Near-infrared observations of galaxies in Pisces-Perseus V. On the origin of bulges

We investigate the scaling relations of bulge and disk structural parameters for a sample of 108 disk galaxies. Structural parameters of individual galaxies are obtained from two-dimensional bulge/disk decomposition of their H-band surface brightness distributions . Bulges are modelled with a generalized exponential (Sérsic) with variable integer shape index n. We find that bulge effective scalelengths rB e and luminosity M B increase with increasing n, but disk properties are independent of bulge shape. As Hubble type T increases, bulges become less luminous and their mean effective surface brightness < μe > gets fainter; disk < μe > shows a similar, but much weaker, trend. When bulge parameters (< μ B e >, r B e , M B) are compared with disk ones (< μe >, r D e , M D), they are tightly correlated for n = 1 bulges. The correlations gradually worsen with increasing n such that n = 4 bulges appear virtually independent of their disks. The Kormendy relation, < μe > vs. r B e , is shown to depend on bulge shape n; the two parameters are tightly correlated in n = 4 bulges (r = 0.8), and increasingly less so as n decreases; disk < μe > and r D e are well correlated (r = 0.7). Bulge-to-disk size ratios r B e /r D e are independent of Hubble type, but smaller for exponential bulges; the mean rB e /r D e for n = 1 bulges is 4 times smaller than that for n = 4, with a spread which is 9 times smaller. Strongly barred SB galaxies with exponential bulges are more luminous than their unbarred counterparts. Exponential bulges appear to be closely related to their underlying disks, while bulges with higher n values are less so; n = 4 bulges and their disks apparently have no relation. We interpret our results as being most consistent with a secular evolutionary scenario, in which dissipative processes in the disk are responsible for building up the bulges in most spirals.