First Detection of a Counterrotating Molecular Gas Disk in a Spiral Galaxy: NGC 3626

Abstract

In this paper we report the detection of a massive counterrotating molecular gas disk in the early-type spiral NGC 3626, observed in the 1-0 and 2-1 lines of 12CO, mapped with the 30 m telescope and Bure interferometer.12CO emission is concentrated in a compact nuclear disk of average radius r ~ 12'' (1.2 kpc). In the outer disk, from r = 20'' to r = 100'' (2-10 kpc),12CO is not detected, and the neutral gas content is largely dominated by H I. The observed 12CO velocity field pattern corresponds to a gaseous disk with a sense of rotation opposite to that of the stars. Counterrotation is shared by molecular and ionized gas in the center. There is no strong evidence of 12CO emission from gas in direct rotation. The estimated molecular mass in the 12CO nuclear disk is M(H2) ~ 0.3 × 109 M☉, 3 times lower than the mass of the H I disk. Within the nuclear disk,12CO is distributed in a central source of ~1'' (100 pc) radius, where the derived 12CO rotation curve reaches ~240 km s-1, surrounded by a pseudoring of average radius ~6'' (600 pc) characterized by strong noncircular motions. The dynamics of molecular gas, characterized by a regular counterrotating pattern and streaming motions typical of a steady density-wave driven flow and normal 12CO line widths, preclude the occurrence of violent large-scale shocks or of a nonequilibrium dynamical state for the gas. The available optical (Hα, N [II] and S [II]) and radio continuum data (at 12.6 cm and 21 cm) 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 12CO observations suggest that we are probably seeing a late stage of a merger happened in NGC 3626.