Supermassive black holes may stunt galaxy growth
Researchers have found that the strong cosmic winds emerging from supermassive black holes at the centre of dwarf galaxies impact the expansion and evolution of these galaxies by suppressing star formation -- an advance that sheds more light on how galaxies grow and evolve.
The researchers, including those from the University of California (UC) Riverside in the US, used data from the Sloan Digital Sky Survey that maps more than 35 per cent of the sky to find 50 dwarf galaxies of which 29 showed signs of having black holes in their centres.
The study, published in The Astrophysical Journal, noted that six of the 29 galaxies showed evidence of cosmic winds -- specifically, high-velocity flow of ionized gas -- emerging from their active black hole centres.
In a first, the astronomers measured specific properties of these winds, such as their kinematics, distribution, and power source, the study noted.
"We found some evidence that these winds may be changing the rate at which the galaxies are able to form stars," said Gabriela Canalizo, lead author of the study from UC Riverside.
Canalizo added that supermassive blackholes may even have a profound influence, "if not more dramatic," on the way large galaxies grow and age, since the larger ones formed from the merging of dwarf galaxies.
"Dwarf galaxies are small because after they formed, they somehow avoided merging with other galaxies. Thus, they serve as fossils by revealing what the environment of the early universe was like," said co-author Christina Manzano-King from UC Riverside.
According to Manzano-King, this was the first time the cosmic winds were seen in galaxies of such small size.
"Dwarf galaxies are the smallest galaxies in which we are directly seeing winds -- gas flows up to 1,000 kilometers per second -- for the first time," she added.
As material is sucked into a black hole, it heats up due to friction and strong gravitational fields, and releases radiative energy, the study noted.
This energy, according to the researchers, pushed the cosmic winds outward from the centre of the galaxy into intergalactic space.
"What's interesting is that these winds are being pushed out by active black holes in the six dwarf galaxies, rather than by stellar processes such as supernovae," Manzano-King said.
Typically, as this wind emerges from a black hole, it is pushed out and the gas ahead of the wind is compressed, bringing the gas together and increasing the rate of star formation, the study noted.
However, the researchers said that if all the wind gets expelled outwards from the galaxy's centre, the gas becomes less available for coalescing into a star.
The research team believes that the latter phenomenon may be occurring in the six dwarf galaxies it identified.
"In these six cases, the wind has a negative impact on star formation," co-author Laura Sales from UC Riverside said.
The researchers mentioned that theoretical models used by scientists to explain the formation and evolution of galaxies did not include the impact of black holes in dwarf galaxies.
"We are seeing evidence, however, of suppression of star formation in these galaxies. Our findings show that galaxy formation models must include black holes as important, if not dominant, regulators of star formation in dwarf galaxies," Sales said.
