Michael Zemcov

The Sunyaev-Zeldovich Effect

The describes the scattering of cold cosmic microwave background photons off hot gas in the atmospheres of galaxy clusters. The SZ signal changes brightness with the mass of the cluster, but does not change as a function of redshift. This makes it an excellent tracer of large structures in the Universe, since it can be used to characterize the properties of a cluster at any distance.

An important property of the SZ effect is that, because on average the CMB photons gain energy when scattered, the spectral shape of the SZ effect is a decrement in the temperature of the CMB at low frequencies, and an increment at high frequencies, crossing through a null at around 217 GHz. This behaviour is exhibited in the figure below.

The Spectrum of the SZ Effect.

I have concentrated much of my SZ research on making measurements of the spectrum of the SZ effect, particularly in the sub-mm regime where it is an increment in the CMB temperature. These are challenging measurements, since there are very few ways to make small cold spots on the CMB background, but many to make small hot spots (including all of the sub-mm and radio sources in the Universe). However, the potential payoff is rewarding: subtle changes in the scattering physics due to the state of the electrons in the galaxy cluster atmosphere cause changes in the electromagnetic spectrum of the SZ effect. Though small, these changes can allow measurement of the temperature and velocity flows within the cluster medium, and even a measurement of the velocity of the cluster with respect to the CMB rest frame. These are shown in the figure above in the gold and blue lines; the region where this effect is largest is in the SZ increment. Though still an emerging field, these kind of measurements provide tight constraints on structure formation in the Universe which are independent of other measures.

I am currently leading efforts to measure the SZ effect using instruments like Herschel-SPIRE, Z-Spec, and SCUBA-2. The ultimate goal of this work is to map the SZ effect spectrum in many clusters, which will allow measurements of the spectral corrections to the SZ shape. Not only will this yield astrophysical and cosmological information, but also inform new instruments and measurement strategies as we progress.

SZ-Related Photos

Group photo for the SZ2011 conference in Santander, Spain.

A photograph of the Dark Sector at the Amundsen-Scott South Pole Station taken circa 2007 by Iceman. Many historic measurements of the SZ effect and CMB were taken with the various telescopes visible in this image, including QUaD in the upper middle, with Viper just below it, both part of the Martin A. Pomerantz Observatory. Further up from that is what remains of the old AST/RO telescope buried by snow. At the bottom is the new dark sector lab which houses BICEP and SPT.