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That means that the atoms that make up planets,
plants and people may have taken several trips to
circumgalactic space before becoming part of us.
Over hundreds of millions of years, the atoms that
eventually became part of you traveled hundreds
of thousands of light-years.
“This is my favorite thing,” Tumlinson says. “At
some point, your carbon, your oxygen, your nitrogen, your iron was out in intergalactic space.”
How galaxies die
But not all galaxies get their CGM gas back. Losing
the gas could shut off star formation in a galaxy
for good. No one knows how star formation shuts
off, or quenches. But the answer is probably in
the CGM.
Galaxies come in two main forms: young
spiral galaxies that are making stars and old
blobby galaxies where star formation is quenched
(SN Online: 4/23/18).
“How galaxies quench and why they stay that
way is one of the most important questions in galaxy formation generally,” Tumlinson says. “It just
has to have something to do with the gas supply.”
One possibility, suggested in a paper posted
online February 20 at arXiv.org, is that sprays of
supernova-heated gas could get stripped from galaxies. Physicist Chad Bustard of the University of
Wisconsin–Madison and colleagues simulated the
Large Magellanic Cloud, a satellite galaxy of the
Milky Way, and found that the small galaxy’s outflowing gas was swept away by the slight pressure
of the galaxy’s movement around the Milky Way.
Alternatively, a dead galaxy’s CGM gas could be
too hot to sink into the galaxy and form stars. If
so, star-forming galaxies should have CGMs full
of cold gas, and dead galaxies should be shrouded
in hot gas. Hot gas would stay floating above the
galactic disk like a hot air balloon, too buoyant to
sink in and form stars.
But Hubble saw the opposite. Star-forming
galaxies had CGMs chock-full of oxygen-VI —
meaning that the gas was so hot (a million degrees
Celsius or more) that oxygen atoms lost five of
their original electrons. Dead galaxies had surprisingly little oxygen-VI.
“That was puzzling,” Tumlinson says. “If theory
told us anything, it should have gone the other way.”
All that hot gas could potentially explain why
those galaxies died — except that these galaxies
were full of star-forming cold gas, too.
“The dead galaxies have plenty of fuel left in the
tank,” Tumlinson says. “We don’t know why they’re
not using it. Everybody’s chasing that problem.”
Grabbing at the elephant
The chase comes at a good time. Until recently,
observers had no way to map a single galaxy’s
CGM. Researchers have had to add up dozens of
Reading what’s not there Using light from a
quasar (QSO), researchers can “see” CGMs. In this example, spectra from two galaxies, G1 and G2 (at left),
have certain wavelengths missing (red, at bottom)
where the CGMs’ atoms are absorbing light.
G1
N
E
QSO G2
1,200
1,202 1,292 1,296 1,300 1,206 1,210
8
10 10
55
00
6
4
2
0
1,300 1,400 1,500 1,600 1,700 1,800
P
hot
o
n
s( 10
– 4
p
e
rs
e
c
on
d
)
The European Southern Observatory’s Medusa-like MUSE
instrument was installed on the Very Large Telescope in
Chile in 2014 to take spectra across a full galaxy.
G1 G2
