Laura C. Parker

McMaster University, Hamilton, Ontario, Canada

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Publications (25)108.03 Total impact

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    ABSTRACT: We present an analysis of galaxies in groups and clusters at 0.8 < z < 1.2, from the GCLASS and GEEC2 spectroscopic surveys. We compute a ‘conversion fraction’ fconvert that represents the fraction of galaxies that were prematurely quenched by their environment. For massive galaxies, Mstar > 1010.3 M⊙, we find fconvert ∼ 0.4 in the groups and ∼0.6 in the clusters, similar to comparable measurements at z = 0. This means the time between first accretion into a more massive halo and final star formation quenching is tp ∼ 2 Gyr. This is substantially longer than the estimated time required for a galaxy's star formation rate to become zero once it starts to decline, suggesting there is a long delay time during which little differential evolution occurs. In contrast with local observations we find evidence that this delay time-scale may depend on stellar mass, with tp approaching tHubble for Mstar ∼ 109.5 M⊙. The result suggests that the delay time must not only be much shorter than it is today, but may also depend on stellar mass in a way that is not consistent with a simple evolution in proportion to the dynamical time. Instead, we find the data are well-matched by a model in which the decline in star formation is due to ‘overconsumption’, the exhaustion of a gas reservoir through star formation and expulsion via modest outflows in the absence of cosmological accretion. Dynamical gas removal processes, which are likely dominant in quenching newly accreted satellites today, may play only a secondary role at z = 1.
    Preview · Article · Nov 2015 · Monthly Notices of the Royal Astronomical Society
  • Ian D. Roberts · Laura C. Parker · Ananthan Karunakaran
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    ABSTRACT: Galaxy morphologies and star-formation rates depend on environment. Galaxies in under-dense regions are generally star-forming and disky whereas galaxies in overdense regions tend to be early-type and not actively forming stars. The mechanism(s) responsible for star-formation quenching and morphological transformation remain unclear, although many processes have been proposed. We study the dependence of star-formation and morphology on X-ray luminosity for galaxies in Sloan Digital Sky Survey Data Release 7 (SDSS-DR7) groups and clusters. While controlling for stellar and halo mass dependencies, we find that galaxies in X-ray strong groups and clusters have preferentially low star-forming and disk fractions -- with the differences being strongest at low stellar masses. The trends that we observe do not change when considering only galaxies found within or outside of the X-ray radius of the host group. When considering central and satellite galaxies separately we find that this dependence on X-ray luminosity is only present for satellites, and we show that our results are consistent with "galaxy stangulation" as a mechanism for quenching these satellites. We investigate the dynamics of the groups and clusters in the sample, and find that the velocity distributions of galaxies beyond the virial radius in low X-ray luminosity halos tend to be less Gaussian in nature than the rest of the data set. This may be indicative of low X-ray luminosity groups and clusters having enhanced populations of star-forming and disk galaxies as a result of recent accretion.
    No preview · Article · Nov 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: It has been shown that galaxy properties depend strongly on their host environment. In order to understand the relevant physical processes driving galaxy evolution it is important to study the observed properties of galaxies in different environments. Mass segregation in bound galaxy structures is an important indicator of evolutionary history and dynamical friction time-scales. Using group catalogues derived from the Sloan Digital Sky Survey Data Release 7 (SDSS DR7), we investigate mass-segregation trends in galaxy groups at low redshift. We investigate average galaxy stellar mass as a function of group-centric radius and find evidence for weak mass segregation in SDSS groups. The magnitude of the mass segregation depends on both galaxy stellar mass limits and group halo mass. We show that the inclusion of low-mass galaxies tends to strengthen mass-segregation trends, and that the strength of mass segregation tends to decrease with increasing group halo mass. We find the same trends if we use the fraction of massive galaxies as a function of group-centric radius as an alternative probe of mass segregation. The magnitude of mass segregation that we measure, particularly in high-mass haloes, indicates that dynamical friction is not acting efficiently.
