The 5 Gyr evolution of sub-M* galaxies

TL;DR

This study investigates the evolution of sub-M* galaxies over 5 billion years, analyzing their morphology, structure, and merger activity, revealing persistent peculiarities and low surface brightness features indicative of hierarchical assembly.

Contribution

It provides a consistent analysis of sub-M* galaxy morphology and structure at two epochs, highlighting their ongoing evolution and the significance of minor mergers in their development.

Findings

Sub-M* galaxies follow a similar Hubble sequence as massive galaxies but with more peculiar and low surface brightness types.

A high fraction of peculiar and low surface brightness galaxies persists at z=0.5, indicating ongoing hierarchical assembly.

Over 10% of sub-M* galaxies at z=0.5 show signs of merging, with disc sizes comparable to M31.

Abstract

We gathered two complete samples of $M_{AB}(r)<-18$ ($M_{star} > 10^{9} M_{\odot}$) galaxies, which are representative of the present-day galaxies and their counterparts at 5 Gyr ago. We analysed their 2D luminosity profiles and carefully decomposed them into bulges, bars and discs. This was done in a very consistent way at the two epochs, by using the same image quality and same (red) filters at rest. We classified them into elliptical, lenticular, spiral, and peculiar galaxies on the basis of a morphological decision tree. We found that at $z=0$, sub-M* ($10^{9} M_{\odot} < M_{star} < 1.5 \times 10^{10} M_{\odot}$) galaxies follow a similar Hubble Sequence compared to their massive counterparts, though with a considerable larger number of (1) peculiar galaxies and (2) low surface brightness galaxies. These trends persist in the $z\sim0.5$ sample, suggesting that sub-M* galaxies have not reached yet a virialised state, conversely to their more massive counterparts. The fraction of peculiar galaxies is always high, consistent with a hierarchical scenario in which minor mergers could have played a more important role for sub-M* galaxies than for more massive galaxies. Interestingly, we also discovered that more than 10\% of the sub-M* galaxies at z=0.5 are low surface brightness galaxies with clumpy and perturbed features that suggest merging. Even more enigmatic is the fact that their disc scale-lengths are comparable to that of M31, while their stellar masses are similar to that of the LMC.