Despite being incredibly poor in heavy elements, LAP1-B has an unusually high amount of carbon; its carbon-to-oxygen ratio is higher than our Sun’s. The researchers think the answer might lie in how these massive first-generation stars died.
According to our models, when a massive, Population III star reaches the end of its life, its core collapses into a black hole, but the resulting supernova explosion isn’t energetic enough to blow the entire star apart. “Their bounding energy of gravity is stronger that in the usual massive stars,” Nakajima said.
Instead, the collapse results in a faint supernova with significant fallback, in which the heavier elements from the star’s core, such as oxygen, are sucked back past the event horizon and trapped in the black hole beneath. At the same time, the lighter outer layers, which are rich in carbon, escape and are expelled into the surrounding gas. LAP1-B’s chemical makeup, with low oxygen but elevated carbon, looks like a fingerprint of a gas cloud produced by Population III stars supernovae.
But there was one other clue hidden in the gas in LAP1-B, and it was all about its speed.
The dark matter
By looking at how the emission lines in the spectrum were broadened by the Doppler effect, Nakajima and his colleagues measured that the gas is swirling around inside the galaxy at roughly 58 kilometers per second, a rather typical value for dwarf galaxies.
Using the laws of gravity, the team calculated how much mass must be present to keep gas moving at that speed from flying off into intergalactic space. “We estimated the amount of material at 10 million solar masses,” Nakajima said.
Because the stars account for less than 3,300 solar masses and the gas adds just a tiny bit more, the team concluded that the rest of the galaxy must be made up of dark matter.