The Fundamental Manifold of Spheroids

We present a unifying empirical description of the structural and kinematic properties of all spheroids embedded in dark matter halos. We find that the stellar spheroidal components of galaxy clusters, which we call cluster spheroids (CSphs) and which are typically one hundred times the size of normal elliptical galaxies, lie on a “fundamental plane” as tight as that defined by ellipticals (rms in effective radius of ∼ 0.07), but that has a different slope. The slope, as measured by the coefficient of the log σ term, declines significantly and systematically between the fundamental planes of ellipticals, brightest cluster galaxies (BCGs), and CSphs. We attribute this decline primarily to a continuous change in Me/Le, the mass-to-light ratio within the effective radius re, with spheroid scale. The magnitude of the slope change requires that it arises principally from differences in the relative distributions of luminous and dark matter, rather than from stellar population differences such as in age and metallicity. By expressing the Me/Le term as a function of σ in the simple derivation of the fundamental plane and requiring the behavior of that term to mimic the observed nonlinear relationship between logMe/Le and log σ, we simultaneously fit a 2-D manifold to the measured properties of dwarf ellipticals, ellipticals, BCGs, and CSphs. The combined data have an rms scatter in log re of 0.114 (0.099 for the combination of Es, BCGs, and CSphs), which is modestly larger than each fundamental plane has alone, but which includes the scatter introduced by merging different studies done in different filters by different investigators. This “fundamental manifold” fits the structural and kinematic properties of spheroids that span a factor of 100 in σ and 1000 in re. While our mathematical form is neither unique nor derived from physical principles, the tightness of the fit leaves little room for improvement by other unification schemes over the range of observed spheroids. Subject headings: galaxies: formation— galaxies: elliptical and lenticular, cD— galaxies: fundamental parameters — galaxies: structure