New light on galaxy formation The epoch of reionization began roughly 200 million years after the Big Bang, as photons from the first stars broke up hydrogen atoms into their constituent protons and electrons. Researchers in the Delphi project are investigating the relationship between galaxy formation and reionization, while also addressing other major questions in physical cosmology, as Dr Pratika Dayal explains.
According to our current understanding, immediately after its inception in the Big Bang, the Universe underwent a period of accelerated expansion (“inflation”) after which it cooled adiabatically. Roughly 400,000 years later, for the first time the Universe became cool enough that electrons and protons recombined to form hydrogen and helium (“recombination”); at this point matter and radiation also decoupled (“decoupling”) giving rise to the Cosmic Microwave Background (CMB; shown by the coloured dots). This was followed by the cosmic “Dark Ages” when no significant radiation sources existed. These cosmic dark ages ended with the formation of the first stars a few hundred million years after the Big Bang. These first stars started producing the first photons that could reionize hydrogen into electrons and protons, starting the ``Epoch of cosmic Reionization” which had three main stages: the ``pre-overlap phase” where each source produced an ionized region around itself, the ``overlap phase” when nearby ionized regions started overlapping and the ``post-overlap phase” when all of the hydrogen in the Universe was effectively completely ionized.
The first stars are thought to have formed around 200 million years after the Big Bang, following the end of the cosmic dark ages. The first galaxies produced photons which led to the start of the epoch of cosmic reionization, when hydrogen was broken apart into protons and electrons, marking an important point in the evolution of the universe. “As the universe was re-ionized, it also heated up. When reionization started, the universe had an ambient average temperature on the order of around 60 kelvin. But in regions which were reionized, the temperature rose to up to 20,000 kelvin,” explains Dr Pratika Dayal. Based at the University of Groningen in the Netherlands, Dr Dayal is the Principal Investigator of the Delphi project, an ERC-backed initiative developing a model to investigate the relationship between cosmic reionization and the rate of galaxy formation. “If there are small or low-mass galaxies inside these ionized regions, their gas may be boiled out. These low-mass galaxies are the most numerous in the universe, so we think in general that they were one of the key drivers of reionization,” she continues.
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These tiny galaxies which formed in the first billion years of cosmic history are the key building blocks of all structure, and essentially the major sources of reionization. The question of how these tiny galaxies evolved into the galaxies that we see today is another major area of interest to Dr Dayal. “Galaxies form hierarchically, and over time smaller galaxies merged to form larger and larger systems,” she outlines. The third question being addressed in the project surrounds the nature of dark matter, which is thought to account for the majority of matter in the universe. “Everything done in this area currently is based on the cold dark matter paradigm (CDM), where dark matter is very cold and made up of heavy particles. This is an assumption though – we’re not sure whether dark matter is cold, or if it’s actually warm,” explains Dr Dayal. “If these galaxies that formed in the first billion years of cosmic history are the key building blocks of all structure and the key sources of reionization, then changing the kind of dark matter that they are embedded in would probably change the formation of structure and the history of reionization.”
Cosmic reionization The processes of galaxy formation and reionization were closely inter-linked during the epoch of reionization, with galaxies forming and in turn driving reionization. However, reionization may also have hindered star formation to some degree. “We think that when galaxies formed, they reionized the surrounding space. That could essentially have stopped some or all of the star formation in the reionized space,” outlines Dr Dayal. The epoch of reionization ended when all the hydrogen had been reionized, now Dr Dayal and her colleagues are looking back into cosmic history to learn more about how reionization affected galaxy formation. “We have data from a number of telescopes, for example the Hubble Space telescope, which allows us to look at these low-mass galaxies in the first billion years of cosmic history,” she says. “Mapping out the epoch of reionization is one of the key aims of cuttingedge facilities including Lofar (Low frequency Array), MWA (Murchison widefield Array) and the forthcoming SKA (Square Kilometre
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