A recent study, led by the Center for Astrobiology (CAB), CSIC-INTA and using modelling techniques developed at the University of Oxford, has uncovered an unprecedented richness of small organic molecules in the deeply obscured nucleus of a nearby galaxy, thanks to observations made with the James Webb Space Telescope (JWST). The work, published in Nature Astronomy, provides new insights into how complex organic molecules and carbon are processed in some of the most extreme environments in the Universe.
The study focuses on IRAS 07251–0248, an ultra-luminous infrared galaxy whose nucleus is hidden behind vast amounts of gas and dust. This material absorbs most of the radiation emitted by the central supermassive black hole, making it extremely difficult to study with conventional telescopes. However, the infrared wavelength range penetrates the dust and provides unique information about these regions, revealing the dominant chemical processes in this extremely dusty nucleus.
State-of-the-art instruments
The team used spectroscopic observations from the JWST space telescope covering the 3–28 micron wavelength range, combining data from the NIRSpec and MIRI instruments. These observations allow the detection of chemical signatures from gas-phase molecules, as well as features from ices and dust grains. Thanks to these data, the researchers were able to characterize the abundance and temperature of numerous chemical species in the nucleus of this buried galaxy.
The observations reveal an extraordinarily rich inventory of small organic molecules, including benzene (C₆H₆), methane (CH₄), acetylene (C₂H₂), diacetylene (C₄H₂), and triacetylene (C₆H₂), and, detected for the first time outside the Milky Way, the methyl radical (CH₃). In addition to gas-phase molecules, a large abundance of solid molecular materials was found, such as carbonaceous grains and water ices.
"We found an unexpected chemical complexity, with abundances far higher than predicted by current theoretical models," explains lead author Dr Ismael García Bernete formerly of Oxford University and now a researcher at CAB. "This indicates that there must be a continuous source of carbon in these galactic nuclei fuelling this rich chemical network."
These molecules could play a key role as fundamental building blocks for complex organic chemistry, of interest for processes relevant to life. Co-author Professor Dimitra Rigopoulou (Department of Physics, University of Oxford) adds: " Although small organic molecules are not found in living cells, they could play a vital role in prebiotic chemistry representing an important step towards the formation of amino acids and nucleotides."
Factories of organic molecules in the Universe
The analysis, involving techniques and theoretical polycyclic aromatic hydrocarbons (PAHs) models developed by the Oxford group, suggests that the observed chemistry cannot be explained solely by high temperatures or turbulent gas motions. Instead, the results point to cosmic rays, abundant in these extreme nuclei, as fragmenting PAHs and carbon-rich dust grains, releasing small organic molecules into the gas phase.
The study also finds a clear correlation between hydrocarbon abundance and the intensity of cosmic-ray ionization in similar galaxies, supporting this scenario. These results suggest that deeply obscured galactic nuclei could act as factories of organic molecules, playing a key role in the chemical evolution of galaxies.
This work opens new avenues to study the formation and processing of organic molecules in space extreme environments and demonstrates the enormous potential of JWST to explore regions of the Universe that have remained hidden until now.
In addition to CAB, the following institutions also contributed to this work: Instituto de Física Fundamental (CSIC; M. Pereira-Santaella, M. Agúndez, G. Speranza), University of Alcalá (E. González-Alfonso) and University of Oxford (D. Rigopoulou, F.R. Donnan, N. Thatte).
