Signs of the First Stars in a Distant Galaxy

The galaxy RX J2129–z8He II is remarkable for its high redshift, sloped spectrum, and strong emission lines. New research suggests that the elusive first stars could be responsible for the galaxy’s properties.

The Hunt for the First Stars

Long ago, the universe was substantially less picturesque than it is today; neutral hydrogen gas absorbed all visible light and rendered the universe opaque. When the first stars glimmered into existence and assembled into the first galaxies, they began to chip away at the opaque universe, carving out increasingly large bubbles of transparent gas.

The nature of the first stars, also called Population III stars, is of great interest to astronomers, but tracking them down is easier said than done. While galaxies composed only of the first generation of stars may be out of reach, shielded from view by clouds of neutral gas, galaxies in which Population III stars mingle with later stellar generations might be easier to spot.

Thirteen Billion Years Ago in a Galaxy Far, Far Away

JWST image of the galaxy RX J2129–z8He II

JWST image of the galaxy RX J2129–z8He II, which is gravitationally lensed by the foreground galaxy cluster RX J2129. [Wang et al. 2024]

Recently, a research team led by Xin Wang (University of Chinese Academy of Sciences) may have identified such a galaxy. Using images from JWST, Wang and collaborators picked out the galaxy RX J2129–z8He II, which even in the eyes of the world’s most powerful infrared telescope is a barely discernible red smudge. JWST spectra of this galaxy place it at a redshift of z = 8.1623, just 613 million years after the Big Bang. At this time, stars and galaxies were still busily ionizing the universe, transforming it from opaque to transparent.

RX J2129–z8He II’s spectrum is tilted sharply toward short wavelengths, more so than any other known galaxy beyond a redshift of z = 7, and it’s marked by several prominent emission lines, including one from singly ionized helium atoms. These factors suggest that the galaxy contains a powerful source of ultraviolet radiation. In the local universe, emission from singly ionized helium is somewhat rare, arising from massive stars that have lost their atmospheres, binary systems containing a star and either a black hole or a neutron star, and galaxies with accreting supermassive black holes. None of these sources are likely to be the cause of the galaxy’s helium emission line. Instead, Wang’s team posits, massive Population III stars could be the source of the ionizing ultraviolet photons.

plot of the spectrum of the galaxy RX J2129–z8He II

JWST spectrum of RX J2129–z8He II, with an inset image showing the He II line. The flux increases toward shorter wavelengths before being attenuated by the intergalactic medium (IGM). Click to enlarge. [Wang et al. 2024]

Are Population III Stars Responsible?

To test this theory, Wang’s team used photoionization models to simulate the properties of a galaxy containing Population III stars as well as stars from later generations. They found that a collection of Population III stars with a total mass of 780,000 solar masses could reproduce the observed emission lines in the galaxy’s spectrum. Previous modeling suggests that this quantity of Population III stars is reasonable for a galaxy at that time period.

As for how a galaxy could host Population III stars alongside their stellar descendants, Wang’s team suggests that new Population III stars could form belatedly in pockets of pristine gas that weren’t swept up into stars in the first round of star formation, or in gas that spilled into the galaxy from the surrounding circumgalactic medium.

This study marks a promising advance in the search for the first generation of stars. In addition to identifying RX J2129–z8He II as a possible home for Population III stars, Wang’s team gained a better sense for the spectral signatures of these stars, which can be used to identify other galaxies that may host them.


“A Strong He II λ1640 Emitter with an Extremely Blue UV Spectral Slope at z = 8.16: Presence of Population III Stars?” Xin Wang et al 2024 ApJL 967 L42. doi:10.3847/2041-8213/ad4ced