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

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

Junzhi Wang, an astronomer at Shanghai Astronomical Observatory in China, andhis colleagues spotted the molecules calling card in a galaxy named Markarian231. Lying 560 million light-years away in the constellation Ursa Major, Markarian 231is the nearest galaxy to Earth that contains a quasar, where gaswhirls around a supermassive black hole and gets so hot that it glowsbrilliantly.(SN: 8/31/15).

Using radio telescopes in Spain and France, the astronomers saw radiation at awavelength of 2.52 millimeters, a signatureof O2s presence, the team reports in the Feb. 1 Astrophysical Journal.This is the first detection of molecular oxygen in an extragalactic object,Wang says.

Its also the most molecular oxygen ever seen outside the solarsystem. Previously, astronomers had seen the molecule in just two star-formingclouds within the Milky Way, the Orion Nebula and the Rho Ophiuchicloud(SN: 1/28/20). Astronomers think the shortage of interstellar O2 isdue to oxygen atoms and water molecules freezing onto dust grains, locking up theoxygen. In these stellar nurseries, though, shocks from bright newborn starscan rip water ice from the dust, freeing oxygen atoms to find each other andform molecules.

But even in the Orion Nebula, molecular oxygen is rare, withhydrogen molecules outnumbering oxygen molecules a million to one. Hydrogenalso dominates in Markarian 231. But molecular oxygen spans the outskirts ofthe galactic disk at abundances more than 100 times greater than in the OrionNebula.

Thats very high, says Gary Melnick, an astrophysicist at theHarvard-Smithsonian Center for Astrophysics in Cambridge, Mass., who was notinvolved in the work. There is no known explanation for an abundance of molecularoxygen that high. To confirm that the radiation really arises from O2,Melnick says the observers should look for a second wavelength from themolecule.

That wont be easy, Wang says, because other molecules also emit radiation atthose wavelengths. To shore up the case for O2, the scientists wentthrough the many molecules that give off wavelengths similar to the onedetected 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 teammember Paul Goldsmith, an astronomer at the Jet Propulsion Laboratory inPasadena, Calif.One possibleexplanation for all the O2 is that Markarian 231 goes through a morevigorous version of the Orion Nebulas oxygen-forming process. The galaxy is aprolific star factory, spawning new stars 100 times as fast as the Milky Wayand spewing out 700 solar masses of gas per year. High-speed gas from thegalaxys center may slam into gas in the disk, shaking water ice from dust grainsso that molecular oxygen can form.

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

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Molecular oxygen has been spotted beyond the Milky Way for the first time - Science News