    Preview · Article · Nov 2014 · Monthly Notices of the Royal Astronomical Society Letters
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    ABSTRACT: 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 ∼ 1013 M⊙ 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 > 1010.3 M⊙. 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 the GCLASS sample of more massive clusters at the same redshift, we find the total stellar mass is strongly correlated with the dynamical mass, with log M200 = 1.20(log Mstar − 12) + 14.07. This stellar fraction of ∼1 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.
    Preview · Article · Jun 2014 · Monthly Notices of the Royal Astronomical Society
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    Annie Hou · Laura C. Parker · William E. Harris
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    ABSTRACT: We investigate pre-processing using the observed quenched fraction of group and cluster galaxies in the Yang et al. Sloan Digital Sky Survey (SDSS)-seventh data release (DR7) group catalogue in the redshift range of 0.01 < z < 0.045. We categorize group galaxies as virialized, infall or backsplash and we apply a combination of the Dressler–Shectman statistic and group member velocities to identify subhaloes. On average, the fraction of galaxies that reside in subhaloes is a function of host halo mass, where more massive systems have a higher fraction of subhalo galaxies both in the overall galaxy and infall populations. Additionally, we find that within the range 2 ≲ r200 < 3 the quiescent fraction is higher in the subhalo population with respect to both the field and non-subhalo populations. At these large radii (2 ≲ r200 < 3), the majority of galaxies (∼80 per cent) belong to the infall population and therefore, we attribute the enhanced quenching to infalling subhalo galaxies, indicating that pre-processing has occurred in the subhalo population. We conclude that pre-processing plays a significant role in the observed quiescent fraction, but only for the most massive (Mhalo > 1014.5 M⊙) systems in our sample.
    Preview · Article · Apr 2014 · Monthly Notices of the Royal Astronomical Society
  • Annie Hou · L. C. Parker · W. E. Harris
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    ABSTRACT: We investigate the importance of pre-processing in the observed quenched fraction of rich group and cluster galaxies from the SDSS-DR7 Yang et al. (2007) group catalogue in the redshift range of 0:01 < z < 0:045. A combination of the Dressler-Shectman statistic and the group member velocity distribution is used to identify subhaloes within the group. On average the fraction of galaxies that reside in subhaloes is a strong function of host halo mass where more massive systems, such as clusters, have a higher fraction of subhaloes both in the overall galaxy and infall populations. Comparison of the properties of galaxies that reside in subhaloes to those that do not shows that beyond the virial radius (r > 2r200) galaxies in the subhalo population differ from the non-subhalo population. In particular, the quiescent fraction is higher in subhalo galaxies with respect to both the field and non-subhalo galaxies. At these large radii, we find that the majority of galaxies 80%) belong to the infall population. Therefore, we attribute the enhanced quenching to infalling subhalo galaxies, indicating that pre-processing has occurred in the subhalo population. We conclude that pre-processing plays a signicant role in the observed quiescent fraction, but only for the most massive (Mhalo > 10^14.5 Msun) systems in our sample.
    No preview · Article · Jan 2014
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    ABSTRACT: We present new analysis from the 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 $\sim5$ 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 timescales and delays between satellite accretion and the onset of quenching. We find that with no delay, the quenching timescale 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 timescale ($\sim0.25$ Gyr), but this model also predicts intermediate galaxies H{\delta} 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 ($\lesssim0.25$ Gyr).
    Preview · Article · Dec 2013 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We examine galaxy groups from the present epoch to z = 1 to explore the impact of group dynamics on galaxy evolution. We use group catalagues from the Sloan Digital Sky Survey (SDSS), the Group Environment and Evolution Collaboration (GEEC) and the high redshift GEEC2 sample to study how the observed member properties depend on 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 10^13-10^14.5 M_sun, or with dynamical sate. However, at a redshift of approximately 0.4 we do see some evidence that the quiescent fraction in low mass galaxies (log(M_star/M_sun) < 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 roughly z = 0.4, while the amount of detected substructure remains constant to z = 1. Based on these results, we conclude that for massive galaxies (log(M_star/M_sun_ > 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 dynamical state. For low mass galaxies, we do see 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 the environment in galaxy evolution.
