Destruction and multiple ionization of PAHs by X-rays in circumnuclear regions of AGNs

Abstract

The infrared signatures of polycyclic aromatic hydrocarbons (PAHs) are observed in a variety of astrophysical objects, including the circumnuclear medium of active galactic nuclei (AGNs). These are sources of highly energetic photons (0.2 to 10 keV), exposing the PAHs to a harsh environment. In this work, we examined experimentally the photoionization and photostability of naphthalene (C 10 H 8 ), anthracene (C 14 H 10 ), 2methyl-anthracene (C 14 H 9 CH 3 ) and pyrene (C 16 H 10 ) upon interaction with photons of 275, 310 and 2500 eV. The measurements were performed at the Brazilian Synchrotron Light Laboratory using time-of-flight mass-spectrometry (TOF-MS). We determined the absolute photoionization and photodissociation cross sections as a function of the incident photon energy; the production rates of singly, doubly and triply charged ions; and the molecular half-lives in regions surrounding AGNs. Even considering moderate X-ray optical depth values (τ = 4.45) due to attenuation by the dusty torus, the halflives are not long enough to account for PAH detection. Our results suggest that a more sophisticated interplay between PAHs and dust grains should be present in order to circumvent molecular destruction. We could not see any significant difference in the half-life values by increasing the size of the PAH carbon backbone, N C , from 10 to 16. However, we show that the multiple photoionization rates are significantly greater than the single ones, irrespective of the AGN source. We suggest that an enrichment of multiply charged ions caused by X-rays can occur in AGNs.

Cold molecular gas and PAH emission in the nuclear and circumnuclear regions of Seyfert galaxies

“…In addition to the photodissociation cross-section of naphthalene, we used the nuclear H 2 column densities (Table 5), 2−10 keV intrinsic luminosities, and distances from the AGN. The main difference with similar calculations reported by Monfredini et al (2019) is that in our case, the column densities are measured over the same region as is probed by the ground-based mid-IR slit.…”

“…We chose the naphthalene molecule (C 10 H 8 ) as an illustrative example to compute the half-lives of PAHs in the nuclear regions of our Seyfert galaxies. This PAH molecule among those studied by Monfredini et al (2019) has the smallest photodissociation cross-section when exposed to hard X-ray photons at E = 2.5 keV. Thus, naphthalene provides upper limits to the halflives (see Eq.…”

“…We investigated whether an additional dependency of the detection of nuclear PAH emission on the AGN luminosity and the distance from the AGN might be established (see also Esquej et al 2014;Alonso-Herrero et al 2014). Following Monfredini et al (2019), at a given distance from the AGN (r AGN ), the X-ray photon flux on a PAH molecule for a given photon energy is…”

“…However, Voit (1992) postulated that when PAH emission is detected in the nuclear or circumnuclear regions of AGN, the PAH molecules must be shielded by high column densities of X-ray absorbing gas. Recently, Monfredini et al (2019) presented experimental work to evaluate the photoionization and photodissociation properties of some simple (number of carbon atoms between 10 and 16) PAH molecules when exposed to X-ray photons. They demonstrated that this process results in dissociation of the molecules and enrichment of multiply charged ions caused by X-ray photoselection.…”

Starbursts versus active galactic nuclei: Studying astrochemistry of dusty galaxies

“…We suggest that AGN can impact the composition of the galaxy ISM by altering the PAH population, without destroying these molecules. This is in agreement with experimental (Monfredini et al 2019;Quitián-Lara et al 2018) and observational works (Lambrides et al 2019;Minsley et al 2020). This is also consistent with Alonso-Herrero et al (2020), who points out that PAH molecules are being shielded from AGN radiation by the large content of molecular cold gas.…”