reported that every time they looked within
490,000 light-years of a galaxy, they saw spectra
dappled with blank spots from atoms absorbing
light. That meant that CGMs weren’t odd cloaks
worn by just a few galaxies. They were everywhere.
Tumlinson’s team spent the first few years after
Hubble’s upgrade like 19th century naturalists
describing new species. The group measured the
mass and the chemical makeup of the galaxies’
CGMs and found they were huge cisterns of heavy
elements. CGMs contain 10 million times the mass
of the sun in oxygen alone. In many cases, the mass
of a CGM is comparable to the mass of the entire
visible part of its galaxy.
The finding offers an answer to a long-standing
cosmic mystery: How do galaxies have enough
star-forming fuel to keep going for billions of
years? Galaxies build stars from collapsing
clouds of cool gas at a constant rate; the Milky
Way, for example, makes one to two solar masses’
worth of stars every year. But there isn’t enough
cool gas within the visible part of a galaxy, the
disk containing its stars, to support observed
rates of star formation.
“We think that gas probably comes from the
CGM,” Werk says. “But exactly how that gas is get-
ting into galaxies, where it gets in, the timescale
on which it gets in, are there things that prevent it
from getting in? Those are big questions that keep
us all awake at night.”
Werk and Peeples realized that all that mass
could help solve two other cosmic bookkeep-
ing problems. All elements heavier than helium
(which astronomers lump together as “metals”)
are forged by nuclear fusion in the hearts of stars.
When stars use up their fuel and explode as super-
novas, they scatter those metals around to be
folded into the next generation of stars.
But if you add up all the metals in the stars, gas
and dust in a given galaxy’s disk, it’s not enough to
account for all the metals the galaxy has ever made.
The mismatch gets even worse if you include
the hydrogen, helium, electrons and protons —
basically all the ordinary matter that should have
collected in the galaxy since the Big Bang. Astronomers call all those bits baryons. Galaxies seem to
be missing 70 to 95 percent of that stuff.
So Peeples and Werk led a comprehensive effort
to tally all the ordinary matter in about 40 galaxies
observed with Hubble’s new spectrometer. The
researchers published the results in two 2014
papers in the Astrophysical Journal.
At the time, Werk reported that at least half
of galaxies’ missing ordinary matter can be
accounted for in their CGMs. In a 2017 update,
Werk and colleagues found that the mass of
baryons just in the form of cool gas in a galaxy’s
CGM could be nearly 90 billion solar masses.
“Obviously, this mass could resolve the galactic
missing baryons problem,” the team wrote.
“It’s a classic science story,” Schawinski says.
The researchers had a hypothesis about where
the missing material should be and made predictions. The group made observations to test those
predictions and found what it sought.
In a separate study, Peeples showed that
although metals are born in galaxies’ starry disks,
those metals don’t stay there. Only 20 to 25 percent
of the metals a galaxy has ever produced remains in
the stars, gas and dust in the disk, where the metals
can be incorporated into new stars and planets. The
rest probably ends up in the CGM.
“If you look at all the metals the galaxies ever
produced in their whole lifetime, more of them are
outside the galaxy than are still inside the galaxy,”
Tumlinson says, “which was a huge shock.”
So how did the metals get into the CGM? Quasars’
spectra couldn’t help with that question. Their
light shows only a slice through a single galaxy at a
single moment in time. But astronomers can track
galaxies’ growth and development with computer
simulations based on physical rules for how stars
and gas behave.
This strategy revealed the churning, ever-chang-ing nature of gas in galaxies’ CGMs. Simulations
such as EAGLE, or Evolution and Assembly of
GaLaxies and their Environments, which is run
out of Leiden University in the Netherlands,
showed that metals can reach CGMs through
stars’ violent lives: in powerful winds of radiation blowing away from massive young stars, and
in the death throes of supernovas spraying metals
far and wide.
Once the metals are in the CGM, though, they
don’t always stay put. In simulations, galaxies
seem to use the same gas over and over again.
“It’s basically just gravity,” Peeples says. “Throw
a baseball up, and it’ll come back to the ground.”
The same goes for gas flowing out of galaxies:
Unless the gas travels fast enough to escape the
galaxy’s gravity altogether, those atoms will eventually fall back into the disk — and form new stars.
Some simulations show discrete gas parcels making the trip from a galaxy’s disk out into the CGM
and back again several times. Together, CGMs and
their galaxies are giant recycling devices.
FEATURE | A GALAXY’S ECOSYSTEM
This EAGLE simulation
shows that, over time,
metals (colors) move
away from the center of
a galaxy to the circumgalactic medium.