Confronting semi-analytic galaxy models with galaxy-matter correlations observed by CFHTLenS

Testing predictions of semi-analytic models of galaxy evolution against observations help to understand the complex processes that shape galaxies. We compare predictions from the Garching and Durham models implemented on the Millennium Run with observations of galaxy-galaxy lensing (GGL) and galaxy-galaxy-galaxy lensing (G3L) for various galaxy samples with stellar masses in the range 0.5 < (M_* / 10^10 M_Sun) < 32 and photometric redshift range 0.2 < z < 0.6 in the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS). We find that the predicted GGL and G3L signals are in qualitative agreement with CFHTLenS data. Quantitatively, the models succeed in reproducing the observed signals in the highest stellar mass bin (16 < ( M_* / 10^10 M_Sun) < 32) but show different degrees of tension for the other stellar mass samples. The Durham models are strongly excluded at the 95% confidence level by the observations as they largely over-predict the amplitudes of the GGL and G3L signals, probably because they predict too many satellite galaxies in massive halos.Comment: 9 pages, 8 figures, submitted to A&A. Comments welcom

Comparing galaxy-galaxy(-galaxy) lensing in semi-analytic models and observations to study galaxy evolution

One of the main challenges in cosmology is to understand the properties of dark matter, its distribution in the Universe, and its connection with baryonic matter. An ideal method to study the relation between baryonic matter and dark matter is the so-called "gravitational lensing". It relies on the fact that the light emitted from a background source in the distant Universe is deflected by the foreground matter distribution or "lens'', leading to distortions in the observed image of the source. By studying these image distortions, one can obtain information about the mass distribution associated with the lens. In the weak gravitational lensing regime, the lensing effect is too small to create a detectable lensing signal from a single image. One thus needs to examine the distortions in a large number of sources in order to derive statistical properties about the lenses mass. In the case where both the source and the lens are galaxies, this technique is known as ``galaxy-galaxy lensing'' (GGL). Distortion patterns around lens galaxy pairs instead of individual galaxies can also be analysed, a method known as "galaxy-galaxy-galaxy lensing'' (G3L) which gives information on the matter environment of galaxy pairs. In order to be able to interpret GGL and G3L measurements, a theoretical understanding of these statistics is required. A common approach is to use semi-analytic models (SAMs) which combine the results from dark matter N-body simulations with analytical prescriptions for the physical processes governing galaxy formation and evolution. Comparing the outcomes of SAMs with observations therefore offers an opportunity to connect observed properties of galaxies with the underlying physical processes leading to those features. In this thesis, we first use synthetic galaxy catalogs from two SAMs, the Garching and Durham models, and their predictions of GGL and G3L for various galaxy populations. These SAMs are all implemented on one of the largest dark matter simulations, the Millennium Simulation. However, they differ in several details which lead to different predictions of GGL and G3L. Therefore, comparing the SAMs predictions against each other allows us to gain information on the physical processes involved and how the different treatments used in the models impact the signal. Moreover, comparisons between the SAMs predictions of GGL and G3L suggest that G3L provides new information which cannot be obtained from the second-order GGL statistics alone. In order to identify shortcomings of the SAMs and obtain valuable information on how to improve the models, one needs to compare the SAMs results with observational measurements. Therefore, in the second part of this thesis, we investigate the ability of three SAMs, the Garching and Durham models as well as an updated version of the Garching model, to reproduce observations of GGL and G3L. For this purpose, we use measurements from the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS) which is a multi-color optical survey optimised for weak lensing analysis. We study the GGL and G3L signals for galaxy samples selected according to their stellar mass and redshift, and analyze the clustering properties of galaxies and galaxy pairs of these samples. Our results indicate that not all models can quantitatively reproduce the GGL and G3L observations although there is an overall qualitative agreement between the models and CFHTLenS data

A comparison of the excess mass around CFHTLenS galaxy-pairs to predictions from a semi-analytic model using galaxy-galaxy-galaxy lensing

The matter environment of galaxies is connected to the physics of galaxy formation and evolution. Utilising galaxy-galaxy-galaxy lensing as a direct probe, we map out the distribution of correlated surface mass-density around galaxy pairs for different lens separations in the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS). We compare, for the first time, these so-called excess mass maps to predictions provided by a recent semi-analytic model, which is implanted within the dark-matter Millennium Simulation. We analyse galaxies with stellar masses between $10^9-10^{11}\,{\rm M}_\odot$ in two photometric redshift bins, for lens redshifts $z\lesssim0.6$, focusing on pairs inside groups and clusters. To allow us a better interpretation of the maps, we discuss the impact of chance pairs, i.e., galaxy pairs that appear close to each other in projection only. Our tests with synthetic data demonstrate that the patterns observed in the maps are essentially produced by correlated pairs that are close in redshift ($\Delta z\lesssim5\times10^{-3}$). We also verify the excellent accuracy of the map estimators. In an application to the galaxy samples in the CFHTLenS, we obtain a $3\sigma-6\sigma$ significant detection of the excess mass and an overall good agreement with the galaxy model predictions. There are, however, a few localised spots in the maps where the observational data disagrees with the model predictions on a $\approx3.5\sigma$ confidence level. Although we have no strong indications for systematic errors in the maps, this disagreement may be related to the residual B-mode pattern observed in the average of all maps. Alternatively, misaligned galaxy pairs inside dark matter halos or lensing by a misaligned distribution of the intra-cluster gas might also cause the unanticipated bulge in the distribution of the excess mass between lens pairs.Comment: 21 pages, 12 figures; abridged abstract; revised version for A&A after addressing all comments by the refere

