2.5.2. Bulge formation

The physics of bulge formation in spirals is of great interest. In our own Galaxy the observational evidence for early bulge formation seems established, mainly since the main tracers of bulge/halo populations (eg. globular clusters and RR Lyrae stars) are known to be old. The notion that bulges form well before the discs in the first stage of galactic collapse is the essential component in both the classic Eggen, Lynden-Bell, & Sandage (ELS) scenario [34] and more modern variants [18]. Hierarchical galaxy formation scenarios also require old bulges formed by mergers [49, 11], with visible discs built-up gradually from gas accreted onto these merger remnants. Recently, however, the issue of the relative age of the bulge and disc in extragalactic systems has become controversial. N-body simulations indicate that bulges form naturally from bar instabilities in discs [67, 19], and recent observations now seem to indicate that bulges display the morphological and dynamical characteristics of such a formation scenario (eg. triaxiality or ``peanut'' shapes [55], cylindrical rotation [78], and disky kinematics [53]). Most remarkably, recent observations indicate that the inner discs and bulges in local galaxies cannot be distinguished in terms of colour [10, 25], which is surprising given the visual impression from ``true colour'' deep HST images of distant galaxies, in which bulges seem generally redder than disks [84]. Recent quantitative modelling of low-redshift spirals in the Hubble Deep Field [4] also seems to suggest that bulges are generally the oldest parts of galaxies ⁽²⁾ .

² However, if secular activity somehow generates bulges from disks without any star-formation activity (ie. by simply reorganizing existing galaxy populations without forming any new ones), then it is possible that red bulges could still be formed, provided they were built-up exclusively from the oldest disk populations in the centres of the galaxies. The plausibility of this hypothesis could perhaps be tested by hydrodynamical simulations. I am grateful to Mike Merrifield for pointing out this possibility.