Supermassive black holes grow faster than expected

Supermassive black holes in the Chandra Deep Field-South (Image NASA/CXC/Penn. State/G. Yang et al and NASA/CXC/ICE/M. Mezcua et al.; Optical: NASA/STScI; Illustration: NASA/CXC/A. Jubett)
Supermassive black holes in the Chandra Deep Field-South (Image NASA/CXC/Penn. State/G. Yang et al and NASA/CXC/ICE/M. Mezcua et al.; Optical: NASA/STScI; Illustration: NASA/CXC/A. Jubett)

Two articles currently being published in the journal “Monthly Notices of the Royal Astronomical Society” describe two researches on the connection between the development of supermassive black holes and the galaxies that host them. Two separate teams used observations from NASA’s Chandra X-ray Observatory and other telescopes, concluding that supermassive black holes grow faster than new stars form in their host galaxies. This contradicts previous models that suggested a growth proportional to star formation in the galaxies.

One of the two teams was led by Guang Yang of Penn State University and in addition to Chandra’s observations used those from the Hubble Space Telescope and other telescopes. This includes data collected during the Chandra Deep Field-South / North and COSMOS-Legacy surveys. The galaxies considered for this research are very far away, between 4.3 to 12.2 billion light years from Earth.

The analysis of the data collected indicates that the relationship between the growth of a supermassive black hole and star formation in a galaxy is much higher in the most massive galaxies. In galaxies with a total stellar mass of 100 billion solar masses, that ratio is ten times higher than the one in galaxies with an overall star mass of 10 billion solar masses.

According to Niel Brandt, one of the authors of the research and also of Penn State University, it’s possible that the most massive galaxies supply their supermassive black holes of cold gas more efficiently. However, the mechanism isn’t clear so the research must continue.

The other team was led by Mar Mezcua of the Institute of Space Sciences in Spain and studied 72 galaxies in the center of distant clusters of up to 3.5 billion light from Earth, meaning that they’re much closer than the ones studied by their colleagues. In addition to Chandra’s observations, this team used the ones from Australia Telescope Compact Array, Karl G. Jansky Very Large Array (VLA) and Very Long Baseline Array.

Among the galaxies observed there’s the one at the center of the Hercules Cluster, also known as Abell 2152. The galaxy is called Hercules A and is an active galaxy precisely because the supermassive black hole at its center generates strong electromagnetic emissions. The image below includes data of Chandra in purple, of the VLA in blue and of the Hubble space telescope in white.

This team estimated the masses of the supermassive black holes in the galaxies observed through a relationship that links them to the radio and X-ray emissions associated with the black hole. The result is that the estimates are ten times higher than those made by another method which assumes that within a galaxy there’s a growth of a supermassive black hole proportional to star formation.

According to estimates, almost half of the supermassive black holes studied have a mass of at least 10 billion times the Sun’s. If those estimates are correct, they are extreme objects even among black holes, in a category that some astronomers call ultra-massive black holes. Also for this team there are only hypotheses to explain their masses.

The influence that supermassive black holes have on the galaxies that host them is at the center of a lot of research. What are called feedback mechanisms that regulate that influence are not yet clear but it’s certain that they’re important in favoring or inhibiting star formation. For this reason, it’s important to understand these extreme objects’ development mechanisms.

Hercules A (Image X-ray: NASA/CXC/SAO, Optical: NASA/STScI, Radio: NSF/NRAO/VLA))
Hercules A (Image X-ray: NASA/CXC/SAO, Optical: NASA/STScI, Radio: NSF/NRAO/VLA))

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