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Showing papers by "Warrick J. Couch published in 2013"


Journal ArticleDOI
TL;DR: In this article, the authors search for observational evidence that major mergers do play a significant role in the build up of stellar mass in distant galaxy clusters, and they find a major merger rate of 0.38 ± 0.14 mergers per Gyr at z ∼ 1.2.
Abstract: Recent independent results from numerical simulations and observations have shown that brightest cluster galaxies (BCGs) have increased their stellar mass by a factor of almost 2 between z ∼ 0.9 and z ∼ 0.2. The numerical simulations further suggest that more than half this mass is accreted through major mergers. Using a sample of 18 distant galaxy clusters with over 600 spectroscopically confirmed cluster members between them, we search for observational evidence that major mergers do play a significant role. We find a major merger rate of 0.38 ± 0.14 mergers per Gyr at z ∼ 1. While the uncertainties, which stem from the small size of our sample, are relatively large, our rate is consistent with the results that are derived from numerical simulations. If we assume that this rate continues to the present day and that half of the mass of the companion is accreted on to the BCG during these mergers, then we find that this rate can explain the growth in the stellar mass of the BCGs that is observed and predicted by simulations. Major mergers therefore appear to be playing an important role, perhaps even the dominant one, in the build up of stellar mass in these extraordinary galaxies.

123 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used N-body simulations to predict the clustering of WiggleZ galaxies with respect to dark matter, and marginalize over them to obtain constraints on σ8(z), the variance of perturbations on a scale of 8 h^−1 Mpc and its evolution with redshift.
Abstract: Higher order statistics are a useful and complementary tool for measuring the clustering of galaxies, containing information on the non-Gaussian evolution and morphology of large-scale structure in the Universe. In this work we present measurements of the three-point correlation function (3PCF) for 187 000 galaxies in the WiggleZ spectroscopic galaxy survey. We explore the WiggleZ 3PCF scale and shape dependence at three different epochs z = 0.35, 0.55 and 0.68, the highest redshifts where these measurements have been made to date. Using N-body simulations to predict the clustering of dark matter, we constrain the linear and non-linear bias parameters of WiggleZ galaxies with respect to dark matter, and marginalize over them to obtain constraints on σ8(z), the variance of perturbations on a scale of 8 h^−1 Mpc and its evolution with redshift. These measurements of σ_8(z), which have 10–20 per cent accuracies, are consistent with the predictions of the Λ cold dark matter concordance cosmology and test this model in a new way.

110 citations


Journal ArticleDOI
TL;DR: GNOSIS as mentioned in this paper is a prototype astrophotonic instrument that utilizes "OH suppression fibers" consisting of fiber Bragg gratings and photonic lanterns to suppress the 103 brightest atmospheric emission doublets between 1.47 and 1.7µm.
Abstract: The near-infrared is an important part of the spectrum in astronomy, especially in cosmology because the light from objects in the early universe is redshifted to these wavelengths. However, deep near-infrared observations are extremely difficult to make from ground-based telescopes due to the bright background from the atmosphere. Nearly all of this background comes from the bright and narrow emission lines of atmospheric hydroxyl (OH) molecules. The atmospheric background cannot be easily removed from data because the brightness fluctuates unpredictably on short timescales. The sensitivity of ground-based optical astronomy far exceeds that of near-infrared astronomy because of this long-standing problem. GNOSIS is a prototype astrophotonic instrument that utilizes “OH suppression fibers” consisting of fiber Bragg gratings and photonic lanterns to suppress the 103 brightest atmospheric emission doublets between 1.47 and 1.7µm. GNOSIS was commissioned at the 3.9m Anglo-Australian Telescope with the IRIS2 spectrograph to demonstrate the potential of OH suppression fibers, but may be potentially used with any telescope and spectrograph combination. Unlike previous atmospheric suppression techniques GNOSIS suppresses the lines before dispersion and in a manner that depends purely on wavelength. We present the instrument design and report the results of laboratory and on-sky tests from commissioning. While these tests demonstrated high throughput (� 60%) and excellent suppression of the skylines by the OH suppression fibers, surprisingly GNOSIS produced no significant reduction in the interline background and the sensitivity of GNOSIS+IRIS2 is about the same as IRIS2. It is unclear whether the lack of reduction in the interline background is due to physical sources or systematic errors as the observations are detector noise dominated. OH suppression fibers could potentially impact ground-based astronomy at the level of adaptive optics or greater. However, until a clear reduction in the interline background and the corresponding increasing in sensitivity is demonstrated optimized OH suppression fibers paired with a fiber-fed spectrograph will at least provide a real benefit at low resolving powers. Subject headings: atmospheric effects – infrared: diffuse background – instrumentation: miscellaneous

