The APEX-SZ Experiment
The APEX-SZ experiment is designed to study and discover galaxy clusters via the small distortion they imprint on the Cosmic Microwave Background (CMB) radiation, called the Sunyaev-Zel'dovich effect (SZe). The amplitude of the SZe signal is directly related to the cluster mass, and largely independent of the distance to the cluster. This makes the SZe an ideal tool for doing cosmology--and in particular studying the expansion history of the universe--because the most distant and oldest clusters do not fade away as they would if we were looking at light (such as optical, x-ray, or radio) being produced in the cluster itself.
The APEX-SZ focal plane consists of 320 Transition Edge Sensor bolometers operating at 150 GHz and read out with a novel frequency domain multiplexed readout system. It has 1 arcminute FWHM resolution and 0.4 degree field of view. The receiver is shown at right.
The instrument makes its home on APEX, a 12m Cassegrain telescope built by the Max Planck Institute for Radioastronomy (MPIfR), the European Southern Observatory, and the Swedish Onsala Space Observatory. APEX is situated on the Atacama Plateau in Chile at an altitude of 5100m (16,700 feet), one of the driest places on earth. This allows APEX to look out at the cosmos with minimal interference from the atmosphere. The mean atmospheric transmittance is better than 95% at 150 GHz, amounting to less than about 1mm of precipitable water vapor. APEX-SZ receives about a month of observation time per year on APEX.
APEX-SZ saw first light mid-2005 (the deployment team is shown at right), making it the first operational large format array of Transition Edge Sensors bolometers. In many ways it is a technology pathfinder for other CMB experiments, as it implements three key new technologies:
These technologies have been ear-marked for nearly every next-generation CMB instrument (such as The South Pole Telescope, ACT, CLOVER, and POLARBEAR), so their development and field-demonstration has been one of the most important milestones for the APEX-SZ team.
- large array (320 detectors) of transition edge sensor bolometers
- SQUID-based multiplex readout system (frequency domain)
- a cryogenics system that cools the focal plane to 250 mK without the use of expendable cryogens
APEX-SZ is a collaboration between U.C. Berkeley (lead institution, PI Adrian Lee), Lawrence Berkeley National Labs, U.C. Boulder, McGill University, Max Planck Institute, Bonn University, and Onsala Sweden.
At McGill, our team consists of Prof. Matt Dobbs, postdoc Dr. Trevor Lanting (left for a new position in 2008), and M.Sc student James Kennedy.
- N. Halverson, T. Lanting et. al., "Sunyaev-Zel'dovich Effect Observations of the Bullet Cluster (1E 0657-56) with APEX-SZ", submitted to ApJ, July 2008 [arXiv:astro-ph/0807.4208]. The SZe map of the Bullet system is shown with white contours (interval of 100 uK) in the image to the right, overlaid on an X-ray map from XMM observations. The green contours show the weak lensing surface mass density reconstructed from Clowe et al., 2006.
- M. Dobbs, N. Halverson et. al., "APEX-SZ First-Light and Instrument Status", New Astronomy Reviews 50 (2006) 960-968. (PDF Format)
View looking into the APEX-SZ cryostat. The focal plane has been removed and the 3He, 3He, 4He absorption fridge is visible in the center. The hand belongs to Berkeley grad student Dan Schwan, who designed the cryostat.