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AKARI carried out all-sky surveys at wavelengths of 9, 18, 65, 90, 140 and 160 μm. Among them, the 9μm map can efficiently trace the emission features of polycyclic aromatic hydrocarbons (PAHs). Utilizing this unique all-sky PAH map, we investigate the distribution of the PAH emission for the whole Galactic plane. In particular, the Galactic center regions suffer foreground contamination by the zodiacal light, and therefore we have carefully removed the zodiacal light component by modifying the Kelsall model. As a result, we find that the PAH emission is extended very widely ( ∼ 30◦) toward the north direction from the Galactic center region. Since it is known that there are very nearby ( ∼150 pc) Ophiuchus clouds toward similar directions, we subtracted the foreground component associated with these clouds using the HI data. As a result, we confirm that there still remains a significant fraction of the widely-extended PAH emission toward this direction, suggesting that the excess PAH emission originates in the Galactic center region. The spatial distribution resembles that of the Fermi bubbles, well-known gamma-ray structures, which are thought to be created by a past outflow in the Galactic center. Moreover, the PAH/dust ratio is significantly enhanced in the region showing the excess PAH emission. The high PAH/dust ratio is possibly created by shattering of carbonaceous grains in the past outflow from the Galactic center.

Large-scale extraplanar PAH emission in the Galactic center region found by AKARI mid-infrared all-sky survey