Bulge n and B/T in High-Mass Galaxies: Constraints on the origin of Bulges in Hierarchical models

Tim Weinzirl, Shardha Jogee, Sadegh Khochfar, Andreas Burkert, John Kormendy

Research output: Contribution to journalArticlepeer-review


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 sstarf ≥ 1.0 × 1010 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.
Original languageEnglish
Pages (from-to)411-447
JournalAstrophysical Journal
Issue number1
Publication statusPublished - 1 May 2009


  • galaxies: bulges
  • galaxies: evolution
  • galaxies: formation


Dive into the research topics of 'Bulge n and B/T in High-Mass Galaxies: Constraints on the origin of Bulges in Hierarchical models'. Together they form a unique fingerprint.

Cite this