2016A&A...587A.137M

We present a pilot study of ∼3'' resolution observations of low CO transitions with the Submillimeter Array in three nearby Seyfert galaxies, which are part of the low-luminosity quasi-stellar object (LLQSOs) sample consisting of 99 nearby (z=0.06) type-1 active galactic nuclei (AGN) taken from the Hamburg/ESO quasi-stellar object (QSO) survey. Two sources were observed in ¹²CO(2-1) and ¹³CO(2-1) and the third in ¹²CO(3-2) and HCO⁺(4-3). None of the sources is detected in continuum emission. More than 80% of the ¹²CO detected molecular gas is concentrated within a diameter (FWHM)<1.8kpc. ¹³CO is tentatively detected, while HCO⁺ emission could not be detected. All three objects show indications of a kinematically decoupled central unresolved molecular gas component. The molecular gas masses of the three galaxies are in the range M_mol=(0.7-8.7)x10⁹M_☉. We give lower limits for the dynamical masses of M_dyn>1.5x10⁹M_☉ and for the dust masses of M_dust>1.6x10⁶M_☉. The R₂₁=¹²CO/¹³CO(2-1) line luminosity ratios show Galactic values of R₂₁∼5-7 in the outskirts and R₂₁>20 in the central region, similar to starbursts and (ultra)luminous infrared galaxies ((U)LIRGs; i.e. LIRGs and ULIRGs), implying higher temperatures and stronger turbulence. All three sources show indications of ¹²CO(2-1)/¹²CO(1-0) ratios of ∼0.5, suggesting a cold or diffuse gas phase. Strikingly, the ¹²CO(3-2)/(1-0) ratio of ∼1 also indicates a higher excited phase. Since these galaxies have high infrared luminosities of L_IR≥10¹¹L_☉ and seem to contain a circumnuclear starburst with minimum surface densities of gas and star formation rate (SFR) around Σ_mol=50-550M_☉/pc2 and Σ_SFR=1.1-3.1M_☉/kpc2/yr, we conclude that the interstellar medium in the centers of these LIRG Seyferts is strongly affected by violent star formation and better described by the ULIRG mass conversion factor.