Sky and Telescope - July 2017 - 25

ger starbirth by compressing the surrounding gas, ultimately
they heat and clear out gas in superbubbles spanning hundreds
of light-years. In large galaxies such as our Milky Way, this
can create pockets of infertile space. And in smaller galaxies,
superbubbles may completely remove all potentially starforming material, sterilizing the entire system.
If ram-pressure stripping only operates under certain
circumstances, and stellar feedback has only limited range in
larger galaxies, then only one alternative remains to explain
giant, red-and-dead ellipticals: black holes.

STA BURST WIND: J. G E ACH & R. CR AIN; BLOWING BUBBLES: N ASA / ESA / HUBBLE HERITAG E TE A M (STSCI / AUR A);
BL ACK DE ATH: ESA / ATG MEDIA L A B

Death by Black Hole
At the hub of most large galaxies is a supermassive black hole
millions or billions of times more massive than the Sun.
Some are dormant, while others are raging beasts consuming
huge amounts of gas. What they don't swallow, they shoot
out along magnetic ﬁeld lines coiled tightly as gun barrels.
The material bursts out of the galaxy as powerful jets of
charged particles and radiation. While superheated matter
spirals inward to await being swallowed or spat out, it settles
into a disk, which may launch its own wind. The jets and disk
together provide sufﬁcient power to blow a huge bubble into a
galaxy's stores of molecular gas - heating or sweeping up gas
that might otherwise have formed stars. Molecular hydrogen
must be at most a few tens of degrees above absolute zero to
condense into stars, so it doesn't take much for the black hole
to disturb the gas's delicate thermal condition.
At least, that's the theory. Now, there's some evidence to
back it up, from the SDSS project Mapping Nearby Galaxies at
Apache Point Observatory (MANGA) that's mapping 10,000
nearby galaxies. Among this immense collection, an international team of scientists led by Edmond Cheung (University of
Tokyo) identiﬁed a new, rare type of galaxy: red geysers. They
contain supermassive black holes that are only nibbling on surrounding gas; nevertheless, their low feeding rates are enough
to launch a wind that spews into the galaxy.
The prototype red geyser observed by Cheung's group,
nicknamed Akira, is ripping cool gas from a small companion
dubbed Tetsuo. Normally, cool gas would turn into stars, but
the red geyser's wind, revealed in velocity measurements of
ionized gas billowing out from the central black hole, heats
the gas and blows it away, leading the galaxy to turn red.
Many large elliptical galaxies are ensconced within a gaseous halo that, like the intracluster medium, is hot and emits
X-rays. Similar halos have been discovered encapsulating
spiral galaxies, too - in 2012, for example, Chandra detected
an X-ray halo around our Milky Way with a temperature
between 1 million and 2.5 million kelvin and a mass between
10 and 60 billion Suns. The halos hold material left over from
galaxy formation, as well as gas accumulated from the intergalactic medium. Feedback processes also feed and heat the
halos and can maintain a red-and-dead state even after the
black hole has become largely inactive.
"Once it becomes too strong, the black hole begins to
quench its own gas supply," explains Megan Donahue

STARBURST WIND A millimeter-wavelength image from the IRAM
Plateau de Bure Interferometer shows cool gas ﬂowing out of a
small galaxy at speeds up to 2 million mph. Winds from regions of
intense star formation appear to be the culprit.

BLOWING BUBBLES A star 45 times the mass of our Sun
produces powerful winds to create this ionized bubble. Stellar
particles and radiation sweep into the interstellar medium, heating and pushing aside the cooler gas. (The star, the bright purple
source at 10 o'clock, appears offset from the center because its
winds encountered denser gas on one side than on the other.)

BLACK HOLE ANNIHILATION This artist's impression portrays
a feeding black hole and the powerful wind it generates, which
may heat and blow out a galaxy's potentially star-forming gas.

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* J U LY 2 017

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Sky and Telescope - July 2017

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