Galaxy Scale Interstellar Infrared Spectrum Reproduced By A Hydrocarbon Pentagon-Hexagon Combined Molecule

TL;DR

This study demonstrates that a specific hydrocarbon pentagon-hexagon combined molecule can accurately reproduce the interstellar infrared spectra observed in multiple galaxies, suggesting it as a common carrier of interstellar dust IR features.

Contribution

The paper extends previous quantum chemistry models to multiple galaxies, showing that a single hydrocarbon molecule can replicate galaxy-scale IR spectra.

Findings

Successfully reproduced major IR bands of NGC6946 and M83.

Predicted weaker IR bands aligned well with observations.

Calculated intensity ratios matched observed ranges, especially in M83.

Abstract

Interstellar dust shows ubiquitous interstellar infrared spectrum (IR) due to polycyclic aromatic hydrocarbon (PAH). By our previous quantum chemistry calculation, it was suggested that a molecule group having hydrocarbon pentagon-hexagon combined skeleton could reproduce observed IR of dust clouds in Milky Way galaxy. This paper extends to other many galaxies. Typical galaxies are NGC6946 and M83. Those infrared spectrum were compared with that of a model molecule (C23H12)2+ having hydrocarbon two pentagons combined with five hexagons. Observed major infrared bands of 6.2, 7.7, 8.6, and 11.3 micrometer were successfully reproduced as 6.4, 7.7, 8.5, and 11.2 micrometer. Even observed weaker bands of 12.0, 12.7, 14.2 micrometer were predicted well by computed bands as 12.0, 12.6, and 13.9 micrometer. IR intensity ratio was compared to check theoretical validity. Calculated intensity ratio between 7.7 versus 11.3 micrometer (PAH7.7/11.3) was 4.0, whereas observed ratio was in a range of 2~6, also calculated PAH6.2/11.3 was 1.4 for observed range of 0.9~2.6. Especially, every calculated ratio was so close to that of M83 arm region. It should be noted that both calculated wavelength and intensity could reproduce observed galaxy scale infrared spectrum. Hydrocarbon pentagon-hexagon molecule would be general carrier in many galaxies including Milky Way.