Galaxies are born, they evolve, and they age. They go from blue star-forming spiral galaxies to “red and dead” elliptical ones. For a long time astronomers have speculated that supermassive black holes (SMBHs) stopped older galaxies from making new stars, and thanks to state-of-the-art observations, we might now know how they do it.
According to an international team of researchers, SMBHs generate an hourglass shaped outflow (wind) that aligns itself with the rest of the galactic gas, heating it up. For stars to form, the gas needs to be cool enough for it to condense and then collapse. SMBHs make sure that is not the case and quench the star formation.
“We knew that there had to be a way to prevent star formation in these galaxies, and now we have a good idea of what it is,” said Edmond Cheung, the lead author of the research, in a statement.
Galaxies evolve from a spiral to elliptical shape by colliding with each other. These galaxy mergers change the shape of the galaxy and usually start a period of intense star formation, followed by the SMBH becoming active. Most astronomers envisioned SMBHs producing galaxy-wide winds to snuff out new stars, but the exact mechanism was not clear.
“You can think of these winds as super-heating the atmospheres of galaxies,” explains Cheung. “As soon as any gas starts to cool, it gets blasted by this wind, like water droplets turning to steam.”
The study, published in Nature, looked at a specific pair of galaxies, nicknamed Akira and Tetsuo, observed in the MaNGA survey. The observation was incredibly detailed, looking at both the outflow from the black hole and how gas moves inside Akira, the larger of the two galaxies.
Akira, which has a mass of 60 billion suns, is part of a new class of galaxies called “red geysers” producing a few quenching episodes every billion years. The fuel for this process comes from cannibalizing smaller galaxies, which is what is happening to Tetsuo.
The researchers think about five percent of galaxies are red geysers, and while they might not represent all the old, red, quiet galaxies in the universe, they represent a step towards this peculiar phase of galaxy evolution.