83 citations


Journal ArticleDOI
TL;DR: In this article, the authors study the growth rate of galaxy structure to redshift z = 0.9 using more than 162,000 galaxy redshifts from the WiggleZ Dark Energy Survey and determine the pairwise velocity dispersion of the sample in a nonparametric manner.
Abstract: The growth history of large-scale structure in the Universe is a powerful probe of the cosmological model, including the nature of dark energy. We study the growth rate of cosmic structure to redshift z = 0.9 using more than 162 000 galaxy redshifts from the WiggleZ Dark Energy Survey. We divide the data into four redshift slices with effective redshifts z = [0.2, 0.4, 0.6, 0.76] and in each of the samples measure and model the two-point galaxy correlation function in parallel and transverse directions to the line of sight. After simultaneously fitting for the galaxy bias factor we recover values for the cosmic growth rate which are consistent with our assumed Λcold dark matter (ΛCDM) input cosmological model, with an accuracy of around 20 per cent in each redshift slice. We investigate the sensitivity of our results to the details of the assumed model and the range of physical scales fitted, making close comparison with a set of N-body simulations for calibration. Our measurements are consistent with an independent power-spectrum analysis of a similar data set, demonstrating that the results are not driven by systematic errors. We determine the pairwise velocity dispersion of the sample in a non-parametric manner, showing that it systematically increases with decreasing redshift, and investigate the Alcock–Paczynski effects of changing the assumed fiducial model on the results. Our techniques should prove useful for current and future galaxy surveys mapping the growth rate of structure using the two-dimensional correlation function.

64 citations


Journal ArticleDOI
TL;DR: In this paper, the kinematic properties of 10 nearby brightest cluster galaxies (BCGs) and 4 BCG companion galaxies located within a redshift of z = 0.1.
Abstract: Using the VIMOS integral field unit (IFU) spectrograph on the Very Large Telescope, we have spatially mapped the kinematic properties of 10 nearby brightest cluster galaxies (BCGs) and 4 BCG companion galaxies located within a redshift of z = 0.1. In the hierarchical formation model, these massive galaxies (1010.5 M ☉ < M dyn < 1011.9 M ☉) are expected to undergo more mergers than lower mass galaxies, and simulations show that dry minor mergers can remove angular momentum. We test whether BCGs have low angular momenta by using the λ Re parameter developed by the SAURON and ATLAS3D teams and combine our kinematics with Sloan Digital Sky Survey photometry to analyze the BCGs' merger status. We find that 30% (3/10) of the BCGs and 100% of the companion galaxies (4/4) are fast rotators as defined by the ATLAS3D criteria. Our fastest rotating BCG has a λ Re = 0.35 ± 0.05. We increase the number of BCGs analyzed from 1 in the combined SAURON and ATLAS3D surveys to 11 BCGs total and find that above M dyn ~ 11.5 M ☉, virtually all galaxies, regardless of environment, are slow rotators. To search for signs of recent merging, we analyze the photometry of each system and use the G – M 20 selection criteria to identify mergers. We find that 40% ± 20% of our BCGs are currently undergoing or have recently undergone a merger (within 0.2 Gyr). Surprisingly, we find no correlation between galaxies with high angular momentum and morphological signatures of merging.

