# Assembly bias evidence in close galaxy pairs

## TL;DR

This study investigates assembly bias in galaxy formation by analyzing close galaxy pairs from SDSS data, revealing that satellites around more massive primaries are older, indicating environment-driven differences in galaxy evolution.

## Contribution

It provides new evidence of one-halo assembly bias by analyzing stellar populations in satellite galaxies across different primary galaxy masses using SDSS data.

## Key findings

- Satellites of more massive primaries are older at fixed velocity dispersion.
- Older satellite populations are associated with smaller mass ratios, indicating environmental effects.
- The age difference can be up to approximately 0.5 Gyr in lower mass satellites.

## Abstract

The growth channel of massive galaxies involving mergers can be studied via close pairs as putative merger progenitors, where the stellar populations of the satellite galaxies will be eventually incorporated into the massive primaries. We extend our recent analysis of the GAMA-based sample of close pairs presented in Ferreras et al. to the general spectroscopic dataset of SDSS galaxies (DR14), for which the high S/N of the data enables a detailed analysis of the differences between satellite galaxies with respect to the mass of the primary galaxy. A sample of approximately two thousand satellites of massive galaxies is carefully selected within a relatively narrow redshift range (0.07<z<0.14). Two main parameters are considered as major drivers of the star formation history of these galaxies, namely: the stellar velocity dispersion of the satellite ($\sigma$), as a proxy of "local" drivers, and the ratio between the stellar mass of the satellite and the primary, $\mu=M_{\rm SAT}/M_{\rm PRI}$, meant to serve as an indicator of environment. Consistently with the independent, GAMA-based work, we find that satellites around the most massive primaries appear older, at fixed velocity dispersion, with respect to satellites of lower mass primaries. This trend is more marked in lower mass satellites ($\sigma$~100 km/s), with SSP-equivalent age differences up to ~0.5 Gyr, and can be interpreted as a one-halo assembly bias, so that satellites corresponding to smaller values of the mass ratio $\mu$ represent older structures, akin to fossil groups.