Sky and Telescope - July 2017 - 26
FERMI BUBBLES Two giant bubbles on either side of the Milky
Way's disk, seen in microwaves, X-rays, and gamma-rays (pictured here), point to a violent event in our galaxy's past: either the
ancient antics of the now-quiescent supermassive black hole or a
previous burst of star formation and stellar feedback.
(Michigan State University), who has a particular interest in
how black holes inﬂuence star formation.
Cheung estimates that up to 10% of galaxies with quiescent black holes are red geysers - indeed, even our own Milky
Way Galaxy ﬁts the bill. Two gigantic cavities above and
below its disk, called the Fermi bubbles, are ancient geysers
that might be relics of our black hole's rollicking past (S&T:
April 2014, p. 26).
Astronomers have witnessed similar behavior in elliptical
galaxies using the European Space Agency's retired Herschel
Space Observatory, which observed the universe at far-infrared wavelengths. Herschel detected cold gas reservoirs in a
half-dozen giant elliptical galaxies. Yet the gas wasn't able to
cool enough to form stars. Observations by NASA's Chandra X-ray Observatory show that the central black hole was
agitating the gas.
SPEEDING STARBIRTH Astronomers found stars forming within
the galactic wind pouring out of the galaxy IRAS F23128−5919,
depicted here in an artist's conception. The stars, forming at a
rate of 15 Suns' worth per year, are ﬂying out of the galaxy along
with the outﬂow, though at a somewhat slower pace.
HOT HALO A huge X-ray-emitting halo of gas surrounds the Milky
Way Galaxy and many other spiral and elliptical galaxies.
J U L Y 2 0 1 7 * SK Y & TELESCOPE
But Donahue had suspicions that there was more to the story.
She began to apply for time on Hubble to search elliptical
galaxies for ultraviolet light from hot, young stars.
Her requests for Hubble time were politely turned down.
"It was hard," she says of her attempts. "The received wisdom
was that [elliptical galaxies] are red and dead, so why would
you waste time pointing an ultraviolet telescope at them?"
As it happens, the Cluster Lensing and Supernova Survey
with Hubble (CLASH) was studying how 25 massive clusters
act as gravitational lenses, magnifying the light of much
more distant galaxies. The CLASH project was interested in
these magniﬁed galaxies and so observed each cluster - and
the ellipticals within them - through 16 different ﬁlters,
"I was extremely happy because I could have proposed for
years and never gotten that kind of coverage," says Donahue.
It was worth the wait. Hubble's observations of many of the
brightest cluster galaxies (BCGs), the most luminous, largest,
and exclusively elliptical galaxies at the center of each cluster,
revealed a delightful variety of ultraviolet-emitting knots and
ﬁlaments. Stars are forming at a rate of up to 80 solar masses
per year - in galaxies that were presumed to be red and dead
- and simulations suggest the central black holes are contributing to the rebirth. Like a phoenix arising from the ashes,
the galaxies were being brought back to life. So were they ever
really dead, or were they just faking it?
"Quenching [of star formation] might be too simplistic a
concept," Donahue acknowledges.
Instead she describes a scenario in which feedback engages
in a subtle interplay with a galaxy's gas reservoir. Whether
feedback comes from an active black hole or from newborn
and dying stars, it may act as a galactic thermostat. First it
heats surrounding gas, preventing it from falling onto the
black hole or condensing into more stars. But as activity shuts
down, the gas has a chance to cool and fall inward, which
in turn re-ignites the black hole's activity or forms stars (or
both). And so on.
FER MI BUBBLES: N ASA / DOE / FER MI L AT / D. FINK BEINER E T A L.; SPEEDING
STA RBIRTH: ESO; HOT H A LO: N ASA / CXC / OHIO STATE UNIV. / A . GUPTA E T
A L. / M. WEISS
The Galactic Phoenix