[Paper Review] A low-luminosity type-1 QSO sample: IV. Molecular gas contents and conditions of star formation in three nearby Seyfert galaxies

L. Moser, Melanie Krips+9 more|Kölner Universitäts PublikationsServer (Universität zu Köln)|Jul 4, 2017

Galaxies: Formation, Evolution, Phenomena142 references5 citations

TL;DR

This study presents high-resolution Submillimeter Array observations of low-J CO transitions in three nearby Seyfert galaxies from the low-luminosity type-1 QSO sample, revealing molecular gas masses of (0.7–8.7)×10⁹ M☉, kinematically decoupled central components, and elevated 12CO(2–1)/13CO(2–1) line ratios (R₂₁ ≳ 20) indicating high-temperature, turbulent gas. The findings suggest these LIRG-like Seyferts host intense circumnuclear starbursts, requiring ULIRG-like CO-to-H₂ mass conversion factors due to extreme gas excitation and dynamical conditions.

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) x 10^9 M_sun. We give lower limits for the dynamical masses of M_dyn > 1.5 x 10^9 M_sun and for the dust masses of M_dust > 1.6 x 10^6 M_sun. 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 > 10^11 L_sun and seem to contain a circumnuclear starburst with minimum surface densities of gas and star formation rate (SFR) around Σ_mol = 50 - 550 M_sun pc^-2 and Σ_SFR = 1.1 - 3.1 M_sun 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 ...

Motivation & Objective

• To investigate the molecular gas content and excitation conditions in the central regions of three nearby low-luminosity type-1 QSOs with Seyfert activity.
• To determine the spatial distribution and kinematics of molecular gas using high-resolution submillimeter interferometry.
• To assess the implications of gas excitation for CO-to-H₂ mass conversion factors in low-luminosity AGN hosts.
• To evaluate whether the interstellar medium in these galaxies is better described by Milky Way-like or ULIRG-like physical conditions.

Proposed method

• Submillimeter Array (SMA) observations at ~3″ resolution of 12CO(2–1), 13CO(2–1), 12CO(3–2), and HCO⁺(4–3) transitions in three nearby Seyfert galaxies.
• Analysis of line luminosity ratios R₂₁ = L′(12CO 2–1)/L′(13CO 2–1) to infer gas temperature, density, and excitation conditions.
• Estimation of molecular gas mass M_mol from integrated line fluxes using standard CO-to-H₂ conversion factors.
• Dynamical mass M_dyn estimated from gas kinematics assuming virial equilibrium in the central regions.
• Dust mass M_dust derived from lower limits based on non-detection of continuum emission.
• Comparison of observed CO line ratios (e.g., 12CO(2–1)/12CO(1–0) ≈ 0.5 and 12CO(3–2)/12CO(1–0) ≈ 1) to infer multiple ISM phases.

Experimental results

Research questions

• RQ1 What is the molecular gas mass and spatial distribution in the central regions of these low-luminosity Seyfert galaxies?
• RQ2 How do the excitation conditions of molecular gas, as traced by CO line ratios, compare to those in the Milky Way and ULIRGs?
• RQ3 What is the dynamical mass and dust mass in the circumnuclear regions, and how do they constrain the gas mass and star formation efficiency?
• RQ4 Is the CO-to-H₂ mass conversion factor in these galaxies better described by Milky Way or ULIRG values, given the observed line ratios?

Key findings

• The molecular gas mass in the three galaxies ranges from (0.7–8.7)×10⁹ M☉, with over 80% concentrated within a diameter of less than 1.8 kpc.
• A kinematically decoupled, unresolved central molecular gas component is detected in all three galaxies, indicating complex dynamics.
• The 12CO(2–1)/13CO(2–1) line luminosity ratio R₂₁ is ∼5–7 in the outer regions and >20 in the center, indicating higher gas temperatures and turbulence, similar to ULIRGs.
• The 12CO(2–1)/12CO(1–0) ratio is ∼0.5, suggesting a cold or diffuse gas phase, while the 12CO(3–2)/12CO(1–0) ratio of ∼1 indicates a higher-excited gas component.
• The dynamical mass exceeds 1.5×10⁹ M☉ and the dust mass is >1.6×10⁶ M☉, supporting a low CO-to-H₂ mass conversion factor.
• The observed excitation patterns imply that the interstellar medium in these galaxies is best described by ULIRG-like CO-to-H₂ mass conversion factors due to high turbulence and temperature in the central regions.