How Galaxies Die: New Insights Into The Quenching of Star Formation
How Galaxies Die: New Insights Into The Quenching of Star Formation
How Galaxies Die: New Insights Into The Quenching of Star Formation
1/3
stellar mass before their central black holes can is in is revealed by its star formation rate.
grow large enough to quench star formation. Thus,
small-radius galaxies quench at lower masses than One of the study's conclusions is that the growth
large-radius galaxies. rate of black holes must change as galaxies evolve
from one stage to the next. The observational
"That is the new insight, that if galaxies with large evidence suggests that most of the black hole
radii have smaller black holes at a given stellar growth occurs in the green valley when galaxies
mass, and if black hole feedback is important for are beginning to quench.
quenching, then large-radius galaxies have to
evolve further," she said. "If you put together all "The black hole seems to be unleashed just as star
these assumptions, amazingly, you can reproduce formation slows down," Faber said. "This was a
a large number of observed trends in the structural revelation, because it explains why black hole
properties of galaxies." masses in star-forming galaxies follow one scaling
law, while black holes in quenched galaxies follow
This explains, for example, why more massive another scaling law. That makes sense if black hole
quenched galaxies have higher central stellar mass grows rapidly while in the green valley."
densities, larger radii, and larger central black
holes. Faber and her collaborators have been discussing
these issues for many years. Since 2010, Faber
Based on this model, the researchers concluded has co-led a major Hubble Space Telescope galaxy
that quenching begins when the total energy survey program (CANDELS, the Cosmic Assembly
emitted from the black hole is approximately four Near-infrared Deep Extragalactic Legacy Survey),
times the gravitational binding energy of the gas in which produced the data used in this study. In
the galactic halo. The binding energy refers to the analyzing the CANDELS data, she has worked
gravitational force that holds the gas within the halo closely with a team led by Joel Primack, UCSC
of dark matter enveloping the galaxy. Quenching is professor emeritus of physics, which developed the
complete when the total energy emitted from the Bolshoi cosmological simulation of the evolution of
black hole is twenty times the binding energy of the the dark matter halos in which galaxies form. These
gas in the galactic halo. halos provide the scaffolding on which the theory
builds the early star-forming phase of galaxy
Faber emphasized that the model does not yet evolution before quenching.
explain in detail the physical mechanisms involved
in the quenching of star formation. "The key The central ideas in the paper emerged from
physical processes that this simple theory evokes analyses of CANDELS data and first struck Faber
are not yet understood," she said. "The virtue of about four years ago. "It suddenly leaped out at me,
this, though, is that having simple rules for each and I realized if we put all these things together—if
step in the process challenges theorists to come up galaxies had a simple trajectory in radius versus
with physical mechanisms that explain each step." mass, and if black hole energy needs to overcome
halo binding energy—it can explain all these slanted
Astronomers are accustomed to thinking in terms of boundaries in the structural diagrams of galaxies,"
diagrams that plot the relations between different she said.
properties of galaxies and show how they change
over time. These diagrams reveal the dramatic At the time, Faber was making frequent trips to
differences in structure between star-forming and China, where she has been involved in research
quenched galaxies and the sharp boundaries collaborations and other activities. She was a
between them. Because star formation emits a lot visiting professor at Shanghai Normal University,
of light at the blue end of the color spectrum, where she met first author Zhu Chen. Chen came
astronomers refer to "blue" star-forming galaxies, to UC Santa Cruz in 2017 as a visiting researcher
"red" quiescent galaxies, and the "green valley" as and began working with Faber to develop these
the transition between them. Which stage a galaxy ideas about galaxy quenching.
2/3
"She is mathematically very good, better than me,
and she did all of the calculations for this paper,"
Faber said.
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no
part may be reproduced without the written permission. The content is provided for information purposes only.
3/3