Scientists have known for decades that the evolution of galaxies is linked to the growth and changes of supermassive black holes at their center. While this connection is particularly important in explaining how star formation in galaxies is regulated, only now have researchers found direct observational evidence that this is the case.
According to a new Nature paper, the quenching of star formation takes place earlier and more efficiently in galaxies with a bigger black hole. When the team of astronomers looked at how this feedback between black holes and star formation changed throughout the life of a galaxy, they found a continuous relationship between the two.
"For galaxies with the same mass of stars but different black hole mass in the center, those galaxies with bigger black holes were quenched earlier and faster than those with smaller black holes," lead author Ignacio Martín-Navarro of UC Santa Cruz said in a statement. "So star formation lasted longer in those galaxies with smaller central black holes."
Supermassive black holes can, when they feed, emit a large amount of energetic radiation. All this energy ends up heating the interstellar gas, which can get so hot that it can’t condense into clouds to form stars. The emission from the black hole can also push gas out of the galaxy. These observations help astronomers figure out the details of these processes to construct more realistic simulations.
"We've been dialing in the feedback to make the simulations work out, without really knowing how it happens," added co-author Professor Jean Brodie, also from UC Santa Cruz. "This is the first direct observational evidence where we can see the effect of the black hole on the star formation history of the galaxy."
To do this, the team used the light spectrum of the galaxies – the contributions of photons of different wavelengths – to work out these histories. Stars of different ages will emit different light and this can be used to reconstruct how a galaxy has changed over billions of years. The team showed that the changes in these histories were only correlated to the black hole mass and not to other properties such as the type of galaxy or its size.
However, more work is needed. Astronomers will have to include these observations in simulations of galaxy evolution, in addition to more observations. There are many details in the quenching feedback mechanism that are still unclear.