Fast radio burst reveals the Milky Way is emptier than we thought
The intriguing cosmic mystery of fast radio bursts (FRBs) has now shed light (pun intended) on another mystery. By studying the signature of an FRB from a nearby galaxy, astronomers have found that the Milky Way has far less matter than expected.
FRBs are exactly as the name suggests – they’re bursts of radio signals from deep space that last mere milliseconds. Some are one-hit wonders while others repeat either randomly or periodically. Exactly what causes them remains unexplained, but with hundreds recorded since the first detection in 2007 we’re getting closer to an answer with each event.
For the new study, astronomers examined an FRB first detected in March 2022, by the Deep Synoptic Array (DSA) in California. It was traced back to a galaxy about 163 million light-years away, and knowing that distance and the direction it came from, the team was able to measure just how much the signal had dispersed before it reached the observatory. That in turn allows astronomers to work out how much matter the radio signals have passed through on their journey.
In doing so, the researchers were able to calculate how much matter there is in the Milky Way’s circumgalactic medium (CGM) – the halo of dust and gas that surrounds our entire galaxy. The team describes it as like shining a light through fog to figure out how thick it is.
From this, the scientists calculated that the mass of the Milky Way’s CGM is less than 100 billion Suns – far lighter than expected. In fact, when combined with all the regular matter in the rest of the galaxy, the Milky Way’s total mass weighs in under 60% of the average for galaxies.
So where is all this matter? The team says that this new evidence backs up previous hypotheses that matter is often flung out of galaxies, thanks to a range of processes, like stellar winds, supernovae, and supermassive black holes.
“These results strongly support scenarios predicted by galaxy-formation simulations where feedback processes expel matter from the halos of galaxies,” said Vikram Ravi, lead author of the study. “This is fundamental to galaxy formation, whereby matter is funneled in and blown out of galaxies in cycles.”
With the DSA set to become even more powerful, new insights are inevitable. Currently only 63 out of an eventual 110 dishes are in operation, and longer term, a follow-up observatory will be made up of 2,000 dishes. This will enable far more FRBs to be detected, giving us a better shot at an answer to the mystery as well as other discoveries like this new revelation about our Milky Way.
The research has been submitted for publication in The Astrophysical Journal.