The poorest oxygen-forming star-forming galaxy has found evidence that the first galaxies to emerge after the birth of the universe shone with supermassive stars that left large black holes behind.
Such galaxies are rare now because almost as soon as a galaxy begins star formation, massive stars produce huge amounts of oxygen, which is the most abundant element in the cosmos after hydrogen and helium. Astronomers reward the few galaxies of this type found near home because they offer an idea of what conditions were like in the very early universe, before the stars made much oxygen (SN: 8/7/19).
The oxygen-hydrogen ratio of the new galaxy – a standard measure of the relative abundance of oxygen in the cosmos – is well below 2 percent of the sun, researchers said in an article published in the Astrophysical Journal published online on March 22 on arXiv .org.
“It’s pretty hard to pick up such a rare object,” says astrophysicist Takashi Kojima, who, along with colleagues, made the discovery while at the University of Tokyo.
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Called HSC J1631 + 4426, the record galaxy, found using the Subaru telescope in Hawaii, is 430 million light-years from Earth in the constellation of Hercules. The galaxy is a dwarf, with far fewer stars to create oxygen than the Milky Way has. Those relatively few stars gave the runt just a pinch of oxygen – one oxygen atom for every 126,000 hydrogen atoms. This is only 1.2 to 1.6 percent of the level of oxygen in the sun.
“Any new galaxy is good,” says Trinh Thuan, an astronomer at the University of Virginia in Charlottesville who helped find the former champion four years ago. "We're counting the number of (very oxygen-poor galaxies) in the palm of our hand." The oxygen / hydrogen ratio of the new galaxy is 83 percent that of the previous record, J0811 + 4730, which is 620 million light-years away in the constellation Lynx.
NASA, ESA, A. Aloisi / Institute of Space Telescope Sciences and European Space Agency.
In HSC J1631 + 4426, Kojima and colleagues also find strange abundances of another chemical element: iron. Although the total amount of iron in the galaxy is low, “we found that the iron-oxygen abundance ratio is surprisingly high,” he says.
The same pattern also appears in Lynx's oxygen-poor galaxy. In contrast, ancient Milky Way stars tend to have little iron relative to oxygen. That’s because newborn stars get most of the iron from long-lived star explosions. Those explosions had not occurred when the oldest stars in the Milky Way formed. But in the two almost virgin galaxies, the amount of iron relative to oxygen is as high as that of the sun, which acquired large amounts of both elements from previous generations of stars.
“This is a very unusual pattern and it’s not obvious how to explain it,” says Volker Bromm, an astrophysicist at the University of Texas at Austin who did not participate in the discovery.
Just before Kojima got his doctorate. in 2020, he found a possible explanation: high-mass stars in dense star clusters merged to make stellar goliaths more than 300 times more massive than the sun. These superstars exploded and watered their galactic homes with both iron and oxygen, resulting in high iron-oxygen ratios in the two primitive galaxies, as well as a source of the little oxygen there.
It is not known that there are such massive stars in the modern Milky Way. But Kojima says their presence in the two oxygen-poorest star galaxies suggests that the primordial galaxies had them as well.
When superstars died, they should leave behind black holes of intermediate mass, which are more than 100 times more massive than the sun (SN: 2/9/20). This is about 10 times larger than typical black holes, which can form when bright stars die.
Kojima's team sees evidence of these large black holes in the newly discovered galaxy. The gas that revolves around such large black holes should be so hot that it emits high-energy photons or light particles. Due to their high energy, these photons would snatch electrons even from helium atoms, which cling firmly to their electrons and convert the atoms into positively charged ions. Indeed, the galaxy of Hercules emits a wavelength of blue light that comes from such helium ions.
The record-breaking galaxy is “an exciting preview of things to come,” Bromm says. In the coming years, he says, huge telescopes will open that will find even more extreme galaxies (SN: 1/10/20). "Then we will have a wonderfully complementary way of learning about the primitive universe."