Supermassive Black Holes 'Spawn' Dwarf Galaxies --"Outnumber Larger Galaxies Like Our Milky Way 50 to One"
"Dwarf galaxies outnumber larger galaxies like the Milky Way 50 to one," says lead researcher Dr Samantha Penny, of the University of Portsmouth Institute of Cosmology and Gravitation. "So if we want to tell the full story of galaxies, we need to understand how dwarf systems work."
The results, presented today at a meeting of the American Astronomical Society, are particularly important because dwarf galaxies (those composed of up to 100 million to several billion stars) are far more numerous than bigger systems and what happens in these is likely to give a more typical picture of the evolution of galaxies.
In any galaxy stars are born when clouds of gas collapse under the force of their own gravity. But stars don't keep being born forever - at some point star formation in a galaxy shuts off. The reason for this differs in different galaxies but sometimes a supermassive black hole is the culprit.
Supermassive black holes can regulate their host galaxy's ability to form new stars through a heating process. The black hole drives energy through powerful winds. When this wind hits the giant molecular clouds in which stars would form, it heats the gas, preventing its collapse into new stars.
Size comparison of a dwarf galaxy (right inset, bottom) with a larger galaxy in the center. Top inset: Dwarf galaxy overlain with some of the MaNGA data, revealing the winds from the supermassive black hole.
Size comparison of a dwarf galaxy (right inset, bottom) with a larger galaxy in the centre. Top inset: Dwarf galaxy overlain with some of the MaNGA data, revealing the winds from the supermassive black hole.
Previous research has shown that this process can prevent star formation in larger galaxies containing hundreds of billions of stars - but it was believed a different process could be responsible for dwarf galaxies ceasing to produce stars. Scientists previously thought that the larger galaxies could have been interacting gravitationally with the dwarf systems and pulling the star-making gas away.
Data, however, showed the researchers that the dwarf galaxies under observation were still accumulating gas which should re-start star formation in a red, dead galaxy but wasn't. This led the team to the supermassive black hole discovery.
Dr Penny said: "Our results are important for astronomy because they potentially impact how we understand galaxy evolution. Supermassive black holes weren't thought to influence dwarf systems but we've shown that isn't the case. This may well have a big influence on future research as simulations of galaxy formation don't usually include the heating effect of supermassive black holes in low-mass galaxies, including the dwarf systems we have examined in this work."
The team of international scientists used data from the Sloan Digital Sky Survey (SDSS), which has a telescope based in New Mexico, to make their observations. Using SDSS's Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey, they were able to map the processes acting on the dwarf galaxies through the star systems' heated gas, which could be detected. The heated gas revealed the presence of a central supermassive black hole, or active galactic nucleus (AGN), and through MaNGA the team were able to observe the effect that the AGN had on their host dwarf galaxies.
The NASA/ESA Hubble Space Telescope image at the top of the page shows NGC 5477, is a dwarf galaxies in the Messier 101 group. Without obvious structure, but with visible signs of ongoing starbirth, NGC 5477 looks much like an archetypal dwarf irregular galaxy.
The Daily Galaxy via University of Portsmouth