In the search for the elusive Epoch of Reionisation (EoR), one of the fundamental questions that needs answering is “where do I point my telescope?”. June 2019’s Monthly Media from PhD student Ronniy Joseph answers exactly that, as well as the equally important question of “when do I look?”.

To observe the faint signal from neutral hydrogen atoms during the EoR we need to observe the sky for over 1000 hours. Unfortunately the sky is full of bright objects that also emit radio waves. In earlier Monthly Media’s we have discussed the impact the bright radio structures have, and why we need to stay well clear of those to be able to detect the EoR with the Murchison Widefield Array (MWA).

The Milky Way as seen in the radio spectrum [Haslam et al. 2014/Skyview], with the three different EoR search fields for the MWA telescope overlayed. Each EoR field is in a region of the sky where there is little diffuse radio signal such that the EoR might be detected.

In practice this means we only take observations during 3 months in the year: September (EoR0), November (EoR1) and January (EoR2). The reason why is shown in the above figure. Here’s a map of the sky, similar to a map where we unfold the the map of the Earth onto a square. The circles show which parts of the sky we look at, at specific months in the year. So why only these parts?

Like Dr Cathryn Trott showed, the Milky Way is a problem. By looking at these parts of the sky we can stay well clear of that. But there are also other bright radio sources in the Universe.

These other spots on the map are radio galaxies that host supermassive black holes, some of them even emit bright radio jets. It’s especially those with complicated structures and jets that make it hard to observe the EoR, shown by Dr Christene Lynch.

This means we want to look for the EoR on locations of the sky that are free from the Milky Way and other complicated radio sources. However we do need a few bright sources around, that ideally look like points to our telescope. These bright point-like radio sources are needed to fine-tune the telescope’s antennas, this was discussed in Ronniy Joseph’s recent paper.

Finding the right field-of-view window for searching for the Epoch of Reionisation is a balancing act. Luckily there are three such windows throughout the year and across the sky in which the search can be conducted for the EoR with the MWA telescope.