A low-luminosity type-1 QSO sample: IV. Molecular gas contents and conditions of star formation in three nearby Seyfert galaxies
Abstract
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) < 1.8 kpc. 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 Mmol = (0.7 - 8.7) x 109 Msun. We give lower limits for the dynamical masses of Mdyn > 1.5 x 109 Msun and for the dust masses of Mdust > 1.6 x 106 Msun. 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 LIR > 1011 Lsun and seem to contain a circumnuclear starburst with minimum surface densities of gas and star formation rate (SFR) around mol = 50 - 550 Msun pc-2 and SFR = 1.1 - 3.1 Msun kpc-2 yr-1, we conclude that the interstellar medium in the centers of these LIRG Seyferts is strongly affected by violent star formation and better ...
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