The precision array for probing the epoch of re-ionization: Eight station results

We are developing the Precision Array for Probing the Epoch of Re-ionization (PAPER) to detect 21cm emission from the early universe, when the first stars and galaxies were forming. We describe the overall experiment strategy and architecture and summarize two PAPER deployments: a four-antenna array in the low radio frequency interference (RFI) environment of Western Australia and an eight-antenna array at a prototyping site at the NRAO facilities near Green Bank, WV. From these activities we report on system performance, including primary beam model verification, dependence of system gain on ambient temperature, measurements of receiver and overall system temperatures, and characterization of the RFI environment at each deployment site. We present an all-sky map synthesized between 139MHz and 174MHz using data from both arrays that reaches down to 80mJy (4.9K, for a beam size of 2.15e-5 sr at 156MHz), with a 10mJy (620mK) thermal noise level that indicates what would be achievable with better foreground subtraction. We calculate angular power spectra (C _ℓ) in a cold patch and determine them to be dominated by point sources, but with contributions from galactic synchrotron emission at lower radio frequencies and angular wavemodes. Although the sample variance of foregrounds dominates errors in these power spectra, we measure a thermal noise level of 310mK at ℓ = 100 for a 1.46MHz band centered at 164.5MHz. This sensitivity level is approximately 3 orders of magnitude in temperature above the level of the fluctuations in 21cm emission associated with re-ionization.