Sky and Telescope - July 2017 - 27
NR AO / AUI / NSF / D. BERRY / SK Y WORKS / A LM A (ESO / N AOJ / NR AO)
That's what Donahue observed in the BCGs: With feedback stalled, the gas was able to cool and fall back onto the
galaxy, some of it forming new stars, the rest heading towards
the black hole where it will kick off the next round of activity.
Furthermore, although this rebirth was only observed in the
BCGs, Donahue suspects that it's at work in other galaxies,
too, though harder to observe.
"The physics of cooling, precipitation, infall, and the creation of cold molecular gas is not unique to clusters. It's just
that we can see it all in a cluster," she says. "We have all the
pieces of the puzzle in front of us."
Perhaps rather than being truly dead, many galaxies
instead enter a state of hibernation. Or it may be even more
complicated than that: Some forms of feedback may actually resuscitate galaxies rather than kill them. Recent evidence from the European Southern Observatory's Very Large
Telescope in Chile has shown new stars born within the very
winds that ought to halt their formation. Observing a galaxy
collision 600 million light-years away, Roberto Maiolino
(University of Cambridge, UK) and colleagues found evidence
of infant stars in cold gas that had been swept up by outﬂows
moving through one of the colliding galaxies.
Different feedback mechanisms are also important for
different galaxies, so looking for a single cosmic killer may be
the wrong way to go about things. In smaller galaxies, stellar
outﬂows and supernovae play a dominant role, and black
holes only have to step in once in a while, says Donahue. In
larger galaxies, stellar feedback isn't enough. "The black holes
have to step up almost all the time."
So are external processes, such as ram-pressure stripping or interactions with other galaxies, ever important? To
answer that question, astronomers surveyed 70,000 galaxies in the Cosmological Evolution Survey (COSMOS), where
some of the most powerful telescopes on Earth and space
have imaged a 2-square-degree patch of sky in the constellation Sextans. The COSMOS team found that internal processes, such as black hole and stellar feedback, quenched most
star formation until about 8 billion years ago. Since then,
external forces have come to the fore. This result isn't surprising, since star formation and quasar activity were reaching
their peaks in the early universe, whereas most clusters took
longer to fully form and inﬂuence galaxies.
Further evidence for the importance of internal feedback
processes in the early universe comes from research led by
Sandro Tacchella (Swiss Federal Institute of Technology,
Zurich) in 2015. Using the Hubble Space Telescope and the
Very Large Telescope, Tacchella and colleagues mapped the
distribution of old and new stars in 22 young elliptical galaxies that existed about 10 billion years ago, discovering that
their star formation was ending from the inside out, rather
than being brought to an end by external processes.
While the identity of the galactic killer may vary in time
and space, it's the galaxies themselves that get the last laugh:
They're capable of coming back to life, albeit in subdued fashion, hundreds of millions of years after the cosmic crime has
been committed. The interplay between quenching mechanisms is going to be crucial to a greater understanding of
how galaxies evolve.
¢ KEITH COOPER, a British freelance science journalist, was
editor of Astronomy Now magazine from 2006 to 2015. You
can follow him via @21stCenturySETI on Twitter.
REBIRTH An artist depicts cold gas clumps that have condensed out of hot intergalactic surroundings to rain back onto a galaxy's
supermassive black hole, such as was observed in the brightest galaxy of the cluster Abell 2597. The clumps are fueling both star formation and black hole activity.
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