We use two new hydrodynamical simulations of Λ cold dark matter (ΛCDM) and f(R) gravity to test the methodology used by Wilcox et al. (W15) in constraining the effects of a fifth force on the ...profiles of clusters of galaxies. We construct realistic simulated stacked weak lensing and X-ray surface brightness cluster profiles from these cosmological simulations, and then use these data projected along various lines of sight to test the spherical symmetry of our stacking procedure. We also test the applicability of the NFW profile to model weak lensing profiles of clusters in f(R) gravity. Finally, we test the validity of the analytical model developed in W15 against the simulated profiles. Overall, we find our methodology is robust and broadly agrees with these simulated data. We also apply our full Markov Chain Monte Carlo analysis from W15 to our simulated X-ray and lensing profiles, providing consistent constraints on the modified gravity parameters as obtained from the real cluster data, e.g. for our ΛCDM simulation we obtain |f
R0| < 8.3 × 10−5 (95 per cent CL), which is in good agreement with the W15 measurement of |f
R0| < 6 × 10−5. Overall, these tests confirm the power of our methodology which can now be applied to larger cluster samples available with the next generation surveys.
The chameleon gravity model postulates the existence of a scalar field that couples with matter to mediate a fifth force. If it exists, this fifth force would influence the hot X-ray emitting gas ...filling the potential wells of galaxy clusters. However, it would not influence the clusters weak lensing signal. Therefore, by comparing X-ray and weak lensing profiles, one can place upper limits on the strength of a fifth force. This technique has been attempted before using a single, nearby cluster (Coma, z = 0.02). Here we apply the technique to the stacked profiles of 58 clusters at higher redshifts (0.1 < z < 1.2), including 12 new to the literature, using X-ray data from the XMM Cluster Survey and weak lensing data from the Canada–France–Hawaii–Telescope Lensing Survey. Using a multiparameter Markov chain Monte Carlo analysis, we constrain the two chameleon gravity parameters (β and ϕ∞). Our fits are consistent with general relativity, not requiring a fifth force. In the special case of f(R) gravity (where
$\beta = \sqrt{1/6}$
), we set an upper limit on the background field amplitude today of |f
R0| < 6 × 10−5 (95 per cent CL). This is one of the strongest constraints to date on |f
R0| on cosmological scales. We hope to improve this constraint in future by extending the study to hundreds of clusters using data from the Dark Energy Survey.
An accepted explanation for the accelerated expansion of the late-time Universe is to modify the Einstein equation, either by adding a component to the energy-momentum tensor via dark energy, or to ...the Einstein tensor via a modification to gravity. The second of these options often involves the introduction of a scalar field, which couples to the matter components of the Universe and gives rise to a fifth force, of the same order of magnitude as gravity. Through a variety of experiments and astronomical observations, this fifth force has been demonstrated to be negligible on Terrestrial and Solar System scales. Therefore if it does act on large scales, it must be suppressed, or `screened', on small scales. In this thesis, I place constraints upon one of these screening methods, chameleon gravity. Chameleon gravity postulates the existence of a scalar field that couples with matter to mediate a fifth force. If it exists, this fifth force would influence the hot X-ray emitting gas filling the potential wells of galaxy clusters. However, it would not influence the cluster's weak lensing signal. Therefore, by comparing X-ray and weak lensing profiles, upper limits can be placed on the strength of a fifth force. To do so I first present two hydrodynamical simulations, one evolved under CDM+GR and the other under f(R). From these two simulations I generate X-ray surface brightness and weak lensing profiles for a number of simulated clusters. Using these profiles I test many of the assumptions of the technique used to constraint IfR0I. I then use these profiles to test the analytic pipelines developed to constrain f(R) gravity by applying a full MCMC analysis. From doing so I find constraints on the modified gravity parameters of IfR0I< 8:3 x10-5. Next I outline the creation of a sample of 58 clusters, including 12 new to the literature, with high quality weak lensing data from CFHTLenS and X-ray data from XCS. By stacking these clusters I use a multi-parameter MCMC analysis to constrain the two chameleon gravity parameters ( ß and φ∞). The fits are consistent with general relativity, not requiring a fifth force. In the special case of f(R) gravity (where ß = √1/6), I set an upper limit on the background field amplitude today of IfR0I< 6 x 10-5 (95% CL). This is one of the strongest constraints to date on IfR0I on cosmological scales. These fits are also found to be consistent with those recovered from the f(R) simulations. Finally I look at the future of this method, beginning with forecasting the constraints that this technique will be able to place on f(R) gravity using the Dark Energy Survey, finding IfR0I> 2 x 10-5. Next I discuss how the X-ray surface brightness profiles might be improved by removing contaminating point sources from the X-ray images and find that doing so leads to a reduction in the error bars of 5%. I end this thesis by detailing how the techniques discussed within can be applied to constrain other modified gravity theories, namely the Vainshtein mechanism. Doing so I am able to place competitive constraints upon Vainshtein gravity, including the first ever constraint on a particular parametrisation.
