For the first time, astronomers have found molecular oxygen — the
same gas humans need to breathe — in a galaxy outside the Milky Way.

Oxygen is the third most common element in the cosmos, after hydrogen and
helium. So astronomers once thought molecular oxygen, O2, would be
common in the space between the stars. But despite repeated searches, no one
had ever seen molecular oxygen beyond our galaxy — until now.

Junzhi Wang, an astronomer at Shanghai Astronomical Observatory in China, and
his colleagues spotted the molecule’s calling card in a galaxy named Markarian
231. Lying 560 million light-years away in the constellation Ursa Major, Markarian 231
is the nearest galaxy to Earth that contains a quasar
, where gas
whirls around a supermassive black hole and gets so hot that it glows
brilliantly.
(SN: 8/31/15).

Using radio telescopes in Spain and France, the astronomers saw radiation at a
wavelength of 2.52 millimeters, a signature
of O2’s presence
, the team reports in the Feb. 1 Astrophysical Journal.
“This is the first detection of molecular oxygen in an extragalactic object,”
Wang says.

It’s also the most molecular oxygen ever seen outside the solar
system. Previously, astronomers had seen the molecule in just two star-forming
clouds within the Milky Way, the Orion Nebula and the Rho Ophiuchi
cloud

(SN: 1/28/20). Astronomers think the shortage of interstellar O2 is
due to oxygen atoms and water molecules freezing onto dust grains, locking up the
oxygen. In these stellar nurseries, though, shocks from bright newborn stars
can rip water ice from the dust, freeing oxygen atoms to find each other and
form molecules.

But even in the Orion Nebula, molecular oxygen is rare, with
hydrogen molecules outnumbering oxygen molecules a million to one. Hydrogen
also dominates in Markarian 231. But molecular oxygen spans the outskirts of
the galactic disk at abundances more than 100 times greater than in the Orion
Nebula.

That’s “very high,” says Gary Melnick, an astrophysicist at the
Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., who was not
involved in the work. “There is no known explanation for an abundance of molecular
oxygen that high.” To confirm that the radiation really arises from O2,
Melnick says the observers should look for a second wavelength from the
molecule.

That won’t be easy, Wang says, because other molecules also emit radiation at
those wavelengths. To shore up the case for O2, the scientists went
through the many molecules that give off wavelengths similar to the one
detected and found that nobody had ever seen any of those molecules in space —
except for O2. “It is guilt by elimination, if you will,” says team
member Paul Goldsmith, an astronomer at the Jet Propulsion Laboratory in
Pasadena, Calif.

One possible
explanation for all the O2 is that Markarian 231 goes through a more
vigorous version of the Orion Nebula’s oxygen-forming process. The galaxy is a
prolific star factory, spawning new stars 100 times as fast as the Milky Way
and spewing out 700 solar masses of gas per year. High-speed gas from the
galaxy’s center may slam into gas in the disk, shaking water ice from dust grains
so that molecular oxygen can form.

In turn, that oxygen could
keep the galaxy hyperactive: Radiation the molecule emits helps cool the
gas so that some of it can collapse and create even more new stars in the
galaxy.