Processing of polycyclic aromatic hydrocarbons in molecular-loop regions near the Galactic center revealed by AKARI

TL;DR

This study uses AKARI mid-infrared observations to analyze polycyclic aromatic hydrocarbons in molecular-loop regions near the Galactic center, revealing their processing and destruction in these environments.

Contribution

First detailed mid-infrared mapping of aromatic hydrocarbons in Galactic center molecular loops, linking IR emission suppression to grain processing and destruction.

Findings

Detection of 9 um emission structures extending from the Galactic plane

Suppressed 9 um brightness near CO loop foot points

Absence of 3.3 um aromatic hydrocarbon emission in loop regions

Abstract

We present the AKARI mid-infrared diffuse map of an area of about 4 deg x 3 deg near the Galactic center in the 9 um band. The band intensity is mostly attributed to the aromatic hydrocarbon infrared emissions of carbonaceous grains at wavelengths of 6.2, 7,7, 8.6, and 11.3 um. We detect the 9 um emission structures extending from the Galactic plane up to the latitude of about 2.5 deg, which have spatial correspondence to the molecular loops revealed by the NANTEN 12CO (J=1-0) observations. We find that the surface brightness at 9 um is suppressed near the foot points of the CO loops. The ratios of the 9 um to the IRAS 100 um brightness show significant depression near such bright regions in the CO emission. With the AKARI near-IR 2.5--5 um spectroscopy, we find that the 3.3 um aromatic hydrocarbon emission is absent in the region associated with the loop. These suggest the processing and destruction of carbonaceous grains in the CO molecular loops.