The ERC project ‘M2C’ aims to test the ΛCDM paradigm of structure formation in the Universe by using the evolution of the dark matter profiles in the most massive galaxy clusters as cosmic laboratoires. We will also assess the dynamical behavior of the baryons as they collect in the halo potential. This ambitious 5-year programme combines multi-wavelength (optical, sub-mm, X-ray) observations and large-scale, state-of-the-art, cosmological numerical simulations.
A key aspect of the project is to develop the capacity to find and characterise massive galaxy clusters (M ≥ 5 × 1014 M?) up to high redshift (z~1). The strategy adopted builds on the advent of cluster detection via the Sunyaev-Zeldovich (SZ) effect in the sub-mm band; specifically, it expands on the published Planck SZ survey by extending object detection to lower signal-to-noise. Here the team has established two complementary approaches. Firstly, it has developed the first algorithm that undertakes combined detection and characterisation in the X-ray and sub-mm bands. This has been tested on known objects in the Planck SZ survey in combination with ROSAT X-ray Survey (RASS) data, as well as on simulated data. It has been shown to yield excellent results in recovered parameters. Application of the algorithm to the blind detection of clusters in Planck and RASS is ongoing. Secondly, because the cluster mass function is dominated by low mass, low redshift objects, and because low signal-to-noise detections are potentially subject to Malmquist and Eddington bias effects, the team has explored the use of ancillary data in optical (Sloan Digital Sky Survey; SDSS) and infrared (Wide-Field Infrared Survey Explorer; WISE) to pre-select likely high redshift candidates. Deep optical follow-up of these candidates using MegaCam on the Canada-France Hawaii Telescope (CFHT) has underlined the excellent success rate of this new approach to pre-selection. The net result is a significant increase in the number of known high mass, high-redshift objects.
Intensive X-ray follow-up of the newly-discovered systems has been undertaken via three XMM-Newton Large Programmes totalling more than 2.5 Ms. To exploit these data, the team has developed a number of new X-ray analysis tools, notably a suite of programs for morphological analysis, and a tool to combine the radial profiles from Chandra and XMM-Newton. These observations have yielded data of sufficient quality to enable individual analysis of the thermodynamic properties (density, temperature, pressure, entropy) of the intra-cluster medium (ICM) up to z~1. As a result, we are now in a position to probe ICM and dark matter evolution using a sizeable number of 51 objects.
The team has become increasingly involved in the observing campaign with the New IRAM KIDs Array (NIKA2). This innovative instrument will deliver SZ observations with a resolution similar to that of XMM-Newton, opening the way to new and novel ways to probe the ICM and dark matter content. The team has made significant contributions to a number of papers using the NIKA pathfinder instrument, most notably showing the exceptional synergy of X-ray and SZ imaging of similar resolution for 1 and 2D studies of the ICM. The team was also heavily involved in defining the NIKA2 Guaranteed Time SZ Programme to observe 45 distant (z>0.5) clusters. Future work will involve studying how to optimally combine these upcoming SZ images with X-ray data.
Cosmological numerical simulations have been undertaken to accompany the observational effort. The challenge has been to simulate enough volume to generate a sizeable sample of high mass systems (essentially the volume of the visible Universe), while maintaining the necessary resolution to compare with observations. The team has now completed simulations of three 1 Gpc boxes, with the re-zooming technique used to obtain high-resolution (~5 kpc) data on 425 systems with M ≥ 5 × 1014 M?, including 50 at z=1. This is the first time such a combination of volume and resolution has been achieved. The resulting simulations probe a thus-far unexplored range in mass and redshift, and will provide the ideal theoretical comparison to the observational constraints that are being obtained in other parts of the project.
