August 31, 2016

Planck sheds a light on reionization history

ESA's Planck satellite has revealed that the first stars in the Universe started forming later than previous observations of the Cosmic Microwave Background indicated. This new analysis also shows that these stars were the only sources required to account for reionizing atoms throughout the cosmos, having completed half of this process when the Universe had reached an age of 700 million years.


Cosmic Reionization. Copyright: ESA – C. Carreau

Observations of very distant galaxies hosting supermassive black holes indicate that the Universe had been completely reionized by the time it was about 900 million years old. The starting point of this process, however, is much harder to determine and has been a hotly debated topic in recent years.

"The CMB can tell us when the epoch of reionization started and, in turn, when the first stars formed in the Universe," explains Jan Tauber, Planck project scientist at ESA.
"It is in the tiny fluctuations of the CMB polarisation that we can see the influence of the reionization process and deduce when it began."


Polarisation of the Cosmic Microwave Background. Credit: ESA and the Planck Collaboration

A first estimate of the epoch of reionization came in 2003 from NASA's Wilkinson Microwave Anisotropy Probe (WMAP), suggesting that this process might have started early in cosmic history, when the Universe was only a couple of hundred million years old. This first estimate was soon corrected, as subsequent data from WMAP pushed the starting time to later epochs, indicating that the Universe had not been significantly reionized until at least some 450 million years into its history.

In 2015, the Planck Collaboration provided new data to tackle the problem, moving the reionization epoch even later in cosmic history and revealing that this process was about half-way through when the Universe was around 550 million years old. The result was based on Planck's first all-sky maps of the CMB polarisation, obtained with its Low-Frequency Instrument (LFI).

Now, a new analysis of data from Planck's other detector, the High-Frequency Instrument (HFI), which is more sensitive to this phenomenon than any other so far, shows that reionization started even later – much later than any previous data have suggested.

"The highly sensitive measurements from HFI have clearly demonstrated that reionization was a very quick process, starting fairly late in cosmic history and having half-reionized the Universe by the time it was about 700 million years old," says Jean-Loup Puget from the Institut d'Astrophysique Spatiale (IAS) in Orsay, France, principal investigator of Planck's HFI.
"These results are now helping us to model the beginning of the reionization phase."

"We have also confirmed that no other agents are needed, besides the first stars, to reionize the Universe," adds Matthieu Tristram, a Planck Collaboration scientist at Laboratoire de l'Accélérateur Linéaire in Orsay, France.

The new study dates the formation of the first stars much later on the cosmic timeline than previously thought, suggesting that the first generation of galaxies are well within the observational reach of future astronomical facilities, and possibly even some current ones.

In fact, it is likely that some of the very first galaxies have already been detected with long exposures, such as the Hubble Ultra Deep Field observed with the NASA/ESA Hubble Space Telescope, and it will be easier than expected to catch many more with future observatories such as the NASA/ESA/CSA James Webb Space Telescope.


Planck intermediate results. XLVI. Reduction of large-scale systematic effects in HFI polarization maps and estimation of the reionization optical depth
Planck Collaboration
A&A, Forthcoming article, Accepted: 14 July 2016, doi: 10.1051/0004-6361/201628890

Planck intermediate results. XLVII. Planck constraints on reionization history
Planck Collaboration
A&A, Forthcoming article, Accepted: 23 July 2016, doi: 10.1051/0004-6361/201628897

Read the article on ESA's website