State-of-the-art observations show that within approximately 1 billion years after the Big Bang, all of the primordial hydrogen that permeated the immense space between galaxies had been destroyed by high energy photons in a process known as reionization. This time period in the history of the Universe, also known as ‘the dawn of galaxies’ remains one of the most active frontiers in modern astrophysics from both an observational and theoretical point of view.
Much remains unknown about the sources which provided the high energy photons needed for reionization. The standard picture suggests it was likely the first generation of galaxies with a possible additional contribution coming from radiating supermassive black holes. More exotic solutions have been proposed as well, including annihilating dark matter. Identifying the sources responsible for reionization as well as understanding their properties is key to constraining the conditions which governed all subsequent galaxy formation in the Universe.
Unfortunately, our current telescopes only probe the tail end of this epoch. For this reason, theoretical and computer models are relied upon for insight into the physics during the reionization. Simulating this regime is both complicated and computationally expensive; however, over the past two years, researchers at the Institute for Astronomy (IoA) and Kavli Institute for Cosmology Cambridge (KICC), have designed a new, state-of-the-art computer algorithm which they are exploiting on DiRAC supercomputers at the Universities of Cambridge, Leicester and Durham to better understand the epoch of reionization. This new algorithm (presented in a recently submitted paper by Katz et al. 2016) follows the flow of high energy photons which are directly and on-the-fly coupled to the chemical and thermodynamic state of the gas at different speeds depending on the density of the medium. This has overcome many issues relating to standard algorithms in the field at much reduced computational cost, allowing us to simulate significantly more complex systems than previously possible.
The team at IoA and KICC has thus for the first time generated a multi-scale view of galaxy formation during this epoch – from the large scale cosmic web all the way down to individual star forming regions inside of primordial galaxies. Because the flow of radiation is followed explicitly, the researchers have used an inhomogeneous radiation field to compute the distribution of emission lines coming from ionized metals (such as [CII] and [OIII]) which can be observed by some of our most sensitive telescopes. Likewise, they have self-consistently tracked the location and mass of ionized, neutral, and molecular gas to make testable predictions about these quantities which control star formation in the Early Universe.
In the coming years, new observations from the James Webb Space Telescope, Atacama Large Millimeter Array, and Square Kilometer Array will shed light on this exciting frontier and be compared to these simulated models to give insight into the physical processes which drive ‘the dawn of galaxy’ formation.