The role of minor mergers in the recent star formation history of early-type galaxies

TL;DR

This paper shows that minor mergers can explain the observed UV colour scatter and recent star formation in early-type galaxies, aligning with predictions from the standard LCDM cosmology.

Contribution

It demonstrates through simulations that minor mergers with specific mass ratios and gas fractions reproduce observed UV colours and star formation in early-type galaxies.

Findings

Minor mergers with mass ratios ≤ 1:4 explain UV colour scatter.

Early-type galaxies with (NUV-r) < 3.8 likely experienced recent mergers.

Color distributions are consistent with minor merger activity in LCDM cosmology.

Abstract

We demonstrate that the large scatter in the ultra-violet (UV) colours of intermediate-mass early-type galaxies in the local Universe and the inferred low-level recent star formation in these objects can be reproduced by minor mergers in the standard LCDM cosmology. Numerical simulations of mergers with mass ratios less than or equal to 1:4, with reasonable assumptions for the ages, metallicities and dust properties of the merger progenitors, produce good agreement to the observed UV colours of the early-type population, if the infalling satellites are assumed to have (cold) gas fractions greater than 20%. Early-types that satisfy (NUV-r) < 3.8 are likely to have experienced mergers with mass ratios between 1:4 and 1:6 within the last ~1.5 Gyrs, while those that satisfy 3.8<(NUV-r)<5.5 are consistent with either recent mergers with mass ratios < 1:6 or mergers with higher mass ratios that occurred more than ~1.5 Gyrs in the past. We demonstrate that the early-type colour-magnitude relations and colour distributions in both the UV and optical spectral ranges are consistent with the expected frequency of minor merging activity in the standard LCDM cosmology at low redshift. We present a strong plausibility argument for minor mergers to be the principal mechanism behind the large UV scatter and associated low-level recent star formation observed in early-type galaxies in the nearby Universe.