A simple model explains a wide range of observations by describing a competition between galaxy halos and their central black holes that eventually deactivates star formation.
Astronomers studying the evolution of galaxies have long struggled to understand what causes star formation to shut down in massive galaxies. Although many theories have been put forward to explain this process, known as “cooling,” there is still no consensus on a satisfactory model.
Now, an international team led by Sandra Faber, professor emeritus of astronomy and astrophysics at UC Santa Cruz, has proposed a new model that successfully explains a wide range of observations about the structure of the galaxy, supermassive black holes, and cooling. of star formation. The researchers presented their findings in an article published on July 1, 2020, in the Astrophysical magazine.
The model supports one of the main ideas about cooling that it attributes dungeon “Feedback,” the energy released in and around a galaxy from a central supermassive black hole as matter falls into the black hole and fuels its growth. This energetic feedback heats, ejects, or disrupts the galaxy’s gas supply, preventing gas from falling out of the galaxy’s halo to fuel star formation.
“The idea is that in star-forming galaxies, the central black hole is like a parasite that eventually grows and kills the host,” Faber explained. “That has been said before, but we have had no clear rules for saying when a black hole is large enough to shut down star formation in its host galaxy, and we now have quantitative rules that really work to explain our observations.”
Size and mass
The basic idea involves the relationship between the mass of stars in a galaxy (stellar mass), how scattered those stars are (the radius of the galaxy) and the mass of the central black hole. For star-forming galaxies with a given stellar mass, the density of the stars in the center of the galaxy correlates with the radius of the galaxy, so that galaxies with larger radii have lower central star densities. Assuming that the mass of the central black hole scales with the central stellar density, star-forming galaxies with larger radii (at a given stellar mass) will have lower black hole masses.
What that means, Faber explained, is that larger galaxies (those with larger radii for a given stellar mass) have to evolve more and build a higher stellar mass before their central black holes can grow large enough to extinguish star formation. Therefore, small radius galaxies fade to lower masses than large radius galaxies.
“That is the new idea, that if galaxies with large radii have smaller black holes in a given stellar mass, and if feedback from black holes is important for cooling, then large radius galaxies have to evolve even further. “, said. “If you put all these assumptions together, surprisingly, you can reproduce a large number of observed trends in the structural properties of galaxies.”
This explains, for example, why more massive dull galaxies have higher central stellar densities, larger radii, and larger central black holes.
Based on this model, the researchers concluded that cooling begins when the total energy emitted by the black hole is approximately four times the gravitational binding energy of the gas in the galactic halo. Bond energy refers to the gravitational force that gas maintains within the halo of dark matter that envelops the galaxy. Cooling is complete when the total energy emitted by the black hole is twenty times the binding energy of the gas in the galactic halo.
Physical processes
Faber emphasized that the model has not yet explained in detail the physical mechanisms involved in cooling star formation. “The key physical processes that this simple theory evokes are not yet understood,” he said. “The virtue of this, however, is that having simple rules for each step in the process challenges theorists to find physical mechanisms that explain each step.”
Astronomers are used to thinking in terms of diagrams that plot relationships between the different properties of galaxies and show how they change over time. These diagrams reveal the dramatic differences in structure between star-forming and fading galaxies and the sharp boundaries between them. Because star formation emits a lot of light at the blue end of the color spectrum, astronomers refer to “blue” star-forming galaxies, “red” quiescent galaxies, and the “green valley” as the transition between them . What stage a galaxy is in is revealed by its star formation rate.
One of the study’s conclusions is that the growth rate of black holes must change as galaxies evolve from one stage to the next. Observational evidence suggests that most of the growth of the black hole occurs in the green valley when galaxies begin to cool.
“The black hole appears to be unleashing just as star formation slows down,” Faber said. “This was a revelation, because it explains why the masses of black holes in star-forming galaxies follow one scale law, while the black holes in temperate galaxies follow another scale law. That makes sense if the mass of the black hole grows rapidly in the green valley. “
Candles
Faber and his collaborators have been discussing these issues for many years. Since 2010, Faber has co-directed an important Hubble space telescope Galaxy Prospecting Program (CANDELS, Cosmic Assembly Deep Infrared Extragalactic Deep Legacy Survey), which produced the data used in this study. Analyzing the CANDELS data, he has worked closely with a team led by Joel Primack, UCSC professor emeritus of physics, who developed the Bolshoi cosmological simulation of the evolution of dark matter halos in which galaxies form. These halos provide the scaffolding upon which the theory builds the early star-forming phase of the galaxy’s evolution before it fades.
The central ideas in the document emerged from CANDELS data analyzes and hit Faber for the first time about four years ago. “It suddenly jumped at me, and I realized that if we put all these things together, if the galaxies had a simple path in radius versus mass, and if the energy of the black hole needs to exceed the halo bond energy, it can explain all these sloped limits on the structural diagrams of galaxies, “he said.
At the time, Faber made frequent trips to China, where he participated in research collaborations and other activities. She was a visiting professor at Shanghai Normal University, where she met first author Zhu Chen. Chen came to UC Santa Cruz in 2017 as a visiting researcher and began working with Faber to develop these ideas about galaxy cooling.
“She is mathematically very good, better than me, and did all the calculations for this job,” said Faber.
Faber also credited former collaborator David Koo, UCSC professor emeritus of astronomy and astrophysics, for first focusing attention on the central densities of galaxies as the key to the growth of central black holes.
Among the puzzles explained by this new model there is a notable difference between our Milky Way galaxy and its very similar neighbor Andromeda. “The Milky Way and Andromeda have almost the same stellar mass, but the Andromeda black hole is almost 50 times larger than the Milky Way,” said Faber. “The idea that black holes grow a lot in the green valley largely explains this mystery. The Milky Way is just entering the green valley and its black hole is still small, while Andromeda has just come out, so its black hole has gotten much bigger and is also duller than the Milky Way. “
Reference: “Quenching as a contest between the Halos Galaxy and its central black holes” by Zhu Chen, SM Faber, David C. Koo, Rachel S. Somerville, Joel R. Primack, Avishai Dekel, Aldo Rodríguez-Puebla, Yicheng Guo, Guillermo Barro, Dale D. Kocevski, A. van der Wel, Joanna Woo, Eric F. Bell, Jerome J. Fang, Henry C. Ferguson, Mauro Giavalisco, Marc Huertas-Company, Fangzhou Jiang, Susan Kassin, Lin Lin, FS Liu, Yifei Luo, Zhijian Luo, Camilla Pacifici, Viraj Pandya, Samir Salim, Chenggang Shu, Sandro Tacchella, Bryan A. Terrazas and Hassen M. Yesuf, July 7, 2020, Astrophysical magazine.
DOI: 10.3847 / 1538-4357 / ab9633
In addition to Faber, Chen, Koo, and Primack, the article’s co-authors include researchers at some two dozen institutions in seven countries. This work was funded by grants from POT and the National Science Foundation.
