The Atlas3D project - XX. Mass-size and Mass-sigma projections of the Virial Plane of early-type galaxies: variation of morphology, kinematics, mass-to-light ratio and stellar initial mass function

In the companion Paper XIX we derive accurate total (M/L)_JAM within a sphere of radius r=Re, as well as stellar (M/L)_stars for the volume-limited Atlas3D sample of 260 early-type galaxies. Here we study the two projections (M_JAM,sigma_e) and (M_JAM,R_e^max) of the thin Virial Plane (VP)(M_JAM,sigma_e,R_e^max) which describes the distribution of the galaxy population. The distribution of galaxy properties on both projections of the VP is characterized by (i) a boundary in the galaxy distribution, described by two power-laws, joined by a break at a characteristic mass M_JAM ~ 3*10^10 Msun, which corresponds to the minimum Re and maximum stellar density, and (ii) a characteristic mass M_JAM ~ 2*10^11 Msun which separates a population dominated by fast rotator with disks at lower masses, from one dominated by quite round slow rotators at larger masses. The distribution of ETGs properties on the two projections of the VP tends to be constant along lines of constant sigma_e, and forms a continuous and parallel sequence with the distribution of spiral galaxies. This applies to the dynamical (M/L)_JAM and to other indicators of the (M/L)_pop of the stellar population, like Hbeta and colour, as well as to galaxy concentration, which we show is tracing the bulge mass. A similar variation along contours of sigma_e is also observed for the stellar Initial Mass Function (IMF), which was recently shown to vary systematically within the ETGs galaxy population. Our preferred relation has the form log [(M/L)_stars/(M/L)_Salp] = a + b*log (sigma_e/130kms) with a=-0.11+/-0.01 and b=0.36+/-0.06. This trend implies a transition of the mean IMF from Kroupa to Salpeter in the interval log(sigma_e) ~ 1.9-2.4, with a smooth variation in between. [Abridged]