Galaxies on a collision course may feed supermassive black holes

New research seeks to reveal the secrets behind these celestial objects.
Loukia Papadopoulos
A black hole.jpg
A black hole.


New research out of Newcastle University published on Thursday reveals that supermassive black holes may be pushed to grow when they find themselves in galaxies that are about to collide with each other.

“Our novel approach looks at hundreds of thousands of distant galaxies with a statistical approach and asks how likely any two galaxies are to be close together and so likely to be on a collision course,” said Sean Dougherty, a postgraduate student at Newcastle University and lead author of the paper.

The researchers produced a new method for determining how likely it is that two galaxies are very close together and are expected to collide in the future, providing the ideal conditions for a black hole to grow. They then applied this technique to hundreds of thousands of galaxies in the distant universe.

Many challenges

The hope is that through this analysis, they may find an insight into the supermassive black hole situated inside the Milky Way and how our galaxy evolved over time. Past attempts to understand this topic have been filled with obstacles.

“These supermassive black holes are very challenging to find because the X-ray light, which astronomers have typically used to find these growing black holes, is blocked, and not detected by our telescopes. But these same black holes can be found using infrared light, which is produced by the hot dust surrounding them,” said in the statement Dr. Chris Harrison, a researcher at Newcastle University’s School of Mathematics, Statistics, and Physics and co-author of the study.

“The difficulty in finding these black holes and in establishing precise distance measurements explains why this result has previously been challenging to pin down these distant ‘cosmic noon’ galaxies. With JWST we are expecting to find many more of these hidden growing black holes. JWST will be far better at finding them, therefore we will have many more to study, including ones that are the most difficult to find. From there, we can do more to understand the dust that surrounds them, and find out how many are hidden in distant galaxies.”

Now, the researchers are waiting on more data arriving from the James Webb Space Telescope they hope will reveal even more secrets about how these black holes emerge and grow.

The study is published in the journal Monthly Notices of the Royal Astronomical Society.

Study abstract:

Observations of the nearby universe reveal an increasing fraction of active galactic nuclei (AGN) with decreasing projected separation for close galaxy pairs, relative to control galaxies. This implies galaxy interactions play a role in enhancing AGN activity. However, the picture at higher redshift is less established, partly due to limited spectroscopic redshifts. We combine spectroscopic surveys with photometric redshift probability distribution functions for galaxies in the CANDELS and COSMOS surveys, to produce the largest ever sample of galaxy pairs used in an AGN fraction calculation for cosmic noon (0.5 < z < 3). We present a new technique for assessing galaxy pair probability (based on line-of-sight velocities ±1000 km s−1) from photometric redshift posterior convolutions and use these to produce weighted AGN fractions. Over projected separations 5–100 kpc we find no evidence for enhancement, relative to isolated control galaxies, of X-ray (LX > 1042 erg s−1) or infrared-selected AGN in major (mass ratios up to 4:1) or minor (4:1 to 10:1) galaxy pairs. However, defining the most obscured AGN as those detected in the infrared but not in X-rays, we observe a trend of increasing obscured AGN enhancement at decreasing separations. The peak enhancement, relative to isolated controls, is a factor of 2.08 ± 0.61 for separations <25 kpc. Our simulations with mock data, indicates this could be a lower limit of the true enhancement. If confirmed with improved infrared imaging (e.g., with JWST) and redshifts (e.g., with forthcoming multi-object spectrograph surveys), this would suggest that galaxy interactions play a role in enhancing the most obscured black hole growth at cosmic noon.