Formation of elongated galaxies with low masses at high redshift

TL;DR

This study uses cosmological simulations to identify elongated, low-mass galaxies at high redshift, revealing their formation linked to dark matter halo shapes and feedback effects, and explaining their observed asymmetric shapes.

Contribution

It demonstrates that elongated galaxy morphologies at high redshift are driven by dark matter halo shapes and feedback, a novel insight into early galaxy formation.

Findings

Low-mass galaxies at z~2 are elongated and prolate.

Dark matter halo shape influences galaxy elongation.

Elongated galaxies produce asymmetric projected shapes.

Abstract

We report the identification of elongated (triaxial or prolate) galaxies in cosmological simulations at $z\simeq2$. These are preferentially low-mass galaxies ($M_s \le 10^{9.5} \ M_\odot$), residing in dark-matter (DM) haloes with strongly elongated inner parts, a common feature of high-redshift DM haloes in the $\Lambda$CDM cosmology. Feedback slows formation of stars at the centres of these halos, so that a dominant and prolate DM distribution gives rise to galaxies elongated along the DM major axis. As galaxies grow in stellar mass, stars dominate the total mass within the galaxy half-mass radius, making stars and DM rounder and more oblate. A large population of elongated galaxies produces a very asymmetric distribution of projected axis ratios, as observed in high-z galaxy surveys. This indicates that the majority of the galaxies at high redshifts are not discs or spheroids but rather galaxies with elongated morphologies.