The Structure & Dynamics of Massive Early-type Galaxies: On Homology, Isothermality and Isotropy inside one Effective Radius

TL;DR

This study analyzes 58 massive early-type galaxies, revealing they are structurally close to homologous, with near-isothermal density profiles and mild radial anisotropy, providing constraints on galaxy formation over four billion years.

Contribution

It offers the first detailed observational evidence that massive early-type galaxies are nearly homologous with isothermal density profiles inside one effective radius.

Findings

Galaxies have an average density slope of ~2.085 with low intrinsic scatter.

No significant correlation between density slope and galaxy mass or redshift.

Results support near-isothermality and mild radial anisotropy in massive ellipticals.

Abstract

Based on 58 SLACS strong-lens early-type galaxies with direct total-mass and stellar-velocity dispersion measurements, we find that inside one effective radius massive elliptical galaxies with M_eff >= 3x10^10 M_sun are well-approximated by a power-law ellipsoid with an average logaritmic density slope of <gamma'_LD> = -dlog(rho_tot)/dlog(r)=2.085^{+0.025}_{-0.018} (random error on mean) for isotropic orbits with beta_r=0, +-0.1 (syst.) and sigma_gamma' <= 0.20^{+0.04}_{-0.02} intrinsic scatter (all errors indicate the 68 percent CL). We find no correlation of gamma'_LD with galaxy mass (M_eff), rescaled radius (i.e. R_einst/R_eff) or redshift, despite intrinsic differences in density-slope between galaxies. Based on scaling relations, the average logarithmic density slope can be derived in an alternative manner, fully independent from dynamics, yielding <gamma'_SR>=1.959 +- 0.077. Agreement between the two values is reached for <beta_r> =0.45 +- 0.25, consistent with mild radial anisotropy. This agreement supports the robustness of our results, despite the increase in mass-to-light ratio with total galaxy mass: M_eff ~ L_{V,eff}^(1.363+-0.056). We conclude that massive early-type galaxies are structurally close-to homologous with close-to isothermal total density profiles (<=10 percent intrinsic scatter) and have at most some mild radial anisotropy. Our results provide new observational limits on galaxy formation and evolution scenarios, covering four Gyr look-back time.