The search for the first stars to light up the Universe

Title: A hide-and-seek game: Looking for Population III stars during the Epoch of Reionization through the HeIIλ1640 line

Authors: Alessandra Venditti , Volker Bromm , Steven L. Finkelstein , Antonello Calabrò , Lorenzo Napolitano , Luca Graziani , and Raffaella Schneider

First Author’s Institution: Dipartimento di Fisica, Sapienza, Universita` di Roma, Piazzale Aldo Moro 5, 00185, Roma, Italy

Status: Submitted to APJL [open access]

The Dawn of the Universe

A long time ago, in a galaxy far, far away… the very first stars lit up the Universe, ending the Cosmic Dark Ages and ushering in the Cosmic Dawn. In our current best model of the Universe, this happened around 13.4 billion years ago, around 100 million years after the Big Bang. Finding evidence of these very first stars, also called Population III (Pop III) stars, is one of the ultimate treasure hunts in astronomy, and one that the James Webb Space Telescope (JWST) was specifically designed for.

These Pop III stars were formed from highly pristine gas, i.e. from clouds that are almost exclusively hydrogen and helium (the lightest elements in the periodic table). It’s these first stars that then began to form and release heavier elements, and after many billions of years of star formation, the Universe today is a lot more chemically evolved. However, some pockets of pristine gas reservoirs are predicted to hang around even after Cosmic Dawn, meaning that Pop III stars could still exist as late as 12.5 billion years ago (which corresponds to a redshift of 6).

The authors of today’s paper carry out a very interesting experiment to predict just how many Pop III stars we might be able to find at these later times (from a redshift 6-10, which corresponds to ~ 12.5 to 13 billion years ago). The first important question they address is how do we even go about finding these stars?

 Oh look, a clue!

Formation within a pristine environment is a key characteristic of Pop III stars, and actually helps us find them. Thanks to their hydrogen-rich composition, they are theorised to be much more massive than modern stars, and capable of powering very energetic (hard) radiation fields. With this huge amount of energy, they are able to double ionise the Helium in the surrounding gas, which then causes emission of the HeII recombination line at 1640 Angstrom. This emission line is therefore a great clue for finding Pop III stars!

We can even predict how many Pop III systems should exist at each point in time, and predict how strong this emission line should be. To do this, the authors use the dustyGadget cosmological simulations. In summary, they simulate the evolution of a Universe of a certain size (volume), and include as much physics as possible (for example star formation recipes). These simulations are currently the largest volume simulations that also include models for Pop III stars. In the top panels of Figure 1, 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 this emission is faint and difficult to observe, so next the authors need to consider the capabilities of our current best instrument (JWST).

Figure 1: Top panels: Number density of Pop III systems expected at a given redshift in haloes within a given range of stellar mass. The total number density is shown as a grey, solid line, while the numbers observable by JWST NIRSpec IFU are shown by the golden lines, and the NIRSpec MOS by the brown lines. The solid/dashed linestyle refers to the best/worst-case observations. Bottom panels: The fraction of Pop IIIs missed in JWST/NIRSpec observations. The authors find that a significant number of Pop III systems can be overlooked by JWST. This Figure 4 of the paper.

Is JWST up to the task?

JWST hosts a variety of instruments, and for this work we are mainly interested in the NIRSpec Integral Field Unit and the NIRSpec Multi-Object Spectroscopy modes. We can make a pretty good estimate of just how capable these instruments are at detecting HeII 1640 by calculating their sensitivity limits, i.e. how strong does the emission need to be for us to detect it? The authors calculate the sensitivity limits of these instruments for a variety of observing times and set ups. They then compare the sensitivity limit to the predicted emission line strength of the simulated Pop III systems to work out how many of those systems they would be able to observe (coloured lines in top panels), and what fraction are missed (bottom panels).

These results indicate that only the brightest Pop III systems (within the most massive haloes) can be observed. Very low luminosity systems might be missed even with ~50 hour exposures. However, there’s still hope! Even with these limitations, the authors predict that more than 400 Pop III systems could be discovered within current JWST surveys – although spectroscopic follow-ups would be necessary to identify them.

Overall, today’s paper makes some very exciting predictions about finding Population III stars using JWST, which would help astronomers understand the very first light in the Universe.

Astrobite edited by Nathalie Korhonen Cuestas

Featured image credit: NAOJ

About Lucie Rowland

I'm a first year PhD student at Leiden Observatory in the Netherlands, studying massive, star forming galaxies in the early Universe with ALMA and JWST. It's a really exciting time to be interested in astronomy, so I hope to make groundbreaking new research more accessible!

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