Revisiting the Bulge–Halo Conspiracy. I. Dependence on Galaxy Properties and Halo Mass

Abstract

We carry out a systematic investigation of the total mass density profile of massive ( log M star / M ⊙ ≳ 11.3 ) early-type galaxies and its dependence on galactic properties and host halo mass with the aid of a variety of lensing/dynamical data and large mock galaxy catalogs. The latter are produced via semi-empirical models that, by design, are based on just a few basic input assumptions. Galaxies with measured stellar masses, effective radii, and Sérsic indices, are assigned, via abundance matching relations, host dark matter halos characterized by a typical ΛCDM profile. Our main results are as follows. (1) In line with observational evidence, our semi-empirical models naturally predict that the total, mass-weighted density slope at the effective radius γ ′ is not universal, steepening for more compact and/or massive galaxies, but flattening with increasing host halo mass. (2) Models characterized by a Salpeter or variable initial mass function (IMF) and uncontracted dark matter profiles are in good agreement with the data, while a Chabrier IMF and/or adiabatic contractions/expansions of the dark matter halos are highly disfavored. (3) Currently available data on the mass density profiles of very massive galaxies ( log M star / M ⊙ ≳ 12 ), with M halo ≳ 3 × 10 14 M ⊙ , favor instead models with a stellar profile flatter than a Sérsic one in the very inner regions ( r  ≲ 3–5 kpc), and a cored NFW or Einasto dark matter profile with median halo concentration a factor of ∼2 or ≲1.3, respectively, higher than those typically predicted by N -body numerical simulations.