Formation of galactic bulges from the cold gas filaments in high-redshift dark matter halos

TL;DR

This paper proposes that galactic bulges in high-mass galaxies form from cold gas filaments inflowing into hot halos at high redshifts, aligning with observed mass and age trends.

Contribution

It demonstrates that bulge formation from cold gas inflows explains observed stellar age and mass relations, especially in high-mass galaxies.

Findings

Bulge-to-total mass ratio increases with galaxy mass.

Mean stellar age of bulges increases with galaxy mass.

Age gradient across bulges decreases with galaxy mass.

Abstract

Formation process(es) of galactic bulges are not yet clarified although several mechanisms have been proposed. In a previous study, we suggested one possibility that galactic bulges have been formed from the cold gas inflowing through surrounding hot halo gas in massive dark matter halos at high redshifts. It was shown that this scenario leads to the bulge-to-total stellar mass ratio increasing with the galaxy mass, in agreement with the well-known observed trend. We here indicate that it also reproduces recent observational results that the mean stellar age of the bulge increases with the galaxy mass while the age gradient across the bulge decreases. We infer that this formation path applies mainly to high-mass galaxies and the bulges in lower-mass galaxies have different origins such as secular formation from the disc material.