A short time after the Big Bang, the Cosmic Dawn emerged and featured the formation of the first stars, black holes, and galaxies. Neutral hydrogen gas dominated the Universe during this epoch and is now detectable in the radio bands through its redshifted 21-cm line, initially manifest as an absorption line in the cosmic microwave background. As the life cycle of the first stars ended and the first black holes formed, matter accreting onto the black holes produced X-rays, which heated the intergalactic medium (IGM). As the IGM was heated, the 21-cm line of the neutral hydrogen gas moved from producing absorption lines to emission lines until the gas was ionized during the epoch of reionization, thus removing the detectable signal.
This two-year research project will employ a novel observing strategy to detect this redshifted 21-cm line signal using the existing first Long Wavelength Array station (LWA1) in New Mexico, which consists of 256 dual-polarization dipole antennas that are digitally combined to form multiple beams on the sky. While the time history of the absorption and emission features is imprinted on the all-sky radio spectrum, the observational challenge is to separate the approximately 100 mK signal from the dominant Galactic foreground emission. The investigators' strategy exploits the unique beam forming capability to remove calibration uncertainties that plague other instruments by simultaneously targeting science and calibrator fields. The investigators expect to their efforts to lead to the first detection of the 21-cm absorption signal at a redshift of z=25 that will open a new window on early star formation and the IGM.
Educationally, as both institutions have high minority enrollment, this project will actively seek to involve underrepresented student groups, and it will fund a graduate student for two years and will provide opportunities for student training and mentoring in an interdisciplinary environment including principles of digital signal processing, software design, statistical analysis, model fitting, and astrophysics.
Information about the formation and evolution of the first stars, galaxies, and black holes in the early Universe is imprinted in the all-sky radio spectrum obsrevable today. The cosmological signal should be apparent as a "global 21cm signature" at low radio frequencies. However, the predicted signal is much fainter than other astronomical radio sources and has yet to be detected. This project addressed the significant observational challenges by investigating experimental techniques that exploit the unique instrumental properties of the Long Wavelength Array (LWA) in New Mexico. The telescope consists of 256 dual-polarization dipole antennas, operating between 10 and 88 MHz, that are digitally combined to form multiple beams on the sky. During the project, we acquired and analyzed 95 hours of observations with the telescope (16 TB of data) and helped to characterize the telescope, focusing on its primary field of view (also called its "beampattern"). These data were used to produce maps of the LWA beampattern based on raster scans over bright sources and to explore the ability of the telescope to detect the cosmological signal. The project also produced a simulation framework to investigate the effects of variations in the telescope beampattern on future observations to detect the cosmological signal. Using the simulations, we found that changes in the beampattern with frequency across the LWA band are able to disrupt the desired measurement by coupling angular variations in the radio sky--due to emission from our own Milky Way galaxy--into the observed spectrum. We identify observing strategies that could minimize this effect. These strategies should form the basis for future attempts to detect the cosmological signal. Several efforts to detect the cosmological "global 21cm" signal are underway. This project contributed to the identification of frequency-dependent beampatterns as potentially significant sources of error or uncertainty in these experiments and has helped to identify techniques to mitigate them.
