Supermassive black holes, with masses more than a million times that of the Sun, reside at the centers of all massive galaxies. A new study published in the journal Nature shows that the star formation history of a massive galaxy depends on the mass of its central black hole.
Supermassive black holes at the cores of galaxies blast radiation and ultra-fast winds outward, as illustrated in this artist’s conception. Image credit: NASA / JPL-Caltech.
Every massive galaxy has a central supermassive black hole, revealing its presence through its gravitational effects on the galaxy’s stars and sometimes powering the energetic radiation from an active galactic nucleus (AGN).
The energy pouring into a galaxy from an AGN is thought to turn off star formation by heating and dispelling the gas that would otherwise condense into stars as it cooled.
This idea has been around for decades, and scientists have found that simulations of galaxy evolution must incorporate feedback from the black hole in order to reproduce the observed properties of galaxies.
But observational evidence of a connection between supermassive black holes and star formation has been lacking, until now.
The new study, led by University of California Observatories astronomer Ignacio Martín-Navarro, focused on massive galaxies for which the mass of the central black hole had been measured in previous studies by analyzing the motions of stars near the center of the galaxy.
To determine the star formation histories of the galaxies, the researchers analyzed detailed spectra of their light obtained by the Hobby-Eberly Telescope Massive Galaxy Survey.
They used computational techniques to analyze the spectrum of each galaxy and recover its star formation history by finding the best combination of stellar populations to fit the spectroscopic data.
“It tells you how much light is coming from stellar populations of different ages,” Dr. Martín-Navarro noted.
When the team compared the star formation histories of galaxies with black holes of different masses, they found striking differences. These differences only correlated with black hole mass and not with galactic morphology, size, or other properties.
“For galaxies with the same mass of stars but different black hole mass in the center, those galaxies with bigger black holes were quenched earlier and faster than those with smaller black holes,” Dr. Martín-Navarro said.
“So star formation lasted longer in those galaxies with smaller central black holes.”
A supermassive black hole is only luminous when it is actively gobbling up matter from its host galaxy’s inner regions.
AGNs are highly variable and their properties depend on the size of the black hole, the rate of accretion of new material falling onto the black hole, and other factors.
“We used black hole mass as a proxy for the energy put into the galaxy by the AGN, because accretion onto more massive black holes leads to more energetic feedback from active galactic nuclei, which would quench star formation faster,” Dr. Martín-Navarro explained.
“The precise nature of the feedback from the black hole that quenches star formation remains uncertain,” said Dr. Aaron Romanowsky, an astronomer at San Jose State University and the University of California Observatories.
“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.”
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Ignacio Martín-Navarro et al. Black-hole-regulated star formation in massive galaxies. Nature, published online January 1, 2018; doi: 10.1038/nature24999
