The CO velocity field, derived from the position-velocity diagram taken along the galaxy major axis, reveals the existence of a compact nuclear disk rotating at high velocities which has no HI counterpart. The derived rotation curve reaches v 130 kms in less than 250 pc. This high velocity regime for molecular gas is satisfactorily explained by an inner bar scenario. The bulge of NGC4013 is box-shaped at optical wavelengths, which suggests the existence of a non-axisymmetric potential in the inner disk. The distribution of CO emissivity itself towards the center suggests that the orbits of some molecular clouds are inclined with respect to the plane of the galaxy.
NGC4013 presents a distorted vertical distribution of matter (stars and gas): it has a box-shaped bulge, a thick optical disk with a warped plane, and a spectacular HI warp in the outer disk. The molecular gas disk vertical structure is not resolved in the 2-1 line. The slight inclination of the galaxy allows us detecting non-axisymmetric structures that are probably spiral arms. If so the estimated inclination angle is i=86. 5, in excellent agreement with the derived by Guthrie (1992) from optical data. We show that the warp of the stellar disk reported from optical measurements might be an artifact due to spiral arms seen in projection. We have not detected a CO counterpart of the HI warp.
Figure 2: Position-velocity diagram, observed along the kinematical major axis of NGC4013, in the CO(2-1) line (solid contours and grey scale). The location of the nuclear disk (ND), the high velocity gas (W and W ) and the CO depleted regions (H and H ) are indicated by the arrows. Countour levels and gray scale are equally spaced from 10% to 90% by steps of 10% of the maximum value (0.075 K). The cross in the bottom left quarter indicates the resolution in both axis (6.5 km s , 12")