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Probing the Cosmic Dawn – Further Information

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 (SDSS).

The emerging structure of the Universe. Image: SKA Organisation / Swinburne astronomy productions.

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.

Here comes the science bit!

Observations from the Sloan Sky Survey 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.

quasar” or quasi-stellar radio source is a very energetic and distant active galactic nucleus. They are exceptionally bright, and very far away.

First identified as being high redshift (z) objects, they emit strongly in the radio end of the spectrum.

While the nature of these objects was controversial until as recently as the early 1980s, there is now a consensus in the science community that they are compact regions nearby black holes in the central areas of massive galaxies. As they occur at such vast distances, they are part of the jigsaw aiming to piece together the formation of the early Universe from its Hydrogen rich beginnings, through to the time period when then first stars and galaxies started to form.

The fraction of ionised hydrogen left over from the Big Bang provides evidence for the time of formation of the first stars and quasar black holes in the early Universe. Think of the neutral fraction as the percentage of neutral Hydrogen present when things star to form in the early universe.

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 ionise 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.

Interesting facts

  • In the early part of the twentieth century, Slipher, Hubble and others made the first measurements of the redshifts and blueshifts of galaxies beyond our own Galaxy
  • The largest observed redshift, corresponding to the greatest distance and furthest back in time, is that of the cosmic microwave background the numerical value of its redshift is about z = 1089 (z = 0 corresponds to present time), and it shows the state of the Universe about 13.8 billion years ago, about 379,000 years after the Big Bang