The quest to observe the first luminous objects in the universe has long been an important driver of astronomy in general and cosmology in particular. Interest in these objects has only grown recently with new observations by the Wilkinson Microwave Anisotropy Probe (WMAP) and high-redshift quasars selected from the Sloan Digital Sky Survey.

The Square Kilometre Array will provide detailed pictures of structure formation and reionisation through observations of the redshifted 21 cm line of neutral hydrogen. Unlike constraints from the CMB, line radiation allows us to separate the contributions from different redshifts.

Through multifrequency observations, we can therefore construct fully three-dimensional maps of neutral gas in the universe. Such maps are crucial for studying the time dependence of reionisation.

The latter have shown evidence for a sharp rise in the neutral fraction of the intergalactic medium (IGM) at z~6, implying that epoch of reionization ends at this time. On the other hand, WMAP has found a surprisingly large electron scattering optical depth to the cosmic microwave background (CMB) radiation, implying that reionization began at z~20.

Reconciling these observations requires reionisation to be a complex process, with the ionising sources having qualitatively different (and time-dependent) characteristics from all galaxies that we can currently observe and with feedback from protogalaxies playing a crucial role in regulating the formation of subsequent generations of objects.

The emission from a patch of the IGM depends on its density, temperature, and neutral fraction. When the first sources of light turn on, the IGM will be visible first in absorption and then in emission as these sources heat their surroundings. Fluctuations across the sky will show us how structure grows (through density variations) and how the heating occurs (whether through shocks or radiation from the first objects).

The protogalaxies will also ionize surrounding pockets of gas, shutting off 21 cm emission around bright objects. The pattern of ionised and neutral gas, and its evolution with time, will teach us about the sources responsible for reionisation.

Moreover, the SKA will have the sensitivity to make high-resolution spectra of high-redshift radio sources. These spectra will yield detailed information about the early evolution of the cosmic web, the growth of ionised regions around protogalaxies, and even provide the only known direct way to observe minihalos, small clumps of dark matter and gas in the IGM that are predicted by many structure formation theories.

Find out more:

Probing the Dark Ages with the Square Kilometre Array – C.L. Carilli, S. Furlanetto, F. Briggs, M. Jarvis, S. Rawlings, H. Falcke – in Science with the Square Kilometre Array, 2004.