Prof. Dimitra Rigopoulou

Abstract:

This Letter presents an analysis of the infrared (∼3–28 μm) spectra extracted from the nuclear (r < 150 pc) regions of four low-luminosity active galactic nuclei (AGN), observed by JWST NIRSpec/integral field unit and MIRI/Medium Resolution Spectroscopy as an extension of the Galaxy Activity, Torus, and Outflow Survey. We find that, compared to higher-luminosity AGN, these low-luminosity AGN exhibit distinct properties in their emission of ionized gas, polycyclic aromatic hydrocarbons (PAHs), and molecular hydrogen (H2). Specifically, the low-luminosity AGN exhibit relatively weak high ionization potential lines (e.g., [Ne V] and [O IV]), and the line ratios suggest that fast radiative shocks (with vs of ∼100s km s−1) are the primary excitation source of ionized gas therein. Under the low-excitation conditions of their nuclear regions, these low-luminosity AGN generally exhibit a higher fraction of PAHs with large size (NC ≳ 200), reflecting the preferential destruction of smaller PAH molecules by AGN feedback. Furthermore, the H2 transitions in these low-luminosity AGN are not fully thermalized, with slow, plausibly jet-driven molecular shocks (with vs ≤ 10 km s−1) likely being the extra excitation source. Taken together with results from the literature, these findings indicate that feedback operates in both low- and high-luminosity AGN, although its impact varies with AGN luminosity. In particular, systematic variations in PAH band ratios are found across AGN, demonstrating the differing influence of feedback in AGN of varying luminosities and highlighting the potential of PAH band ratios as diagnostics for distinguishing kinetic- and radiative-mode AGN feedback.

Abstract:

Hydrocarbons play a key role in shaping the chemistry of the interstellar medium, but their enrichment and relation with carbonaceous grains and polycyclic aromatic hydrocarbons still lack clear observational constraints. Here we report on JWST NIRSpec + MIRI/MRS infrared observations (~3–28 μm) of the local ultra-luminous infrared galaxy (ULIRG) IRAS 07251−0248, which revealed the extragalactic detection of small gas-phase hydrocarbons, such as benzene (C<inf>6</inf>H<inf>6</inf>), triacetylene (C<inf>6</inf>H<inf>2</inf>), diacetylene (C<inf>4</inf>H<inf>2</inf>), acetylene (C<inf>2</inf>H<inf>2</inf>), methane (CH<inf>4</inf>) and methyl radical (CH<inf>3</inf>), as well as deep amorphous C–H absorptions in the solid phase. The unexpectedly high abundance of these molecules indicates an extremely rich hydrocarbon chemistry not explained by high-temperature gas-phase chemistry, ice desorption or oxygen depletion. Instead, the most plausible explanation is the erosion and fragmentation of carbonaceous grains and polycyclic aromatic hydrocarbons. This scenario is supported by the correlation between the abundance of one of their main fragmentation products, C<inf>2</inf>H<inf>2</inf>, and the cosmic-ray ionization rate for a sample of local ULIRGs. These hydrocarbons are outflowing at ~160 km s⁻¹, which may represent a potential formation pathway for hydrogenated amorphous grains. Our results indicate that IRAS 07251−0248 might not be unique but represents an extreme example of the commonly rich hydrocarbon chemistry prevalent in deeply obscured galactic nuclei.