Molecular Gas in NUclei of GAlaxies (NUGA)

Abstract

We present 12 CO(1-0) and 12 CO(2-1) maps of the interacting barred LINER/Seyfert 2 galaxy NGC 3627 obtained with the IRAM interferometer at resolutions of 2. 1 × 1. 3 and 0. 9 × 0. 6, respectively. We also present single-dish IRAM 30 m 12 CO(1-0) and 12 CO(2-1) observations used to compute short spacings and complete interferometric measurements. These observations are complemented by IRAM 30 m measurements of HCN(1-0) emission detected in the center of NGC 3627. The molecular gas emission shows a nuclear peak, an elongated bar-like structure of ∼18 (∼900 pc) diameter in both 12 CO maps and, in 12 CO(1-0), a two-arm spiral feature from r ∼ 9 (∼450 pc) to r ∼ 16 (∼800 pc). The inner ∼18 bar-like structure, with a north/south orientation (PA = 14 • ), forms two peaks at the extremes of this elongated emission region. The kinematics of the inner molecular gas shows signatures of non-circular motions associated both with the 18 bar-like structure and the spiral feature detected beyond it. The 1.6 μm H-band 2MASS image of NGC 3627 shows a stellar bar with a PA = −21 • , different from the PA (= 14 • ) of the 12 CO bar-like structure, indicating that the gas is leading the stellar bar. The far-infrared Spitzer-MIPS 70 and 160 μm images of NGC 3627 show that the dust emission is intensified at the nucleus and at the ansae at the ends of the bar, coinciding with the 12 CO peaks. The GALEX far-ultraviolet (FUV) morphology of NGC 3627 displays an inner elongated (north/south) ring delimiting a hole around the nucleus, and the 12 CO bar-like structure is contained in the hole observed in the FUV. The torques computed with the HST-NICMOS F160W image and our PdBI maps are negative down to the resolution limit of our images, ∼60 pc in 12 CO(2-1). If the bar ends at ∼3 kpc, coincident with corotation (CR), the torques are negative between the CR of the bar and the nucleus, down to the resolution limit of our observations. This scenario is compatible with a recently-formed rapidly rotating bar which has had insufficient time to slow down because of secular evolution, and thus has not yet formed an inner Lindblad resonance (ILR). The presence of molecular gas inside the CR of the primary bar, where we expect that the ILR will form, makes NGC 3627 a potential smoking gun of inner gas inflow. The gas is fueling the central region, and in a second step could fuel directly the active nucleus.

The resolved star-formation relation in nearby active galactic nuclei

“…In contrast, the Hi emission exhibits a spiral morphology without signatures of a bar in the atomic gas (Haan et al 2008;Walter et al 2008). The derived gravity torque budget shows that NGC 3627 is a potential smoking gun of inner gas inflow at a resolution of ∼60 pc (Casasola et al 2011). In addition to 12 CO lines, other molecular transitions have been detected in NGC 3627, including HCN(1−0), HCN(2−1), HCN(3−2), HCO + (1−0), and HCO + (3−2), suggesting the presence of high density gas (e.g., Gao & Solomon 2004;Krips et al 2008).…”

“…NGC 3627 also possesses X-ray properties of a galaxy with a recent starburst (Dahlem et al 1996). Both the radio continuum (2.8 cm and 20 cm, Urbanik et al 1985;Paladino et al 2008) and the CO emissions (e.g., Regan et al 2001;Paladino et al 2008;Haan et al 2009;Casasola et al 2011;Watanabe et al 2011) show a nuclear peak, extending along the leading edges of the bar that forms two broad maxima at the bar ends, and then the spiral arms trail off from the bar ends. In contrast, the Hi emission exhibits a spiral morphology without signatures of a bar in the atomic gas (Haan et al 2008;Walter et al 2008).…”

Gas inflows towards the nucleus of the Seyfert 2 galaxy NGC 1667

“…Recent ALMA observations releaved streaming motions along nuclear spirals in NGC 1433 (Combes et al 2013) and NGC 1566 (Combes et al 2014). Gas inflows have been observed in CO in NGC 2782 , NGC 3147 (Casasola et al 2008), NGC 3627 (Casasola et al 2011), NGC 4579 (García-Burillo et al 2009) and NGC 6574 (Lindt-Krieg et al 2008).…”

Interactions of the Galactic bar and spiral arm in NGC 3627

“…These active bar/arm interaction zones resemble the structures found in W43 in our Milky Way well. Furthermore, the location of NGC3627 is very favorable for our study: the orientation is almost face-on (inclination angle of 65 deg with a position angle θ PA =170 o , Chemin et al 2003), the object is relatively nearby at a distance of 11.1 Mpc (e.g., Saha et al 1999) where spatial resolution elements of 1 correspond to linear scales of ∼54 pc, and the systemic velocity is V sys =744 km s −1 (Casasola et al 2011). More details will be given in section 4.…”

“…Circles mark the locations and areas of the PdBI(2-1) primary beams for NGC3627N and NGC3627S. The central star and white line indicate the bar location following Casasola et al (2011). A linear scale-bar assuming 11.1 Mpc distance is shown in the bottom-left corner.…”