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Balogh M.L.,University of Waterloo | Balogh M.L.,Leiden University | McGee S.L.,Leiden University | Mok A.,University of Waterloo | And 7 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2014

We present the data release of the Gemini-South GMOS spectroscopy in the fields of 11 galaxy groups at 0.8 < z < 1, within the COSMOS field. This forms the basis of the Galaxy Environment Evolution Collaboration 2 (GEEC2) project to study galaxy evolution in haloes with M ̃ 1013M⊙ across cosmic time. The final sample includes 162 spectroscopically confirmed members with R < 24.75, and is >50 per cent complete for galaxies within the virial radius, and with stellar mass Mstar > 1010.3M⊙. Including galaxies with photometric redshifts, we have an effective sample size of ̃400 galaxies within the virial radii of these groups.We present group velocity dispersions, dynamical and stellar masses. Combining with theGCLASS sample of more massive clusters at the same redshift,we find the total stellarmass is strongly correlated with the dynamical mass, with logM200 = 1.20(logMstar - 12) + 14.07. This stellar fraction of̃1 per cent is lower than predicted by some halo occupation distribution models, though the weak dependence on halo mass is in good agreement. Most groups have an easily identifiable most massive galaxy (MMG) near the centre of the galaxy distribution, and we present the spectroscopic properties and surface brightness fits to these galaxies. The total stellar mass distribution in the groups, excluding the MMG, compares well with an NFW (Navarro Frenk & White) profile with concentration 4, for galaxies beyond ̃0.2R200. This is more concentrated than the number density distribution, demonstrating that there is some mass segregation. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Source


Tyler K.D.,University of Arizona | Rieke G.H.,University of Arizona | Wilman D.J.,Max Planck Institute for Extraterrestrial Physics | McGee S.L.,University of Waterloo | And 7 more authors.
Astrophysical Journal | Year: 2011

Galaxy star formation rates (SFRs) are sensitive to the local environment; for example, the high-density regions at the cores of dense clusters are known to suppress star formation. It has been suggested that galaxy transformation occurs largely in groups, which are the intermediate step in density between field and cluster environments. In this paper, we use deep MIPS 24 μm observations of intermediate-redshift (0.3 ≲ z ≲ 0.55) group and field galaxies from the Group Environment and Evolution Collaboration (GEEC) subset of the Second Canadian Network for Observational Cosmology (CNOC2) survey to probe the moderate-density environment of groups, wherein the majority of galaxies are found. The completeness limit of our study is log (L TIR(L ∞)) ≳ 10.5, corresponding to SFR ≳2.7 M ∞ yr-1. We find that the group and field galaxies have different distributions of morphologies and mass. However, individual group galaxies have star-forming properties comparable to those of field galaxies of similar mass and morphology; that is, the group environment does not appear to modify the properties of these galaxies directly. There is a relatively large number of massive early-type group spirals, along with E/S0 galaxies, that are forming stars above our detection limit. These galaxies account for the nearly comparable level of star-forming activity in groups as compared with the field, despite the differences in mass and morphology distributions between the two environments. The distribution of specific SFRs (SFR/M *) is shifted to lower values in the groups, reflecting the fact that groups contain a higher proportion of massive and less active galaxies. Considering the distributions of morphology, mass, and SFR, the group members appear to lie between field and cluster galaxies in overall properties. © 2011. The American Astronomical Society. All rights reserved. Source


Balogh M.L.,University of Waterloo | Mcgee S.L.,University of Waterloo | Mcgee S.L.,Durham University | Wilman D.J.,Max Planck Institute for Extraterrestrial Physics | And 8 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2011

We introduce our survey of galaxy groups at 0.85 < z < 1, as an extension of the Group Environment and Evolution Collaboration. Here we present the first results, based on Gemini GMOS-S nod-and-shuffle spectroscopy of seven galaxy groups selected from spectroscopically confirmed, extended XMM detections in COSMOS. We use photometric redshifts to select potential group members for spectroscopy, and target galaxies with r < 24.75. In total, we have over 100 confirmed group members, and four of the groups have >15 members. The dynamical mass estimates are in good agreement with the masses estimated from the X-ray luminosity, with most of the groups having 13 < logMdyn/M⊙ < 14. We compute stellar masses by template-fitting the spectral energy distributions; our spectroscopic sample is statistically complete for all galaxies with Mstar≳ 1010.1M⊙, and for blue galaxies we sample masses as low as Mstar∼ 108.8M⊙. The fraction of total mass in galaxy starlight spans a range of 0.25-3per cent, for the six groups with reliable mass determinations. Like lower redshift groups, these systems are dominated by red galaxies, at all stellar masses Mstar > 1010.1M⊙. A few group galaxies inhabit the 'blue cloud' that dominates the surrounding field; instead, we find a large and possibly distinct population of galaxies with intermediate colours. The 'green valley' that exists at low redshift is instead well populated in these groups, containing ∼30per cent of the galaxies. These do not appear to be exceptionally dusty galaxies, and about half show prominent Balmer absorption lines. Furthermore, their Hubble Space Telescope morphologies appear to be intermediate between those of red-sequence and blue-cloud galaxies of the same stellar mass. Unlike red-sequence galaxies, most of the green galaxies have a disc component, but one that is smaller and less structured than discs found in the blue cloud. We postulate that these are a transient population, migrating from the blue cloud to the red sequence, with a star formation rate that declines with an exponential time-scale 0.6 < τ < 2Gyr. Such galaxies may not be exclusive to the group environment, as we find examples also amongst the non-members. However, their prominence among the group galaxy population, and the marked lack of blue, star-forming galaxies, provides evidence that the group environment either directly reduces star formation in member galaxies or at least prevents its rejuvenation during the normal cycle of galaxy evolution. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS. Source


Mok A.,University of Waterloo | Balogh M.L.,University of Waterloo | Balogh M.L.,Leiden University | Mcgee S.L.,Leiden University | And 8 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2014

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). © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Source


Hou A.,McMaster University | Parker L.C.,McMaster University | Wilman D.J.,Max Planck Institute for Extraterrestrial Physics | Mcgee S.L.,Durham University | And 6 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

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 ∼525kms -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. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS. Source

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