Molecular gas in NUclei of GAlaxies (NUGA)

Abstract

Within the NUclei of GAlaxies (NUGA) project we have obtained IRAM PdBI and 30 m 12 CO(1−0) and 12 CO(2−1) observations of the spiral galaxy NGC 6951. Previous work shows that there is indirect evidence of gas inflow from 3 kpc down to small radii: a largescale stellar bar, a prominent starburst ring (r ≈ 580 pc) and a LINER/Seyfert 2 nucleus. In this paper we study the gas kinematics as traced by the CO line emission in detail. We quantify the influence of the large-scale stellar bar by constructing an analytical model of the evolution of gas particles in a barred potential. From this model gravitational torques and mass accumulation rates are computed. We compare our model-based gravitational torque results with previous observationally-based ones. The model also shows that the large-scale stellar bar is indeed the dominant force for driving the gas inward, to the starburst ring. Inside the ring itself a nuclear stellar oval might play an important role. Detailed analysis of the CO gas kinematics there shows that emission arises from two co-spatial, but kinematically distinct components at several locations. The main emission component can always be related to the overall bar-driven gas kinematics. The second component exhibits velocities that are larger than expected for gas on stable orbits, has a molecular gas mass of 1.8 × 10 6 M , is very likely connected to the nuclear stellar oval, and is consistent with inflowing motion towards the very center. This may form the last link in the chain of gas inflow towards the active galactic nucleus in NGC 6951.

How does star formation proceed in the circumnuclear starburst ring of NGC 6951?

“…Barth et al (1995) used a distance modulus of 31.2, which corresponds to a distance of 17.4 Mpc. Assuming a distance of 24.1 Mpc to NGC 6951, as in Van der Laan et al (2011), increases the distance modulus to 31.9, or correspondingly increases the magnitudes by -0.7 mag. The absolute magnitudes are given in Table 4.…”

“…We extracted them from the archive calibrated with the "best" reference files. We also utilize NICMOS F110W and F160W images from the HST archive already used in previous work (Van der Laan et al 2011, original: Hunt & Malkan 2004. Finally, we obtained a Paα image, which was observed during the same campaign and reduced using the same approach as for NGC 1097 in Hsieh et al (2011).…”

“…Their rms noise corresponds to M H 2 ∼ 10 5 M /beam. We refer to Van der Laan et al (2011) for further details on the data reduction. A total H 2 molecular gas mass of 2.2 × 10 9 M was derived from the CO(1-0) observations, assuming a Galactic conversion factor and corrected for helium abundance.…”

“…However, the intensity maxima are at slightly different positions in the ring. From modeling the orbits in the circumnuclear region, Van der Laan et al (2011) derived that the inflow rate of gas onto the ring is ∼2.0 M per year.…”

“…Its molecular gas dynamics in and around the ring have already been studied in detail by some of the authors (Van der Laan et al 2011), as well as by others (Kohno et al 1999;Krips et al 2007;Storchi-Bergmann et al 2007). The gravitational torques acting on the cold molecular gas have also been analyzed (Haan et al 2009; Van der Laan et al 2011). Mazzuca et al (2008) have investigated the Hα regions in the ring and found young stellar ages (<10 Myr) in the northern segment of the ring and older ages (∼10 Myr for an instantaneous, or ∼100 Myr for a continuous star formation scenario) in the rest of the circumnuclear ring.…”