Apr 18 2018
Although relatively young galaxies have been shining with new, bright stars formed at a fast rate, the formation of stars ultimately ceases when a galaxy evolves. An innovative research has demonstrated that the mass of the black hole at the galaxy’s center governs the timeframe for this “quenching” of star formation. The study has been published in the Nature journal on January 1, 2018.
Each huge galaxy has a supermassive black hole positioned at the center—where the mass of the black hole is million times greater than that of the sun—disclosing its presence with its gravitational impacts on the stars in the galaxy and at times powering the energetic radiation from an active galactic nucleus (AGN). It is considered that the energy transferred into a galaxy from an AGN arrests the formation of stars by heating and dissipating the gas that would otherwise condense into stars upon being cooled.
Researchers have believed this concept for many years, and astrophysicists have discovered that simulations of evolution of galaxies ought to include feedback from the black hole to replicate the observed characteristics of galaxies. However, to date, there has been no observational proof of a relation between supermassive black holes and star formation.
We’ve been dialing in the feedback to make the simulations work out, without really knowing how it happens. This is the first direct observational evidence where we can see the effect of the black hole on the star formation history of the galaxy.
Jean Brodie, Professor of Astronomy and Astrophysics, UC Santa Cruz
The new findings disclose a constant interplay between star formation and black hole activity throughout the life time of a galaxy, thereby having an impact of each generation of stars being formed during the evolution of the galaxy.
Headed by Ignacio Martín-Navarro, first author of the study who is a postdoctoral researcher at UC Santa Cruz, the research concentrated on massive galaxies for which the central black hole mass had been evaluated in earlier researches by investigating the movement of stars close to the center of the galaxies. In order to ascertain the star formation background of the galaxies, Martín-Navarro investigated the detailed spectra of their light captured by the Hobby-Eberly Telescope Massive Galaxy Survey.
Spectroscopy allows astronomers to isolate and evaluate the distinct wavelengths of light emitted by an object. Martín-Navarro adopted computational methods to investigate the spectrum of every galaxy and retrieve its star formation background by zeroing in on the ideal combination of stellar populations to match the spectroscopic data. “It tells you how much light is coming from stellar populations of different ages,” stated Martín-Navarro.
Upon comparing the star formation backgrounds of galaxies that had black holes of distinct masses, he discovered obvious differences. These differences were analogous only with the mass of the black hole but not with galactic size, morphology, or other characteristics.
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. So star formation lasted longer in those galaxies with smaller central black holes.
Ignacio Martín-Navarro, Postdoctoral Researcher, UC Santa Cruz
Other scientists have tried to find out correlations between the luminosity of AGNs and star formation, but in vain. Martín-Navarro stated that this may be due to the fact that the time scales are very distinct, where star formation takes place over hundreds of millions of years, whereas eruptions from AGNs take place over comparatively short time periods.
A supermassive black hole is luminous only if it is vigorously devouring matter from the inner regions of its host galaxy. AGNs are largely variable and their characteristics are reliant on the black hole’s size, the rate of accretion of new material falling onto the black hole, and several 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.
Ignacio Martín-Navarro, Postdoctoral Researcher, UC Santa Cruz
Aaron Romanowsky, an astronomer at San Jose State University and UC Observatories, and coauthor of the study, stated that the accurate characteristic of the feedback from the black hole that arrests the formation of stars is still undetermined.
“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,” stated Romanowsky.