Stellar kinematics

We have measured the velocity curve from the two main lines of the Caii absorption triplet at 24#24 Å, by means of cross-correlating the galaxy frames with seven spectra of five different K giant stars, observed during the same night and with the same setup. These stars are HD132737, HD171232, HD208817, HD198858, and HD1918. The cross-correlation is performed with a 2D procedure we have implemented that makes use of the IRAF function CROSSCORR. The position and amplitude of the peak of the cross-correlation function is measured, and the resulting velocity curves are weight averaged at each position angle. The signal-to-noise ratio of the data allows measurements only within a 20 arcsec radius at PA=84 and 48, and 10 arcsec at PA=138. The resulting curves are shown in Fig.  (expanded for the central region in the insets) for the three position angles, together with the corresponding H1#1 emission velocity curves for comparison.

First, we inspect the stellar velocity curves. Inwards of the circumnuclear ring, in the 17#172 arcsec radius resolved nuclear zone, there seems to be a normal stellar rotation curve, with a measured gradient of 41 and 26 km s^-1 arcsec^-1 at PA=138 and 84, and flat at PA=48. Outside this radius it is possible to partially resolve the absorption line profile into two components, that are seen in the cross-correlation function by corresponding local peaks; in most spatial increments one component fully dominates the profile, while in some increments the position of the two components can be more easily measured. The velocities corresponding to these two components are plotted with filled and open symbols in the figure. On either side of the nucleus, these two stellar components have their velocity of the same sign, although their amplitude difference reaches 50 km s^-1.

We now look into the comparison with the gas H1#1 velocity curves. Where only one stellar component is measured, i.e. within the nuclear 2 arcsec radius, the velocity curves of the gas and of the stars at PA=84 and 138 follow each other approximately. This is not the case at PA=48, where the stellar velocity curve is flat, while the H1#1 curve presents a slope (c.f. previous section 4.3) with a peculiar structure in the central 17#171.5 arcsec.

At PA=84 the nuclear 35#35 arcsec part of the gaseous and stellar velocity curves both follow the same similar pattern; however, when the two stellar components can be kinematically resolved, neither of them two match the gas kinematics. One component is very flat with a mean velocity of 1440 km s^-1 from 4.5 arcsec eastwards of the nucleus, and a velocity of 1400 km s^-1 from 5 arcsec westwards of the nucleus. This stellar component matches the velocity of the gas entrained in the bar both at 30 arcsec east (at 1440 km s^-1) and at 30 arcsec west (at 1400 km s^-1; c.f. previous section 4.2 and Fig. ). We shall refer to this component as the stellar bar component. The other stellar component is steeper but not as much as the gas component between 17#173 arcsec and 17#177 arcsec, where the gas presents a significantly larger rotation amplitude than the stellar disk rotation at the same projected distance (a similar case to that reported for the barred galaxy NGC 6701 by Márquez et al. 1996, cf. their fig. 12). The extrapolation of this stellar component meets the gas rotation in the disk at 40 arcsec east (at 1490 km s^-1) and at 40 arcsec west (at 1330 km s^-1); we shall refer to this component as the stellar disk component.

At PA=138 the stellar velocity presents a similar behaviour. Only one component is apparent within a 3 arcsec radius; this generally follows the gas velocity, although with less structure in the nucleus. Outwards of this radius two stellar velocity components can be distinguished. At the south-east the steeper of the two components reaches a maximum velocity of  1553 km s^-1, that extrapolates to match the disk gas velocity. The stars and the gas kinematics are most different at PA=48. The gas presents an apparent counter-rotating component within a 6 arcsec radius with respect to its outer disk rotation, and an additional peculiar velocity structure in the nuclear 17#171.5 arcsec. The stars show a slowly rising kinematics from north-east to south-west, compatible with the disk rotation, and there is no indication of any peculiar velocity structure in the nucleus similar to the gaseous one. This peculiar structure in the gas velocity along the minor axis could be due to an off-centered slit, to a gaseous nuclear outflow, or to a structure related to the iILR.