Variations in the 6.2 $\mu$m emission profile in starburst-dominated galaxies: a signature of polycyclic aromatic nitrogen heterocycles (PANHs)?

TL;DR

This study analyzes the 6.2 μm PAH emission profiles in starburst galaxies, revealing a significant presence of nitrogen-containing heterocycles (PANHs) that may serve as an important nitrogen reservoir in the universe.

Contribution

It provides the first large-scale classification of PAH profiles in starburst galaxies, linking specific spectral features to the presence of PANHs and their role in interstellar nitrogen.

Findings

67% of galaxies show class A profiles near 6.22 μm

Class A profiles are explained by PANHs presence

PANHs may be a significant nitrogen reservoir in the universe

Abstract

Analyses of the polycyclic aromatic hydrocarbon (PAH) feature profiles, especially the 6.2 $\mu$m feature, could indicate the presence of nitrogen incorporated in their aromatic rings. In this work, 155 predominantly starburst-dominated galaxies (including HII regions and Seyferts, for example), extracted from the Spitzer/IRS ATLAS project (Hern\'an-Caballero & Hatziminaoglou 2011), have their 6.2 $\mu$m profiles fitted allowing their separation into the Peeters' A, B and C classes (Peeters et al. 2002). 67% of these galaxies were classified as class A, 31% were as class B and 2% as class C. Currently class A sources, corresponding to a central wavelength near 6.22 $\mu$m, seem only to be explained by polycyclic aromatic nitrogen heterocycles (PANH, Hudgins et al. 2005), whereas class B may represent a mix between PAHs and PANHs emissions or different PANH structures or ionization states. Therefore, these spectra suggest a significant presence of PANHs in the interstellar medium (ISM) of these galaxies that could be related to their starburst-dominated emission. These results also suggest that PANHs constitute another reservoir of nitrogen in the Universe, in addition to the nitrogen in the gas phase and ices of the ISM.