AKARI Detections of Hot Dust in Luminous Infrared Galaxies

TL;DR

This paper reports the detection of hot dust in luminous infrared galaxies using AKARI's mid-infrared survey, revealing obscured AGNs missed by optical surveys and highlighting the survey's effectiveness in identifying buried active galactic nuclei.

Contribution

It introduces a new method combining AKARI MIR excess selection and follow-up spectroscopy to identify obscured AGNs with hot dust, expanding the known population of such objects.

Findings

Detected hot dust (~500K) in two luminous infrared galaxies.

Identified AGNs with small host galaxy stellar masses (~4-6 x 10^9 M_sun).

Demonstrated AKARI's effectiveness in finding heavily obscured AGNs missed by previous surveys.

Abstract

We present a new sample of active galactic nuclei (AGNs) identified using the catalog of the AKARI Mid-infrared(MIR) All-Sky Survey. Our MIR search has an advantage in detecting AGNs that are obscured at optical wavelengths due to extinction. We first selected AKARI 9micron excess sources with F(9micron)/F(K_S)>2 where K_S magnitudes were taken from the Two Micron All Sky Survey. We then obtained follow-up near-infrared spectroscopy with the AKARI/IRC, to confirm that the excess is caused by hot dust. We also obtained optical spectroscopy with the Kast Double Spectrograph on the Shane 3-m telescope at Lick Observatory. On the basis of on these observations, we detected hot dust with a characteristic temperature of ~500K in two luminous infrared galaxies. The hot dust is suspected to be associated with AGNs that exhibit their nonstellar activity not in the optical, but in the near- and mid-infrared bands, i.e., they harbor buried AGNs. The host galaxy stellar masses of 4-6 x 10^9 M_sun are small compared with the hosts in optically-selected AGN populations. These objects were missed by previous surveys, demonstrating the power of the AKARI MIR All-Sky Survey to widen AGN searches to include more heavily obscured objects. The existence of multiple dusty star clusters with massive stars cannot be completely ruled out with our current data.