Rotational spectroscopy of interstellar PAHs

TL;DR

This paper proposes a new method for detecting interstellar PAHs by analyzing their rotational line emission, using the characteristic 'comb' pattern of quasi-symmetric PAHs to enable unambiguous identification.

Contribution

It analytically computes the rotational spectra of quasi-symmetric PAHs and demonstrates their potential detectability via radio observations using matched-filtering techniques.

Findings

Rotational spectra of quasi-symmetric PAHs form a recognizable 'comb' pattern.

Matched-filtering can significantly improve detection sensitivity.

Detection of PAH 'combs' would confirm specific interstellar PAHs.

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are believed to be ubiquitous in the interstellar medium. Yet, to date no specific PAH molecule has been identified. In this paper, a new observational avenue is suggested to detect individual PAHs, using their rotational line emission at radio frequencies. Previous PAH searches based on rotational spectroscopy have only targeted the bowl-shaped corannulene molecule, with the underlying assumption that other polar PAHs are triaxial and have a complex and diluted spectrum unusable for identification purposes. In this paper the rotational spectrum of quasi-symmetric PAHs is computed analytically. It is shown that the asymmetry of planar, nitrogen-substituted symmetric PAHs is small enough that their rotational spectrum, when observed with a resolution of about a MHz, has the appearance of a "comb" of evenly spaced stacks of lines. The simple pattern of these "comb" spectra allows for the use of matched-filtering techniques, which can result in a significantly enhanced signal-to-noise ratio. Detection forecasts are discussed for regions harbouring "anomalous microwave emission", believed to originate from the collective PAH rotational emission. A systematic search for PAH lines in various environments is advocated. If detected, PAH "combs" would allow to the conclusive and unambiguous identification of specific, free-floating interstellar PAHs.