Interferometric 12CO(J=2-1) image of the Nuclear Region of Seyfert 1 Galaxy NGC 1097

We have mapped the central region of the Seyfert 1 galaxy NGC 1097 in 12CO(J=2-1) with the Submillieter Array (SMA). The 12CO(J=2-1) map shows a central concentration and a surrounding ring, which coincide respectively with the Seyfert nucleus and a starburst ring. The line intensity peaks at the nucleus, whereas in a previously published 12CO(J=1-0) map the intensity peaks at the starburst ring. The molecular ring has an azimuthally averaged 12CO(J=2-1)/(J=1-0) intensity ratio (R21) of about unity, which is similar to those in nearby active star forming galaxies, suggesting that most of the molecular mass in the ring is involved in fueling the starburst. The molecular gas can last for only about 1.2\times10^8 years without further replenishment assuming a constant star formation rate and a perfect conversion of gas to stars. The velocity map shows that the central molecular gas is rotating with the molecular ring in the same direction, while its velocity gradient is much steeper than that of the ring. This velocity gradient of the central gas is similar to what is usually observed in some Seyfert 2 galaxies. To view the active nucleus directly in the optical, the central molecular gas structure can either be a low-inclined disk or torus but not too low to be less massive than the mass of the host galaxy itself, be a highly-inclined thin disk or clumpy and thick torus, or be an inner part of the galactic disk. The R21 value of ~1.9 of the central molecular gas component, which is significantly higher than the value found at the molecular gas ring, indicates that the activity of the Seyfert nucleus may have a significant influence on the conditions of the molecular gas in the central component.Comment: 22 pages, 4 figures, accepted by Ap

Molecular gas in NUclei of GAlaxies (NUGA). XI. A complete gravity torque map of NGC4579: new clues on bar evolution

We create a complete gravity torque map of the disk of the LINER/Seyfert 1.9 galaxy NGC4579. We quantify the efficiency of angular momentum transport and search for signatures of secular evolution in the fueling process from r~15kpc down to the inner r~50pc around the Active Galactic Nucleus (AGN). We use both the 1-0 and 2-1 line maps of CO obtained with the Plateau de Bure Interferometer (PdBI) as part of the NUclei of Galaxies-(NUGA)-project. We derive the stellar potential from a NIR (K band) wide field image of the galaxy. The K-band image, which reveals a stellar bar, together with a high resolution HI map of NGC4579 obtained with the Very Large Array (VLA), allow us to extend the gravity torque analysis to the outer disk. The bulk of the gas response traced by the CO PdBI maps follows the expected gas flow pattern induced by the bar potential in the presence of two Inner Lindblad Resonances (ILR). We also detect an oval distortion in the inner r~200pc of the K-band image. The oval is not aligned with the large-scale bar, a signature of dynamical decoupling. The morphology of the outer disk suggests that the neutral gas is currently piling up in a pseudo-ring formed by two winding spiral arms that are morphologically decoupled from the bar structure. In the outer disk, the decoupling of the spiral allows the gas to efficiently produce net gas inflow on intermediate scales. The corotation barrier seems to be overcome due to secular evolution processes. The gas in the inner disk is efficiently funneled by gravity torques down to r~300pc. Closer to the AGN, the two m=2 modes (bar and oval) act in concert to produce net gas inflow down to r~50pc, providing a clear smoking gun evidence of fueling with associated short dynamical time-scales.Comment: Submitted for publication in A&A. 21 pages, 21 figure

Molecular gas in NUclei of GAlaxies (NUGA)

Within the frame of the NUclei of GAlaxies (NUGA) project, we have determined the distribution and kinematics of the molecular gas within the central kpc with high spatial resolution (100-150 pc), for a sample of active galaxies. The goal is to study the gas-fueling mechanisms in AGN.
We present interferometric observations of 12CO(1-0) and 12CO(2-1) line emission from the Seyfert 2 galaxy NGC 6574, obtained with the IRAM Plateau de Bure Interferometer (PdBI). These data have been combined with 30 m mapping data in these lines to correct for the flux resolved by the interferometer. At an angular resolution of 0.7″ (≡110 pc), the 12CO(2-1) emission is resolved into an inner disk with a radius of 300 pc.
The molecular gas in NGC 6574 is primarily distributed in four components: nucleus, bar, spiral arms – winding up into a pseudo-ring – and an extended underlying disk component. For the overall galaxy host, we find a 12CO(2-1) to 12CO(1-0) line ratio of ~0.3 indicative of cold or sub-thermally excited gas. For the nucleus, this ratio is close to unity, indicating emission from dense and warm molecular gas. Modeling the gas kinematics with elliptical orbits shows that the molecular gas in the differentially rotating disk of NGC 6574 is strongly influenced by the presence of a stellar bar. The nuclear component shows an extension toward the southeast that may be an indication of the lopsidedness of the nuclear gas distribution.
We computed the gravity torques exerted from the stellar bar on the gas, deriving the gravitational potential from near-infrared images, and weighting the torques by the CO distribution. We find negative torques for the gas inside the ring, since the gas aligned with the bar has a slight advance phase shift, leading the bar. This means that gas is flowing in towards the center, at least down to 400 pc in radius, which can explain the observed high nuclear gas concentration. This concentration corresponds to a possible inner Lindblad resonance of the bar, according to the measured rotation curve. The gas has been piling up in this location quite recently, since no startburst has been observed yet