Replenishment from outside: Black holes at the center of galaxies not only feed on the material of their galactic home – a large part of their “feed” could come from outside, as astronomers have found out. Gas streams from other galaxies, which have been consumed by past collisions and nearby passages, often supply active galactic cores. This provides valuable clues about the mechanisms by which such supermassive black holes grow.
At the heart of all galaxies, including our Milky Way, are supermassive black holes. The enormous gravity of these giants shapes the movements of the galactic gases and stars, while at the same time their activity influences star formation and thus galaxy growth. When active, black holes devour matter, releasing enormous amounts of energy in the form of radiation and particle jets visible over billions of light-years.
But where do such black holes get the nourishment for their own growth? Astronomers have often observed how the giants devour gas and individual stars. But that alone cannot explain the rapid growth of these galaxy cores – especially not in the early cosmos.
Telltale Deviations
An observation by astronomers led by Sandra Raimundo from the University of California in Los Angeles now provides a possible answer. They used the Anglo-Australian Telescope in Australia to study gas and stellar motion in 3,068 galaxies. They looked for cases in which the star disk and the galactic gases do not rotate in the same direction, but rather offset from each other or even in opposite directions.
“Larger deviations of more than 30 degrees are considered a clear indication of a past interaction of the galaxy, for example a galaxy merger, the swallowing of a dwarf galaxy or the accretion of extragalactic gases,” explain Raimundo and her colleagues. The latter can occur when a galaxy sucks gas from nearby neighbors or grabbed it from a past galaxy passage. Our Milky Way also has large extragalactic parts.
Alien gases in young galaxies
The exciting thing about it: Astronomers have long suspected that such alien, non-conformally rotating gases play an important role in the material replenishment of the central black holes. “The presence of aberrantly rotating structures promotes the influx of gas into the galactic center,” the researchers explain. This gives the central black hole more “feed” than would normally be present in the core region of growing galaxies.
In fact, the observational data showed that discrepancies between gas and stellar motion are common, especially in young galaxies: in as many as 25 percent of these elliptical or irregular galaxies, the direction of the gas and the stellar disk were tilted more than 45 degrees towards each other. In many cases, the gases formed streams stretching tens of thousands of light-years outward, sometimes reaching as far as neighboring galaxies.
Extragalactic food increases activity
And not only that: In the galaxies with extragalactic gas components, the central black holes were also significantly more active. 20 percent of all misaligned galaxies had active galactic nuclei, 43 percent showed a LINER signature, the astronomers found. Such LINER galaxies have a particularly large amount of ionized gas in their center and are therefore considered a transition stage between inactive and active galaxy nuclei.
“Our study thus proves that the presence of gas shifted against the stars is linked to a higher proportion of active supermassive black holes,” says Raimundo. Because of galaxies without such incorporated foreign gases, only seven percent had active galaxy nuclei and 15 percent were LINER galaxies. “It is evident that the conditions in the misaligned galaxies are particularly favorable for active galactic nuclei and LINERs,” the team said.
More supplies for the black hole
The astronomers suspect that the alien gas is well suited for “feeding” for two reasons: First, it replenishes the internal gas supplies of the home galaxy, which simply means that more supplies are available for the black hole. On the other hand, such components with deviating motion reduce the angular momentum of the galactic material. The gas is thereby slowed down and falls faster towards the center of the galaxy.
According to the researchers, their results confirm that mergers and extragalactic gas flows play an important role in the “nourishment” of supermassive black holes. “This is the first time that a direct link between the presence of foreign gas and the replenishment of food for active black holes has been demonstrated,” says co-author Marianne Vestergaard from the Niels Bohr Institute in Copenhagen. (Nature Astronomy, 2023; doi: 10.1038/s41550-022-01880-z)
Quelle: Nature, University of Southampton
