The role of major mergers in the size growth of intermediate-mass spheroids

TL;DR

This study investigates how major mergers contribute to the size increase of intermediate-mass spheroidal galaxies at high redshift, revealing that such mergers could account for a significant portion of their size evolution.

Contribution

It provides observational evidence linking major mergers to size growth in a previously underexplored mass and redshift range of spheroidal galaxies.

Findings

Disturbed SGs are larger than relaxed ones, with a size ratio of about 2.

Approximately two-thirds of size growth in these galaxies can be attributed to major mergers.

Major mergers likely influence size evolution of more massive SGs at lower redshifts.

Abstract

We study of the role of major mergers (mass ratios >1:4) in driving size growth in high-redshift (1<z<2) spheroidal galaxies (SGs) with stellar masses between 10^9.5 MSun and 10^10.7 MSun. This is a largely unexplored mass range at this epoch, containing the progenitors of more massive SGs on which the bulk of the size-evolution literature is based. We visually split our SGs into systems that are relaxed and those that exhibit tidal features indicative of a recent merger. Numerical simulations indicate that, given the depth of our images, only tidal features due to major mergers will be detectable at these epochs (features from minor mergers being too faint), making the disturbed SGs a useful route for estimating major-merger-driven size growth. The disturbed SGs are offset in size from their relaxed counterparts, lying close to the upper envelope of the local size -- mass relation. The mean size ratio of the disturbed SGs to their relaxed counterparts is ~2. Combining this observed size growth with empirical major-merger histories from the literature suggests that the size evolution of a significant fraction (around two-thirds) of SGs in this mass range could be driven by major mergers. If, as is likely, our galaxies are progenitors of more massive M > 10^10.7 MSun SGs at z<1, then major mergers are also likely to play an important role in the size growth of at least some massive SGs in this mass range.