Cold molecular gas in the inner two kiloparsecs of NGC 4151

We present the first spatially resolved spectroscopic imaging observations of the ¹²CO (1-0) line emission in the central 2.5 kpc of the Seyfert 1 galaxy NGC 4151, obtained with the IRAM Plateau de Bure Interferometer (PdBI). Most of the cold molecular gas is distributed along two curved gas lanes about 1 kpc north and south of the active nucleus, coincident with the circumnuclear dust ring noted by previous authors. These CO arcs lie within the Inner Lindblad Resonance of the large scale oval bar and have kinematics consistent with those derived from neutral hydrogen observations of the disk and the bar. Two additional gas clumps are detected that show non-circular motion-one associated with the southern gas lane and the other lying ∼600 pc north of the nucleus. Closer to the nucleus, no cold molecular gas is detected in the central 300 pc where abundant near-IR H₂ line emission arises. This suggests that the H₂ line emission is not a good indicator of a cold gas reservoir in NGC 4151 and that the H₂ is likely photo-excited by the active galactic nucleus (AGN). The upper limit of the CO mass in the central 300 pc is sufficient to support the AGN activity at its current level for 10⁷ yr. The total cold molecular mass detected by PdBI is 4.3 × 10⁷M_⊙. Finally, 3 mm continuum emission arising from the location of the AGN is detected with a flux of S _3mm ∼ 14 mJy and appears to be unresolved at an angular resolution of 2. ″8 (∼180 pc).