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How Galaxies Die: New Insights Into Galaxy Halos, Black Holes, and Quenching of Star Formation

A easy mannequin explains a variety of observations by describing a contest between galaxy halos and their central black holes that ultimately turns off star formation.

Astronomers finding out galaxy evolution have lengthy struggled to know what causes star formation to close down in huge galaxies. Though many theories have been proposed to clarify this course of, often known as “quenching,” there’s nonetheless no consensus on a passable mannequin.

Now, a global workforce led by Sandra Faber, professor emerita of astronomy and astrophysics at UC Santa Cruz, has proposed a brand new mannequin that efficiently explains a variety of observations about galaxy construction, supermassive black holes, and the quenching of star formation. The researchers offered their findings in a paper revealed on July 1, 2020, within the Astrophysical Journal.

The mannequin helps one of the main concepts about quenching which attributes it to black gap “feedback,” the power launched right into a galaxy and its environment from a central supermassive black gap as matter falls into the black gap and feeds its progress. This energetic suggestions heats, ejects, or in any other case disrupts the galaxy’s fuel provide, stopping the infall of fuel from the galaxy’s halo to feed star formation.

“The idea is that in star-forming galaxies, the central black hole is like a parasite that ultimately grows and kills the host,” Faber defined. “That’s been said before, but we haven’t had clear rules to say when a black hole is big enough to shut down star formation in its host galaxy, and now we have quantitative rules that actually work to explain our observations.”

Dimension and mass

The essential thought entails the connection between the mass of the celebrities in a galaxy (stellar mass), how unfold out these stars are (the galaxy’s radius), and the mass of the central black gap. For star-forming galaxies with a given stellar mass, the density of stars within the heart of the galaxy correlates with the radius of the galaxy in order that galaxies with larger radii have decrease central stellar densities. Assuming that the mass of the central black gap scales with the central stellar density, star-forming galaxies with bigger radii (at a given stellar mass) may have decrease black-hole plenty.

What meaning, Faber defined, is that bigger galaxies (these with bigger radii for a given stellar mass) must evolve additional and construct up a better stellar mass earlier than their central black holes can develop massive sufficient to quench star formation. Thus, small-radius galaxies quench at decrease plenty than large-radius galaxies.

“That is the new insight, that if galaxies with large radii have smaller black holes at a given stellar mass, and if black hole feedback is important for quenching, then large-radius galaxies have to evolve further,” she mentioned. “If you put together all these assumptions, amazingly, you can reproduce a large number of observed trends in the structural properties of galaxies.”

This explains, for instance, why extra huge quenched galaxies have larger central stellar densities, bigger radii, and bigger central black holes.

Primarily based on this mannequin, the researchers concluded that quenching begins when the entire power emitted from the black gap is roughly 4 instances the gravitational binding power of the fuel within the galactic halo. The binding power refers back to the gravitational drive that holds the fuel throughout the halo of darkish matter enveloping the galaxy. Quenching is full when the entire power emitted from the black gap is twenty instances the binding power of the fuel within the galactic halo.

Bodily processes

Faber emphasised that the mannequin doesn’t but clarify intimately the bodily mechanisms concerned within the quenching of star formation. “The key physical processes that this simple theory evokes are not yet understood,” she mentioned. “The virtue of this, though, is that having simple rules for each step in the process challenges theorists to come up with physical mechanisms that explain each step.”

Astronomers are accustomed to considering in phrases of diagrams that plot the relations between completely different properties of galaxies and present how they alter over time. These diagrams reveal the dramatic variations in construction between star-forming and quenched galaxies and the sharp boundaries between them. As a result of star formation emits lots of gentle on the blue finish of the colour spectrum, astronomers discuss with “blue” star-forming galaxies, “red” quiescent galaxies, and the “green valley” because the transition between them. Which stage a galaxy is in is revealed by its star formation charge.

One of the examine’s conclusions is that the expansion charge of black holes should change as galaxies evolve from one stage to the following. The observational proof suggests that almost all of the black gap progress happens within the inexperienced valley when galaxies are starting to quench.

“The black hole seems to be unleashed just as star formation slows down,” Faber mentioned. “This was a revelation, because it explains why black hole masses in star-forming galaxies follow one scaling law, while black holes in quenched galaxies follow another scaling law. That makes sense if black hole mass grows rapidly while in the green valley.”


Faber and her collaborators have been discussing these points for a few years. Since 2010, Faber has co-led a significant Hubble Area Telescope galaxy survey program (CANDELS, the Cosmic Meeting Close to-infrared Deep Extragalactic Legacy Survey), which produced the info used on this examine. In analyzing the CANDELS knowledge, she has labored intently with a workforce led by Joel Primack, UCSC professor emeritus of physics, which developed the Bolshoi cosmological simulation of the evolution of the darkish matter halos wherein galaxies kind. These halos present the scaffolding on which the speculation builds the early star-forming part of galaxy evolution earlier than quenching.

The central concepts within the paper emerged from analyses of CANDELS knowledge and first struck Faber about 4 years in the past. “It suddenly leaped out at me, and I realized if we put all these things together—if galaxies had a simple trajectory in radius versus mass, and if black hole energy needs to overcome halo binding energy—it can explain all these slanted boundaries in the structural diagrams of galaxies,” she mentioned.

On the time, Faber was making frequent journeys to China, the place she has been concerned in analysis collaborations and different actions. She was a visiting professor at Shanghai Regular College, the place she met first writer Zhu Chen. Chen got here to UC Santa Cruz in 2017 as a visiting researcher and started working with Faber to develop these concepts about galaxy quenching.

“She is mathematically very good, better than me, and she did all of the calculations for this paper,” Faber mentioned.

Faber additionally credited her longtime collaborator David Koo, UCSC professor emeritus of astronomy and astrophysics, for first focusing consideration on the central densities of galaxies as a key to the expansion of central black holes.

Among the many puzzles defined by this new mannequin is a placing distinction between our Milky Approach galaxy and its very comparable neighbor Andromeda. “The Milky Way and Andromeda have almost the same stellar mass, but Andromeda’s black hole is almost 50 times bigger than the Milky Way’s,” Faber mentioned. “The idea that black holes grow a lot in the green valley goes a long way toward explaining this mystery. The Milky Way is just entering the green valley and its black hole is still small, whereas Andromeda is just exiting so its black hole has grown much bigger, and it is also more quenched than the Milky Way.”

Reference: “Quenching as a Contest between Galaxy Halos and Their Central Black Holes” by Zhu Chen, S. M. 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-Firm, Fangzhou Jiang, Susan Kassin, Lin Lin, F. S. Liu, Yifei Luo, Zhijian Luo, Camilla Pacifici, Viraj Pandya, Samir Salim, Chenggang Shu, Sandro Tacchella, Bryan A. Terrazas and Hassen M. Yesuf, 7 July 2020, Astrophysical Journal.
DOI: 10.3847/1538-4357/ab9633

Along with Faber, Chen, Koo, and Primack, the coauthors of the paper embrace researchers at some two dozen establishments in seven nations. This work was funded by grants from NASA and the Nationwide Science Basis.

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