Properties of dust at the Galactic center probed by AKARI far-infrared spectral mapping - Detection of a dust feature

Aims. We investigate the properties of interstellar dust in the Galactic center region toward the Arches and Quintuplet clusters. Methods. 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 � × 10 � that includes both clusters to obtain a low-resolution (R = 1. 2c m −1 ) spectrum at every spatial bin of 30 �� × 30 �� . Results. We derive the spatial variations in the dust continuum emission at different wavenumbers, which are compared with those of the [OIII] 88 μm (113 cm −1 ) emission and the OH 119 μ m( 84 cm −1 ) absorption. The spectral fitting shows that two 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 is a significant excess on top of a modified blackbody continuum around 80–90 cm −1 (110–130 μm). Conclusions. The warmer dust component is spatially correlated with the [OIII] 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 quite closely reproduce the observed spectrum with the dust feature.