The changing properties of massive, quiescent galaxies with cosmic time are understood in the framework of a two-phase history (Naab et al. 2009, Oser et al. 2010). The first phase is dominated by dissipative gas accretion accompanied by efficient in-situ star formation, leading to galaxies that are compact, flattened, rotation-supported and metal-rich. As the galaxy transitions to quiescence, its mass growth becomes dominated by dissipationless dry mergers (second phase). While major dry mergers can and do happen, they do not explain, alone, the round shapes and lack of rotation (Bois et al. 2011), the large size (van der Wel et al. 2014) and compact cores (Bezanson et al. 2009) of local, massive quiescent galaxies.
Isotropic dry mergers are predicted to lead to high fractions of radial orbits in the local Universe, which we can efficiently measure using the Gauss-Hermite parametrisation of the velocity distribution proposed by van der Marel & Franx (1993). In particular, high fractions of radial (circular) orbits lead to positive (negative) values of the coefficient of the 4th-order Legendre polynomial. The current generation of large, ultra-deep surveys of the distant Universe (MAGPI, Foster et al. 2021; LEGA-C, van der Wel et al. 2016) enables us to measure h4 up to z=1, when the Universe was only half its current age. The large sample size and integral-field spatial coverage of the SAMI Galaxy Survey enables us to create mock observations of the LEGA-C galaxies in the local Universe, thereby providing the necessary local anchor to gauge the cosmic evolution of h4. By combining data from the SAMI, MAGPI and LEGA-C surveys, we propose to measure if h4 increases with cosmic time, which provides a test for the theory of isotropic minor dry mergers.