52 citations


Journal ArticleDOI
TL;DR: In this article, Chandra observations were used to understand the nature of a sharp edge previously characterized as a cold front and reveal that the edge is coherent over a larger opening angle and is significantly more bow-shock-shaped than previously thought.
Abstract: We present a 250 ks Chandra observation of the cluster merger A2034 with the aim of understanding the nature of a sharp edge previously characterized as a cold front. The new data reveal that the edge is coherent over a larger opening angle and is significantly more bow-shock-shaped than previously thought. Within ~27° about the axis of symmetry of the edge, the density, temperature, and pressure drop abruptly by factors of , , and , respectively. This is inconsistent with the pressure equilibrium expected of a cold front and we conclude that the edge is a shock front. We measure a Mach number and corresponding shock velocity v shock 2057 km s–1. Using spectra collected at the MMT with the Hectospec multi-object spectrograph, we identify 328 spectroscopically confirmed cluster members. Significantly, we find a local peak in the projected galaxy density associated with a bright cluster galaxy that is located just ahead of the nose of the shock. The data are consistent with a merger viewed within ~23° of the plane of the sky. The merging subclusters are now moving apart along a north-south axis approximately 0.3 Gyr after a small impact parameter core passage. The gas core of the secondary subcluster, which was driving the shock, appears to have been disrupted by the merger. Without a driving piston, we speculate that the shock is dying. Finally, we propose that the diffuse radio emission near the shock is due to the revival of pre-existing radio plasma that has been overrun by the shock.

39 citations


Journal ArticleDOI
TL;DR: In this article, the authors presented single dish HI 21-cm emission line measurements of a sample of eleven E+A galaxies at redshifts z < 0.05 and detected H I emission in six of these galaxies.
Abstract: Post-starburst galaxies, or E+A galaxies, are characterized by optical spectra showing strong Balmer absorption lines, indicating a young stellar population, and little or no emission lines, implying no active star formation. These galaxies are interpreted as a transitional population between star-forming, disk-dominated galaxies and spheroidal quiescent, non-star forming galaxies. Here, we present single dish HI 21-cm emission line measurements of a sample of eleven of these galaxies at redshifts z<0.05. We detect H I emission in six of the E+A galaxies. In combination with earlier studies, the total number of E+A galaxies with measured cold gas components is now eleven. Roughly half of the E+As studied so far have detectable HI. The gas fractions of these galaxies, measured with respect to their stellar mass, are between 1 and 10 percent and are at the high end of the gas fractions measured in gas-bearing early type galaxies and typically lower than seen in late-type galaxies with comparable stellar masses. This finding is consistent with the idea that E+As are currently evolving from the blue cloud to the red sequence. However, the question of why the star formation has ceased in these galaxies while a significant gas reservoir is still present can only be answered by higher spatial resolution observations of the cold gas.

37 citations


Journal ArticleDOI
TL;DR: In this article, the authors used deep integral field spectroscopy obtained with the GMOS instrument on Gemini-North to determine the spatial distribution of the post-starburst stellar population in four luminous E+A galaxies at z < 0.04.
Abstract: We have used deep integral field spectroscopy obtained with the GMOS instrument on Gemini-North to determine the spatial distribution of the post-starburst stellar population in four luminous E+A galaxies at z<0.04. We find all four galaxies have centrally-concentrated gradients in the young stellar population contained within the central ~1 kpc. This is in agreement with the Balmer line gradients found in local low luminosity E+A galaxies. The results from higher redshift (z~0.1) samples of luminous E+A galaxies have been varied, but in general have found the post-starburst signature to be extended or a galaxy-wide phenomenon or have otherwise failed to detect gradients in the stellar populations. The ubiquity of the detection of a centrally concentrated young stellar population in local samples, and the presence of significant radial gradients in the stellar populations when the E+A galaxy core is well resolved raises the possibility that spatial resolution issues may be important in interpreting the higher redshift results. The two early type E+A galaxies in our sample that can be robustly kinematically classified, using the LambdaR parameter, are fast-rotators. Combined with previous measurements, this brings the total number of E+A galaxies with measurements of LambdaR to twenty-six, with only four being classified as slow-rotators. This fraction is similar to the fraction of the early-type population as a whole and argues against the need for major mergers in the production of E+A galaxies, since major mergers should result in an increased fraction of slow rotators.

