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

TL;DR

This study uses HST near-infrared imaging to analyze the surface brightness profiles of galaxy bulges, finding most do not follow the traditional r^{1/4} law and suggesting limited merger-driven growth.

Contribution

It provides high-resolution evidence that galaxy bulges typically have Sersic indices below 2, challenging the merger-driven formation paradigm based on classical r^{1/4} profiles.

Findings

Most bulges have Sersic index n < 2.

Ground-based profiles overestimate n due to seeing effects.

Results suggest limited role of mergers in bulge formation.

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 S\'ersic r^{1/n} functional form for the bulge, none of the bulges have an r^{1/4} behaviour; derived S\'ersic shape-indices are <n> = 1.7 \pm 0.7. For the same sample, fits to NIR ground-based profiles yield S\'ersic 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 S\'ersic indices are not expected from merger violent relaxation, and argue against significant merger growth for most bulges.