We have analyzed V, H, and J Hubble Space Telescope (HST) images for a sample of early- to late-type spiral galaxies and have reported elsewhere the statistical frequency of R1/4-law and exponential bulges in our sample as a function of Hubble type and the frequency of occurrence and structural properties of the resolved central nuclei hosted by intermediate- to late-type bulges and disks (see references in the text). Here we use these data to show the following: 1. The V - H color distribution of the R1/4 bulge peaks around 〈V-H〉 ∼ 1.3, with a sigma Δ(V - H) ∼ 0.1 mag. Assuming a solar metallicity, these values correspond to stellar ages of ≈6 + 3 Gyr. In contrast, the V - H color distribution of the exponential bulges peaks at 〈V - H ∼ 0.9〉 and has a sigma Δ(V - H) ∼ 0.4 mag. This likely implies significantly smaller ages and/or lower metallicities for (a significant fraction of the stars in) the exponential bulges compared to the R1/4-law spheroids. 2. Most of the central nuclei hosted by the exponential bulges have V - H and J - H colors that are compatible with relatively unobscured stellar populations. Assuming no or little dust effects, ages ≳ 1 Gyr are suggested for these nuclei, which in turn imply masses of about a few 106 to a few 107 M⊙, i.e., sufficient to dissolve progenitor bars with masses consistent with those inferred for the exponential bulges by their luminosities. 3. While different bulge-nucleus pairs cover a large range of V - H colors, each bulge-nucleus pair has quite similar V - H colors and thus possibly similar stellar populations. The HST photometric analysis suggests that exponential-type bulge formation is taking place in the local universe and that this process is consistent with being the outcome of secular evolution processes within the disks. The structures that are currently formed inside the disks are quite dissimilar from the old elliptical-like spheroids that are hosted by the early-type disks.
Copyright 2015 Elsevier B.V., All rights reserved.
- Galaxies: evolution
- Galaxies: formation
- Galaxies: nuclei
- Galaxies: structure