The 21cm signal at high redshifts

Wednesday, April 28, 2010

To be updated soon…

Although most of the hydrogen in the Universe is ionized, we still don’t exactly how this happened. The basic idea is that as matter started to collapse and stars to form the emitted radiation ionized the existing hydrogen gas. In light of the recent indications from observations by WMAP of an extended period of reionization, the details of the reionization process might be more complex than we previously thought.

Together with my collaborators we’ve investigated the effects of such an extended reionization on the CMB. The free electrons in the reionized (or reionizing) intergalactic medium with peculiar velocities lead to new anisotropies as the CMB photons scatter off of them. We have studied the contributions from inhomogeneous reionization to arcminute scale CMB anisotropies and shown that inhomogeneities in the ionization fraction, rather than in the mean density, dominate both the temperature and the polarization power spectra. Reionization can significantly bias cosmological parameter estimates and degrade gravitational lensing potential reconstruction from temperature maps but not from polarization maps. Fortunately a simple modeling of the reionization temperature power spectrum may be sufficient to remove the parameter bias.

Unfortunately, the CMB observables are mostly sensitive only to the integral history of reionization. Another way to study the reionization process in detail is through the observation of the 21cm radiation. Unlike the CMB, the advantage is that, by selecting the observed frequency, one can probe the neutral content of the Universe at a given redshift, thereby obtaining a tomographic view of the reionization process which can then be tested against the theoretical models. Plans for upcoming low-frequency radio experiments have motivated the study of the 21cm background, during and even prior to, reionization. A major concern for all the 21cm experiments, besides the experimental challenges, involving observations at low radio frequencies (100 - 200 MHz), is the fact that the background itself is highly contaminated by foreground radio emission. Fortunately, it has been shown that it should be possible to remove these foregrounds due mainly to their smoothness in frequency space, while the 21cm signal varies rapidly.

Square Kilometre Array (SKA)
Over the last fifty years our knowledge of the Universe has been revolutionized by the opening of observable windows outside the visible region of the spectrum, from radio waves to gamma rays. One of the few spectral regions still to be explored is at the low radio frequencies. Radio telescopes can probe the neutral hydrogen (HI) in the Universe through observation of the hydrogen spin-flip hyperfine transition at a rest wavelength of 21cm. This allowed the study of nearby galaxies, up to redshift z ~ 0.2. To go beyond this redshift was hard due to the poor resolving power of telescopes and the contamination of the ionosphere so that the field of low frequency radio astronomy remained stagnant for the past 25 years. There have been a number of recent technological developments that make the building of low frequency radio telescopes more viable, namely, the huge increase in computing and networking power, improvement of calibration algorithms and progress in antenna design. These new digital telescopes are based on radio interferometers with large collecting areas spread over a large surface.

Two experiments should start operation in 2008: LOFAR and MWA. These first generation experiments can be considered as pathfinders to the SKA and will certainly influence its final design. SKA will be a second generation experiment aimed at providing over two orders of magnitude increase in sensitivity over existing facilities. This requires a total collecting area of about 1 square Km which will be distributed over an area of 3000 Km or more. Completion is targeted for late in the next decade, with a smaller but still formidable instrument, (10% of SKA) planned for 2011. SKA will probe the gaseous component of the early Universe through observations of the HI 21cm line, thereby addressing fundamental questions on the origin and evolution of the Universe.

  • Posted by msantos at 15:48:37 in 21cm