Low-resolution spectroscopy of the Sunyaev-Zel’dovich effect and estimates of cluster parameters

¹ Dipartimento di FisicaUniversità di Roma “La Sapienza”, Roma, Italy
e-mail: paolo.debernardis@roma1.infn.it
² INFN Sezione di Roma 1, Roma, Italy
³ INAF – Osservatorio Astronomico di Roma, Monte Porzio Catone, Italy
⁴ School of Physics, University of the Witwatersrand, Johannesburg Wits 2050, South Africa

Received: 9 September 2011
Accepted: 8 November 2011

Abstract

Context. The Sunyaev-Zel’dovich (SZ) effect is a powerful tool for studying clusters of galaxies and cosmology. Large mm-wave telescopes are now routinely detecting and mapping the SZ effect in a number of clusters, measure their comptonisation parameter and use them as probes of the large-scale structure and evolution of the universe.

Aims. We show that estimates of the physical parameters of clusters (optical depth, plasma temperature, peculiar velocity, non-thermal components etc.) obtained from ground-based multi-band SZ photometry can be significantly biased, owing to the reduced frequency coverage, to the degeneracy between the parameters and to the presence of a number of independent components larger than the number of frequencies measured. We demonstrate that low-resolution spectroscopic measurements of the SZ effect that also cover frequencies  >270 GHz are effective in removing the degeneracy.

Methods. We used accurate simulations of observations with lines-of-sight through clusters of galaxies with different experimental configurations (4-band photometers, 6-band photometer, multi-range differential spectrometer, full coverage spectrometers) and different intracluster plasma stratifications.

Results. We find that measurements carried out with ground-based few-band photometers are biased towards high electron temperatures and low optical depths, and require coverage of high frequency and/or independent complementary observations to produce unbiased information; a differential spectrometer that covers 4 bands with a resolution of  ~6   GHz eliminates most if not all bias; full-range differential spectrometers are the ultimate resource that allows a full recovery of all parameters.