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A new window on the Universe: The formation and evolution of galaxy clusters and proto-clusters

Mid-Term Report Summary - NEWCLUSTERS (A new window on the Universe: The formation and evolution of galaxy clusters and proto-clusters)

The main aim of the ERC-A project is to study the formation and evolution of clusters and proto-clusters of galaxies. This would be predominantly done with the unique diagnostic tools provided by the new pan-European radio telescope LOFAR as a basis.

LOFAR The Low-Frequency Array (LOFAR) is a 150M euro pan-European radio telescope, consisting of 7,000 static antennas whose signals are combined in software. The antennas are grouped in stations, 38 in the Netherlands and 10 spread over 5 different countries in Europe (UK, France, Sweden, Germany and Poland). Through its beamforming and large collecting area, LOFAR offers transformational increases in survey speed at unprecedented spatial resolution and sensitivity. The LOFAR Surveys Key Science Project (LSKSP; PI Röttgering) plans to use the Dutch array to produce deep, high-resolution images of a significant fraction of the northern sky.

However, reducing LOFAR radio data is a major challenge. This is due to the enormous data rates, bright sources far away from the pointing center dominating the signal, radio frequency interference from radio, TV and planes and the corrupting influence of the ionosphere. Especially the latter is very hard to handle. The issue is that movements in the ionosphere make radio sources twinkle on the sky – very much like atmospheric seeing limits the quality of images coming from optical telescopes. (This is the reason why the Hubble space telescope is such a success: it is above the atmosphere). For LOFAR this twinkling varies from station to station. It also varies over the field of view of an individual station. To solve all these problems was really hard as it involved first understanding of both the mathematics and physics of the issues at hand and subsequent implementation of algorithms into software. The break-through idea was what is now called ‘facet-calibration’. Within a single LOFAR field of view 30 – 50 of the brightest sources are used to calibrate both the ionosphere and the LOFAR beam. An elaborate solving scheme has produced for the first time ever thermal noise limited maps at low frequencies with an angular resolution of ~5 arcsec from 8 hours of data.

Subsequently, sensitive and high-resolution low-frequency images of individual galaxy clusters that contain shocks were produced. Through our detailed studies of the galaxy clusters Abell 2034 (Shimwell et al. MNRAS); the `Sausage cluster’ (Hoang et al. in preparation); and Abell 1914 (Mandal et al. in preparation) we have routinely created LOFAR images that are approximately a factor of 10 more sensitive than those that can be created with other telescopes, whilst also having higher spatial higher resolution. Our observations have revealed: a plethora of previously unseen diffuse radio emission associated with the ICM of Abell 2034 making it one of the richest environments of shocks and turbulence known; tentative evidence of a new shock in the cluster Abell 1914 which we aim to confirm with follow-up observations; and the most precise characterization of a cluster shock ever achieved in the `Sausage cluster’.

The ancestors of local galaxy clusters (`protoclusters’) are powerful laboratories for tracing the emergence of large scale structure and studying the evolution of galaxies in dense environments. It is these structures that at z < 2 virialise into massive X-ray emitting clusters. Every star forming galaxy emits radio emission and very deep LOFAR surveys can detect sufficient number of very distant star forming galaxies that proto-clusters can be found and studied. Our first searches of the deep LOFAR Boötes field presented in Williams et al. (2016) yielded very good candidates at z >2. An optical spectroscopy proposal aimed at confirming two of these LOFAR selected protocluster candidates was submitted and successfully awarded time. With the observations now taken (June 2016) and their processing underway, we hope to soon confirm this novel technique for identifying protoclusters at this critical period in their early history.

Significant progress has been made on the search for the most distant radio loud AGN — known beacons for galaxy protoclusters –, with progress in both the modelling and theoretical understanding of these objects and in the observational searches to find them. Detailed modelling of distant radio-loud AGN has yielded the most robust predictions to-date on the numbers and properties of the z > 6 radio-loud AGN LOFAR surveys will find (Saxena et al. MNRAS in prep.). These models predict that LOFAR can directly probe the surrounding neutral inter-galactic medium through studies of redshifted 21cm absorption. A second project applying and testing the various search techniques required to find these z > 6 AGN is also underway (PI: Saxena). While still in its early stages, this project has already resulted in an extremely successful VLA telescope application.