Journal
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Volume 467, Issue 4, Pages 4131-4149Publisher
OXFORD UNIV PRESS
DOI: 10.1093/mnras/stx344
Keywords
gravitational lensing: weak; methods: data analysis; methods: statistical; galaxies: groups: general; galaxies: haloes
Categories
Funding
- TFC Ernest Rutherford Research Grant [ST/L00285X/1]
- European Research Council under FP7 [279396]
- STFC Ernest Rutherford Fellowship [ST/J004421/1]
- Deutsche Forschungsgemeinschaft [TR33]
- European Research Council under the FP7 [240185]
- European Research Council [647112]
- Emmy Noether grant [Hi 1495/2-1]
- Alexander von Humboldt Foundation
- German Federal Ministry for Economic Affairs and Energy (BMWi) provided via DLR [50QE1103]
- Netherlands Organisation for Scientific Research (NWO) [614.001.103]
- ESO Telescopes at the La Silla Paranal Observatory [177.A-3016, 177.A-3017, 177.A-3018]
- Anglo-Australian Telescope
- STFC [ST/M001334/1, ST/L00285X/1, ST/J004421/1] Funding Source: UKRI
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We constrain the average halo ellipticity of similar to 2600 galaxy groups from the Galaxy And Mass Assembly (GAMA) survey, using the weak gravitational lensing signal measured from the overlapping Kilo Degree Survey (KiDS). To do so, we quantify the azimuthal dependence of the stacked lensing signal around seven different proxies for the orientation of the dark matter distribution, as it is a priori unknown which one traces the orientation best. On small scales, the major axis of the brightest group/cluster member (BCG) provides the best proxy, leading to a clear detection of an anisotropic signal. In order to relate that to a halo ellipticity, we have to adopt a model density profile. We derive new expressions for the quadrupole moments of the shear field given an elliptical model surface mass density profile. Modelling the signal with an elliptical Navarro-Frenk-White profile on scales R < 250 kpc, and assuming that the BCG is perfectly aligned with the dark matter, we find an average halo ellipticity of epsilon(h) = 0.38 +/- 0.12, in fair agreement with results from cold dark matter only simulations. On larger scales, the lensing signal around the BCGs becomes isotropic and the distribution of group satellites provides a better proxy for the halo's orientation instead, leading to a 3 sigma-4 sigma detection of a non-zero halo ellipticity at 250 < R < 750 kpc. Our results suggest that the distribution of stars enclosed within a certain radius forms a good proxy for the orientation of the dark matter within that radius, which has also been observed in hydrodynamical simulations.
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