Absence of nuclear polycyclic aromatic hydrocarbon emission from a compact starburst: The case of the type-2 quasar Mrk 477

TL;DR

This study uses high-resolution mid-infrared observations of the nearby type-2 quasar Mrk 477 to show that PAH emission, a star formation tracer, is absent near the nucleus, likely due to destruction by active galactic nucleus feedback.

Contribution

It provides the first detailed mid-IR spectroscopic evidence that PAH molecules are destroyed near an active nucleus, challenging their reliability as star formation indicators in such regions.

Findings

No PAH features detected in the nuclear region.

Presence of a young nuclear starburst with no PAH emission.

PAH molecules likely destroyed by AGN activity despite high X-ray column density.

Abstract

Mrk 477 is the closest type-2 quasar (QSO2), at a distance of 163 Mpc. This makes it an ideal laboratory for studying the interplay between nuclear activity and star formation with a great level of detail and signal-to-noise. In this Letter we present new mid-infrared (mid-IR) imaging and spectroscopic data with an angular resolution of 0.4 arcsec (~300 pc) obtained with the Gran Telescopio Canarias (GTC) instrument CanariCam. The N-band (8-13 micron) spectrum of the central ~400 pc of the galaxy reveals [S IV]10.51 micron emission, but no 8.6 or 11.3 micron PAH features, which are commonly used as tracers of recent star formation. This is in stark contrast with the presence of a nuclear starburst of ~300 pc in size, an age of 6 Myr, and a mass of 1.1x10^8 Msun, as constrained from ultraviolet Hubble Space Telescope observations. Considering this, we argue that even the more resilient, neutral molecules that mainly produce the 11.3 micron PAH band are most likely being destroyed in the vicinity of the active nucleus despite the relatively large X-ray column density, of log N_H=23.5 cm^-2, and modest X-ray luminosity, of 1.5x10^43 erg/s. This highlights the importance of being cautious when using PAH features as star formation tracers in the central region of galaxies to evaluate the impact of feedback from active galactic nuclei.