Science Results

We normally think of black holes as destructive objects, but new research suggests that the supermassive black hole in the central galaxy of a galaxy cluster may actually help other galaxies in the cluster to continue growing.

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Figure 1: An example of a galaxy cluster used in this study. Star-forming galaxies are indicated by blue circles, and "dead" galaxies by orange circles. Other objects in the image are galaxies and stars unrelated to the galaxy cluster. The pink and light blue shaded areas represent the "aligned" and "perpendicular" directions relative to the central galaxy of the cluster. The upper right image is a zoom-in view of the central galaxy. (Credit: University of Tokyo)

Galaxy clusters contain hundreds to thousands of galaxies. But this crowding may be detrimental to the galaxies. Compared to isolated galaxies, clusters include a higher percentage of "dead" galaxies which have ceased star formation. This is possibly due to the presence of intracluster gas in clusters. Intracluster gas can strip the material for new stars out of galaxies, thus preventing new star formation.

Previous studies on nearby galaxies have found that dead galaxies are more likely to be located along the line extrapolated from the longest axis of the central galaxy, and less likely to be located near its shortest axis. But it was unknown when this pattern started to emerge. Knowing when the pattern started to become apparent would give insights into what might be causing it, and how it has affected the development of galaxy clusters over time.

Because of the finite speed of light, it takes time for the light from distant clusters to reach us, sometimes billions of years. By observing that light, we can see an image of what the cluster looked like when the light was first emitted, in a very real sense looking back in time.

A team of astronomers, led by Makoto Ando at the University of Tokyo, investigated the distribution of star-forming galaxies and "dead" galaxies within a cluster, using data for more than 5000 clusters across a span of 7 billion years obtained with the Subaru Telescope. The team found that the pattern of more dead galaxies being found near the longest axis of the central galaxy remained consistent as far back in time as the current data set could see.

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Figure 2: The detected anisotropy of galaxies that have stopped growing (left) and a conceptual illustration of the data (right). This figure is based on measurements of clusters from about 6 billion years ago made by Hyper-Suprime Cam mounted on the Subaru Telescope. The white dots in the left figure show the fraction of galaxies that have stopped growing for each direction measured from the orientation of the central galaxy. The thick black lines are fitted to show the trends in the data. We can see that the fraction of galaxies that have stopped growing in the "direction aligned with the orientation (long axis) of the central galaxy," indicated by the pink shades, is higher than in the "direction perpendicular to the orientation of the central galaxy," indicated by the light blue shades. (Credit: University of Tokyo)

This distribution matches simulations where the activity of the supermassive black hole in the central galaxy blows away the intracluster gas which suppresses the galaxies. The activity of the supermassive black hole is more efficient around the shortest axis of the central galaxy than around the longest axis. So the black hole activity may actually be protecting the other galaxies.

Makoto Ando, the lead researcher, cautions, "We have not detected direct evidences of this mechanism, such as black hole activity or the anisotropic distribution of intracluster gas. These are expected to be detected by X-ray and radio observations in the future."


These results appeared as Ando et al. "Detection of anisotropic satellite quenching in galaxy clusters up to z~1" in Monthly Notices of the Royal Astronomical Society on December 22, 2022.