First detection of a counter-rotating molecular gas disk in a spiral galaxy: NGC 3626

S. Garcıa-Burillo M.J. Sempere , D. Bettoni
Observatorio Astronómico Nacional-OAN, Apartado 1143, 28800 Alcalá de Henares, , Madrid, Spain,
Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy
Abstract: In this paper we report the detection of a massive counter-rotating molecular gas disk in the early-type spiral NGC 3626, observed in the 1-0 and 2-1 lines of CO mapped with the 30m telescope and Bure interferometer.

CO emission is concentrated in a compact nuclear disk of average radius r 12  (1.2kpc). In the outer disk, from r=20 to r=100  (2-10kpc), CO is not detected and the neutral gas content is largely dominated by HI. The observed CO velocity field pattern corresponds to a gaseous disk with a sense of rotation opposite to that of the stars. Counter-rotation is shared by molecular and ionized gas in the center. There is no strong evidence of CO emission from gas in direct rotation. The estimated molecular mass in the CO nuclear disk is M(H ) 0.3 10 M , 3 times lower than the mass of the HI disk.

Within the nuclear disk, CO is distributed in a central source of 1  (100pc) radius where the derived CO rotation curve reaches 240kms , surrounded by a pseudo-ring of average radius 6  (600pc) characterized by strong non-circular motions. Dynamics of molecular gas, characterized by a regular counter-rotating pattern, streaming motions typical of a steady density-wave driven flow and normal CO line widths preclude the occurrence of violent large scale shocks or of a non-equilibrium dynamical state for the gas.

The available optical-(H , N[II] and S[II]) and radio-continuum data (at 12.6cm and 21cm) indicate that no violent burst of star formation is associated with the nuclear molecular gas. This is confirmed by the lack of IRAS flux record for the nucleus of NGC 3626. The present CO observations suggest that we are probably seeing a late stage of a merger happened in NGC 3626.

  
Figure: a(top) The CO(1-0) position-velocity plot (grey scale) taken along the major axis of NGC 3626, showing counter-rotation with respect to the stars. Stars kinematics is derived from H absorption lines (star like markers). Superposed are the radial velocities of ionized gas observed using several tracers: H (filled circles) and N[II](open squares). The anomalous CO component (see text) is indicated by the arrows. b(bottom) The CO(1-0) position-velocity diagram along the minor axis of NGC 3626. Emission at anomalous velocities is indicated by the arrows.