Sky & Telescope - February 2021 - 66

Fermi Bubbles Reprise

75°
60°

Milky Way

Fermi Bubbles

45°
30°
15°
150°

120°

90°

60°

30°

330°

300°

270°

Leading
a
arm
240°
210°

Leading Arm

LMC

-15°
15

SMC

-30°
30
-45°
45

SMC
C

-60°
60
Stream

LMC
L
Bridge

eam
Magellanic Str

-75°

S CENTRAL FLASH Left: Observations of the gas trailing from the Magellanic Clouds reveal ionized hydrogen (red) and carbon (green). Larger circles
indicate sightlines with higher levels of ionization. Right: Astronomers suspect a bright event in the galactic center ionized gas in the part of the Magellanic Stream above the galactic south pole.

Molecular gas requires dense, cold

Other observations are also raising questions. Astronomers
conditions, and based on the data, the
have found more than 200 cool gas clouds in the lower
team thinks there's a lot of it. That's a
reaches of the Fermi bubbles, moving a few hundred kilometers per second and speeding up as they whiz away from the
problem.
galactic center. Recently, follow-up observations by Enrico Di
Teodoro (Johns Hopkins University) and colleagues detected
molecular gas in two of these clouds. Molecular gas requires
jets. She and her collaborators are developing the code now.
dense, cold conditions, and based on the data, the team
For Mertsch, the need is spectral modeling: simulations
thinks there's a lot of it.
that look at the range of energies that each scenario would
That's a problem, because it's hard to bring this much
give to protons or electrons, and what fraction of the
molecular gas to such high velocities. The star-formation
particles would be at each energy level. Then astronomers
rate in the galactic center doesn't seem to be high enough
could compare the predictions to observations. Not enough
to push that much cold gas out, even if it had been higher
theoretical work has tracked how the particles' energies and
in the past, says team member Naomi McClure-Grifﬁths
locations depend on each other, he says. " I still believe that
(Australian National University), who has also worked on
spectral modeling holds the key to deciphering the origin of
the cyclic star-formation simulations. And even though
the Fermi bubbles. "
the researchers think the clouds would
On the observational side, more
survive less than 10 million years - at
gamma-ray data from ground- and spaceNo Mean Feat
face value, a vote for the black hole - it's
based detectors will reﬁne what we know
To create the Fermi
unclear to her whether, even in tantrum
about the particles that create the Fermi
bubbles, Sgr A* would
mode, Sgr A* would have the gusto to
bubbles' highest-energy photons. And
have to have increased
drive the clouds out.
catching more clouds blowing through the
its current feeding rate
Perhaps it's a both/and situation, she
bubble breeze will tell us about the condiby at least a factor of
says. Star formation might drive a slow
tions inside and in the surrounding halo,
10 million. Although
wind that " loosens up " the surrounding
and thus about the outﬂow that created
astronomers often see
gas, which the black hole could then drive
the still-expanding cavities.
the black hole's radiation
out when it powered a short-lived upheaval.
In a few years, astronomers may reach
ﬂicker and have found
" That's the type of combined simulation
an accord on the Fermi bubbles' origin.
signs of past ﬂares, none
that I would like to see! " she says.
Well. You might not want to bet on it.
of those comes remotely
Yang also wants more simulations -
close to the level needed
in her case, 3D ones that look carefully
Science Editor CAMILLE M. CARLISLE
to explain the Fermi
at how shocks and turbulence driven by
would tell you what her vote is for the
bubbles.
winds that came off the active black hole
culprit, but regular readers can probably
might energize the particles, instead of
guess that she's biased.
66

FEBRUARY 2 021 * SK Y & TELESCOPE

STRE A M IM AG E: DAVID L. NIDE V ER E T A L. (NR AO / AUI / NSF ), A . MELLING ER, LEIDEN / A RG EN TINE / BONN SURV E Y,
PA RKES OBSERVATORY, WESTERBORK OBSERVATORY, ARECIBO OBSERVATORY; STRE AM DATA: J. BL AND -HAW THORN
E T A L. / ASTR OPHYSICAL JOUR NAL 2019; SE Y FERT FL A RE: N ASA / ESA / L. HUSTA K (STSCI)

The Trail Goes Cold

Sky & Telescope - February 2021

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