34 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used the VIMOS Integral Field Unit (IFU) spectrograph on the Very Large Telescope (VLT) to spatially map the kinematic properties of 10 nearby Brightest Cluster Galaxies (BCGs) and 4 BCG companion galaxies located within a redshift of 0.35 ± 0.05.
Abstract: Using the VIMOS Integral Field Unit (IFU) spectrograph on the Very Large Telescope (VLT), we have spatially mapped the kinematic properties of 10 nearby Brightest Cluster Galaxies (BCGs) and 4 BCG companion galaxies located within a redshift of $z=0.1$. In the hierarchical formation model, these massive galaxies $(10^{10.5} M_{\odot} < M_{dyn} < 10^{11.9} M_{\odot})$ are expected to undergo more mergers than lower mass galaxies, and simulations show that dry minor mergers can remove angular momentum. We test whether BCGs have low angular momenta by using the $\lambda_{Re}$ parameter developed by the SAURON and ATLAS\textsuperscript{3D} teams and combine our kinematics with Sloan Digital Sky Survey (SDSS) photometry to analyze the BCGs' merger status. We find that 30% (3/10) of the BCGs and 100% of the companion galaxies (4/4) are fast rotators as defined by the ATLAS\textsuperscript{3D} criteria. Our fastest rotating BCG has a $\lambda_{Re}=0.35\pm0.05$. We increase the number of BCGs analyzed from 1 in the combined SAURON and ATLAS\textsuperscript{3D} surveys to 11 BCGs total and find that above $M_{dyn}\sim11.5 M_{\odot}$, virtually all galaxies regardless of environment are slow rotators. To search for signs of recent merging, we analyze the photometry of each system and use the $G-M_{20}$ selection criteria to identify mergers. We find that $40\pm20$% of our BCGs are currently undergoing or have recently undergone a merger (within 0.2 Gyrs). Surprisingly, we find no correlation between galaxies with high angular momentum and morphological signatures of merging.

32 citations


Journal ArticleDOI
TL;DR: In this paper, Chandra observations of the A2034 cluster merger were used to understand the nature of a sharp edge previously characterized as a cold front and reveal that the edge is coherent over a larger opening angle and is significantly more bow-shock-shaped than previously thought.
Abstract: We present a $250\,$ks Chandra observation of the cluster merger A2034 with the aim of understanding the nature of a sharp edge previously characterized as a cold front. The new data reveal that the edge is coherent over a larger opening angle and is significantly more bow-shock-shaped than previously thought. Within $\sim 27\,$degrees about the axis of symmetry of the edge the density, temperature and pressure drop abruptly by factors of $1.83^{+0.09}_{-0.08}$, $1.85^{+0.41}_{-0.41}$ and $3.4^{+0.8}_{-0.7}$, respectively. This is inconsistent with the pressure equilibrium expected of a cold front and we conclude that the edge is a shock front. We measure a Mach number $M = 1.59^{+0.06}_{-0.07}$ and corresponding shock velocity $v_{\rm shock}\simeq 2057\,$km/s. Using spectra collected at the MMT with the Hectospec multi-object spectrograph we identify 328 spectroscopically confirmed cluster members. Significantly, we find a local peak in the projected galaxy density associated with a bright cluster galaxy which is located just ahead of the nose of the shock. The data are consistent with a merger viewed within $\sim 23\,$degrees of the plane of the sky. The merging subclusters are now moving apart along a north-south axis approximately $0.3\,$Gyr after a small impact parameter core passage. The gas core of the secondary subcluster, which was driving the shock, appears to have been disrupted by the merger. Without a driving 'piston' we speculate that the shock is dying. Finally, we propose that the diffuse radio emission near the shock is due to the revival of pre-existing radio plasma which has been overrun by the shock.

