Sky & Telescope - February 2021 - 60

FERMI BUBBLES REPRISE

by Camille M. Carlisle

Double Bubb
Astronomers may be closing in on the culprit behind the structure that sprouts
from our galaxy's center.

O

ne of the largest structures in our galaxy is invisible
galactic disk. We can " see " them in radio waves, microwaves,
to the human eye. This structure is a dumbbell of
X-rays, and gamma rays. The X-rays show us bright edges
dazzling proportions - two humongous bubbles, each
and a dim interior, indicating the bubbles are less dense than
roughly 30,000 light-years tall, that balloon out of the Milky
their surroundings. In radio, we detect signs of orderly magWay's core and loom above and below the spiral disk.
netic fields. In microwaves, the structures look like a barelyThe Fermi bubbles have puzzled astronomers since observthere haze that fades with height.
ers found hints of them in the early 2000s. Long controAnd in gamma rays, the bubbles are at their strangest:
versial, their existence is now a certainty, but their origin
smooth and unvarying in brightness, with sharp edges likely
is not (S&T: Apr. 2014, p. 26). Did the Milky Way's central
created by a magnetic sheath that encases the cavities and
black hole throw a raging fit? Or did the explosive deaths of a
traps speedy particles.
passel of stars together blast out their
These particles fill the bubbles and
surroundings? The community split
create much of the radiation we see.
on this question.
Electrons corkscrew along magnetic
Electrons corkscrew
Looking at other galaxies for confield lines and spit out the radio and
along magnetic field
text hasn't helped much. Astronomers
microwave photons. Either electrons or
lines and spit out the
see signs of nuclear outflows in many
protons create the gamma rays - and
of them, powered either by black holes
which particle is to blame remains a
radio and microwave
or starbursts (or with fingerprints
key point of contention.
photons.
of both). Hints of a dumbbell in our
Hsiang-Yi Karen Yang (National
closest big neighbor, Andromeda, have
Tsing Hua University, Taiwan) lists a
been called into question - we're just
battery of observations that the right
too far away to make out what's there. To solve the Fermi
formation scenario must account for: the sharp edges, the
bubbles' origin mystery, we must stay near the Milky Way.
energies of the electrons emitting the microwaves, and the
And in the last several years, astronomers have come
different appearances at different wavelengths, among othcloser to an answer. They've changed our view of the bubbles
ers. " Any successful model would need to satisfy all these
from 2D to 3D, essentially turning what was a flat photo
stringent observational constraints, " she says.
into a moving picture of an expanding exhaust carrying
If the gamma rays and microwaves are made by the same
clouds out of the galactic center. Many of these observations
population of electrons - and that's a reasonable possibility,
indicate that whatever happened, it happened a few million
because electrons moving fast enough to emit microwaves
years ago - the same time frame needed to peg the black hole
would also knock into nearby photons and kick them up to
as the culprit. Although astronomers still don't agree that the
gamma-ray energies - then the Fermi bubbles likely formed
black hole is the answer, some say that it's becoming harder
a few million years ago, recently enough that the electrons
to argue for anything else.
haven't had time to slow down. The best way to shoot a tor " I wouldn't say it's 100%, " says Andrew Fox (Space
rent of relativistic electrons thousands of light-years within
Telescope Science Institute), who's worked on several of the
such a short span of time is a major flare-up from Sagitrecent studies. " But just looking at the different observations
tarius A*, our galaxy's supermassive black hole. If a load of
and seeing that they all paint this consistent picture, to me
gas dumped onto Sgr A* all those years ago, it could have
that's quite convincing. "
created a big fluffy disk that fueled jets and a powerful wind,
shooting electrons up and out and inflating the bubbles.
The scenario might also explain the massive stars clustered
Blowin' in the Wind
around the black hole that are a few million years old: They
The Fermi bubbles begin about 300 light-years above the

60

FEBRUARY 2 021 * SK Y & TELESCOPE



Sky & Telescope - February 2021

Table of Contents for the Digital Edition of Sky & Telescope - February 2021

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Sky & Telescope - February 2021 - Cover2
Sky & Telescope - February 2021 - 1
Sky & Telescope - February 2021 - Contents
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