GAMA+KiDS: Alignment of galaxies in galaxy groups and its dependence on galaxy scale

Christos Georgiou, Nora Elisa Chisari, Maria Cristina Fortuna, Henk Hoekstra, Konrad Kuijken, Benjamin Joachimi, Mohammadjavad Vakili, Maciej Bilicki, Andrej Dvornik, Thomas Erben, Benjamin Giblin, Catherine Heymans, Nicola R. Napolitano, HuanYuan Shan

Published 2019-05-01Version 1

Intrinsic galaxy alignments are a source of bias for weak lensing measurements as well as a tool for understanding galaxy formation and evolution. In this work, we measure the alignment of shapes of satellite galaxies, in galaxy groups, with respect to the brightest group galaxy (BGG), as well as alignments of the BGG shape with the satellite positions, using the highly complete Galaxy And Mass Assembly (GAMA) spectroscopic survey and deep imaging from the Kilo Degree Survey. We control systematic errors with dedicated image simulations and measure accurate shapes using the DEIMOS shape measurement method. We find a significant satellite radial alignment signal, which vanishes at large separations from the BGG. We do not identify any strong trends of the signal with galaxy absolute magnitude or group mass. The alignment signal is dominated by red satellites. We also find that the outer regions of galaxies are aligned more strongly than their inner regions, by varying the radial weight employed during the shape measurement process. This behaviour is evident for both red and blue satellites. BGGs are also found to be aligned with satellite positions, with this alignment being stronger when considering the innermost satellites, using red BGGs and the shape of the outer region of the BGG. Lastly, we measure the global intrinsic alignment signal in the GAMA sample for two different radial weight functions and find no significant difference.