We present a direct measurement of the mean halo occupation distribution (HOD) of galaxies taken from the eleventh data release (DR11) of the Sloan Digital Sky Survey-III Baryon Oscillation ...Spectroscopic Survey (BOSS). The HOD of BOSS low-redshift (LOWZ: 0.2 < z < 0.4) and Constant-Mass (CMASS: 0.43 < z < 0.7) galaxies is inferred via their association with the dark matter haloes of 174 X-ray-selected galaxy clusters drawn from the XMM Cluster Survey (XCS). Halo masses are determined for each galaxy cluster based on X-ray temperature measurements, and range between log sub( 10)(M sub( 180)/M...) = 13 and 15. Our directly measured HODs are consistent with the HOD-model fits inferred via the galaxy-clustering analyses of Parejko et al. for the BOSS LOWZ sample and White et al. for the BOSS CMASS sample. Under the simplifying assumption that the other parameters that describe the HOD hold the values measured by these authors, we have determined a best-fitting alpha-index of 0.91 plus or minus 0.08 and 1.27... for the CMASS and LOWZ HOD, respectively. These alpha-index values are consistent with those measured by White et al. and Parejko et al. In summary, our study provides independent support for the HOD models assumed during the development of the BOSS mock-galaxy catalogues that have subsequently been used to derive BOSS cosmological constraints. (ProQuest: ... denotes formulae/symbols omitted.)
We measure the evolution of the velocity dispersion–temperature (σv–T
X) relation up to z = 1 using a sample of 38 galaxy clusters drawn from the XMM Cluster Survey. This work improves upon previous ...studies by the use of a homogeneous cluster sample and in terms of the number of high-redshift clusters included. We present here new redshift and velocity dispersion measurements for 12 z > 0.5 clusters observed with the Gemini Multi Object Spectographs instruments on the Gemini telescopes. Using an orthogonal regression method, we find that the slope of the relation is steeper than that expected if clusters were self-similar, and that the evolution of the normalization is slightly negative, but not significantly different from zero (σv ∝ T
0.86±0.14
E(z)−0.37±0.33). We verify our results by applying our methods to cosmological hydrodynamical simulations. The lack of evolution seen in our data is consistent with simulations that include both feedback and radiative cooling.
A comprehensive tactical narrative of the fight on the first day of the Battle of Gettysburg (July 1, 1863) from Harry W. Pfanz, a former historian at Gettysburg National Military Park and author of ...two previous books on the battle
An accepted explanation for the accelerated expansion of the late-time Universe is to modify the Einstein equation, either by adding a component to the energy-momentum tensor via dark energy, or to ...the Einstein tensor via a modification to gravity. The second of these options often involves the introduction of a scalar field, which couples to the matter components of the Universe and gives rise to a fifth force, of the same order of magnitude as gravity. Through a variety of experiments and astronomical observations, this fifth force has been demonstrated to be negligible on Terrestrial and Solar System scales. Therefore if it does act on large scales, it must be suppressed, or `screened', on small scales. In this thesis, I place constraints upon one of these screening methods, chameleon gravity. Chameleon gravity postulates the existence of a scalar field that couples with matter to mediate a fifth force. If it exists, this fifth force would influence the hot X-ray emitting gas filling the potential wells of galaxy clusters. However, it would not influence the cluster's weak lensing signal. Therefore, by comparing X-ray and weak lensing profiles, upper limits can be placed on the strength of a fifth force. To do so I first present two hydrodynamical simulations, one evolved under CDM+GR and the other under f(R). From these two simulations I generate X-ray surface brightness and weak lensing profiles for a number of simulated clusters. Using these profiles I test many of the assumptions of the technique used to constraint IfR0I. I then use these profiles to test the analytic pipelines developed to constrain f(R) gravity by applying a full MCMC analysis. From doing so I find constraints on the modified gravity parameters of IfR0I< 8:3 x10-5. Next I outline the creation of a sample of 58 clusters, including 12 new to the literature, with high quality weak lensing data from CFHTLenS and X-ray data from XCS. By stacking these clusters I use a multi-parameter MCMC analysis to constrain the two chameleon gravity parameters ( ß and φ∞). The fits are consistent with general relativity, not requiring a fifth force. In the special case of f(R) gravity (where ß = √1/6), I set an upper limit on the background field amplitude today of IfR0I< 6 x 10-5 (95% CL). This is one of the strongest constraints to date on IfR0I on cosmological scales. These fits are also found to be consistent with those recovered from the f(R) simulations. Finally I look at the future of this method, beginning with forecasting the constraints that this technique will be able to place on f(R) gravity using the Dark Energy Survey, finding IfR0I> 2 x 10-5. Next I discuss how the X-ray surface brightness profiles might be improved by removing contaminating point sources from the X-ray images and find that doing so leads to a reduction in the error bars of 5%. I end this thesis by detailing how the techniques discussed within can be applied to constrain other modified gravity theories, namely the Vainshtein mechanism. Doing so I am able to place competitive constraints upon Vainshtein gravity, including the first ever constraint on a particular parametrisation.
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