Astronomers have found a relationship between how stars
form in a galaxy and the mass of its central 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
this is the case. A paper published today in Nature changes that.
“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.
“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
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.