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A high-resolution tex2html_wrap_inline773 CO(1-0) study of the nucleus of NGC 5907- Observations and modelling

S.Garcıa-Burillo tex2html_wrap_inline933 , M.Guélin tex2html_wrap_inline935 , N.Neininger tex2html_wrap_inline937
tex2html_wrap_inline843 Observatorio Astronómico Nacional(OAN), Apartado 1143, 28800- Alcalá de Henares, Spain, (
tex2html_wrap_inline845 Institut de Radioastronomie Millimétrique, IRAM, 38406-St Martin D'Hères, France
tex2html_wrap_inline847 Max-Planck-Institut fur Radioastronomie, Auf dem Hugel 69, D-5321 Bonn, Germany.
Abstract: We have studied with the IRAM interferometer the tex2html_wrap_inline773 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 ( tex2html_wrap_inline949 , tex2html_wrap_inline951 ).

Within tex2html_wrap_inline953 from the center, we detect molecular gas with velocities of tex2html_wrap_inline955 kms tex2html_wrap_inline781 (relative to the source systemic velocity tex2html_wrap_inline959 kms tex2html_wrap_inline781 (LSR)), and, within tex2html_wrap_inline963 from the center, velocities of up to tex2html_wrap_inline965 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 ( tex2html_wrap_inline967 =70 kmskpc tex2html_wrap_inline781 ) and of an Outer Inner Linblad Resonance (oILR) at r tex2html_wrap_inline971 14''. The dynamical mass inside the R< 300 pc region, estimated from the steepness of the apparent rotation curve, is tex2html_wrap_inline975 . 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 tex2html_wrap_inline979 of the plane of the galaxy. The thickness of the arms, perpendicularly to this plane, is found to be tex2html_wrap_inline981 ( tex2html_wrap_inline983 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): tex2html_wrap_inline773 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 tex2html_wrap_inline781 per 3.6'' beam and 1 to 9 Jy.kms tex2html_wrap_inline781 beam tex2html_wrap_inline781 , by steps of 1 Jy.kms tex2html_wrap_inline781 beam tex2html_wrap_inline781 . The maximum is I tex2html_wrap_inline793 = 9.4Jy kms tex2html_wrap_inline781 beam tex2html_wrap_inline781 ). b)(bottom): tex2html_wrap_inline773 CO(1-0) peak brigthness temperature contours. Contours correspond to 10 tex2html_wrap_inline801 , 20%.. 90% of the peak maximum of (0.17 Jy.beam tex2html_wrap_inline781 ). 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 tex2html_wrap_inline809 and O tex2html_wrap_inline811 .

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Next: Molecular gas in the Up: Scientific results Previous: The molecular envelope around