Cosmological studies of the Sunyaev-Zel'dovich Effect with the APEX-SZ instrument

We have built the APEX-SZ instrument to observe galaxy clusters with the Sunyaev-Zel'dovich Effect (SZE), in which Cosmic Microwave Background (CMB) photons are scattered off of the hot electron gas within a galaxy cluster. Galaxy clusters are the largest collapsed objects in the universe and make excellent tracers of the distribution and evolution of matter. Galaxy cluster surveys can be used to measure the matter density of the universe O m , the equation of state of dark energy w, the amplitude of matter fluctuations at 8 h -1 Mpc s 8 , and the Hubble constant H 0 . Galaxy cluster observations with the SZE also provide a unique probe of galaxy cluster physics.

I will describe the design and fabrication of the 330-element Transition-Edge Sensor (TES) bolometer array used in APEX-SZ. CMB power is measured with an aluminum-titanium bilayer TES centered on a silicon-nitride spiderweb covered with a thin-film gold absorber. Frequency-domain multiplexing is used to read out the TESs. An integration cavity is formed behind a smooth-walled conical horn by the silicon wafer in direct contact with a metal backshort. This cavity couples the gold absorber to incoming millimeter-wave radiation. I will also describe finite element analysis electromagnetic simulations of the integration cavity intended to optimize the optical efficiency of the bolometer array.

The full configuration APEX-SZ instrument was deployed to the 12-meter diameter APEX telescope sited in Chile in March 2007. Observations of the Bullet cluster, a massive galaxy cluster merger perpendicular to the line-of-sight, were made in August 2007. The Bullet cluster was detected in the SZE with a23s significance within the central 1' radius of the source position. We measure a temperature decrement of --880 ± 80mK CMB using an isothermal elliptical b- model fit to the temperature map. We measure a cluster gas mass fraction fg = 0.121±0.038 within a radius 1.42 Mpc, consistent with X-ray and weak lensing results. This work represents the first scientific result from a large array of multiplexed TES bolometers.