The gas that surrounds galaxies appears to be more spread out than previously thought – and this could help solve a mystery over missing matter.
Supermassive black holes appear to be more powerful than our theories suggest, blasting the gas that surrounds galaxies much further away than expected. This finding could help explain several cosmological conundrums, including the fact that the universe seems to be missing large amounts of gas.
 |
| Galaxies like Andromeda have a supermassive black hole at the centre NASA/JPL-Caltech |
Most objects that we can measure in the universe, like stars and galaxies, emit light that we can observe from Earth. But measuring cold or spread-out gas, which is crucial for our understanding of how the universe works, is trickier because it often doesn’t give off as much light.
A new method that astronomers have developed to map this gas makes use of the cosmic microwave background (CMB), which is the leftover light from the big bang. This light has been travelling through the universe for billions of years, and any object it has encountered along the way, such as galaxies and their gas, leaves a detectable imprint on it that can be measured. “It’s as if the galaxies are casting shadows on the CMB map,” says Boryana Hadzhiyska at the University of California, Berkeley. But because these shadows are very faint, you need many millions of galaxies to get a clear signal, she says.
Now, Hadzhiyska and her colleagues have measured these shadows in the highest detail yet and found that the gas causing them appears to be much more spread out than we find in our best simulations of the universe. “It’s finding something that we don’t really expect, which is that the gas is way more extended than, for example, galaxy formation models predict,” says Hadzhiyska.
To measure these shadows, she and her team combined one of the most detailed maps of the CMB, made using the Atacama Cosmology Telescope (ACT) in Chile, with a vast catalogue of relatively nearby galaxies mapped out by the Dark Energy Spectroscopic Instrument (DESI) in Arizona. By measuring how distorted the CMB map was in relation to the galaxies, they could find the average density of space, which told them where the galaxies’ gas was distributed.
While typical simulations of the universe assume that the gas only extends 1 or 2 per cent outside of a galaxy’s outer limit, Hadzhiyska and her team found that it could stretch out as far as two to three times the galaxy’s radius. This is probably due to phenomena associated with supermassive black holes, such as their jets, being more powerful than we thought, though we don’t know the exact mechanisms at play, says Hadzhiyska.
This finding confirms earlier work that found hints of the spread-out gas using the same technique, says Leah Bigwood at the University of Cambridge, but the increased sensitivity boosts astronomers’ confidence that it is real. “It’s providing this evidence that maybe we’re underestimating at the moment how much these supermassive black holes can push matter out, and if we are underestimating that it can mean it would be biasing our cosmology.”
One problem this could help solve is the so-called missing baryon problem. The CMB can be used to estimate how much gas there is in today’s universe, but much of this inferred gas can’t be observed with light-based telescopes. This missing gas might just be spread out beyond where we can see it, says team member Simone Ferraro, who is also at the University of California, Berkeley. “None of the gas has disappeared, it’s just been pushed out way beyond where people could see before. Now we have these new techniques, and this new data, we’re able to see that far out,” he says.
It could also help to resolve a disagreement between two ways of measuring how clumpy the universe is. The first uses the CMB and the standard model of cosmology to work out how spread out the universe is now, which gives a higher result than another technique measuring how much the gravity of galaxies bends light. However, if the CMB calculation is rerun using the new gas distribution data, then the tension between the two measurements appears to go away, says Hadzhiyska. However, a more detailed analysis will be needed to prove it, she says.
Reference: