Forming disk galaxies in wet major mergers. I. Three fiducial examples

TL;DR

This study uses detailed simulations to explore how major galaxy mergers can produce spiral galaxies with diverse bulge types, disk structures, and kinematic features, challenging previous assumptions about galaxy formation.

Contribution

It demonstrates that spiral galaxies with multiple bulge components can form from wet major mergers, with a detailed analysis of their structural and stellar population properties.

Findings

Remnant galaxies develop classical bulges, thick disks, and thin disks.

A new stellar disk forms gradually from halo gas, resulting in a thin, extended disk.

Discy pseudobulges and bars form before the disk is fully established.

Abstract

Using three fiducial Nbody+SPH simulations, we follow the merging of two disk galaxies with a hot gaseous halo component each, and examine whether the merger remnant can be a spiral galaxy. The stellar progenitor disks are destroyed by violent relaxation during the merging and most of their stars form a classical bulge, while the remaining form a thick disk and its bar. A new stellar disk forms subsequently and gradually in the remnant from the gas accreted mainly from the halo. It is vertically thin and well extended in its equatorial plane. A bar starts forming before the disk is fully in place, contrary to what is assumed in idealised simulations of isolated bar-forming galaxies. It has morphological features such as ansae and boxy/peanut bulges. Stars of different ages populate different parts of the box/peanut. A disky pseudobulge forms also, so that by the end of the simulation, all three types of bulges coexist. The oldest stars are found in the classical bulge, followed by those of the thick disk, then by those in the thin disk. The youngest stars are in the spiral arms and the disky pseudobulge. The disk surface density profiles are of type II (exponential with downbending), and the circular velocity curves are flat and show that the disks are submaximum in these examples: two clearly so and one near-borderline between maximum and submaximum. On average, only roughly between 10 and 20% of the stellar mass is in the classical bulge of the final models, i.e. much less than in previous simulations.