Oral Presentation
Comparison between submillimeter and near-infrared observations of IRAS 17208-0014
Presenter: Shunsuke Baba (NAOJ)
The nature of buried active galactic nuclei (AGNs) is still not well understood due to severe obscuration by the surrounding gas and dust. To investigate the distribution of warm gas around such AGNs, we observed the nucleus of the ultra-luminous infrared galaxy IRAS 17208-0014 in CO(6-5) and 435 um dust continuum with ALMA at the highest angular resolution (~0.04 arcsec, corresponding to ~30 pc) in Band 9 in Cycle 4. This galaxy is in a late-stage merger phase and may host a Compton-thick AGN. Its nuclear region was previously observed with ALMA in HCN(4-3) and HCO+(4-3) with one order of magnitude larger beam size, and these lines showed strong self-absorption (Aalto+15). Similarly, in the near-infrared (NIR) region, strong CO ro-vibrational absorption lines (v=1-0, delta-J=+/-1, ~4.7 micron) were spectroscopically detected with the AKARI satellite. From our ALMA observation we found that the dust emission peak coincided with the coordinates of the buried AGN suggested by previous researches and that at the same position the CO(6-5) line was detected in absorption. Converting the equivalent width of this absorption based on the assumption of the LTE partition function indicated by the AKARI spectrum, the CO total column density was calculated to be N_CO~4x10^18 cm^-2. On the other hand, from a spectral fitting to the AKARI spectrum, we obtained CO column density of ~1x10^19 cm^-2. These two column densities measured in the sub-mm and NIR regions coincide within a factor of ~3. This suggests that the detected CO pure-rotational and ro-vibrational absorptions are happening within the same beam. In this presentation, we discuss the usefulness of sub-mm and NIR absorption lines to investigate the nature of buried AGNs.
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