For the first time oxygen has been detected in another galaxy

Although it is ubiquitous on Earth, molecular oxygen (also called oxygen) is not so easy to find outside of our planet. By just two times, astrophysicists have detected them outside the Solar System. But recently, their instruments have identified traces of O2 even further, more than 500 million light years from Earth, outside the Milky Way. This important discovery, the first detection of dioxygen outside our galaxy , should help astrophysicists better understand the complex molecular dynamics of galactic regions.

Oxygen is the third most abundant element in the Universe, behind hydrogen and helium. Thus, its chemistry and its abundance in interstellar clouds are important for understanding the role of molecular gas in galaxies.

Astronomers have searched for oxygen again and again, using millimetre astronomy, which detects the radio wavelengths emitted by molecules; and spectroscopy, which analyses the spectrum to look for wavelengths absorbed or emitted by specific molecules.

But these searches have turned up a puzzling lack of oxygen molecules. Which means "a comprehensive picture of oxygen chemistry in different interstellar environments is still missing," wrote a team of astronomers led by Junzhi Wang of the Chinese Academy of Sciences in a new paper.

Release of dioxygen into space: the molecular shock hypothesis

One place where molecular oxygen has already been detected is the Orion Nebula, it's been hypothesised that out in space, oxygen is bound up with hydrogen in the form of water ice that is clinging to dust grains.

But the Orion nebula is a stellar nursery, and it's possible that the intense radiation from very hot young stars shocks the water ice into sublimation and splits the molecules, releasing the oxygen. Astrophysicists then turned to the Markarian 231 galaxy.

In 2011, data from the Herschel satellite showed the presence of molecular oxygen in the Orion nebula. Credits: ESA

Markarian 231 is special. It is located 561 million light years away and contains a quasar. It is an extremely bright galactic nucleus with an active supermassive black hole in the center. These are the brightest objects in the Universe, and Markarian 231 contains the closest quasar to Earth. In fact, astronomers believe that Markarian 231 could have two active supermassive black holes in its center, swirling around each other.

Galactic molecular oxygen: potential interactions between molecular fluxes and molecular clouds

An active galactic nucleus induces molecular fluxes, producing such continuous shocks, which could release oxygen from the water in molecular clouds. Molecular fluxes from Markarian 231 are particularly rapid, so Wang and his colleagues went there to get oxygen. Using the 30-meter IRAM radio telescope in Spain, they made observations of the galaxy for four days, over several wavelengths.

Dioxygen spectrum in the Markarian 231 galaxy, established thanks to observations from the IRAM telescope for 4 days. Credits: Junzhi Wang et al. 2020

In those data, they found the spectral signature of oxygen, in line with the shock hypothesis.

"With deep observations toward Markarian 231 using the IRAM 30 meter telescope and NOEMA, we detected [molecular oxygen] emission in [an] external galaxy for the first time," the researchers wrote in their paper.

"The detected O2 emission is located in regions about 10 kpc (32,615 light-years) away from the center of Markarian 231 and may be caused by the interaction between the active galactic nucleus-driven molecular outflow and the outer disc molecular clouds."

The team's measurements revealed that the abundance of oxygen compared to hydrogen was around 100 times higher than that found in the Orion nebula, so the galaxy could be undergoing a more intense version of the same molecule-splitting process.

As Markarian is a starburst galaxy, undergoing furious star formation, this could be possible. Just one region in the galaxy is forming new stars at a rate of over 100 solar masses a year. The Milky Way, by contrast, is pretty quiet, with a star formation rate of around 1 to two solar masses.

Better understand the complex dynamics of galactic oxygen

On the other hand, these results could also mean that more observations need to be made to confirm the detection of oxygen. If the results are valid, the phenomenon could be used to better understand both molecular oxygen in galaxies and the molecular flow of an active galactic nucleus.

"This first detection of extragalactic molecular oxygen provides an ideal tool to study active galactic nucleus-driven molecular outflows on dynamic timescales of tens of megayears," they wrote.

"O2 may be a significant coolant for molecular gas in such regions affected by active galactic nucleus-driven outflows." conclude the authors.


Molecular Oxygen in the Nearest QSO Mrk 231

Junzhi Wang, Di Li, Paul F. Goldsmith, Zhi-Yu Zhang, Yu Gao, Yong Shi8, Shanghuo Li, Min Fang, Juan Li, and Jiangshui Zhang

Published 2020 January 30

The Astrophysical Journal, Volume 889, Number 2

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