Webb Detects Polycyclic Aromatic Hydrocarbons in Nuclear Regions of Seyfert Galaxies

by johnsmith

Astronomers have analyzed publicly available data from the Mid-Infrared Instrument (MIRI) on the NASA/ESA/CSA James Webb Space Telescope for three Seyfert galaxies — NGC 6552, NGC 7469, and NGC 7319 — and investigated the properties of their polycyclic aromatic hydrocarbon emission.

This image, taken with Webb’s Mid-InfraRed Instrument (MIRI), shows the barred spiral galaxy NGC 7469. Image credit: Bohn et al., arXiv: 2209.04466.

This image, taken with Webb’s Mid-InfraRed Instrument (MIRI), shows the barred spiral galaxy NGC 7469. Image credit: Bohn et al., arXiv: 2209.04466.

Polycyclic aromatic hydrocarbons (PAHs) are carbon-based molecules that typically include one or more carbon rings.

These molecules absorb a significant fraction of ultraviolet or optical photons from young stars.

PAH features are considered excellent tracers of the star formation activity in star-forming galaxies and active galactic nuclei.

In a new study, University of Oxford astronomer Ismael García-Bernete and colleagues used the spectroscopic data from Webb’s MIRI instrument, which specifically measures light in the 5-28 micron wavelength range, to characterize the PAH properties of the nuclear regions of three galaxies: NGC 6552, NGC 7469, and NGC 7319.

The astronomers then compared the observations with theoretical predictions for these molecules.

Surprisingly, their results overturned those of previous studies that had predicted that PAH molecules would be destroyed in the vicinity of the black hole at the center of an active galaxy.

Instead, the team revealed that PAH molecules can actually survive in this region, even where very energetic photons could potentially rip them apart.

A potential reason could be that the molecules are protected by large amounts of molecular gas in the nuclear region.

“The Webb/MIRI provides us with a fantastic opportunity to observe galaxies in a way that just hasn’t been possible up until now,” Dr. García-Bernete said.

“We were excited to find that these organic molecules can actually survive in extremely harsh conditions.”

However, even where PAH molecules survived, the results showed that the supermassive black holes at the heart of galaxies had a significant impact on their properties.

In particular, the proportion of larger and neutral molecules became greater, indicating that more fragile small and charged PAH molecules may have been destroyed.

This brings severe limitations to using these PAH molecules to probe how rapidly an active galaxy makes new stars.

“This research is of great interest to the wider astronomy community, particularly those focused on the formation of planets and stars in the most distant and faint galaxies,” Dr. García-Bernete said.

“It is incredible to think that we can observe PAH molecules in the nuclear region of a galaxy and the next step is to analyze a larger sample of active galaxies with different properties.”

“This will enable us to better understand how PAH molecules survive and which are their specific properties in the nuclear region.”

“Such knowledge is key to using PAHs as an accurate tool for characterizing the amount of star formation in galaxies, and thus, how galaxies evolve over time.”

The findings appear in the journal Astronomy & Astrophysics.


I. García-Bernete et al. 2022. A high angular resolution view of the PAH emission in Seyfert galaxies using JWST/MRS data. A&A, published online September 30, 2022; doi: 10.1051/0004-6361/202244806

Source link: https://www.sci.news/astronomy/webb-polycyclic-aromatic-hydrocarbons-seyfert-galaxies-11280.html

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