Properties of dust in the Galactic center region probed by AKARI
  far-infrared spectral mapping - detection of a dust feature
  We investigate the properties of interstellar dust in the Galactic center
region toward the Arches and Quintuplet clusters. With the Fourier Transform
Spectrometer of the AKARI/Far-Infrared Surveyor, we performed the far-infrared
(60 - 140 cm^-1) spectral mapping of an area of about 10' x 10' which includes
the two clusters to obtain a low-resolution (R = 1.2 cm^-1) spectrum at every
spatial bin of 30" x 30". We derive the spatial variations of dust continuum
emission at different wavenumbers, which are compared with those of the [O III]
88 micron (113 cm^-1) emission and the OH 119 micron (84 cm^-1) absorption. The
spectral fitting shows that two dust modified blackbody components with
temperatures of ~20 K and ~50 K can reproduce most of the continuum spectra.
For some spectra, however, we find that there exists a significant excess on
top of a modified blackbody continuum around 80 - 90 cm^-1 (110 - 130 microns).
The warmer dust component is spatially correlated well with the [O III]
emission and hence likely to be associated with the highly-ionized gas locally
heated by intense radiation from the two clusters. The excess emission probably
represents a dust feature, which is found to be spatially correlated with the
OH absorption and a CO cloud. We find that a dust model including micron-sized
graphite grains can reproduce the observed spectrum with the dust feature
fairly well.
