A low-luminosity type-1 QSO sample

IV. Molecular gas contents and conditions of star formation in three nearby Seyfert galaxies

¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
e-mail: moser@ph1.uni-koeln.de
² Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, Domaine Universitaire, 38406 Saint-Martin d’Hères, France
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁴ LERMA (CNRS: UMR 8112), Observatoire de Paris, 61 Av. de l’Observatoire, 75014 Paris, France
⁵ German Aerospace Center (DLR), Königswinterer Str. 522-524, 53227 Bonn, Germany
⁶ Göttingen eResearch Alliance, State and University Library Göttingen, Papendiek 14, 37073 Göttingen, Germany

Received: 19 April 2015
Accepted: 15 September 2015

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 M_mol = (0.7−8.7) × 10⁹M_⊙. We give lower limits for the dynamical masses of M_dyn> 1.5 × 10⁹M_⊙ and for the dust masses of M_dust> 1.6 × 10⁶M_⊙. 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 = 50−550 M_⊙pc^-2 and Σ_SFR = 1.1−3.1 M_⊙ 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 described by the ULIRG mass conversion factor.