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.

Below follow few short excerpts summarising work I am involved in.

Top left: false-colour image of the foreground cluster and the two arcs (A1–A3) of the background galaxy RCS0224z5 observed with X-shooter and SINFONI. Other sections show the detected emission lines. Figure 1 from Witstok et al. (2021b).
Measurements of the metallicity offset from the Fundamental Metallicity Relation (FMR), which predicts the amount of heavy elements in galaxies that have reached an equilibrium state. Our results show a tentative departure from the predicted value for RCS0224z5, as well as for several other high-redshift galaxies from the literature, potentially indicating a recent inflow of pristine (metal-poor) gas. Figure 5 from Witstok et al. (2021b).

This is a short article describing our work on X-shooter and SINFONI observations of a unique, gravitationally lensed galaxy (from the KICC 2020 annual report).

One of the main challenges faced in modern astrophysics is explaining the major phase transition that transformed the very early neutral Universe into an (almost) completely ionised state. This process, referred to as reionization, was set in motion with the formation of the first stars and galaxies, around 150 million years after the Big Bang (around redshift 20), and was completed in under a billion years later (redshift around 6). (Read more about galaxies and their place in a timeline of the Universe here.) The current hypothesis is that low-mass galaxies were the main culprit behind Reionisation: at that point in time, pristine gas – largely composed of hydrogen and helium, not yet enriched with heavier elements like carbon and oxygen – abundantly flowed into these galaxies and was rapidly converted into stars. Therefore, a significant fraction of the resulting stellar populations were likely metal-poor, massive (O- and B-type) stars, whose spectra are much ‘harder’ than stars like the Sun: they produce copious amounts of photons with sufficiently high energy to ionise neutral atoms.

The challenge of confirming this hypothesis lies in the fact that as a result of their low stellar mass, these galaxies are necessarily faint, and thus hard to detect. To constrain their contribution to reionisation, an accurate measurement of two quantities is required. Firstly, observations need to reveal their occurrence rate, which can, in principle, be established relatively easily by number counts in deep surveys – this will certainly be achieved by the much-anticipated James Webb Space Telescope (JWST). Secondly, however, there is the slightly more complex question of their average ionising ability: did these systems produce enough ionising photons, and critically, could these escape the galaxy? This measurement is hindered by the very fact the Universe was in a neutral state initially, since intervening neutral gas (which absorbs all ionising photons) precludes us from directly observing the process of reionization. (Moreover, Lyman-alpha – the principal emission line of hydrogen and hence one of the main ways to identify high-redshift galaxies via their spectra – is also absorbed.)

Characterising sources that are ‘leaking’ ionising radiation is therefore crucial to aid JWST in finding the sources responsible for reionisation. To this end, we obtained new VLT/X-shooter spectroscopic observations (PI: Renske Smit) of a unique gravitationally lensed galaxy at redshift 5 (top figure on the left). A massive foreground cluster (RCS 0224–0002) serendipitously magnifies the background galaxy, named RCS0224z5, by a factor of about 30; only two other known sources at such high redshift have a comparably high magnification. Through measurements of rest-frame energetic ultraviolet emission lines, we found the radiation field to be predomi- nantly stellar in origin. Further analysis of previous observations taken with VLT/SINFONI allowed a measurement of neon and oxygen emission lines, which revealed a high number of ionising photons relative to the gas density inside the galaxy, far exceeding what is typically seen in local galaxies and indeed similar to nearby ‘leakers’, a class of extremely rare galaxies from which a significant fraction of ionising photons escapes. Furthermore, these lines are an indicator for the enrichment of heavy elements, which shows a tentative departure from the Fundamental Metallicity Relation (bottom figure on the left). Finally, we detected a magnesium emission line that has been shown to correlate with the same class of leakers. Independently, we also indirectly inferred that RCS0224z5 has a significant Lyman-alpha (and thus likely also ionising photon) escape fraction. Being the most distant galaxy for which this magnesium line has been reported, this demonstrates for the first time its potential as an indirect tracer of ionising photon leakage at high redshift.

The following is a short description of research started in my master’s degree research project, in collaboration with Dr Ewald Puchwein, Dr Girish Kulkarni, Dr Renske Smit, Prof Martin Haehnelt, and Lewis Weinberger.

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 (see figure on the right) 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. (2021a).