Spectral Energy Distribution of Radio Sources in Nearby Clusters of Galaxies: Implications for Sunyaev-Zel'dovich Effect Surveys

TL;DR

This study investigates the high-frequency radio spectra of galaxies in nearby clusters, revealing a significant fraction with flat or inverted spectra that impact Sunyaev-Zel'dovich effect measurements.

Contribution

It provides detailed spectral measurements at 4.9-43 GHz for 139 cluster galaxies, highlighting the prevalence of flat/inverted spectra and their implications for S-Z surveys.

Findings

60-70% of sources have flat or inverted spectra.

Many sources show flux upward turn above 22 GHz.

Results emphasize the importance of source subtraction in S-Z measurements.

Abstract

To explore the high frequency radio spectra of galaxies in clusters, we used NRAO's Very Large Array at four frequencies, 4.9-43 GHz, to observe 139 galaxies in low redshift (z<0.25), X-ray detected, clusters. The clusters were selected from the survey conducted by Ledlow & Owen, who provided redshifts and 1.4 GHz flux densities for all the radio sources. We find that more than half of the observed sources have steep microwave spectra as generally expected (alpha<-0.5, in the convention S \propto nu^alpha). However, 60-70% of the unresolved or barely resolved sources have flat or inverted spectra. Most of these show an upward turn in flux at nu>22 GHz, implying a higher flux than would be expected from an extrapolation of the lower frequency flux measurements. Our results quantify the need for careful source subtraction in increasingly sensitive measurements of the Sunyaev-Zel'dovich effect in clusters of galaxies (as currently being conducted by, for instance, the Atacama Cosmology Telescope and South Pole Telescope groups).