Here’s what I Zwicky 18 can tell us about the first stars in the universe

A blue dwarf galaxy only 59 million light-years away may harbor cousins of the mysterious Population III stars.

The first stars in the universe were unlike any we can see today. Known to astronomers as Population III stars, they were large, massive, and composed almost entirely of hydrogen and helium. Population III stars were important because they enriched the interstellar medium with metals — all the elements heavier than hydrogen and helium — and participated in reionization, an event a few hundred million years after the Big Bang that made the universe more transparent.

Finding Population III stars could confirm important parts of our theories of cosmology and stellar evolution. However, they should all be gone from the Milky Way by now, having exploded as supernova long ago. We can look into the distant universe to search for them at high redshifts — and indeed, the James Webb Space Telescope will do just that — but detecting individual stars at that distance is beyond our current capabilities. So far, telescopes have turned up nothing.

A Hubble Space Telescope image of I Zwicky 18 shows gas illuminated by young blue stars. Image credit: NASA/ ESA/A. Aloisi.

Recent observations of a nearby dwarf galaxy named I Zwicky 18, however, have given us some hope. Only 59 million light-years away, the galaxy seems to contain clouds of hydrogen that are nearly metal-free. What’s more, it’s undergoing a burst of star formation that might be producing stars very similar to Population III stars. If we could learn more about this galaxy, it could provide us with clues as to what the earliest stars and galaxies in the universe were like.

Is the current wave of star formation the first?

The initial HI observations of I Zwicky 18 used the radio interferometer at Westerbork, in the Netherlands. Image credit: Wikipedia user Onderwijsgek, under the Creative Commons Attribution-Share Alike 2.5 Netherlands license.

One of the first studies to draw attention to the possibility that I Zwicky 18 is forming Population III-analog stars was by Lequex & Viallefond 1980. They supplemented existing optical observations of HII regions — clouds of ionized gas that host young, hot, massive stars — with studies of HI regions via the 21-cm emission line, a key tool for mapping neutral hydrogen. They were trying to figure out if the current round of massive star formation in the dwarf galaxy is its first, or if it had been preceded by other events, polluting the hydrogen clouds with metals.

Their radio observations with the Westerbork Synthesis Radio Telescope found a total HI mass of about 70 million solar masses in six separate regions, three of which remained unresolved. They were unable to connect individual components to the maps of HII regions, but radial velocity measurements of the clouds found that the total mass of the galaxy was much greater by about a factor of ten, suggesting that some other sort of mass was present.

There were two possibilities: either the unseen mass was molecular hydrogen — which would not emit 21-cm radiation — or there was a dim population of older stars. The molecular hydrogen hypothesis couldn’t be ruled out, but the idea of an as-yet unseen group of stars was attractive. For one thing, the HI clouds appeared quite similar to the primordial clouds needed for galaxy formation. If these HI regions were actually primordial, then these dim stars could have supported them against gravitational collapse for billions of years.

Figure 5, Lequex & Viallefond 1980. A map of the HI regions in the galaxy show that three (labeled 1, 2 and 5) are large enough to be resolved, while the others are point sources. Regions 1, 4 and 5 are the most massive.

A picture began to emerge. Comparison of Lyman continuum emission with far-ultraviolet emission indicated that the burst of star formation must have begun about a few million years ago, likely due to the collision of several hydrogen clouds. Before this, there would have been formation of dim red stars on a smaller scale, but not enough to enrich the galaxy more than low observed oxygen abundances suggested. Therefore, the stars forming in I Zwicky 18 should indeed be very close to Population III stars.

What sort of stars are we dealing with?

Figure 1, Kehrig et al. 2015. A composite (hydrogen alpha + UV + r’-band) image of luminous knots in the dwarf galaxy that show intense helium emission.

The idea caught on over the next few decades, and astronomers became interested in determining the nature of these young stars. One group (Kehrig et al. 2015) was particularly interested in determining what type of massive stars could best explain the He II λ4686 line, an indicator of hard radiation and hot stars ionizing material in HII star-forming regions. There were a couple possible culprits:

Early-type Wolf-Rayet stars, which are thought to be responsible for much of the He II λ4686 emission in star-forming galaxies.

Shocks and x-ray binaries, which have also been found in extragalactic HII regions.

Extremely metal-poor O stars, or — going one step further — entirely metal-free O stars, similar to Population III stars.

The group ruled out the Wolf-Rayet stars quickly. Key signatures of metal-poor carbon Wolf-Rayet stars were clearly evident in the spectra, but the inferred number based on the C IV λ1550 line was too small to account for all of the helium emission. Similarly, the x-ray binary possibility was discarded because the sole x-ray binary found was too dim by a factor of 100.

Figure 2, Kehrig et al. 2015. A region of high Hα and He II λ4686 emission shows little overlap with [OI] λ6300 emission and low [S II] contrast, ruling out the possibility of x-ray shocks.

However, a group of maybe a dozen or so metal-free stars of a hundred solar masses or more could successful reproduce the observed He II λ4686 line. There are pockets of gas near a knot in the northwest edge of the galaxy that are devoid of metals and would provide a suitable environment for these stars to form, although there are also likely chemically-enriched stars there, too. Certain models of extremely high-mass (~300 solar masses) offer an alternative to these metal-free stars, but in light of the previous observations, the metal-free models remain enticing.

For the time being, our telescopes can’t detect Population III stars. Until they do, we can still learn a lot about the early universe by studying blue compact dwarf galaxies like I Zwicky 18. Low-redshift, metal-free analogs of the first stars in the universe are close enough for us to study today. The most metal-poor galaxy in the universe is a good place to start.