Segregation effects in DEEP2 galaxy groups

TL;DR

This study examines galaxy segregation in groups across redshifts 0.2 to 1, revealing evolving kinematic and spatial segregation patterns that suggest galaxy infall and energy equipartition over cosmic time.

Contribution

It provides new insights into the evolution of galaxy segregation in groups from redshift 0.8 to 0.4 using DEEP2 survey data, highlighting changes in kinematic and spatial distributions.

Findings

Red galaxies are more prevalent at lower redshifts.

Kinematic segregation is statistically significant in low-z groups.

Spatial segregation shows weaker evidence, varying with redshift.

Abstract

We investigate segregation phenomena in galaxy groups in the range of $0.2<z<1$. We study a sample of groups selected from the 4th Data Release of the DEEP2 galaxy redshift survey. We used only groups with at least 8 members within a radius of 4$\;$Mpc. Outliers were removed with the shifting gapper techinque and, then, the virial properties were estimated for each group. The sample was divided into two stacked systems: low($z\leq0.6$) and high($z>0.6$) redshift groups. Assuming that the color index ${(U-B)_0}$ can be used as a proxy for the galaxy type, we found that the fraction of blue (star-forming) objects is higher in the high-z sample, with blue objects being dominant at $M_{B}>-19.5$ for both samples, and red objects being dominant at $M_{B}<-19.5$ only for the low-z sample. Also, the radial variation of the red fraction indicates that there are more red objects with $R<R_{200}$ in the low-z sample than in the high-z sample. Our analysis indicates statistical evidence of kinematic segregation, at the 99%c.l., for the low-z sample: redder and brighter galaxies present lower velocity dispersions than bluer and fainter ones. We also find a weaker evidence for spatial segregation between red and blue objects, at the 70%c.l. The analysis of the high-z sample reveals a different result: red and blue galaxies have velocity dispersion distributions not statistically distinct, although redder objects are more concentrated than the bluer ones at the 95%c.l. From the comparison of blue/red and bright/faint fractions, and considering the approximate lookback timescale between the two samples ($\sim$3 Gyr), our results are consistent with a scenario where bright red galaxies had time to reach energy equipartition, while faint blue/red galaxies in the outskirts infall to the inner parts of the groups, thus reducing spatial segregation from $z\sim0.8$ to $z\sim0.4$.