Project Level: PhD

The peculiar velocity of a galaxy can be measured two main ways: (1)~`Directly', via so-called distance indicators, a difficult and expensive method requiring many observations of the same object at different wavelengths. Consequently the largest homogeneous sample of directly-measured peculiar velocities to date is the 6-degree Field Galaxy Survey of $\sim$9000 galaxies (6dFGSv), covering only the southern hemisphere. (2)~`Indirectly', by measuring the density of the nearby universe and using that to predict the peculiar velocities. This relies on the fact that high-density regions will continue to accrete more matter while low-density regions will lose more matter. This method is known as {\em velocity field reconstruction} and is observationally much cheaper and has higher fidelity than `direct' measurements, and so is our focus here.

The main objective of this project is to use the positions of all galaxies that will be available from the new galaxy surveys to produce a density map that covers the full sky and as deep as possible. Subsequently, convert that map into a full sky velocity map that can be, globally, used as a hub to predict peculiar velocities for any cosmological analysis. This goal will be addressed through linked aims:

  • How to combine redshifts from DESI (optical) with WALLABY (21 cm) to produce a nearly full sky sample that we can use for reconstruction
  • Creating the first-ever all-sky maps of density and peculiar velocity for galaxies in the nearby universe using these datasets.

Project members

Dr Khaled Said Soliman

Postdoctoral Research Fellow