Astronomers have found a relationship between how stars form in a galaxy and the mass of its central supermassive black hole.
Initially, young galaxies form stars rapidly, but as they age the pace of star formation slackens in a process known as “quenching”. For decades, researchers have theorised that the onset of quenching is related to the mass of the supermassive black hole, equivalent to a million suns or more, lurking at the centre of a galaxy. In fact, to accurately simulate the properties of galaxies in their models, astronomers have to build in feedback from these supermassive black holes.
When a supermassive black hole consumes matter such as nearby gas and stars, an accretion disc is formed – a halo of material falling inwards. As matter falls towards the black hole, it heats up and the disc radiates massive amounts of energy. Researchers think that this energy warms up gas far away from the disc, preventing it from cooling and condensing into stars. However, until now, nobody has found any observational evidence that the energy radiated by this disc and ultimately powered by the black hole affects quenching. A paper published today in Nature changes that.
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“We’ve been dialing in the feedback to make the simulations work out, without really knowing how it happens,” said Jean Brodie, a co-author of the study and a professor of astronomy and astrophysics at 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.”
The researchers, led by UC Santa Cruz postdoc Ignacio Martín-Navarro, referenced a large sample of black hole masses and compared them with observations of galactic light spectra made by the Hobby-Eberly Telescope Massive Galaxy Survey. After discarding noisy spectra, or those which could possibly have interference from other sources, they were left with 57 galaxies to analyse.
Spectra can tell researchers what elements are present in a galaxy and thus reveal their star formation history. Just as archaeologists and historians can work out how people might have lived and died based on objects and writing they left behind, astronomers can work out how stars might have lived and died by looking at the light shining from a galaxy. In this study, the team used computer models to estimate the stellar populations (how many old and young stars there might be) from the data.
The team found that star formation history only correlated with black hole mass and not with other factors, such as the size and shape of the galaxy. In particular, galaxies with larger central black holes tended to have older stellar populations, indicating that the quenching of star formation began sooner.
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“For galaxies with the same mass of stars but different black hole mass in the centre, those galaxies with bigger black holes were quenched earlier and faster than those with smaller black holes. So star formation lasted longer in those galaxies with smaller central black holes,” Martín-Navarro explained.
However, the mechanisms behind quenching are still not completely understood. “There are different ways a black hole can put energy out into the galaxy, and theorists have all kinds of ideas about how quenching happens, but there’s more work to be done to fit these new observations into the models,” said Aaron Romanowsky, another co-author and a researcher at San Jose State University. Beyond around 28 trillion light years, we know that galaxies stopped forming new generations of stars, but we can’t obtain good spectra for them.
As Romanowsky says, there is far more that needs to happen before we understand how galaxies stop forming stars. Regardless, there is now direct evidence that supermassive black holes have some kind of correlation with star formation in galaxies.
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