BULGEnANDB/TIN HIGH-MASS GALAXIES: CONSTRAINTS ON THE ORIGIN OF BULGES IN HIERARCHICAL MODELS

We use the bulge Sérsic index n and bulge-to-total mass ratio ( B / T ) to explore the fundamental question of how bulges form. We perform two-dimensional bulge–disk–bar decomposition on H -band images of 143 bright, high-mass ( M ⋆ ⩾ 1.0 × 10 10 M ☉ ) low-to-moderately inclined ( i < 70°) spirals. Our results are as follows. (1) Our H -band bar fraction (∼58%) is consistent with that from ellipse fits. (2) 70% of the stellar mass is in disks, 10% in bars, and 20% in bulges. (3) A large fraction (∼69%) of bright spirals have B / T ⩽ 0.2, and ∼76% have low n ⩽ 2 bulges. These bulges exist in barred and unbarred galaxies across a wide range of Hubble types. (4) About 65% (68%) of bright spirals with n ⩽ 2 ( B / T ⩽ 0.2) bulges host bars, suggesting a possible link between bars and bulges. (5) We compare the results with predictions from a set of ΛCDM models. In the models, a high-mass spiral can have a bulge with a present-day low B / T ⩽ 0.2 only if it did not undergo a major merger since z ⩽ 2. The predicted fraction (∼ 1.6%) of high-mass spirals, which have undergone a major merger since z ⩽ 4 and host a bulge with a present-day low B / T ⩽ 0.2, is a factor of over 30 smaller than the observed fraction (∼66%) of high-mass spirals with B / T ⩽ 0.2. Thus, contrary to common perception, bulges built via major mergers since z ⩽ 4 seriously fail to account for the bulges present in ∼66% of high mass spirals . Most of these present-day low B / T ⩽ 0.2 bulges are likely to have been built by a combination of minor mergers and/or secular processes since z ⩽ 4.