Kaustuv Basu

A Sunyaev–Zel’dovich take on cluster radio haloes – I. Global scaling and bi‐modality using Planck data

Giant radio haloes in galaxy clusters are the primary evidence for the existence of relativistic particles (cosmic rays) and magnetic fields over Mpc scales. Observational tests for the different theoretical models explaining their powering mechanism have so far been obtained through X-ray selection of clusters, e.g. by comparing cluster X-ray luminosities with radio halo power. Here we present the first global scaling relations between radio halo power and integrated Sunyaev–Zel’dovich (SZ) effect measurements, using the Planck all-sky cluster catalogue and published radio data. The correlation agrees well with previous scaling measurements based on X-ray data and offers a more direct probe into the mass dependence inside radio haloes. However, we find no strong indication for a bi-modal cluster population split between radio halo and radio-quiet objects. We discuss the possible causes for this apparent lack of bi-modality and compare the observed slope of the radio–SZ correlation with competing theoretical models of radio halo origin.

CMB observations and the production of chemical elements at the end of the dark ages

The metallicity evolution and ionization history of the universe must leave its imprint on the Cosmic Microwave Background through resonant scattering of CMB photons by atoms, ions and molecules. These transitions partially erase origi- nal temperature anisotropies of the CMB, and also generate new fluctuations. In this paper we propose a method to determine the abundance of these heavy species in low density (over-densities less than 10 4 −10 5 ) optically thin regions of the universe by using the unprecedented sensitivity of current and future CMB experiments. In particular, we focus our analysis on the sensitivity of the Planck HFI detectors in four spectral bands. We also present results for l = 220 and 810 which are of interest for balloon and ground-based instruments, like ACT, APEX and SPT. We use the fine-structure transitions of atoms and ions as a source of frequency dependent optical depth (τν). These transitions give different contributions to the power spectrum of CMB in different observing channels. By comparing results from those channels, it is possible to avoid the limit imposed by the cosmic variance and to extract information about the abundance of corresponding species at the redshift of scattering. For Planck HFI we will be able to get strong constraints (10 −4 −10 −2 solar fraction) on the abundances of neutral atoms like C, O, Si, S, and Fe in the redshift range 1−50. Fine-structure transitions of ions like CII, NII or OIII set similar limits in the very important redshift range 3−25 and can be used to probe the ionization history of the universe. Foregrounds and other frequency dependent contaminants may set a serious limitation for this method.

A giant radio halo in the cool core cluster CL1821+643

Giant radio halos are Mpc-size sources found in some merging galaxy clusters. The synchrotron emitting electrons are thought to be (re)accelerated by plasma turbulence induced by the merging of two massive clusters. Cool core galaxy clusters have a low temperature core, likely an indication that a major merger has not recently occurred. CL1821+643 is one of the strongest cool core clusters known so far. Surprisingly, we detect a giant radio halo with a largest linear size of

The impact of the SZ effect on cm-wavelength (1–30 GHz) observations of galaxy cluster radio relics

\sim

Mpc-scale diffuse radio emission in two massive cool-core clusters of galaxies

1.1 Mpc. We discuss the radio and X-ray properties of the cluster in the framework of the proposed models for giant radio halos. If a merger is causing the radio emission, despite the presence of a cool-core, we suggest that it should be off-axis, or in an early phase, or a minor one.

Planck's view on the spectrum of the Sunyaev–Zeldovich effect

Radio relics in galaxy clusters are believed to be associated with powerful shock fronts that originate during cluster mergers, and are a testbed for the acceleration of relativistic particles in the intracluster medium. Recently, radio relic observations have pushed into the cm-wavelength domain (1–30 GHz) where a break from the standard synchrotron power law spectrum has been found, most noticeably in the famous “Sausage” relic. Such spectral steepening is seen as an evidence for non-standard relic models, such as ones requiring seed electron population with a break in their energy spectrum. In this paper, however, we point to an important effect that has been ignored or considered insignificant while interpreting these new high-frequency radio data, namely the contamination due to the Sunyaev-Zel’dovich (SZ) effect that changes the observed synchrotron flux. Even though the radio relics reside in the cluster outskirts, the shock-driven pressure boost increases the SZ signal locally by roughly an order of magnitude. The resulting flux contamination for some well-known relics are non-negligible already at 10 GHz, and at 30 GHz the observed synchrotron fluxes can be diminished by a factor of several from their true values. At higher redshift the contamination gets stronger due to the redshift independence of the SZ effect. Interferometric observations are not immune to this contamination, since the change in the SZ signal occurs roughly at the same length scale as the synchrotron emission, although there the flux loss is less severe than single-dish observations. Besides presenting this warning to observers, we suggest that the negative contribution from the SZ effect can be regarded as one of the best evidence for the physical association between radio relics and shock waves. We present a simple analytical approximation for the synchrotron-to-SZ flux ratio, based on a theoretical radio relic model that connects the nonthermal emission to the thermal gas properties, and show that by measuring this ratio one can potentially estimate the relic magnetic fields or the particle acceleration efficiency.

Redshift evolution of the 1.4 GHz volume averaged radio luminosity function in clusters of galaxies

Radio halos are diffuse synchrotron sources on scales of ~1 Mpc that are found in merging clusters of galaxies, and are believed to be powered by electrons re-accelerated by the merger-driven turbulence. We present measurements of extended radio emission on similarly large scales in two clusters of galaxies hosting cool cores: Abell 2390 and Abell 2261. The analysis is based on interferometric imaging with the JVLA, VLA and GMRT. We present detailed radio images of the targets, subtract the compact emission components, and measure the spectral indices for the diffuse components. The radio emission in A2390 extends beyond a known sloshing-like brightness discontinuity, and has a very steep in-band spectral slope at 1.5 GHz that is similar to some known ultra-steep spectrum radio halos. The diffuse signal in A2261 is more extended than in A2390 but has lower luminosity. X-ray morphological indicators, derived from XMM-Newton X-ray data, place these clusters in the category of relaxed or regular systems, although some asymmetric features that can indicate past minor mergers are seen in the X-ray brightness images. If these two Mpc-scale radio sources are categorized as giant radio halos, they question the common assumption of radio halos occurring exclusively in clusters undergoing violent merging activity, in addition to commonly used criteria in distinguishing between radio halos and mini-halos.

The SKA Mid-frequency All-sky Continuum Survey: Discovering the unexpected and transforming radio-astronomy

We present a detailed analysis of the stacked frequency spectrum of a large sample of galaxy clusters using Planck data, together with auxiliary data from the AKARI and IRAS missions. Our primary goal is to search for the imprint of relativistic corrections to the thermal Sunyaev-Zeldovich effect (tSZ) spectrum, which allow to measure the temperature of the intracluster medium. We remove Galactic and extragalactic foregrounds with a matched filtering technique, which is validated using simulations with realistic mock data sets. The extracted spectra show the tSZ signal at high significance and reveal an additional far-infrared (FIR) excess, which we attribute to thermal emission from the galaxy clusters themselves. This excess FIR emission from clusters is accounted for in our spectral model. We are able to measure the tSZ relativistic corrections at

Prime-Cam: a first-light instrument for the CCAT-prime telescope

2.2\sigma

CCAT-Prime: science with an ultra-widefield submillimeter observatory on Cerro Chajnantor

by constraining the mean temperature of our cluster sample to