Sky & Telescope - February 2021 - 63

sar required. The observations show ions moving at similar
speeds to what's seen in ultraviolet.
" That's really exciting, " Fox says. Combined, the WHAM
and ultraviolet measurements indicate a range of temperatures (roughly 10,000 to 100,000 kelvin) that's cooler than
the plasma detected with X-rays. " It really points to this very
multiphase, turbulent gas. It's not just a simple bubble that is
ﬁlled full of plasma all at one temperature. "

Chimney Sweeping
Part of the mystery of the Fermi bubbles has been how they
connect to the galactic center. The spectroscopic observations
look at clouds that are at least a few thousand light-years
above the disk, and early studies showed the bubbles clearly at
high latitudes but had trouble tracing the features down into
the messy environment below. Although it was fairly obvious
from the bubbles' shape that they originate in the galactic
center, the connection was an extrapolation.
Not anymore, says Gabriele Ponti (INAF Brera Astronomi-

cal Observatory, Italy). Ponti's team used the XMM-Newton
X-ray telescope to peer deep into the galactic center and see if
they could ﬁnd where the outﬂow comes from. They discovered two " chimneys, " channels ﬁlled with X-ray-emitting
plasma that extend from the center up to the bottom of each
of the Fermi bubbles.
If the Fermi bubbles are balloons, then the chimneys are
the nozzles: These are the channels by which energy and particles traveled from the galactic center into the Fermi bubbles.
" I am convinced of that, " Ponti says.
Unfortunately, the X-rays only permit us to see the pathways; they don't tell us what made them. The chimneys span
about 150 light-years at their bases, and most of the galactic
center's star formation in the last several million years has
also concentrated in this area, with many aging, massive stars
still in the neighborhood. It could be that the X-rays arise
when supernovae's powerful outﬂows hit the surrounding
interstellar medium, meaning that stars' dramatic deaths
power the Fermi bubbles. Conversely, if Sgr A* inﬂated the

Sun

0
Movement toward
Sun (blueshifted
d)

Velocity

Ionized
carbon &
silicon

Visible emission

Ionized
hydrogen

UV absorption

E X PLORING THE BUBBLES: G REGG DINDER M A N / S&T, SOURCES: FER MI BUBBLES A ND U V SIG H TLINES: T. ASHLE Y E T A L. /
ASTR OPHYSICAL JOUR NAL 2020, NEU TR A L CLOUDS: E. M. DI TEODORO E T A L. / NATUR E 2020, MAGELL ANIC STRE AM SIGHTLINES:
A . J. FOX E T A L. / ASTR OPHYSICAL JOUR NAL 2014, CHIMNE YS: G. PON TI E T A L. / NATUR E 2019, H-A LPH A: D. K RISHN A R AO E T A L. /
ASTR OPHYSICAL JOUR NAL LE T TERS 2020; SIG H TLINE: DH A NESH K RISHN A R AO A ND N ASA

Distant quasar

Mo
ovement away
fro
om Sun (redshifted)

S SIGHTLINE Astronomers study gas moving in the Fermi bubbles by looking at the fingerprints the gas leaves in distant quasars' spectra (inset,
bottom spectrum). If the gas is moving toward us, the lines are blueshifted; if it's moving away, the lines are redshifted. Observers also recently
detected emission from ionized hydrogen (top spectrum) near one of the same quasar sightlines, and it's also moving toward us. Combined, the
examples of these shifts suggest we're actually seeing the bubbles expand up and out. (The gray circles mark gas that is not moving toward or
away from us - either gas moving straight up inside the bubbles, or gas much closer that revolves around the galactic center at the same rate
that the Sun does.)

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Sky & Telescope - February 2021

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