A high-resolution CO(1-0) study of the nucleus of NGC 5907- Observations and modelling

S.Garcıa-Burillo , M.Guélin , N.Neininger
Observatorio Astronómico Nacional(OAN), Apartado 1143, 28800- Alcalá de Henares, Spain, (burillo@oan.es)
Institut de Radioastronomie Millimétrique, IRAM, 38406-St Martin D'Hères, France
Max-Planck-Institut fur Radioastronomie, Auf dem Hugel 69, D-5321 Bonn, Germany.
Abstract: We have studied with the IRAM interferometer the CO(1-0) emission around the nucleus of NGC 5907, an edge-on spiral at the center of which we had previously detected fast rotating molecular gas. The high angular resolution (3.1'', or 160 pc along the major axis) allowed an accurate determination of NGC 5907's kinematical center ( , ).

Within from the center, we detect molecular gas with velocities of kms (relative to the source systemic velocity kms (LSR)), and, within from the center, velocities of up to km/s. Part of the gas exhibits non-circular motions. We explain the gas kinematics by the presence of a stellar bar with an angular velocity ( =70 kmskpc ) and of an Outer Inner Linblad Resonance (oILR) at r 14''. The dynamical mass inside the R< 300 pc region, estimated from the steepness of the apparent rotation curve, is . It is comparable to the stellar mass derived from near IR photometry and >10 times larger than the mass of gas in the same region. The CO brightness distribution and, presumably, the molecular gas distribution are highly asymmetrical.

The interferometer, which filters out the smooth disk emission detected by the 30-m telescope, reveals two narrow structures parallel to the major axis. We identify those with spiral arms. From their position and velocities, we determine an inclination of the plane of the galaxy. The thickness of the arms, perpendicularly to this plane, is found to be ( 150 pc).

The large mass concentration near the nucleus and the flat rotation curve of NGC 5907 are more typical of early or intermediate-type spirals than of late type spirals. Yet, the large CO and HI luminosties, the conspicuous molecular arms, and the absence of visible bulge on optical photographs are more characteristic of Sc-Sd galaxies. NGC 5907 may thus be at a critical stage of galactic evolution.

 

 

Figure: a)(top): CO(1-0) integrated intensity contours observed with the Bure interferometer towards the center of NGC 5907. x and y are offsets (in arcsec) with respect to the dynamical center C. Contours are -2 Jy.kms per 3.6'' beam and 1 to 9 Jy.kms beam , by steps of 1 Jy.kms beam . The maximum is I = 9.4Jy kms beam ). b)(bottom): CO(1-0) peak brigthness temperature contours. Contours correspond to 10 , 20%.. 90% of the peak maximum of (0.17 Jy.beam ). The dashed y=0 line represents the major axis and the dashed x=0 line, the minor axis. The central bar, projected on the plane of the sky, lies between O and O .