The MASSIVE Survey - VII. The relationship of angular momentum, stellar mass and environment of early-type galaxies

We analyse the environmental properties of 370 local early-type galaxies (ETGs) in the MASSIVE and ATLAS^3D surveys, two complementary volume-limited integral-field spectroscopic (IFS) galaxy surveys spanning absolute K-band magnitude - 21.5 ≳ M_K ≳ -26.6, or stellar mass 8× 10⁹ ≲ M_* ≲ 2× 10^{12} M_{⊙}. We find these galaxies to reside in a diverse range of environments measured by four methods: group membership (whether a galaxy is a brightest group/cluster galaxy, satellite or isolated), halo mass, large-scale mass density (measured over a few Mpc) and local mass density (measured within the Nth neighbour). The spatially resolved IFS stellar kinematics provide robust measurements of the spin parameter λ_e and enable us to examine the relationship among λ_e, M_* and galaxy environment. We find a strong correlation between λ_e and M_*, where the average λ_e decreases from ∼0.4 to below 0.1 with increasing mass, and the fraction of slow rotators f_slow increase from ∼10 to 90 per cent. We show for the first time that at fixed M_*, there are almost no trends between galaxy spin and environment; the apparent kinematic morphology-density relation for ETGs is therefore primarily driven by M_* and is accounted for by the joint correlations between M_* and spin, and between M_* and environment. A possible exception is that the increased f_slow at high local density is slightly more than expected based only on these joint correlations. Our results suggest that the physical processes responsible for building up the present-day stellar masses of massive galaxies are also very efficient at reducing their spin, in any environment.