My research

My research interests include galaxy formation and evolution in the early Universe: how the first stars and galaxies were formed, and how they subsequently brought about the Reionization of the Universe, a major phase transition that turned the neutral Universe back to a (mostly) ionised state.

In my current research, I am involved in multiple different projects with various collaborators at the Kavli Institute for Cosmology. I analyse both cosmological hydrodynamical simulations specifically aimed at accurately modelling the intergalactic medium, as well as observational data of high-redshift star-forming galaxies from several major observatories.

Together with my PhD supervisors, Dr Renske Smit and Prof Roberto Maiolino, and others, I’m working on data from the Very Large Telescope (VLT; in particular the MUSE and X-shooter instruments) and Atacama Large Millimeter/submillimeter Array (ALMA). With these observations, we are exploring the physical conditions of star formation in the very early Universe, and its implications for the subsequent evolution of galaxies up to the present day.

With Dr Ewald Puchwein, Dr Girish Kulkarni, Dr Renske Smit, Prof Martin Haehnelt, and Lewis Weinberger, I am involved in analysing the Sherwood simulation suite, a set of large-scale cosmological hydrodynamical simulations aiming to accurately model the intergalactic medium. Specifically, we are working on making theoretical predictions of the brightness of this vast reservoir of diffuse gas in the principle emission line of hydrogen, Lyman-alpha.

The gas outside galaxies and their direct environments, collectively called the intergalactic medium, forms the bulk of visible matter in the Universe. Thus far, however, it has mostly eluded direct detection by even the most powerful telescopes (although the first studies are starting to uncover its faint glow). Our predictions for observing this signal with the MUSE spectrograph and similar spectrographs on the next generation of observatories are published as Witstok, J., et al., 2021, A&A, 650, A98.

Lyα surface brightness (SB) in simulated narrowband images in a simulation snapshot at z = 4.8. The SB shown is a combination of recombination emission (of all gas in the simulation) below the mirror limit (indicated on the colour bar as HM12), and collisional excitation of gas below half the critical self-shielding density. Panel a shows a region of 8 x 8 (cMpc/h)^2 (5.2 x 5.2 arcmin^2) on a pixel grid of 1024 × 1024. Panels b-d show Lyα narrowband images the size of 1 x 1 arcmin^2 consisting of 300 x 300 pixels (as the field of view of MUSE). The volume probed by one of these narrowband images at this redshift is 2.84 (cMpc/h)^3. The areas covered by these maps are indicated by the white squares in the overview panel a. Halos with halo mass greater than 10^9.5 solar masses are shown as circles, their size indicating their projected virial radius. The most massive halo in each panel is annotated. Figure 8 from Witstok et al. (2021).