    Preview · Article · Aug 2013 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We present deep GMOS-S spectroscopy for 11 galaxy groups at 0.8<z<1.0, for galaxies with r_{AB}<24.75. Our sample is highly complete (>66%) for eight of the eleven groups. Using an optical-NIR colour-colour diagram, the galaxies in the sample were separated with a dust insensitive method into three categories: passive (red), star-forming (blue), and intermediate (green). The strongest environmental dependence is observed in the fraction of passive galaxies, which make up only ~20 per cent of the field in the mass range 10^{10.3}<M_{star}/M_\odot<10^{11.0} but are the dominant component of groups. If we assume that the properties of the field are similar to those of the `pre-accreted' population, the environment quenching efficiency (\epsilon_\rho) is defined as the fraction of field galaxies required to be quenched in order to match the observed red fraction inside groups. The efficiency obtained is ~0.4, similar to its value in intermediate-density environments locally. While green (intermediate) galaxies represent ~20 per cent of the star-forming population in both the group and field, at all stellar masses, the average sSFR of the group population is lower by a factor of ~3. The green population does not show strong H-delta absorption that is characteristic of starburst galaxies. Finally, the high fraction of passive galaxies in groups, when combined with satellite accretion models, require that most accreted galaxies have been affected by their environment. Thus, any delay between accretion and the onset of truncation of star formation (\tau) must be <2 Gyr, shorter than the 3-7 Gyr required to fit data at z=0. The relatively small fraction of intermediate galaxies requires that the actual quenching process occurs quickly, with an exponential decay timescale of \tau_q<1 Gyr.
    Full-text · Article · Feb 2013 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We present the global group properties of two samples of galaxy groups containing 39 high-quality X-ray-selected systems and 38 optically (spectroscopically) selected systems in coincident spatial regions at 0.12 < z < 0.79. The total mass range of the combined sample is ~(1012-5) × 1014M ☉. Only nine optical systems are associable with X-ray systems. We discuss the confusion inherent in the matching of both galaxies to extended X-ray emission and of X-ray emission to already identified optical systems. Extensive spectroscopy has been obtained and the resultant redshift catalog and group membership are provided here. X-ray, dynamical, and total stellar masses of the groups are also derived and presented. We explore the effects of utilizing different centers and applying three different kinds of radial cut to our systems: a constant cut of 1 Mpc and two r 200 cuts, one based on the velocity dispersion of the system and the other on the X-ray emission. We find that an X-ray-based r 200 results in less scatter in scaling relations and less dynamical complexity as evidenced by results of the Anderson-Darling and Dressler-Schectman tests, indicating that this radius tends to isolate the virialized part of the system. The constant and velocity dispersion based cuts can overestimate membership and can work to inflate velocity dispersion and dynamical and stellar mass. We find LX-σ and M stellar-LX scaling relations for X-ray and optically selected systems are not dissimilar. The mean fraction of mass found in stars, excluding intracluster light, for our systems is ~0.014 with a logarithmic standard deviation of 0.398 dex. We also define and investigate a sample of groups which are X-ray underluminous given the total group stellar mass. For these systems the fraction of stellar mass contributed by the most massive galaxy is typically lower than that found for the total population of groups implying that there may be less intragroup medium contributed from the most massive member in these systems. Eighty percent of 15 underluminous groups have less than 40% of their stellar mass in the most massive galaxy which happens in less than 1% of cases with samples matched in stellar mass, taken from the combined group catalog.
    Full-text · Article · Aug 2012 · The Astrophysical Journal
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    ABSTRACT: The presence of substructure in galaxy groups and clusters is believed to be a sign of recent galaxy accretion and can be used not only to probe 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 catalog. We find that 4 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.
    Full-text · Article · Jan 2012 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: 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.
