Researchers detect numerous supermassive black holes inside of dying galaxies in our local universe

Supermassive black holes may have the power to “quench” star formation in their galaxies, according to a new study that detected numerous supermassive black holes from early on in our local universe’s life.

An international team of astronomers combined data from some of the most powerful telescopes on Earth to piece together an idea of how galaxies with little-to-no star formation might be related to black holes, looking at radio emissions from galaxies more than 10 billion light years away from us for the very first time.

The study, published in the Astrophysical Journal in April, compared data from stagnant galaxies versus star-forming galaxies, and found that among older galaxies, those producing no stars were far more likely to have a supermassive black hole lurking at their centre.

Within the known universe, there are countless galaxies in varying stages of growth.

Many massive elliptical galaxies in our local universe — which refers to a region around our solar system about 1 billion light years in radius — are relatively stagnant, seeing little star formation.

Modelling in previous research has suggested that these galaxies had an intense period of star formation at the beginning of their lives, followed by an abrupt slowing.

But why are some galaxies still producing stars while others are stagnant?

“It is not well understood what physical processes are responsible for the rapid quenching and suppressing subsequent star formation activities,” the study stated.

One theory for the suppression of star formation is that radiation pouring from an active galactic nucleus could be interfering with the formation of new stars in that particular galaxy.

An active galactic nucleus (AGN) is a small, central point of a galaxy that has a significantly higher luminosity and radiation than anything else in the galaxy, sometimes so bright it outshines the entire galaxy completely.

AGNs are believed to surround a supermassive black hole, with the radiation being the byproduct of that black hole devouring material that gets too close.

However, while it’s been theorized before that black holes could be related to the slowing of star formation in galaxies, many questions remain.

In this study, researchers wanted to dive into this theory by looking at a wider spectrum of galaxies at once, including ones that were dimmer and farther away, which may not have been included in research before.

Since it takes a long time for the light from distant galaxies to reach us, what we see when we look in the sky is a snapshot of what those galaxies used to look like millions, or even billions, of years ago.

The older and farther away a galaxy is, the harder it is to study.

Because the signals from individual galaxies were too faint to identify, in this new study researchers stacked X-ray and radio images of galaxies on top of each other to help filter out the noise and get a better idea of the average signal from these galaxies.

Researchers selected the galaxies to look at using the latest data from the Cosmic Evolution Survey (COSMOS), an astronomical survey that focuses on a specific field of the visible universe, covering more than two million galaxies, in order to study the formation and evolution of galaxies.

The team, led by researchers in Japan, looked specifically at galaxies 9.5 - 12.5 billion light years away.

What researchers found was that the X-ray luminosity of stagnant galaxies at least 12 billion light years away from us was higher than that of star-forming galaxies at the same mass and distance. But the X-ray signals and radio emissions of these stagnant galaxies couldn’t be explained by their number of stars, indicating that this high luminosity was coming from an AGN, and thus a supermassive black hole.

Stagnant galaxies were also more common in galaxy clusters or other denser parts of the universe than in the general field of space.

The study also found that the trend of stagnant galaxies having a brighter luminosity than star-forming galaxies was weaker among galaxies that were younger and closer to Earth, compared to those that were much farther away.

The researchers theorized that this could be because other factors are impacting the star formation in these closer galaxies instead of it being driven largely by the presence of a supermassive black hole.

“Our work hints at the evolving role of AGN feedback for galaxy quenching toward higher redshift, and future observations of [stagnant galaxies] may shed further light on the detailed physics,” the study stated in its conclusions. Redshift is a term in astronomy referring to how far away an object in space is, with a higher redshift meaning it is farther away from us.

Researchers noted that there were still aspects of the data that could not be explained purely by AGNs, and that more research is needed into how and why black holes may be connected to the suppression of star formation.