HST Optical-NIR Colors of Nearby $R^{1/4}$ and Exponential Bulges

Abstract

We have analysed $V$, $H$ and $J$ HST images for a sample of early- to late-type spiral galaxies, and reported elsewhere the statistical frequency of $R^{1/4}$-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 that: {\it (i)} The $V-H$ color distribution of the $R^{1/4}$ bulges peaks around $<V-H> \sim 1.3$, with a sigma $\Delta (V-H) \sim 0.1$ magnitudes. Assuming a solar metallicity, these values correspond to stellar ages of $\approx 6 \pm 3$ Gyrs. In contrast, the $V-H$ color distribution of the exponential bulges peaks at $<V-H> \sim 0.9$ and has a sigma $\Delta (V-H) \sim 0.4$ mags. This likely implies significantly smaller ages and/or lower metallicities for (a significant fraction of the stars in) the exponential bulges compared to the $R^{1/4}$-law spheroids. {\it (ii)} Most of the central nuclei hosted by the exponential bulges have $V-H$ and $J-H$ colors which are compatible with relatively unobscured stellar populations. Assuming no or little dust effects, ages $\gta 1$ Gyrs are suggested for these nuclei, which in turn imply masses of about a few 10$^6$ to a few 10$^7$ M$_\odot$, i.e., sufficient to dissolve progenitor bars with masses consistent with those inferred for the exponential bulges by their luminosities. {\it (iii)} 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. (abridged)