    Preview · Article · Sep 2011 · The Astrophysical Journal
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    ABSTRACT: (abridged) We introduce our survey of galaxy groups at 0.85<z<1, as an extension of the Group Environment and Evolution Collaboration (GEEC). 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. 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<log(Mdyn/Msun)<14. Our spectroscopic sample is statistically complete for all galaxies with Mstar>1E10.1 Msun, and for blue galaxies we sample masses as low as Mstar=1E8.8 Msun. Like lower-redshift groups, these systems are dominated by red galaxies, at all stellar masses Mstar>1E10.1 Msun. 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 ~30 per cent of galaxies. These do not appear to be exceptionally dusty galaxies, and about half show prominent Balmer-absorption lines. Furthermore, their HST morphologies appear to be intermediate between those of red-sequence and blue-cloud galaxies of the same stellar mass. 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 timescale 0.6 Gyr< tau < 2 Gyr. 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.
    Full-text · Article · Apr 2011 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We examine the star formation properties of group and field galaxies in two surveys, Sloan Digital Sky Survey (at z∼ 0.08) and Group Environment Evolution Collaboration (GEEC; at z∼ 0.4). Using ultraviolet imaging from the Galaxy Evolution Explorer space telescope, along with optical and, for GEEC, near-infrared photometry, we compare the observed spectral energy distributions to large suites of stellar population synthesis models. This allows us to accurately determine star formation rates and stellar masses. We find that star-forming galaxies of all environments undergo a systematic lowering of their star formation rate between z= 0.4 and 0.08 regardless of mass. None the less, the fraction of passive galaxies is higher in groups than the field at both redshifts. Moreover, the difference between the group and field grows with time and is mass dependent, in the sense the difference is larger at low masses. However, the star formation properties of star-forming galaxies, as measured by their average specific star formation rates, are consistent within the errors in the group and field environment at fixed redshift. The evolution of passive fraction in groups between z= 0.4 and 0 is consistent with a simple accretion model, in which galaxies are environmentally affected 3 Gyr after falling into a ∼1013 M⊙ group. This long time-scale appears to be inconsistent with the need to transform galaxies quickly enough to ensure that star-forming galaxies appear similar in both the group and field, as observed.
    Preview · Article · Dec 2010 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: At low redshift, galaxy groups are hostile environments, gradually shutting down star formation activity; but at z=1 there is evidence that groups actually induce star formation in massive galaxies. While much can be learned about this important process from wide-field, sparsely sampled redshift surveys, a complementary and necessary approach is to study a smaller, pure sample of groups in detail. Only then is it possible to robustly measure the group mass, centre and radius, and the group-to-group variations predicted by simple infall models. We propose to continue a 3 semester programme begun in 09A, to study galaxies in 16 groups at 0.8
    No preview · Article · Feb 2010
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    ABSTRACT: Galaxy groups are evolving environments which display diverse properties even at fixed redshift. The majority of galaxies in the Universe lie in these groups which range from 'poor' systems containing few galaxies (commonly identified via optical selection methods) to massive groups (often identified via X-ray emission from the Intra-Group Medium, IGM). Local studies reveal the galaxy populations of groups vary from being dominated by early (as in typical clusters) to late-type (as in the field population) galaxies. Thus groups are interesting in their own right and also as an environment which affects the evolution of their member galaxies. In order to study groups spanning a significant mass and evolutionary range, we have defined two samples, one via optical spectroscopy and the other via X-ray emission. Both samples contain 25 groups and span a redshift range of 0.04
    No preview · Article · Jan 2010