29 citations


Journal ArticleDOI
TL;DR: In this article, the authors investigated the role of the structure of galaxies and their stellar population in shaping and tilting the Fundamental Plane (FP) with respect to the virial plane (VP) expectation.
Abstract: By exploiting the database of early-type galaxies (ETGs) members of the WINGS survey of nearby clusters, we address here the long debated question of the origin and shape of the Fundamental Plane (FP). Our data suggest that different physical mechanisms concur in shaping and ’tilting’ the FP with respect to the virial plane (VP) expectation. In particular, an “hybrid solution” in which the structure of galaxies and their stellar population are the main contributors to the FP tilt seems to be favoured. We find that the bulk of the tilt should be attributed to structural non-homology, while stellar population effects play an important but less crucial role. In addition, our data indicate that the differential FP tilt between the V - and K-band is due to a sort of entanglement between structural and stellar population effects, for which the inward steepening of color profiles (V K) tends to increase at increasing the stellar mass of ETGs. The same kind of analysis applied to the ATLAS3D and SDSS data in common with WINGS (WSDSS throughout the paper) confirms our results, the only remarkable difference being the less important role that our data attribute to the stellar mass-to-light-ratio (stellar populations) in determining the FP tilt. The ATLAS3D data also suggest that the FP tilt depends as well on the dark matter (DM) fraction and on the rotational contribution to the kinetic energy (Vrot/σ), thus again pointing towards the above mentioned “hybrid solution”. We show that the global properties of the FP, i.e. its tilt and tightness, can be understood in terms of the underlying correlation among mass, structure and stellar population of ETGs, for which, at increasing the stellar mass, ETGs become (on average) ’older’ and more centrally concentrated. Finally, we show that a Malmquist-like selection effect may mimic a differential evolution of the mass-to-light ratio for galaxies of different masses. This should be taken into account in the studies investigating the amount of the so called “downsizing” phenomenon.

Journal ArticleDOI
TL;DR: In this paper, the authors place the most robust constraint to date on the scale of the turnover in the cosmological matter power spectrum using data from the WiggleZ Dark Energy Survey.
Abstract: We place the most robust constraint to date on the scale of the turnover in the cosmological matter power spectrum using data from the WiggleZ Dark Energy Survey. We find this feature to lie at a scale of k_0 = 0.0160^(+ 0.0035)_(− 0.0041) (h Mpc^−1) (68 per cent confidence) for an effective redshift of z_(eff) = 0.62 and obtain from this the first ever turnover-derived distance and cosmology constraints: a measure of the cosmic distance–redshift relation in units of the horizon scale at the redshift of radiation–matter equality (r_H) of D_V(z_(eff) = 0.62)/r_H = 18.3^(+6.3)_(−3.3) and, assuming a prior on the number of extra relativistic degrees of freedom N_(eff) = 3, constraints on the cosmological matter density parameter Ω_M h^2 = 0.136^(+0.026)_(−0.052) and on the redshift of matter–radiation equality z_(eq) = 3274^(+631)_(−1260). We stress that these results are obtained within the theoretical framework of Gaussian primordial fluctuations and linear large-scale bias. With this caveat, all results are in excellent agreement with the predictions of standard ΛCDM models. Our constraints on the logarithmic slope of the power spectrum on scales larger than the turnover are bounded in the lower limit with values only as low as −1 allowed, with the prediction of P(k) ∝ k from standard ΛCDM models easily accommodated by our results. Finally, we generate forecasts to estimate the achievable precision of future surveys at constraining k_0, Ω_M h^2, z_(eq) and N_(eff). We find that the Baryon Oscillation Spectroscopic Survey should substantially improve upon the WiggleZ turnover constraint, reaching a precision on k_0 of ±9 per cent (68 per cent confidence), translating to precisions on Ω_M h^2 and z_(eq) of ±10 per cent (assuming a prior N_(eff) = 3) and on Neff of + 78− 56 per cent (assuming a prior Ω_M h^2 = 0.135). This represents sufficient precision to sharpen the constraints on N_(eff) from WMAP, particularly in its upper limit. For Euclid, we find corresponding attainable precisions on (k_0, Ω_M h^2, N_(eff)) of (3, 4,^(+ 17)_(− 21)) per cent. This represents a precision approaching our forecasts for the Planck Surveyor.

