Natures of clump-origin bulges: similarities to the Milky Way bulge

TL;DR

This study uses high-resolution simulations to explore a third bulge formation scenario, the clump-origin bulge, revealing it shares properties with pseudobulges but contains old, metal-rich stars, challenging traditional classifications.

Contribution

The paper demonstrates through simulations that clump-origin bulges have unique properties, combining features of pseudobulges and classical bulges, providing new insights into bulge formation.

Findings

Clump-origin bulges resemble pseudobulges dynamically.

They consist of old, metal-rich stars.

They cannot be classified solely as classical or pseudobulges.

Abstract

Bulges in spiral galaxies have been supposed to be classified into two types: classical bulges or pseudobulges. Classical bulges are thought to form by galactic merger with bursty star formation, whereas pseudobulges are suggested to form by secular evolution. Noguchi (1998, 1999) suggested another bulge formation scenario, `clump-origin bulge'. He demonstrated using a numerical simulation that a galactic disc forms clumpy structures in the early stage of disc formation, then the clumps merge into a single bulge at the centre. I perform a high-resolution N-body/SPH simulation for the formation of the clump-origin bulge in an isolated galaxy model. I find that the clump-origin bulge resembles pseudobulges in dynamical properties, but this bulge consists of old and metal-rich stars. These natures, old metal-rich population but pseudobulge-like structures, mean that the clump-origin bulge can not be simply classified into classical bulges nor pseudobulges. From these results, I discuss similarities of the clump-origin bulge to the Milky Way bulge.