The missing matter of galaxies

Un team di ricerca italiano ha scovato la quantità di materia barionica mancante nelle galassie che non eravamo riusciti rilevare a fin ora
La galassia a spirale M99, ripresa dal telescopio spaziale Hubble

In most galaxies, a huge percentage of matter appears to be missing. Today, researchers may have found the missing matter of all galaxies, but the discovery contradicts accepted models of their formation.

Observations over the past decade have shown that galaxies the size of the Milky Way seem to have much less matter than they should. This expectation is based on the ratio of dark matter to normal (baryonic) matter in the universe. This ratio suggests that galactic disks contain only 20% of the baryonic matter they should possess.

Baryonic matter in galaxies

While studies in the following years found about half of the missing matter in clouds surrounding galaxies called cold circumgalactic medium, the other half remained well hidden. Fabrizio Nicastro of the National Institute of Astrophysics in Italy and his colleagues used data from three galaxies detected by the XMM-Newton space telescope and the Chandra X-ray Observatory to search for the rest of the missing matter. The research results (ref.) are published on Arvix.

These powerful observers were unable to detect baryonic matter in any of the galaxies. But when the researchers overlaid the observations of the three galaxies, they were able to measure the overall light rather than one at a time.

Among the gaseous signals in the cold circumgalactic medium, they found warmer gas with a mass 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. This evidence reveals most of the baryons in these cosmic structures” says Nicastro.

An extraordinary discovery

There’s only one problem. The matter shouldn’t be thought of as missing. Simulations on young galaxies show that the powerful winds created by energetic processes like star formation should have actually thrown much of the baryonic matter away from them.

But this means that the missing gas should have already been far away and not around the circumgalactic medium. “We thought these baryons should be missing because they were expelled from the galaxy” says Vikram Ravi of the California Institute of Technology. “From this point of view, it’s an extraordinary result if the data are confirmed”. If the missing matter is really there, we’ll have to rethink how galaxies work on a large scale.

“When we look at the halo of the Milky Way, we actually find a good amount of structure and clumps” says Ravi. “I think it will take a comparison with very detailed simulations and the Milky Way itself to interpret these data more deeply”. Comparing other galaxies with the Milky Way will be particularly crucial in the coming years, especially since we are unable to measure the circumgalactic medium 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 to really study the population of these components [of the circumgalactic medium], new instrumentation is needed” says Nicastro. It may take a decade or more before those new telescopes are ready to truly uncover the mystery of this missing matter.

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