Some time ago, in a galaxy far, far away, the first stars lit up the universe, ending the cosmic dark ages and birthing the cosmic dawn. Our current understanding of physics tells us that this happened around 13.4 billion years ago (around 100 million years after the big bang). Finding evidence of these very first stars, also known as Population III (Pop III) stars, is one of the main purposes of the JWST.

These Pop III stars were formed from clouds of gas which are almost entirely comprised of hydrogen and helium (the lightest elements in the periodic table). These first stars formed and began to release heavier elements through fusion, and as a result, the universe today is a lot more chemically evolved. Some reservoirs of these light clouds of gas ('pristine environments') are still predicted to hang around after the cosmic dawn, meaning that Pop III stars could still exist as late as 12.5 billion years ago (corresponding to a redshift of z = 6).

In May, Venditti et al. published a paper detailing a search for these Pop III stars during the 'epoch of reionization' (z ≃ 6 - 10). Formation within a pristine environment is a key characteristic of Pop III stars. Thanks to their hydrogen-rich composition, they are theorised to be much more massive than stars born later on and are thus capable of powering super energetic radiation fields. Due to this huge amount of energy, they can double ionise the helium in the surrounding gas, which then causes the emission of the He-II recombination line at a wavelength of 1640 angstrom (HeIIλ1640). Finding this emission line is a pretty great indicator that a Pop III star could be lurking nearby!

Using a particular type of cosmological simulation (dustyGadget), the authors of the paper predict how many Pop III systems should exist at each point in the universe's history, and also predict how strong the emission line should be. The dustyGadget simulations are currently the largest volume simulations which also include models for Pop III stars. In the top panels of the figure below, you can see how many Pop III systems exist in the simulations as a function of stellar mass and at different redshifts (different points in cosmic history), plotted by the solid grey line.

The downside is that the emission is faint and difficult to observe – the question now is: is JWST up to the task?

JWST hosts a ton of instruments – for this case, we are interested in the NIRSpec IFU and the NIRSpec MOS modes. We can estimate the capability of these instruments by calculating their sensitivity limits (i.e., how strong does the emission need to be for us to detect it?). The authors did exactly that and found that only the brightest Pop III systems can be observed; very low luminosity systems might be missed even with ~50 hour exposures! Despite this, they predict that more than 400 Pop III systems could be discovered within current JWST surveys! Spectroscopic follow-ups would be necessary to identify them, of course :)

published: 27/07/24 by kaan evcimen