Where have all the bulges gone?

TL;DR

This study analyzes HST images of 130 nearby early-type disc galaxies, revealing that only a small fraction have pure, structureless bulges, while most show complex inner substructures or lack bulges entirely.

Contribution

It provides new insights into the diversity of bulge structures in early-type disc galaxies, challenging the traditional view of bulges as simple, relaxed stellar systems.

Findings

Only 12% of galaxies have pure, structureless bulges with n > 2.

Most galaxies exhibit inner substructures like bars, spirals, or rings.

A significant number of galaxies lack bulges altogether.

Abstract

Several recent studies indicate that bulges are more complex than merely structureless relaxed stellar systems. We study the HST images of a sample of 130 nearby early type (S0-Sab) disc galaxies and detect pure structureless bulges with the Sersic index n > 2 for only 12% of the galaxies. Other galaxies show varied substructure in their inner regions (inner bars, inner spiral arms, inner rings) and sometimes contain no bulge at all. Inner substructure is more common for these galaxies, which also display structure at larger scales.