Science

The Sunyaev-Zel'dovich Effect

The Sunyaev-Zel'dovich (SZ) effect is the name given to the process by which the Cosmic Microwave Background (CMB) blackbody spectrum is distorted by the presence of galaxy clusters. Galaxy clusters are the largest gravitationally-collapsed structures in the universe, whose abundance and structure provide powerful probes of cosmology. Gas falling into the gravitational potential well of these clusters is heated to roughly 10⁸ Kelvin and becomes ionized. Photons from the CMB pass through this ionized plasma, and as many as 1-2% of the CMB photons can be inverse Compton scattered by the hot gas. On average the energy of the scattered photons is increased, spectrally distorting the CMB in a characteric manner that is known as the thermal SZ effect. Deviations in intensity from the black-body ideal of the CMB are shown, as a function of frequency, as the blue line in the graph. The intensity spectrum of the CMB is shown for comparison as a black dotted line, scaled by 5x10^-4. Note that our observing frequencies of 150 GHz and 217 GHz correspond to the decrement region and null region of the thermal SZ effect, respectively.

There is a second SZ effect, the kinetic SZ effect, caused by the motion of of the galaxy clusters with respect to the rest frame of the CMB. The net motion of the scattering electrons in the hot intercluster gas imparts a Doppler shift to the scattered photons. This kinetic SZ effect is shown in the graph as the red dotted line. It is a decrement in overall power because the galaxy is moving away from us. A net increment in power can also occur if the galaxy is moving toward us, this effect would appear in the graph as the red dotted line, reflected about the x-axis. This effect in principle allows one to determine the peculiar velocities of the galaxy clusters; peculiar meaning 'with respect to the CMB rest frame'.

The SZ effect has the benefit of being redshift independent, so that it allows for galaxy cluster surveys that look much deeper into space than x-ray, optical, and infrared surveys. The number of clusters APEX can detect is not limited by redshift, but by a minumum cluster mass.

APEX SZ Survey Science Goals

• Discovery and catalog of order 1000 previously unknown galaxy clusters in a mass limited survey
• Observe evolution of structure, and test theories of structure formation
• Constrain mass density of the Universe Ω_m and dark energy equation of state w
• Measure Hubble constant H₀ and acceleration parameter q₀ independent of the distance ladder
• Study CMB secondary anisotropies
  • weak lensing
  • Ostriker-Vishniac effect