Galaxy-galaxy(-galaxy) lensing as a sensitive probe of galaxy evolution

The gravitational lensing effect provides various ways to study the mass environment of galaxies. We investigate how galaxy-galaxy(-galaxy) lensing can be used to test models of galaxy formation and evolution. We consider two semi-analytic galaxy formation models based on the Millennium Run N-body simulation: the Durham model by Bower et al. (2006) and the Garching model by Guo et al. (2011). We generate mock lensing observations for the two models, and then employ Fast Fourier Transform methods to compute second- and third-order aperture statistics in the simulated fields for various galaxy samples. We find that both models predict qualitatively similar aperture signals, but there are large quantitative differences. The Durham model predicts larger amplitudes in general. In both models, red galaxies exhibit stronger aperture signals than blue galaxies. Using these aperture measurements and assuming a linear deterministic bias model, we measure relative bias ratios of red and blue galaxy samples. We find that a linear deterministic bias is insufficient to describe the relative clustering of model galaxies below ten arcmin angular scales. Dividing galaxies into luminosity bins, the aperture signals decrease with decreasing luminosity for brighter galaxies, but increase again for fainter galaxies. This increase is likely an artifact due to too many faint satellite galaxies in massive group and cluster halos predicted by the models. Our study shows that galaxy-galaxy(-galaxy) lensing is a sensitive probe of galaxy evolution.Comment: 11 pages, 8 figures, accepted in A&

An exploration of galaxy-galaxy lensing and galaxy clustering in the Millennium-XXL simulation

The combination of galaxy-galaxy lensing and galaxy clustering data has the potential to simultaneously constrain both the cosmological and galaxy formation models. In this paper, we perform a comprehensive exploration of these signals and their covariances through a combination of analytic and numerical approaches. First, we derive analytic expressions for the projected galaxy correlation function and stacked tangential shear profile and their respective covariances, which include Gaussian and discreteness noise terms. Secondly, we measure these quantities from mock galaxy catalogues obtained from the Millennium-XXL simulation and semi-analytic models of galaxy formation. We find that on large scales (R > 10 h-1 Mpc), the galaxy bias is roughly linear and deterministic. On smaller scales (R ≲ 5 h-1 Mpc), the bias is a complicated function of scale and luminosity, determined by the different spatial distribution and abundance of satellite galaxies present when different magnitude cuts are applied, as well as by the mass dependence of the host haloes on magnitude. Our theoretical model for the covariances provides a reasonably good description of the measured ones on small and large scales. However, on intermediate scales (1 < R < 10 h-1 Mpc), the predicted errors are ˜2-3 times smaller, suggesting that the inclusion of higher order, non-Gaussian terms in the covariance will be required for further improvements. Importantly, both our theoretical and numerical methods show that the galaxy-galaxy lensing and clustering signals have a non-zero cross-covariance matrix with significant bin-to-bin correlations. Future surveys aiming to combine these probes must take this into account in order to obtain unbiased and realistic constraints

KiDS+VIKING+GAMA:Testing semi-analytic models of galaxy evolution with galaxy-galaxy-galaxy lensing

Several semi-analytic models (SAMs) try to explain how galaxies form, evolve and interact inside the dark matter large-scale structure. These SAMs can be tested by comparing their predictions for galaxy-galaxy-galaxy-lensing (G3L), which is weak gravitational lensing around galaxy pairs, with observations. We evaluate the SAMs by Henriques et al. (2015; H15) and by Lagos et al. (2012; L12), implemented in the Millennium Run, by comparing their predictions for G3L to observations at smaller scales than previous studies and also for pairs of lens galaxies from different populations. We compare the G3L signal predicted by the SAMs to measurements in the overlap of the Galaxy And Mass Assembly survey (GAMA), the Kilo-Degree Survey (KiDS), and the VISTA Kilo-degree Infrared Galaxy survey (VIKING), splitting lens galaxies into two colour and five stellar-mass samples. Using an improved G3L estimator, we measure the three-point correlation of the matter distribution for mixed lens pairs with galaxies from different samples, and unmixed lens pairs with galaxies from the same sample. Predictions by the H15 SAM agree with the observations for all colour-selected and all but one stellar-mass-selected sample with 95% confidence. Deviations occur for lenses with stellar masses below $9.5h^{-2}\mathrm{M}_\odot$ at scales below $0.2h^{-1}\mathrm{Mpc}$. Predictions by the L12 SAM for stellar-mass selected samples and red galaxies are significantly higher than observed, while the predicted signal for blue galaxy pairs is too low. The L12 SAM predicts more pairs of small stellar-mass and red galaxies than the H15 SAM and the observations, as well as fewer pairs of blue galaxies. Likely explanations are different treatments of environmental effects by the SAMs and different models of the initial mass function. We conclude that G3L provides a stringent test for models of galaxy formation and evolution.Comment: 14 pages, 8 figures, replaced with version accepted to Astronomy & Astrophysics after considering referees comment