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

Abstract

Analyses of the polycyclic aromatic hydrocarbon (PAH) feature profiles, especially the 6.2 µm 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 (Hernán-Caballero & Hatziminaoglou 2011), have their 6.2 µm 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 µm, 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.

Spitzer’s perspective of polycyclic aromatic hydrocarbons in galaxies

“…While the profile variations of the 7.7 µm band seen in the ISO/SWS spectra were employed as a spectral classification scheme 42 , the Spitzer/IRS observations led to further classifications of the PAH spectra 65,66 . Moreover, the subtle variations in the peak wavelength of the 6.2 µm emission band, commonly attributed to polycyclic aromatic nitrogen heterocycles -PAHs with one or more nitrogen atoms substituted into their carbon skeleton 67 -were recently seen in the Spitzer/IRS spectra of starburst galaxies 68 . 2D spectral and photometric mapping of PAH emission with Spitzer.…”

Theoretical study of infrared spectra of interstellar PAH molecules with N, NH & NH$_2$ incorporation

“…Classes C and D are characterized by a very broad feature at around 8 µm, while Class D also shows a broad 6.2 µm band. Class A objects are found more prevalent in the observations (Canelo et al 2018;Pino et al 2008). Hudgins et al (2005) conclude the viable existence of N-containing PAHs (PANHs); large size endoskeletal PANHs (≥ 50 C atoms) mainly in Class A objects, while small size exoskeletal PANHs (≤ 50 C atoms) for Class B.…”

A systematic survey for z< 0.04 CLAGNs

“…Some objects in our sample do show interesting spectral variability while falling below our CLAGN thresholds, and we retain them in this work while not classifying them as CLAGNs. AGNs such as Mrk 883, which change type by only 0.1 do not meet our CLAGN criteria, however, it is considered a CLAGN by Canelo et al (2018).…”

“…Examples of low-redshift CLAGNs from the literature include Mrk 833 (Canelo et al 2018), which changed from type 1.9 to type 1.8, NGC 7603 (Tohline & Osterbrock 1976), which changed from type 1 to type 1.5, Mrk 372 (Penston & Perez 1984), which changed from type 1.5 to type 1.9 and NGC 1566 (Oknyansky et al 2018), which changed from type 1.9 to type 1.2. While there is not a strict definition in the literature, for consistency with the literature we classify objects as CLAGN if the broad-line components completely disappears, a new broad-line component appears and/or if the Osterbrock (1977) and Osterbrock (1981) spectral type changes by more than 0.1 (that is a change from type 1.8 to 1.9 and 1.9 to 2.0 and vice versa is not significant enough to be classified as a CLAGN).…”