Formation of Galactic Pseudo-bulges via Gas Rich Major Mergers

TL;DR

This study uses high-resolution simulations to show that gas-rich major mergers can form pseudo-bulges in disk galaxies, challenging traditional views and explaining the prevalence of bulge-less galaxies.

Contribution

It introduces a novel mode of pseudo-bulge formation through gas-rich major mergers, supplementing secular evolution theories.

Findings

Pseudo-bulges formed by mergers are rotation-supported with low Sersic indices.

Simulations show bulges formed in mergers are similar to observed pseudo-bulges.

This process explains the high fraction of bulge-less giant disk galaxies.

Abstract

It is widely accepted that within the framework of LCDM a significant fraction of giant-disk galaxies has recently experienced a violent galactic merger. We present numerical simulations of such major mergers of gas-rich pure disk galaxies, and focus on the innermost stellar component (bulge) of the disk remnants. The simulations have high spatial and mass resolutions, and resolve regions deep enough to allow bulge classification according to standard kinematical and structural characteristics. In agreement with recent studies we find that these bulges are dominated by stars formed in the final coalescence process. In contrast to the common interpretation of such components as classical bulges (i.e. similar to intermediate luminosity ellipticals), we find they are supported by highly coherent rotations and have Sersic indices n<2, a result leading to their classification as pseudo-bulges. Pseudo-bulge formation by gas rich major mergers of pure disks is a novel mode of pseudo-bulge formation; It complements pseudo-bulge growth by secular evolution, and it could help explain the high fractions of classically bulge-less giant disk galaxies, and pseudo-bulges found in giant Sc galaxies.