A detection of wobbling brightest cluster galaxies within massive galaxy clusters

A striking signal of dark matter beyond the standard model is the existence of cores in the centre of galaxy clusters. Recent simulations predict that a brightest cluster galaxy ( BCG) inside a cored galaxy cluster will exhibit residual wobbling due to previous major mergers, long after the relaxation of the overall cluster. This phenomenon is absent with standard cold dark matter where a cuspy density profile keeps a BCG tightly bound at the centre. We test this hypothesis using cosmological simulations and deep observations of 10 galaxy clusters acting as strong gravitational lenses. Modelling the BCG wobble as a simple harmonic oscillator, we measure the wobble amplitude, A(w), in the BAHAMAS suite of cosmological hydrodynamical simulations, finding an upper limit for the cold dark matter paradigm of A(w) < 2 kpc at the 95 per cent confidence limit. We carry out the same test on the data finding a non-zero amplitude of A(w) = 11.82(-3.0)(+7.3) kpc, with the observations dis-favouring A(w) = 0 at the 3 sigma confidence level. This detection of BCG wobbling is evidence for a dark matter core at the heart of galaxy clusters. It also shows that strong lensing models of clusters cannot assume that the BCG is exactly coincident with the large-scale halo. While our small sample of galaxy clusters already indicates a non-zero A(w), with larger surveys, e.g. Euclid, we will be able to not only confirm the effect but also to use it to determine whether or not the wobbling finds its origin in new fundamental physics or astrophysical process.

Observable tests of self-interacting dark matter in galaxy clusters: BCG wobbles in a constant density core

David Richard Harvey, Richard Massey

Models of cold dark matter (CDM) predict that the distribution of dark matter in galaxy clusters should be cuspy, centrally concentrated. Constant density cores would be strong evidence for beyond CDM physics, such as self-interacting dark matter (SIDM). An observable consequence would be oscillations of the brightest cluster galaxy (BCG) in otherwise relaxed galaxy clusters. Offset BCGs have indeed been observed - but only interpreted via a simplified, analytic model of oscillations. We compare these observations to the BAryons and HAloes of MAssive Sysmtes (BAHAMAS)-SIDM suite of cosmological simulations, which include SIDM and a fully hydrodynamical treatment of star formation and feedback. We predict that the median offset of BCGs increases with the SIDM cross-section, cluster mass, and the amount of stellar mass within 10 kpc, while CDM exhibits no trend in mass. Interpolating between the simulated cross-sections, we find that the observations (of 10 clusters) are consistent with CDM at the similar to 1.5 sigma level, and prefer cross-section sigma/m < 0.12(0.39) cm(2) g(-1) at 68 per cent (95 per cent) confidence level. This is on the verge of ruling out velocity-independent dark matter self-interactions as the solution to discrepancies between the predicted and observed behaviour of dwarf galaxies, and will be improved by larger surveys by Euclid or Superpressure Balloon-borne Imaging Telescope (SuperBIT).

OXFORD UNIV PRESS2019

Hubble Frontier Fields: a high-precision strong-lensing analysis of galaxy cluster MACSJ0416.1-2403 using similar to 200 multiple images

Hakim Atek, Benjamin Yvan Alexandre Clement, Mathilde Jauzac, Jean-Paul Richard Kneib, Richard Massey, Markus Rexroth, Johan Richard

We present a high-precision mass model of the galaxy cluster MACSJ0416.1-2403, based on a strong-gravitational-lensing analysis of the recently acquired Hubble Space Telescope Frontier Fields (HFF) imaging data. Taking advantage of the unprecedented depth provided by HST/Advanced Camera for Survey observations in three passbands, we identify 51 new multiply imaged galaxies, quadrupling the previous census and bringing the grand total to 68, comprising 194 individual lensed images. Having selected a subset of the 57 most securely identified multiply imaged galaxies, we use the LENSTOOL software package to constrain a lens model comprised of two cluster-scale dark-matter haloes and 98 galaxy-scale haloes. Our best-fitting model predicts image positions with an rms error of 0.68 arcsec, which constitutes an improvement of almost a factor of 2 over previous, pre-HFF models of this cluster. We find the total projected mass inside a 200 kpc aperture to be (1.60 +/- 0.01) x 10(14) M-circle dot, a measurement that offers a three-fold improvement in precision, reaching the per cent level for the first time in any cluster. Finally, we quantify the increase in precision of the derived gravitational magnification of high-redshift galaxies and find an improvement by a factor of similar to 2.5 in the statistical uncertainty. Our findings impressively confirm that HFF imaging has indeed opened the domain of high-precision mass measurements for massive clusters of galaxies.

Oxford University Press2014

Hubble Frontier Fields: a high-precision strong-lensing analysis of the massive galaxy cluster Abell 2744 using similar to 180 multiple images

Hakim Atek, Benjamin Yvan Alexandre Clement, Mathilde Jauzac, Jean-Paul Richard Kneib, Richard Massey, Markus Rexroth, Johan Richard

We present a high-precision mass model of galaxy cluster Abell 2744, based on a strong gravitational-lensing analysis of the Hubble Space Telescope Frontier Fields (HFF) imaging data, which now include both Advanced Camera for Surveys and Wide Field Camera 3 observations to the final depth. Taking advantage of the unprecedented depth of the visible and near-infrared data, we identify 34 new multiply imaged galaxies, bringing the total to 61, comprising 181 individual lensed images. In the process, we correct previous erroneous identifications and positions of multiple systems in the northern part of the cluster core. With the LENSTOOL software and the new sets of multiple images, we model the cluster using two cluster-scale dark matter haloes plus galaxy-scale haloes for the cluster members. Our best-fitting model predicts image positions with an rms error of 0.79 arcsec, which constitutes an improvement by almost a factor of 2 over previous parametric models of this cluster. We measure the total projected mass inside a 200 kpc aperture as (2.162 +/- 0.005) x10(14) M-circle dot, thus reaching 1 per cent level precision for the second time, following the recent HFF measurement of MACSJ0416.1-2403. Importantly, the higher quality of the mass model translates into an overall improvement by a factor of 4 of the derived magnification factor. Together with our previous HFF gravitational lensing analysis, this work demonstrates that the HFF data enables high-precision mass measurements for massive galaxy clusters and the derivation of robust magnification maps to probe the early Universe.

Oxford University Press2015