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    ABSTRACT: X-ray properties of galaxy groups can unlock some of the most challenging research topics in modern extragalactic astronomy: the growth of structure and its influence on galaxy formation. Only with the advent of the Chandra and XMM facilities have X-ray observations reached the depths required to address these questions in a satisfactory manner. Here we present an X-ray imaging study of two patches from the CNOC2 spectroscopic galaxy survey using combined Chandra and XMM data. A state of the art extended source finding algorithm has been applied, and the resultant source catalog, including redshifts from a spectroscopic follow-up program, is presented. The total number of spectroscopically identified groups is 25 spanning a redshift range 0.04-0.79. Approximately 50% of CNOC2 spectroscopically selected groups in the deeper X-ray (RA14h) field are likely X-ray detections, compared to 20% in the shallower (RA21h) field. Statistical modeling shows that this is consistent with expectations, assuming an expected evolution of the Lx-M relation. A significant detection of a stacked shear signal for both spectroscopic and X-ray groups indicates that both samples contain real groups of about the expected mass. We conclude that the current area and depth of X-ray and spectroscopic facilities provide a unique window of opportunity at z~0.4 to test the X-ray appearance of galaxy groups selected in various ways. There is at present no evidence that the correlation between X-ray luminosity and velocity dispersion evolves significantly with redshift, which implies that catalogs based on either method can be fairly compared and modeled. Comment: 15 pages, ApJ in press
    Full-text · Article · Sep 2009 · The Astrophysical Journal
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    Annie Hou · Laura C. Parker · William E. Harris · David J. Wilman
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    ABSTRACT: The dynamical state of galaxy groups at intermediate redshifts can provide information about the growth of structure in the universe. We examine three goodness-of-fit tests, the Anderson--Darling (A-D), Kolmogorov and chi-squared tests, in order to determine which statistical tool is best able to distinguish between groups that are relaxed and those that are dynamically complex. We perform Monte Carlo simulations of these three tests and show that the chi-squared test is profoundly unreliable for groups with fewer than 30 members. Power studies of the Kolmogorov and A-D tests are conducted to test their robustness for various sample sizes. We then apply these tests to a sample of the second Canadian Network for Observational Cosmology Redshift Survey (CNOC2) galaxy groups and find that the A-D test is far more reliable and powerful at detecting real departures from an underlying Gaussian distribution than the more commonly used chi-squared and Kolmogorov tests. We use this statistic to classify a sample of the CNOC2 groups and find that 34 of 106 groups are inconsistent with an underlying Gaussian velocity distribution, and thus do not appear relaxed. In addition, we compute velocity dispersion profiles (VDPs) for all groups with more than 20 members and compare the overall features of the Gaussian and non-Gaussian groups, finding that the VDPs of the non-Gaussian groups are distinct from those classified as Gaussian. Comment: 11 pages, 9 figures, accepted for publication in ApJ
    Preview · Article · Aug 2009 · The Astrophysical Journal

  • No preview · Article · Jun 2009
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    ABSTRACT: (Abridged) We present new optical and near-infrared imaging for a sample of 98 spectroscopically-selected galaxy groups at 0.25<z<0.55. We measure accurate colours for group members and the surrounding field population, statistically complete above a stellar mass limit of M=1E10 Msun. The overall colour distribution is bimodal in both the field and group samples; but at fixed luminosity the fraction of group galaxies populating the red peak is larger, by 20+/-7 per cent, than that of the field. In particular, group members with early-type morphologies, as identified in HST imaging, exhibit a tight red sequence, similar to that seen for more massive clusters. We show that approximately 20-30 per cent of galaxies on the red sequence may be dust-reddened galaxies with non-negligible star formation and early-spiral morphologies. This is true of both the field and group sample, and shows little dependence on near infrared luminosity. Thus, the fraction of bright group members with no sign of star formation or AGN activity is 54+/-6 per cent. Our field sample, which includes galaxies in all environments, contains 35+/-3 per cent of such inactive galaxies, consistent with the amount expected if all such galaxies are located in groups and clusters. This reinforces our earlier conclusions, that dense environments at z<0.5 are associated with a premature cessation of star formation in some galaxies; in particular we find no evidence for significantly enhanced star formation in these environments. Simple galaxy formation models predict a quenching of star formation in groups that is too efficient, overpopulating the red sequence. Attempts to fix this by increasing the timescale of this quenching equally for all group members distorts the colour distribution in a way that is inconsistent with observations. Comment: Resubmitted to MNRAS following referee report
    Full-text · Article · May 2009 · Monthly Notices of the Royal Astronomical Society