Near- to mid-Infrared Observations of Galaxy Mergers: NGC2782 and NGC7727

Abstract

We present the results of near- to mid-infrared (NIR to MIR) imaging and NIR spectroscopic observations of two galaxy mergers, NGC 2782 (Arp 215) and NGC 7727 (Arp 222), with the Infrared Camera on board AKARI. NGC 2782 shows extended MIR emission in the eastern side of the galaxy, which corresponds to the eastern tidal tail seen in the HI 21 cm map, while NGC 7727 shows extended MIR emission in the north of the galaxy, which is similar to the plumes seen in the residual image at the K-band after subtracting a galaxy model. Both extended structures are thought to have formed associated with their merger events. They show excess emission at 7--15 micron, which can be attributed to emission from polycyclic aromatic hydrocarbons (PAHs), while the observed spectral energy distributions decline longward of 24 micron, suggesting that very small grains (VSGs) are deficient. These characteristics of the observed MIR spectral energy distribution may be explained if PAHs are formed by fragmentation of VSGs during merger events. The star formation rate is estimated from the MIR PAH emission in the eastern tail region of NGC 2782 and it is in fair agreement with those estimated from Halpha and [CII] 158 micron. MIR observations are efficient for the study of dust processing and structures formed during merger events.

14 figure, to appear in the Astrophysical Journal

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