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Register a new userMillimeter Interferometric Investigations of the Energy Sources of Three Ultraluminous Infrared Galaxies, UGC 5101, Mrk 273, and IRAS 17208-0014, based on HCN to HCO+ Ratios
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Masatoshi Imanishi n NAO Japan
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We present interferometric observations of three ultraluminous infrared galaxies (ULIRGs; UGC 5101, Mrk 273, and IRAS 17208-0014) in the 3-mm wavelength range, using the Nobeyama Millimeter Array. Both the HCN (J=1-0) and HCO+ (J=1-0) molecular lines were observed simultaneously. HCN emission was clearly detected at the nuclear positions of these ULIRGs, and HCO+ emission was detected at the nuclear positions of UGC 5101 and IRAS 17208-0014. The HCN to HCO+ brightness-temperature ratios toward the nuclei of the three ULIRGs were derived and compared with those of lower luminosity galaxies known to be dominated by active galactic nuclei (AGNs) or starbursts. In UGC 5101 and Mrk 273, where there is evidence for obscured AGNs from previous observations at other wavelengths, we found high HCN/HCO+ ratios (>1.8) that are in the range found for AGN-dominated galaxies. In IRAS 17208-0014, where the presence of a powerful obscured AGN has been unclear, the ratio (1.7) is in between the observed values for starburst- and AGN-dominated galaxies. The high HCN/HCO+ brightness-temperature ratios in UGC 5101 and Mrk 273 could be the consequence of an HCN abundance enhancement, which is expected from chemical effects of the central X-ray emitting AGN on the surrounding dense molecular gas. Our proposed millimeter interferometric method based on HCN/HCO+ ratios may be an effective tool for unveiling elusive buried AGNs at the cores of ULIRGs, especially because of the negligible dust extinction at these wavelengths.
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We present the results of ALMA Cycle 2 observations of the ultraluminous infrared galaxy, IRAS 20551-4250, at HCN/HCO+/HNC J=3-2 lines at both vibrational-ground (v=0) and vibrationally excited (v2=1) levels. This galaxy contains a luminous buried active galactic nucleus (AGN), in addition to starburst activity, and our ALMA Cycle 0 data revealed a tentatively detected vibrationally excited HCN v2=1f J=4-3 emission line. In our ALMA Cycle 2 data, the HCN/HCO+/HNC J=3-2 emission lines at v=0 are clearly detected. The HCN and HNC v2=1f J=3-2 emission lines are also detected, but the HCO+ v2=1f J=3-2 emission line is not. Given the high-energy level of v2=1 and the resulting difficulty of collisional excitation, we compared these results with those of the calculation of infrared radiative pumping, using the available infrared 5-35 micron spectrum. We found that all of the observational results were reproduced, if the HCN abundance was significantly higher than that of HCO+ and HNC. The flux ratio and excitation temperature between v2=1f and v=0, after correction for possible line opacity, suggests that infrared radiative pumping affects rotational (J-level) excitation at v=0 at least for HCN and HNC. The HCN-to-HCO+ v=0 flux ratio is higher than those of starburst-dominated regions, and will increase even more when thederived high HCN opacity is corrected. The enhanced HCN-to-HCO+ flux ratio in this AGN-hosting galaxy can be explained by the high HCN-to-HCO+ abundance ratio and sufficient HCN excitation at up to J=4, rather than the significantly higher efficiency of infrared radiative pumping for HCN than HCO+.
We present our ALMA multi-transition molecular line observational results for the ultraluminous infrared galaxy, IRAS 20551-4250, which is known to contain a luminous buried AGN and shows detectable vibrationally excited (v2=1f) HCN and HNC emission lines. The rotational J=1-0, 4-3, and 8-7 of HCN, HCO+, and HNC emission lines were clearly detected at a vibrational ground level (v=0). Vibrationally excited (v2=1f) J=4-3 emission lines were detected for HCN and HNC, but not for HCO+. Their observed flux ratios further support our previously obtained suggestion, based on J=3-2 data, that (1) infrared radiative pumping plays a role in rotational excitation at v=0, at least for HCN and HNC, and (2) HCN abundance is higher than HCO+ and HNC. The flux measurements of the isotopologue H13CN, H13CO+, and HN13C J=3-2 emission lines support the higher HCN abundance scenario. Based on modeling with collisional excitation, we constrain the physical properties of these line-emitting molecular gas, but find that higher HNC rotational excitation than HCN and HCO+ is difficult to explain, due to the higher effective critical density of HNC. We consider the effects of infrared radiative pumping using the available 5-30 micron infrared spectrum and find that our observational results are well explained if the radiation source is located at 30-100 pc from the molecular gas. The simultaneously covered very bright CO J=3-2 emission line displays a broad emission wing, which we interpret as being due to molecular outflow activity with the estimated rate of ~150 Msun/yr.
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