The matter environment of GAMA galaxy pairs studied with galaxy-galaxy-galaxylensing and KiDS data

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Galaxies of different types are found inside the high-density regions, or halos, of dark matter. The observed composition of galaxy types is not fully understood in detail but very likely depends on the local conditions in the matter density field. This explains the strong correlation between the number density of galaxies and the matter density which can be studied with the well-established effect of gravitational lensing: tidal distortions caused by density inhomogeneities shear the images of background galaxies and thus probe the matter field. Correlations between the observed shapes of background galaxies and positions of foreground galaxies therefore encode statistical information on galaxy physics, more interestingly so if we consider the correlation signal for different galaxy types.As application of such a correlation analysis, we propose to measure the mean gravitational shear from galaxy images in the Kilo-Degree Survey (KiDS) around galaxy pairs in the Galaxy-and-Mass Assembly (GAMA) survey. This three-point statistic, dubbed galaxy-galaxy-galaxy lensing (G3L), quantifies the mean projected matter density around galaxy pairs in excess of that for uncorrelated galaxies; the pairs can either be of the same type (unmixed pairs) or different type (mixed pairs). In contrast to traditional galaxy-galaxy lensing, which focuses on the mean shear around single galaxies, G3L is also sensitive to the correlation of positions of galaxies inside the same halo or the correlation of the type-dependent numbers of galaxies that occupy the halo. In addition, G3L is principally sensitive to the statistical alignment of galaxy pairs with respect to the halo matter distribution, whereas two-point statistics are only sensitive to the radial density profiles of stacked halos. Thus, a significantly measured G3L signal and its interpretation will refine our current understanding of the galaxy halo occupation.The details and applicability of this new tool for galaxy research still have to be established as we will do in this project. A special focus will be given to G3L of mixed pairs. To this end, we have already done a few steps in direction of a new theoretical model for the mixed-pair G3L. We also have demonstrated the feasibility of G3L measurements with certain unmixed pairs in GAMA and KiDS. To now move beyond this early project phase, we ask for DFG funding to investigate and improve the available G3L estimator by utilising the precise GAMA galaxy redshifts, to mature our (unpublished) G3L model for mixed galaxy pairs, and to perform systematic G3L measurements for unmixed and, for the first time, mixed GAMA galaxy pairs. These measurements will then be used to test G3L predictions of existing galaxy models and to refine the halo occupation statistics of galaxies. All in all, this project is a great opportunity for a couple of doctoral theses to work on exciting science in cosmology and galaxy physics with readily availablestate-of-the-art data.