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

S. Garcıa-Burillo tex2html_wrap_inline1968 M.J. Sempere tex2html_wrap_inline1968 , D. Bettoni tex2html_wrap_inline1978
tex2html_wrap_inline1968 Observatorio Astronómico Nacional-OAN, Apartado 1143, 28800 Alcalá de Henares, , Madrid, Spain,
tex2html_wrap_inline1978 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 tex2html_wrap_inline1766 CO mapped with the 30m telescope and Bure interferometer.

tex2html_wrap_inline1766 CO emission is concentrated in a compact nuclear disk of average radius r tex2html_wrap_inline1984 12 tex2html_wrap_inline2151  (1.2kpc). In the outer disk, from r=20 tex2html_wrap_inline2151 to r=100 tex2html_wrap_inline2151  (2-10kpc), tex2html_wrap_inline1766 CO is not detected and the neutral gas content is largely dominated by HI. The observed tex2html_wrap_inline1766 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 tex2html_wrap_inline1766 CO emission from gas in direct rotation. The estimated molecular mass in the tex2html_wrap_inline1766 CO nuclear disk is M(H tex2html_wrap_inline2052 ) tex2html_wrap_inline1984 0.3 tex2html_wrap_inline2158 10 tex2html_wrap_inline2160 M tex2html_wrap_inline1990 , 3 times lower than the mass of the HI disk.

Within the nuclear disk, tex2html_wrap_inline1766 CO is distributed in a central source of tex2html_wrap_inline1984 1 tex2html_wrap_inline2151  (100pc) radius where the derived tex2html_wrap_inline1766 CO rotation curve reaches tex2html_wrap_inline1984 240kms tex2html_wrap_inline1740 , surrounded by a pseudo-ring of average radius tex2html_wrap_inline1984 6 tex2html_wrap_inline2151  (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 tex2html_wrap_inline1766 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 tex2html_wrap_inline1796 , 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 tex2html_wrap_inline1766 CO observations suggest that we are probably seeing a late stage of a merger happened in NGC 3626.

Figure: a(top) The tex2html_wrap_inline1766 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 tex2html_wrap_inline1796 absorption lines (star like markers). Superposed are the radial velocities of ionized gas observed using several tracers: H tex2html_wrap_inline1796 (filled circles) and N[II](open squares). The anomalous tex2html_wrap_inline1766 CO component (see text) is indicated by the arrows. b(bottom) The tex2html_wrap_inline1766 CO(1-0) position-velocity diagram along the minor axis of NGC 3626. Emission at anomalous velocities is indicated by the arrows.

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