A comprehensive investigation of environmental influences on galaxies in group environments
Authors: Van Kempen Wesley , Cluver Michelle , Jarrett Thomas , Croton Darren , Lambert Trystan , Kilborn Virginia V.A. , Taylor Edward N. , Magoulas Christina , Marc Yao Harris Fortune 
Published: October 2024

Environment has long been known to have an impact on the evolution of galaxies, but disentangling its impact from mass evolution requires the careful analysis of statistically significant samples. By implementing cutting-edge visualisation methods to test and validate group-finding algorithms, we utilise a mass-complete sample of galaxies to

$z \lt 0.1$

comprised of spectroscopic redshifts from prominent surveys such as the 2-degree Field Galaxy Redshift Survey and the Galaxy and Mass Assembly Survey. Utilising our group finding methods, we find 1 413 galaxy groups made up of 8 990 galaxies corresponding to 36% of galaxies associated with group environments. We also search for close pairs, with separations of

$r_\mathrm{sep} \lt 50$

$\text{h}^{-1}\text{kpc}$

and

$v_\mathrm{sep} \lt 500 \: \text{km s}^{-1}$

within our sample and further classified them into major (

$M_{sec}/M_{prim} \leq$

0.25) and minor (

$M_{sec}/M_{prim} \gt $

0.25) pairs. To examine the impact of environmental factors, we employ bespoke WISE photometry, which facilitates accurate measurements of stellar mass and star formation rates and hence the best possible description of the variation of galaxy properties as a function of the local environment. Our analysis, employing a derived star-forming main sequence relation, reveals that star-formation (SF) within galaxies are pre-processed as a function of group membership. This is evident from the evolution of the star-forming and quenched population of galaxies. We see an increase in the fraction of quiescent galaxies relative to the field as group membership increases, and this excess of quenched galaxies relative to the field is later quantified through the use of the environmental quenching efficiency (

$\varepsilon_{env}$

) metric. Within the star-forming population, we observe SF pre-processing with the relative difference in specific star formation rates (

$\Delta sSFR$

), where we see a net decrease in SF as group membership increases, particularly at larger stellar masses. We again quantify this change within the SF population with our star formation deficiency (

$\varepsilon_{SFD}$

) metric. Our sample of close pairs at low stellar masses exhibit enhanced star formation efficiencies compared to the field, and at larger stellar mass ranges show large deficiencies. Separating the close pairs into major/minors and primary/secondaries reveals SF enhancements projected separation decreases within the minor pairs, this effect is even more pronounced within minor primaries. This research emphasises the importance of carefully studying the properties of galaxies within group environments to better understand the pre-processing of SF within galaxies. Our results show that the small-scale environments of galaxies influence star-forming properties even when stellar masses are kept constant. This demonstrates that galaxies do not evolve in isolation over cosmic time but are shaped by a complex interaction between their internal dynamics and external influences.