Next Contents Previous


The main conclusion of this paper is that some "bulges" are disk-like in their dynamics and origin. That is, disks can have central brightness profiles that are much steeper than exponentials; in extreme cases, they are as well fitted by r1/4 laws as are ellipticals. Then true bulge-to-total light ratios are smaller than we think. One consequence is that we should be cautious in interpreting evidence for bulge triaxiality; we may be seeing disk effects.

Although the details are far from clear, we believe that all this results from one or more processes of secular dynamical evolution. Bars and oval disks are particularly likely engines, but dissipation and gas infall can happen without them, and a variety of processes may operate. The importance of secular evolution by the interaction of galaxy components has been emphasized by Kormendy (1979, 1980, 1981, 1982a).

The above results are particularly relevant to this meeting because our Galaxy has probably been affected. It is an Sbc (de Vaucouleurs and Pence 1978), so it is unlikely to contain a pristine, pure bulge (Fig. 10). It is barred. The beautiful COBE photograph of the Galaxy (Mather et al. 1990; see the Bahcall Committee report for a color version) shows that its bulge is box-shaped. Massive molecular clouds live near the center. We should be on the lookout for young stars. The age of the bulge has been the most controversial subject discussed at this meeting; surely the stars at large z are old, but does the bulge contain young stars near the disk plane? If external galaxies are any guide, it is likely that our Galaxy has had its bulge augmented by high-density disk material and that active star formation continues near the center. Were this not so, our Galaxy would be quite unusual. When we discuss the stellar population, the dynamics, and the evolution of the Galactic bulge, we should ask: Are some effects that we see properties not of a true bulge but rather of a high central concentration of disk material that may have been heated by instabilities and resonance effects? I believe that it will be important to look for the effects discussed in sections 4 - 6 in our own front yard.


This talk was prepared at the Landessternwarte Königstuhl, Heidelberg, Germany; it is a pleasure to thank I. Appenzeller and R. Bender for their hospitality.

Next Contents Previous