Subaru and Gemini High Spatial Resolution Infrared 18 Micron Imaging Observations of Nearby Luminous Infrared Galaxies

TL;DR

This study uses high-resolution infrared imaging from Subaru and Gemini telescopes to distinguish between starburst and active galactic nucleus energy sources in luminous infrared galaxies, revealing that AGN candidates have compact, high-brightness emission.

Contribution

It provides the first high-resolution 18 micron images of LIRGs, demonstrating the effectiveness of surface brightness as a diagnostic for hidden AGNs versus starbursts.

Findings

AGN candidates show compact, high surface brightness emission.

Starburst LIRGs exhibit extended, modest surface brightness emission.

Infrared imaging and spectroscopy are consistent in diagnosing energy sources.

Abstract

We present the results of a ground-based, high spatial resolution infrared 18 micron imaging study of nearby luminous infrared galaxies (LIRGs), using the Subaru 8.2-m and Gemini South 8.1-m telescopes. The diffraction-limited images routinely achieved with these telescopes in the Q-band (17-23 micron) allow us to investigate the detailed spatial distribution of infrared emission in these LIRGs. We then investigate whether the emission surface brightnesses are modest, as observed in starbursts, or are so high that luminous active galactic nuclei (AGNs; high emission surface brightness energy sources) are indicated. The sample consists of 18 luminous buried AGN candidates and starburst-classified LIRGs identified in earlier infrared spectroscopy. We find that the infrared 18 micron emission from the buried AGN candidates is generally compact, and the estimated emission surface brightnesses are high, sometimes exceeding the maximum value observed in and theoretically predicted for a starburst phenomenon. The starburst-classified LIRGs usually display spatially extended 18 micron emission and the estimated emission surface brightnesses are modest, within the range sustained by a starburst phenomenon. The general agreement between infrared spectroscopic and imaging energy diagnostic methods suggests that both are useful tools for understanding the hidden energy sources of the dusty LIRG population.