Sky & Telescope - February 2021 - 62

Fermi Bubbles Reprise

and even more distant quasars as backlights to ﬁnd clouds
within the bubbles.
These clouds are not the same as the plasma and particles
picked up by the previous studies, stresses Fox. " What we're
really seeing is clouds carried into the Fermi bubbles, " he says.
The clouds are swept up by the hot plasma in the bubbles, like
little leaves blowing around in a wind. " We're using those
leaves as a way of tracing the wind. "
Fox and his collaborators have taken spectra of nearly 70
stars and quasars, almost a third of which overlap with the
Fermi bubbles or their edges and reveal clouds speeding up
through the cavities. The motions imprinted on the clouds'
spectra show that the clouds aren't moving around randomly
but are part of an expanding outﬂow that may have launched
at some 1,000 km/s, Rongmon Bordoloi (North Carolina
State University) reported at the virtual American Astronomical Society meeting last June. Based on the travel times
needed for the clouds to make it from the galactic center
up to their current positions in the bubbles, the wind must

Although it was fairly obvious from
the bubbles' shape that they originate
in the galactic center, the connection
was an extrapolation.
have started around 6 to 9 million years ago. " We are very
conﬁdent from this completely independent measure that
it must be something of [a] short event, " he said during his
presentation.
Such studies are limited to places where astronomers can
ﬁnd a celestial backlight. But Dhanesh Krishnarao (University
of Wisconsin, Madison) and colleagues recently expanded
observers' reach. Reporting in the same AAS session, Krishnarao used the Wisconsin H-Alpha Mapper (WHAM) to look
in the same direction as one of the sightlines that Fox's team
uses. WHAM directly detected the visible-light emission of
ionized hydrogen and nitrogen - no background star or qua-

75°
60°

Background
quasars

45°

Background
stars

30°

Hydrogen
clouds

Microwave haze
H-alpha observations
180°

150°

120°

90°

60°

30°

15°

Chimneys
330°

300°

270°

240°

X-ray edges

210°
-15°

LMC

-30°

SMC
-45°

Mag
ag
ge
gella
elllla
a ic St
anic
Sttrrre
e
ea
eam
am
siightl
sigh
sig
ghtline
gh
ghtl
htl
htlin
in
ne
e
es
s
-60°
-75°

S EXPLORING THE BUBBLES The Fermi bubbles are huge, sharp-edged structures seen in gamma rays, with X-ray-emitting plasma cupping their
bases. Astronomers have probed moving gas in the bubbles' upper reaches using background stars and distant quasars, as well as by detecting
hydrogen-alpha emission along one edge. Closer to the bases, roughly 200 clouds are flying away from the galactic center. The bubbles connect
to the galactic center via two narrow chimneys. Far away, in the debris stream from the Magellanic Clouds, atoms appear to have been ionized by
radiation from the galactic center within the last few million years - the same time scale implied by the outflow's motions. (Note that due to projection
effects, the Magellanic Stream looks much closer to the Fermi bubbles in this illustration than it actually is.)

FEBRUARY 2 021 * SK Y & TELESCOPE

180°

Sky & Telescope - February 2021

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