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Ices in the Galactic Centre : solid ice and gaseous CO in the central parsec

For the past few years, we have observed the central half parsec of our Galaxy in the mid-infrared from 2.8 to 5.1 micron. Our aim is to improve our understanding of the direct environment of SgrA*, the supermassive blackhole at the centre of the Milky Way. This work is described in the present paper and by Moultaka et al. 2015 (submitted). Here, we focus on the study of the spatial distribution of the 12CO ice and gas-phase absorptions. We observed the central half parsec with ISAAC spectrograph located at the UT3/VLT ESO telescope in Chile. The slit was placed along 22 positions arranged parallel to each other to map the region. We built the first data cube in this wavelength range covering the central half parsec. The wavelength interval of the used M-band filter ranges from 4.6 to 5.1 micron. It hosts the P- and R- branches of the ro-vibrational transitions of the gaseous 12CO and 13CO, as well as the absorption band attributed to the 12CO ice at 4.675 micron. Using two calibrators, we could disentangle the local from the line-of-sight absorptions and provide a first-order estimate of the foreground extinction. We find residual ices and gase-phase CO that can be attributed to local absorptions due to material from the interstellar and/or the circumstellar medium of the central parsec. Our finding implies temperatures of the order of 10 to 60K which is in agreement with the presence of water ices in the region highlighted by Moultaka et al. (2004, 2005).