Sky & Telescope - February 2021 - 64

Fermi Bubbles Reprise

There is yet more evidence favoring
an age of a few million years, this
time in gas far, far away from the
galactic center.
bubbles, then the X-ray plasma is a relic of that past activity,
slowly cooling long after the outﬂow ceased.
When asked which scenario he thinks most likely, Ponti
laughs. " I would not bet, " he says.
The X-ray chimneys also show up at radio wavelengths,
identiﬁed in 2019 by Ian Heywood (University of Oxford,
UK) and colleagues using the MeerKAT radio telescope array
in South Africa. Although some astronomers suspect parts of
the radio features lie much closer to us than the galactic center, the edges of the structures MeerKAT detected align fairly
well with those of the X-ray chimneys, Ponti says - which
makes sense, he adds, because when an outﬂow smacks into
the surrounding interstellar medium, the shocked material
should also emit radio waves.
However, the MeerKAT data may tell astronomers something that the X-rays do not. The radio glow comes from corkscrewing electrons, and based on the electrons' properties,
Heywood's team thinks the event that created the outﬂows
happened only a few million years ago.

highly ionized atoms move at about the same speed as the
less-ionized atoms, which only photoionization - that is, being
hit with a really spectacular ﬂash of light - can explain.
The ions appear solely in parts of the stream that are in
view of the galactic center, and stars can't supply the necessary energy. Combine that with the time frame and the photoionization, and the team thinks the most plausible explanation is that about 3.5 million years ago, Sgr A* lit up like a
spotlight. Over the course of a few hundred thousand years,
the gas-guzzling black hole unleashed a surge of ultraviolet
radiation so powerful that it kicked electrons out of atoms
some 250,000 light-years away in the Magellanic Stream.
Simulations by Yang and collaborators also indicate that
Sgr A* is the simplest explanation. If the black hole ate some
10,000 Suns' worth of gas over the course of a few hundred
thousand years, it could have produced enough radiation and
outﬂowing particles to ionize the Magellanic Stream and
inﬂate the Fermi bubbles, she says. Assuming the black hole
threw its tantrum about a million years ago, the simulations
also reproduce the energies of the electrons that would have
created the gamma rays and microwaves, as well as the level
of orderliness in the magnetic ﬁelds, which would be due in
part to what's called magnetic draping. This effect happens
when a solid body (or in this case, an outﬂow) rams into and

Caught in the Headlights

X CHIMNEYS Astronomers have detected much smaller outflow structures emanating from the galactic center, in both X-rays (left) and radio
(center). When overlapped (far right), these two features largely align; the
white ellipses in the overlap are rough outlines for the chimneys. They
connect the galactic center with the bases of the Fermi bubbles and may
have served as the channel by which material entered the bubbles.

FEBRUARY 2 021 * SK Y & TELESCOPE

Galactic latitude

1º

ble
rmi bub
Base of Fe

Galactic plane

Sgr A*

-1º

Base of Fermi bubble

-2º

There is yet more evidence favoring an age of a few million
years, this time in gas far, far away from the galactic center.
The Magellanic Stream is an immense rivulet of gas that
runs through the Milky Way's halo like a ribbon through
an upswept hairdo. It's made of gas torn from the Large and
Small Magellanic Clouds, trailing along with these dwarf
galaxies as they make a daredevil pass around the Milky Way.
In the past several years, Joss Bland-Hawthorn (University of Sydney, Australia) and colleagues have found patches
of ionized hydrogen, carbon, and silicon in the part of the
stream that passes above our galaxy's south pole. They've also
seen hints near the northern pole. Ions are basically heated
atoms; after the energy that ionized the atoms is gone, each
ion will cool back down at a unique rate. Astronomers can
use those rates to determine when the energy source that created all the ions they see in the stream switched off. Based on
the data, the team thinks it happened a few million years ago.
Furthermore, ionized clouds have different patterns of
spectral lines depending on what ionized them. Follow-up
observations reported by Elaine Frazer (Space Telescope Science Institute) at the AAS meeting show that the stream's

1º
160 light-years

1°

Galactic longitude

359º

Sky & Telescope - February 2021

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