physics models in simulations. Additionally, the findings underscore a need for larger simulation volumes to appropriately sample rare, dynamically distinct states analogous to those probed by HSTFF, as further corroborated by the failures to identify comparable clusters in existing simulation volumes.

The implications of this research tread both theoretical and practical grounds. It corroborates the robustness of LCDM in modeling structure at cluster scales while suggesting refinements in simulation strategies to incorporate transient dynamical states more precisely. This work not only offers significant strides in benchmarking cosmological predictions with observational data but also charts pathways for enhancing the accuracy of future investigations in cosmic structure formation.