2013MNRAS.432.1862C

In the companion Paper XV of this series, we derive accurate total mass-to-light ratios (M/L)_JAM~(M/L)(r= R_e) within a sphere of radius r= R_e centred on the galaxy, as well as stellar (M/L)stars (with the dark matter removed) for the volume-limited and nearly mass-selected (stellar mass M_*\gtrsim 6×10^9  M_☉) ATLAS3D sample of 260 early-type galaxies (ETGs, ellipticals Es and lenticulars S0s). Here, we use those parameters to study the two orthogonal projections (M_JAM, σ_e) and (M_JAM, R_e^maj) of the thin Mass Plane (MP) (M_JAM, σ_e, R_e^maj) which describes the distribution of the galaxy population, where M_JAM≡L×(M/L)_JAM~M_*. The distribution of galaxy properties on both projections of the MP is characterized by: (i) the same zone of exclusion (ZOE), which can be transformed from one projection to the other using the scalar virial equation. The ZOE is roughly described by two power laws, joined by a break at a characteristic mass M_JAM~3×10^10  M_☉, which corresponds to the minimum Re and maximum stellar density. This results in a break in the mean M_JAM– σ_e relation with trends M_JAM∝σ_e^2.3 and M_JAM∝σ_e^4.7 at small and large σe, respectively; (ii) a characteristic mass M_JAM~2×10^11  M_☉ which separates a population dominated by flat fast rotator with discs and spiral galaxies at lower masses, from one dominated by quite round slow rotators at larger masses; (iii) below that mass the distribution of ETGs' properties on the two projections of the MP tends to be constant along lines of roughly constant σe, or equivalently along lines with R_e^maj∝M_JAM, respectively (or even better parallel to the ZOE: R_e^maj∝M_JAM^0.75); (iv) it forms a continuous and parallel sequence with the distribution of spiral galaxies; (v) at even lower masses, the distribution of fast-rotator ETGs and late spirals naturally extends to that of dwarf ETGs (Sph) and dwarf irregulars (Im), respectively.

We use dynamical models to analyse our kinematic maps. We show that σe traces the bulge fraction, which appears to be the main driver for the observed trends in the dynamical (M/L)JAM and in indicators of the (M/L)pop of the stellar population like Hβ and colour, as well as in the molecular gas fraction. A similar variation along contours of σe is also observed for the mass normalization of the stellar initial mass function (IMF), which was recently shown to vary systematically within the ETGs' population. Our preferred relation has the form log_10 [(M/L)_stars/(M/L)_Salp]=a+b×log_10(σ_e/130  km s^-1) with a = -0.12±0.01 and b = 0.35±0.06. Unless there are major flaws in all stellar population models, this trend implies a transition of the mean IMF from Kroupa to Salpeter in the interval log_10(σ_e/km  s^-1)~1.9–2.5 (or σ_e~90–290kms-1), with a smooth variation in between, consistently with what was shown in Cappellari et al. The observed distribution of galaxy properties on the MP provides a clean and novel view for a number of previously reported trends, which constitute special two-dimensional projections of the more general four-dimensional parameters trends on the MP. We interpret it as due to a combination of two main effects: (i) an increase of the bulge fraction, which increases σe, decreases Re, and greatly enhance the likelihood for a galaxy to have its star formation quenched, and (ii) dry merging, increasing galaxy mass and Re by moving galaxies along lines of roughly constant σe (or steeper), while leaving the population nearly unchanged.