Journal ArticleDOI
TL;DR: In this paper, the stellar masses and star formation rates in a sample of ∼40 000 spectroscopically confirmed UV-luminous galaxies at 0.3 5σ in the UKIDSS-LAS at all redshifts were characterized.
Abstract: We characterize the stellar masses and star formation rates in a sample of ∼40 000 spectroscopically confirmed UV-luminous galaxies at 0.3 5σ in the UKIDSS-LAS at all redshifts. An even more luminous subset of 15 per cent are also detected in the WISE 3.4 and 4.6 μm bands. In addition, 22 of the WiggleZ galaxies are extremely luminous at 12 and 22 μm and have colours consistent with being star formation dominated. We compute stellar masses for this very large sample of extremely blue galaxies and quantify the sensitivity of the stellar mass estimates to various assumptions made during the spectral energy distribution (SED) fitting. The median stellar masses are log10(M_*/M_⊙) = 9.6 ± 0.7, 10.2 ± 0.5 and 10.4 ± 0.4 for the IR undetected, UKIDSS detected and UKIDSS+WISE detected galaxies, respectively. We demonstrate that the inclusion of NIR photometry can lead to tighter constraints on the stellar masses by bringing down the upper bound on the stellar mass estimate. The mass estimates are found to be most sensitive to the inclusion of secondary bursts of star formation as well as changes in the stellar population synthesis models, both of which can lead to median discrepancies of the order of 0.3 dex in the stellar masses. We conclude that even for these extremely blue galaxies, different SED fitting codes therefore produce extremely robust stellar mass estimates. We find, however, that the best-fitting M/L_K is significantly lower than that predicted by simple optical colour-based estimators for many of the WiggleZ galaxies. The simple colour-based estimator overpredicts M/L_K by ∼0.4 dex on average. The effect is more pronounced for bluer galaxies with younger best-fitting ages. The WiggleZ galaxies have star formation rates of 3–10 M_⊙ yr^(−1) and mostly lie at the upper end of the main sequence of star-forming galaxies at these redshifts. Their rest-frame UV luminosities and stellar masses are comparable to both local compact UV-luminous galaxies as well as Lyman break galaxies at z ∼ 2–3. The stellar masses from this paper will be made publicly available with the next WiggleZ data release.


Journal ArticleDOI
TL;DR: In this article, the ultraviolet (UV) luminosity function of galaxies from the GALEX Medium Imaging Survey with measured spectroscopic redshifts from the first data release of the WiggleZ Dark Energy Survey was presented.
Abstract: We present the ultraviolet (UV) luminosity function of galaxies from the GALEX Medium Imaging Survey with measured spectroscopic redshifts from the first data release of the WiggleZ Dark Energy Survey. Our sample consists of 39 996 NUV M_NUV > −22.5) evolve very rapidly with a number density declining as (1 + z)5±1 from redshift z = 0.9 to 0.6. These starburst galaxies (MNUV 0.55 we find that the bright end of the luminosity function is not well described by a pure Schechter function due to an excess of very luminous (MNUV < −22) galaxies. These luminosity functions can be used to create a radial selection function for the WiggleZ survey or test models of galaxy formation and evolution. Here we test the AGN feedback model in Scannapieco, Silk & Bouwens, and find that this AGN feedback model requires AGN feedback efficiency to vary with one or more of the following: stellar mass, star formation rate and redshift.