Astrophysics from the cosmic 21-cm signal

The cosmic 21-cm signal, corresponding to the spin-flip transition of neutral hydrogen, is set to revolutionize our understanding of the early Universe.  Observations with upcoming radio interferometers will allow us to probe the 3D temperature and ionization structure of the cosmic gas.  This will open a window onto the unseen first galaxies, showing us how their UV and X-ray photons drove the cosmic milestones of the epoch of reionization and epoch of heating.  These final frontiers of astrophysical cosmology will transition from being data-starved to being at the forefront of the “Big Data” revolution.

However, interpreting these observations is still not straightforward.  To help quantify what can we learn from the 21cm line, we developed a Markov chain Monte Carlo (MCMC) analysis tool, entitled 21CMMC.  21CMMC (Greig & Mesinger 2015, 2017, Park et al. 2018) is a massively parallelized MCMC driver of 21cmFAST, allowing us to efficiently and accurately quantify constraints and degeneracies among astrophysical parameters.

The panels below illustrate parameter constraints from a 1000h mock observation with HERA, as well as combining these with current observations of galaxy luminosity functions. The eight astrophysical parameters describe the UV and X-ray properties of the early galaxies, including how their star-formation rates and ionizing photon escape fractions evolve with the mass of their host dark matter halos.  We show the inferred parameters with the corresponding UV luminosity functions and reionization history, using current observations and combined with upcoming 21-cm observations.  For more details on the simulations, including descriptions of the astrophysical parameters, please see Park, Mesinger, Greig & Gillet (2018).

Click to download a cool movie of the MCMC in action! (note for visualization purposes, the “mock” signal at the top was created with the same random seed, while in general we use a different seed than for the MCMC)

The impact of the astrophysical parameters on the 21-cm signal can be seen in the movie below (open in a new window for best results).  movie credit: Bradley Greig and Jaehong Park

Movies varying individual parameters can also be found here.