Probing the Structure and Assembly of Nearby Field Spirals

Abstract

We use the bulge Sérsic index n and bulge-to-total ratio (B∕T) to explore the fundamental question of how bulges form. We perform 2D bulge-disk-bar decomposition on H-band images of 143 bright, high mass ( $$M_{\star } \geq 1.0 \times 10^{10}M_{\odot }$$ ) low-to-moderately inclined (i < 70∘) spirals. Our results are: (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 thirty 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.