Organic Molecules in an Early Galaxy

Einstein ring

The early galaxy’s light was distorted into a ring by gravitational lensing. The orange area in the ring indicates the spectral signature of the polycyclic aromatic hydrocarbons. © J Spilker / S Doyle, NASA, ESA, CSA

Complex organic molecules exist not only on earth, but also in space. There they play an important role, among other things as “midwives” of new stars. Now, with the help of the James Webb telescope, astronomers have for the first time detected such polycyclic aromatic hydrocarbons (PAHs) in a galaxy more than twelve billion light-years away - it existed at a time only around 1.5 billion years after the Big Bang. This is the first evidence for the presence of these organic molecules in such an early galaxy. At the same time, the distribution of PAHs in this cluster of stars raises questions, because they deviate from what astronomers report.

Whether acetic acid, formaldehyde or the football-shaped fullerenes: astronomers have already detected a whole series of organic molecules in space. Most of them are found around young stars, in planetary nebulae or in the glowing remnants of supernovae. Astronomers have also observed numerous more complex organic compounds such as polycyclic aromatic hydrocarbons (PAHs) in the cool, dusty molecular clouds of many stellar cradles. These ring-shaped hydrocarbons linked by double bonds are considered an air pollutant on Earth, but in space these molecules serve as condensation nuclei for interstellar dust and play an important role in the formation of new stars. "These large molecules are actually relatively common in the cosmos," explains first author Justin Spilker from Texas A&M University. "Where these molecules have been found, baby stars usually also shine."

With the MIRI spectrometer and a gravitational lens

Spilker's team has now succeeded for the first time in detecting such organic molecules in the early cosmos. To do this, they targeted the galaxy SPT0418-47, which is around twelve billion light-years away, with the MIRI spectrometer of the James Webb telescope. This galaxy was chosen because its light was amplified and magnified by the phenomenon of gravitational lensing. This effect occurs when a massive foreground object, such as a galaxy cluster or large galaxy, moves in front of a distant object in such a way that its light is deflected and distorted by the gravitational curvature of spacetime. The foreground object acts like a kind of magnifying glass. "By combining the power of the Webb telescope with this natural magnifying glass, we were able to see more detail than is normally possible," explains Spilker.

Using the MIRI spectrometer, the astronomers searched for the spectral signature of polycyclic aromatic hydrocarbons in the galaxy SPT0418-47. These typically leave behind a clearly visible spectral line in the range of 3.3 microns wavelength. "This 3.3 micrometer line - the PAH signature with the shortest wavelength - arises from the vibrational modes of the carbon-hydrogen bonds in the PAH molecule," the researchers explain. "It is only emitted by small neutral PAHs with less than 100 carbon atoms." In fact, the astronomers managed to detect this signature in the distant galaxy SPT0418-47. Closer analysis confirmed that this spectral line could not have come from other possible sources of such spectral signatures, such as the dust around an active galactic core. Instead, the signal most likely comes from organic molecules in the cool gas and dust surrounding star-forming zones, the astronomers report.

PAHs from around 1.5 billion years after the Big Bang

This is the first time the team has detected such complex organic molecules in a galaxy of the early cosmos. They already existed around 1.5 billion years after the Big Bang, when the universe was only ten percent of its current age. "It's amazing that we can identify these molecules found in smoke and smog on Earth even billions of light-years away," says co-author Kedar Phadke of the University of Illinois at Urbana-Champaign. "But discoveries like these are exactly what the James Webb telescope was built for: so that we can learn about and understand the earliest stages of our universe in new and exciting ways." The new data show that there are striking variations in the distribution of PAHs in the distant galaxy - and that the zones of their greatest density are not necessarily where one would expect: "The radiation from PAH molecules, hot dust and large dust grains as well as stars are distributed differently in space," report the astronomers . Some parts of the galaxy had star formation but no PAHs, while others had PAHs but no star formation.

According to Spilker and his colleagues, this could indicate that there were complex, localized processes in such early galaxies that are not yet fully understood. They could have led to the spatial separation of coarse dust and organic molecules, the team explains. However, it is also conceivable that local differences in the ultraviolet radiation produced these differences. "Now that we have demonstrated that it is possible to detect these signatures in the early cosmos, we now want to pursue this question further," says Spilker. In follow-up observations, the astronomers want to clarify whether the polycyclic aromatic hydrocarbons in such galaxies are also closely linked to star formation or not. "The only way to find out is to look at more such galaxies - maybe even more distant than this one," says the researcher.

Source: Justin Spilker (Texas A&M University, College Station) et al., Nature, doi: 10.1038/s41586-023-05998-6

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