Astron. Astrophys. 349, 411-423 (1999)

6. Conclusion

We have shown a high sensitivity HI data cube of NGC 4548 together with 23 pointings in CO(1-0), an H line map, and an optical B image. The HI emission shows a ring-like structure which is symmetric along the minor axis and asymmetric along the major axis. In particular, there is a distorted low intensity arm in the north. The comparison of this data with an optical B image showed that the HI emission follows the spiral arms. The detached northern arm however, has no detectable counterpart in the B image. The dust lanes appearing in the south-west places this side behind the eastern part of the galaxy.

The H line emission shows several bright HII regions at the ends of the bar.

The CO(1-0) data covers centered on the galaxy. The bar appears clearly and joins the HI emission at the end of the bar. The molecular fraction stays constant for galactic radii greater than 40" and rises rapidly between 20" and 40".

We have determined the position angle and the inclination and have fitted a rotation curve to the atomic gas data. The velocity field can be described either by a constant velocity rotation of 250 km s^-1 and a diverging inclination angle for the outer parts or by a constant inclination angle and a diverging rotation velocity for the outer parts or by something in between. The difference in the shape of the rotation curve with the one derived by Guhartakurta et al. (1988) can be explained by the better angular resolution and the much better sensitivity of our data.

We choose a three dimensional visualization of the data to show the detailed velocity field. In three dimensions the constant velocity rotation corresponds to a ring in the restricted phase space (, , heliocentric velocity). The detached northern arm is clearly visible and it appeared that it is distorted in velocity too. The inclusion of the CO data into the atomic gas cube gives insight into the kinematics of the central region and fits nicely with the data of Rubin et al. (1999).

A simple model of a disk with constant rotation velocity is fitted to the data. In the reconstructed emission map we observe an asymmetry in the north. We list the different possibilities for the causes of the distorted velocity field and discuss them in detail.

We conclude that the ram pressure stripping scenario is entirely consistent with all observational constraints and might be the dominating effect. If this is the case there might be the possibility to have a pile-up of the intracluster medium in front of the galaxy in the direction of its motion. X-ray observations with XMM detecting this pile-up may thus confirm our conclusions. Furthermore, we will investigate further this topic using detailed dynamical models of this scenario. The galaxy's velocity with respect to the cluster centre (M87), the orientation of the disk and the detailed shape of the perturbations will enormously constrain the model parameters. Dynamical models are therefore a precious tool to investigate the observed perturbations.

© European Southern Observatory (ESO) 1999

Online publication: September 2, 1999
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