Galactic bulges from Hubble Space Telescope NICMOS observations: the lack of r^{1/4} bulges

Abstract

We use HST near-infrared imaging to explore the shapes of the surface brightness profiles of bulges of S0-Sbc galaxies at high resolution. Modeling extends to the outer bulge via bulge-disk decompositions of combined HST - ground based profiles. Compact, central unresolved components similar to those reported by others are found in ~84% of the sample. We also detect a moderate frequency (~34%) of nuclear components with exponential profiles which may be disks or bars. Adopting the Sérsic r^{1/n} functional form for the bulge, none of the bulges have an r^{1/4} behaviour; derived Sérsic shape-indices are = 1.7 \pm 0.7. For the same sample, fits to NIR ground-based profiles yield Sérsic indices up to n = 4-6. The high-$n$ of ground-based profiles are a result of nuclear point sources blending with the bulge extended light due to seeing. The low Sérsic indices are not expected from merger violent relaxation, and argue against significant merger growth for most bulges.