6. Discussion of the results
The total space velocity and hence specific angular momentum of Pal 5 are significantly different from that to be expected from a tidally stripped fragment of the Sgr dwarf galaxy (eg. Lynden-Bell & Lynden-Bell 1995). Our (mean) angular momentum of Pal 5 is only about half of that of the Sgr dwarf, if we take the Sgr dwarf proper motion obtained by Ibata et al. (1997). Furthermore, our direction of motion of Pal 5, south toward the Galactic Plane, is almost in the opposite sense to that of the Sgr galaxy which at the equivalent point in its orbit would be heading north away from the Galactic Plane (Ibata et al. 1997). Admittedly, if we used the proper motion obtained by Schweitzer, Cudworth & Majewski (1993) then this latter problem would be much reduced. Nonetheless both proper motion results suggest an apogalacticon for Pal 5 of only 17-19 kpc which is virtually impossible to reconcile with a detached satellite of the Sgr dwarf which has a perigalacticon of this order, but whose apogalacticon is likely be greater than 50 kpc or more.
It is possible that Pal 5 could be a detached cluster of an alternative, now defunct, satellite galaxy since that would help to explain how such a low density cluster could have survived for the best part of a Hubble time in such a low attitude orbit. Dynamical friction would have a negligible effect on the orbit of an isolated Pal 5, but could well have significantly decayed the orbit of a previously much more massive host system, leading to capture and eventual tidal disruption.
Considering our results of the Galactic orbit of Pal 5 we may also conclude that the tidal forces along this orbit would lead to a smaller tidal radius compared to the 18.2 arcmin of Webbink (1985) and 15.9 arcmin of Trager, King & Djorgovski (1995). With arcmin and kpc the linear radius is 101 pc. Assuming a mass of Pal 5 of logM = 3.92 (Mandushev et al. 1991) and using common formulae for the classic tidal radius (e.g. formula (4) in Odenkirchen et al. 1997) we get a tidal radius between 12 pc at perigalacticon and 43 pc at apogalacticon. The conservative value of 43 pc corresponds to 7 arcmin which is in good agreement with our Fig. 7a and b and our discussion of possible errors in the measurements of the tidal radius of Pal 5 caused by the galaxy cluster Abell 2050.
Note added in proof: Cudworth (1998) reported that the preliminary result of Schweitzer, Cudworth & Majewski (1993) has been improved so that their final result of the cluster's absolute proper motion is mas/yr. Their revised motion is closer to our result in , but the disagreement in remains.
© European Southern Observatory (ESO) 1998
Online publication: April 20, 1998