Star formation and environmental quenching of GEEC2 group galaxies at z ∼ 1

We present new analysis from the Group Environment Evolution Collaboration 2 (GEEC2) spectroscopic survey of galaxy groups at 0.8 < z < 1. Our previous work revealed an intermediate population between the star-forming and quiescent sequences and a strong environmental dependence in the fraction of quiescent galaxies. Only ∼5 per cent of star-forming galaxies in both the group and field sample show a significant enhancement in star formation, which suggests that quenching is the primary process in the transition from the star-forming to the quiescent state. To model the environmental quenching scenario, we have tested the use of different exponential quenching time-scales and delays between satellite accretion and the onset of quenching. We find that with no delay, the quenching time-scale needs to be long in order to match the observed quiescent fraction, but then this model produces too many intermediate galaxies. Fixing a delay time of 3 Gyr, as suggested from the local Universe, produces too few quiescent galaxies. The observed fractions are best matched with a model that includes a delay that is proportional to the dynamical time and a rapid quenching time-scale (∼0.25 Gyr), but this model also predicts intermediate galaxies Hδ strength higher than that observed. Using stellar synthesis models, we have tested other scenarios, such as the rejuvenation of star formation in early-type galaxies and a portion of quenched galaxies possessing residual star formation. If environment quenching plays a role in the GEEC2 sample, then our work suggests that only a fraction of intermediate galaxies may be undergoing this transition and that quenching occurs quite rapidly in satellite galaxies (≲0.25 Gyr)

Do group dynamics play a role in the evolution of member galaxies?

We examine galaxy groups from the present epoch to z ∼ 1 to explore the impact of group dynamics on galaxy evolution. We use group catalogues from the Sloan Digital Sky Survey (SDSS), the Group Environment and Evolution Collaboration (GEEC) and the high-redshift GEEC2 samples to study how the observed member properties depend on the galaxy stellar mass, group dynamical mass and dynamical state of the host group. We find a strong correlation between the fraction of non-star-forming (quiescent) galaxies and galaxy stellar mass, but do not detect a significant difference in the quiescent fraction with group dynamical mass, within our sample halo mass range of ∼1013–1014.5 M⊙, or with dynamical state. However, at z ∼ 0.4 we do find some evidence that the quiescent fraction in low-mass galaxies [log10(Mstar/M⊙) ≲ 10.5] is lower in groups with substructure. Additionally, our results show that the fraction of groups with non-Gaussian velocity distributions increases with redshift to z ∼ 0.4, while the amount of detected substructure remains constant to z ∼ 1. Based on these results, we conclude that for massive galaxies [log10(Mstar/M⊙) ≳ 10.5], evolution is most strongly correlated to the stellar mass of a galaxy with little or no additional effect related to either the group dynamical mass or the dynamical state. For low-mass galaxies, we do find some evidence of a correlation between the quiescent fraction and the amount of detected substructure, highlighting the need to probe further down the stellar mass function to elucidate the role of environment in galaxy evolution

MUSE sneaks a peek at extreme ram-pressure stripping events – II. The physical properties of the gas tail of ESO137−001

We present a study of the physical properties of the disc and tail of ESO137−001, a galaxy suffering from extreme ram-pressure stripping during its infall into the Norma cluster. With sensitive and spatially resolved MUSE (Multi Unit Spectroscopic Explorer) spectroscopy, we analyse the emission line diagnostics in the tail of ESO137−001, finding high values of [N II]/Hα and [O I]/Hα that are suggestive of the presence of shocks in turbulent gas. However, the observed line ratios are not as strong as commonly seen in pure shock heating models, suggesting that other emission mechanisms may contribute to the observed emission. Indeed, part of the observed emission, particularly at close separations from the galaxy disc, may originate from recombination of photoionized gas stripped from the main body of ESO137−001. We also identify a large number of bright compact knots within in the tail, with line ratios characteristic of H II regions. These H II regions, despite residing in a stripped gas tail, have quite typical line ratios, densities, temperatures, and metallicity (∼0.7 solar). The majority of these H II regions are embedded within diffuse gas from the tail, which is dynamically cool (σ ∼ 25–50 km s− 1). This fact, together with a lack of appreciable gradients in age and metallicity, suggests that these H II regions formed in situ. While our analysis represents a first attempt to characterize the rich physics of the ESO137−001 tail, future work is needed to address the importance of other mechanisms, such as thermal conduction and magnetohydrodynamic waves, in powering the emission in the tail

Galaxy groups at 0.3 ⩽ z ⩽ 0.55 - I. Group properties.

The evolution of galaxies in groups may have important implications for the evolution of the star formation history of the Universe, since many processes which operate in groups may suppress star formation and the fraction of galaxies in bound groups grows rapidly between z= 1 and the present day. In this paper, we present an investigation of the properties of galaxies in galaxy groups at intermediate redshift (z∼ 0.4). The groups were selected from the Canadian Network for Observational Cosmology Redshift Survey (CNOC2) redshift survey as described by Carlberg et al., with further spectroscopic follow-up undertaken at the Magellan telescope in order to improve the completeness and depth of the sample. We present the data for the individual groups, and find no clear trend in the fraction of passive galaxies with group velocity dispersion and group concentration. We stack the galaxy groups in order to compare the properties of group galaxies with those of field galaxies at the same redshift. The groups contain a larger fraction of passive galaxies than the field, this trend being particularly clear for galaxies brighter than graphic in the higher velocity dispersion groups. In addition, we see evidence for an excess of bright passive galaxies in the groups relative to the field. In contrast, the luminosity functions of the star-forming galaxies in the groups and the field are consistent. These trends are qualitatively consistent with the differences between group and field galaxies seen in the local Universe

Substructure in the most massive GEEC groups : field-like populations in dynamically active groups

The presence of substructure in galaxy groups and clusters is believed to be a sign of recent galaxy accretion and can be used to probe not only the assembly history of these structures, but also the evolution of their member galaxies. Using the Dressler–Shectman (DS) test, we study substructure in a sample of intermediate‐redshift (z∼ 0.4) galaxy groups from the Group Environment and Evolution Collaboration (GEEC) group catalogue. We find that four of the 15 rich GEEC groups, with an average velocity dispersion of ∼525 km s−1, are identified as having significant substructure. The identified regions of localized substructure lie on the group outskirts and in some cases appear to be infalling. In a comparison of galaxy properties for the members of groups with and without substructure, we find that the groups with substructure have a significantly higher fraction of blue and star‐forming galaxies and a parent colour distribution that resembles that of the field population rather than the overall group population. In addition, we observe correlations between the detection of substructure and other dynamical measures, such as velocity distributions and velocity dispersion profiles. Based on this analysis, we conclude that some galaxy groups contain significant substructure and that these groups have properties and galaxy populations that differ from groups with no detected substructure. These results indicate that the substructure galaxies, which lie preferentially on the group outskirts and could be infalling, do not exhibit signs of environmental effects, since little or no star formation quenching is observed in these systems