How galaxies die: New insights into the quenching of star formation


Date:
July 16, 2020
Source:
University of California - Santa Cruz
Summary:
Astronomers studying galaxy evolution have long struggled to
understand what causes star formation to shut down in massive
galaxies. Although many theories have been proposed to explain
this process, known as ''quenching,'' there is still no consensus
on a satisfactory model. Now, an international team of scientists
has proposed a new model that successfully explains a wide range
of observations about galaxy structure, supermassive black holes,
and the quenching of star formation.



FULL STORY ========================================================================== Astronomers studying galaxy evolution have long struggled to understand
what causes star formation to shut down in massive galaxies. Although
many theories have been proposed to explain this process, known as
"quenching," there is still no consensus on a satisfactory model.


==========================================================================
Now, an international team led by Sandra Faber, professor emerita
of astronomy and astrophysics at UC Santa Cruz, has proposed a new
model that successfully explains a wide range of observations about
galaxy structure, supermassive black holes, and the quenching of star formation. The researchers presented their findings in a paper published
July 1 in the Astrophysical Journal.

The model supports one of the leading ideas about quenching which
attributes it to black hole "feedback," the energy released into a galaxy
and its surroundings from a central supermassive black hole as matter
falls into the black hole and feeds its growth. This energetic feedback
heats, ejects, or otherwise disrupts the galaxy's gas supply, preventing
the infall of gas 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
explained. "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." The basic idea involves the relationship
between the mass of the stars in a galaxy (stellar mass), how spread out
those stars are (the galaxy's radius), and the mass of the central black
hole. For star-forming galaxies with a given stellar mass, the density
of stars in the center of the galaxy correlates with the radius of the
galaxy so that galaxies with bigger radii have lower central stellar
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 further and build
up a higher stellar mass before their central black holes can grow large
enough to quench star formation. Thus, small-radius galaxies quench at
lower masses 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 said. "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 example, why more massive
quenched galaxies have higher central stellar densities, larger radii,
and larger central black holes.

Based on this model, the researchers concluded that quenching begins
when the total energy emitted from the black hole is approximately
four times the gravitational binding energy of the gas in the galactic
halo. The binding energy refers to the gravitational force that holds
the gas within the halo of dark matter enveloping the galaxy. Quenching
is complete when the total energy emitted from the black hole is twenty
times the binding energy of the gas in the galactic halo.

Faber emphasized that the model does not yet explain in detail the
physical mechanisms involved in the quenching of star formation. "The key physical processes that this simple theory evokes are not yet understood,"
she said.

"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 thinking in
terms of diagrams that plot the relations between different properties
of galaxies and show how they change over time. These diagrams reveal
the dramatic differences in structure between star-forming and quenched 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. Which 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. The observational evidence suggests that most of the black hole growth occurs in the green
valley when galaxies are beginning to quench.

"The black hole seems to be unleashed just as star formation slows down,"
Faber said. "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 issues for many years.

Since 2010, Faber has co-led a major Hubble Space Telescope galaxy survey program (CANDELS, the Cosmic Assembly Near-infrared Deep Extragalactic
Legacy Survey), which produced the data used in this study. In analyzing
the CANDELS data, she has worked closely with a team led by Joel
Primack, UCSC professor emeritus of physics, which developed the Bolshoi cosmological simulation of the evolution of the dark matter halos in which galaxies form. These halos provide the scaffolding on which the theory
builds the early star-forming phase of galaxy evolution before quenching.

The central ideas in the paper emerged from analyses of CANDELS data and
first struck Faber about four years ago. "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 said.

At the time, Faber was making frequent trips to China, where she has
been involved 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 quenching.

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

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

Among the puzzles explained by this new model is a striking
difference between our Milky Way galaxy and its very similar 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 said. "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." In addition to Faber,
Chen, Koo, and Primack, the coauthors of the paper include researchers
at some two dozen institutions in seven countries. This work was funded
by grants from NASA and the National Science Foundation.


========================================================================== Story Source: Materials provided by
University_of_California_-_Santa_Cruz. Original written by Tim
Stephens. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Zhu Chen, S. M. Faber, David C. Koo, Rachel S. Somerville, Joel R.

Primack, Avishai Dekel, Aldo Rodri'guez-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,
F. S. Liu, Yifei Luo, Zhijian Luo, Camilla Pacifici, Viraj Pandya,
Samir Salim, Chenggang Shu, Sandro Tacchella, Bryan A. Terrazas,
Hassen M. Yesuf. Quenching as a Contest between Galaxy Halos and
Their Central Black Holes. The Astrophysical Journal, 2020; 897
(1): 102 DOI: 10.3847/1538-4357/ab9633 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200716101607.htm

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