Galaxies’ missing matter may be found – but now there’s too much of it

The spiral galaxy M99, imaged by the Hubble Space Telescope ESA/Hubble & NASA, M. Kasliwal

Most galaxies seemed to be missing a huge proportion of the matter we expected them to have – now researchers may have found its hiding spot, but the discovery contradicts accepted models of galaxy formation.

Many large galaxies were long thought to be missing a huge proportion of matter – but it has now been found, and that is an even bigger problem for our understanding of the universe.

Observations over the course of the last decade or so have shown that galaxies about the size of the Milky Way appear to have far less baryonic matter – meaning normal matter, not dark matter – than we expect. This expectation is based on the ratio of dark matter to regular matter in the universe, which suggests that galactic discs only had about 20 per cent of the baryonic matter that they should. While studies in following years found about half of that missing matter in clouds surrounding the galaxies called the cool circumgalactic medium, the other half remained elusive.

Now, Fabrizio Nicastro at the National Institute for Astrophysics in Italy and his colleagues have used data on three galaxies from the XMM-Newton space telescope and the Chandra X-ray Observatory to find the rest of their missing matter – and there may actually be way too much of it.

Even those powerful observatories weren’t able to spot the matter in any of the galaxies individually; instead, the researchers had to “stack” the observations of the three galaxies together so they could measure the light of all three cumulatively instead of one at a time.

They found signs that mingling with the cold gas of the circumgalactic medium was hotter gas – a mass of hotter gas equal to hundreds of billions of times the mass of the sun. “The mass we estimate is sufficient to explain the missing mass in galaxies,” says Nicastro. “[This evidence] unveils the majority of baryons in galaxies.”

There is just one problem: the matter shouldn’t be thought of as missing at all. Simulations of young galaxies show that powerful winds created by energetic processes like star formation and supernovae really should have hurled much of the baryonic matter out of these galaxies when they were young. So, they were born with the expected amount of matter, and then it was ejected.

But that means the “missing” matter should be long gone – not hanging out in the circumgalactic medium. “We [thought] these baryons really should be missing, they should have been blown out of the galaxy,” says Vikram Ravi at the California Institute of Technology. “From that perspective, it’s an amazing result if it holds up.” If the missing matter really is there, we will have to rethink how galaxies work on a huge scale.

But before we do that, these results will have to be confirmed. One potential issue is that the data processing and interpretation process tends to smooth things out, so a more precise model of how exactly this matter is distributed could change the amount we think is there. “When we look at the Milky Way’s halo, we actually find a good amount of structure and clumpiness,” says Ravi. “I think that it will take comparison with really detailed simulations and with the Milky Way itself to be able to interpret this data more thoroughly.”

Comparing other galaxies to the Milky Way will be particularly crucial in the coming years, because we just don’t have the ability to measure the circumgalactic media of galaxies beyond our own in detail. “One or two confirmations can still be provided by extremely long observations with the current X-ray satellites (XMM-Newton and Chandra), but in order to really do population studies of these components [of the circumgalactic medium], new instrumentation is needed,” says Nicastro. It could take a decade or longer before those new telescopes are ready to really unravel the mystery of the not-so-missing matter.


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