The potential for cosmological observations in the 21 cm line, using the Universe's phase transition from neutral to ionized hydrogen called the Epoch of Reionization (EoR), has been widely recognized. The EoR, when the first stars, proto-galaxies and quasars emerged, is a very interesting time, because that object formation leads to patchy 21-cm brightness structure, which should be detectable today at redshifts near nine. Several large telescopes sensitive at meter wavelengths are already being used in attempts to detect and eventually map this patchiness. A great deal of theoretical work has been dedicated to predicting 21 cm signatures, and many groups are awaiting the first observational data. This project will use the Giant Meterwave Radio Telescope (GMRT) to observe the patchy neutral gas and thus pin down the timing of the reionization as well as the duration of the partially ionized era.
To achieve these astronomical goals, this study will continue a program of noise reduction at GMRT, including the elimination of terrestrial noise sources and the installation of lower noise amplifiers, which will benefit all users of the telescope. Additional benefits will come from the development of new signal processing techniques, from programs to make astronomy and physics more accessible to students and to the public, and from several programs specifically targeting high school students.
We used the Giant Metrewave Radio Telescope in India to search for a patchy sky-glow that is expected at frequencies in the 100-200 MHz frequency range. When this patchy glow is detected we will be able to use its features to study the first stars and galaxies that began forming just 100 million years after the start of the big bang. The glow we are seeking is due to 21-cm emission by neutral hydrogen gas and the patchiness occurs because stars form first in denser regions. Ultraviolet light produced by these stars unevenly ionizes the gas in the region, and this takes away the 21-cm glow in patches. The period of cosmic history we are studying is called the Epoch of Re-ionization (EoR). When we started at GMRT maps of the sky created with the telescope were contaminated by man-made interference and the glow of the Milky Way. This interference was at the 2 Kelvin amplitude level. We developed novel observing and analysis techniques that allowed use to clean the maps down to the 16 milli-Kelvin level. The techniques we pioneered include the use of pulsars to calibrate the telescope, matrix techniques to remove Milky Way glow, drift scan mosaic techniques and earth-imaging techniques used to localized man-made interference sources. We were able to eliminate the brightest source of man-made interference, which greatly helped in cleaning up the sky images. Now the telescope produces cleaner images, which benefits the entire community of users. While we did not detect the patchy structure we were seeking we did set new upper limits to its amplitude. Our results favor scenarios in which x-ray sources formed very early, long before the universe was reionized. Disfavored are the so-called cold-reionization scenarios that lack X-ray sources. Most groups attempting EoR observations have used arrays of dipole antennas, which behave quite differently form GMRT's array of parabolic dishes. Our attempt is unique in this respect. We published papers describing the advantage and limitations of the dish approach, backed up by on-sky tests. This peer-reviewed and published real-world experience is in practice a requirement before the EoR community could propose any future large array of dishes. Such proposals are now under consideration. Four graduate students (one female) received training under this program as part of their PhD programs. This work was the main PhD thesis topic for one of these students. Two of the graduate students supported under this grant, taught in CMU's Summer Academy for Math and Science, a six-week in-residence program for rising Juniors and Seniors, which draws substantial enrollment from under-represented groups. One gradaute student presented her work at meetings of the Society of Women Engineers.
