The Sloan Lens ACS Survey. III. The Structure and Formation of Early‐Type Galaxies and Their Evolution since z ≈ 1

Abstract

We present a joint gravitational lensing and stellar dynamical analysis of fifteen massive field earlytype galaxies -selected from the Sloan Lens ACS (SLACS) Survey -using Hubble Space Telescope ACS images and luminosity weighted stellar velocity dispersions obtained from the Sloan Digital Sky Survey database. The sample of lens galaxies is well-defined (see Paper I), with a redshift range of z=0.06-0.33 and an average stellar velocity dispersion of σ ap = 263 km s −1 (rms of 44 km s −1 ) inside a 3-arcsec fiber diameter. The following numerical results are found: (i) A joint-likelihood gives an average logarithmic density slope for the total mass density of γ ′ = 2.01 +0.02 −0.03 (68% C.L.; ρ tot ∝ r −γ ′ ) inside R Einst = 4.2 ± 0.4 kpc (rms of 1.6 kpc). The inferred intrinsic rms spread in logarithmic density slopes is σ γ ′ = 0.12, which might still include some minor systematic uncertainties. A range for the stellar anisotropy parameterChanging from a Hernquist to a Jaffe luminosity density profile increases γ ′ by 0.05. (ii) The average position-angle difference between the light distribution and the total mass distribution is found to be ∆θ = 0 ± 3 degrees (rms of 10 degrees), setting an upper limit of γ ext 0.035 on the average external shear. The total mass has an average ellipticity q SIE =0.78±0.03 (rms of 0.12), which correlates extremely well with the stellar ellipticity, q * , resulting in q SIE /q * = 0.99±0.03 (rms of 0.11) for σ 225 km s −1 . At lower velocity dispersions, inclined S0 galaxies dominate, leading to a higher ratio (up to 1.6). This suggests that the dark-matter halo surrounding these galaxies is less flattened than their stellar component. Assuming an oblate mass distribution and random orientations, the distribution of ellipticities implies q 3 ≡ (c/a) ρ = 0.66 with an error of ∼0.2. (iii) The average projected dark-matter mass fraction is inferred to be f DM = 0.25 ± 0.06 (rms of 0.22) inside R E , using the stellar mass-to-light ratios derived from the Fundamental Plane as priors. (iv) Combined with results from the Lenses Structure & Dynamics (LSD) Survey at z 0.3, we find no significant evolution of the total density slope inside one effective radius for galaxies with σ ap ≥ 200 km s −1 : a linear fit gives α γ ′ ≡ d γ ′ /dz = 0.23 ± 0.16 (1 σ) for the range z=0.08-1.01. We conclude that massive early-type galaxies at z=0.06-0.33 on average have an isothermal logarithmic density slope inside half an effective radius, with an intrinsic spread of at most 6 % (1 σ). The small scatter and absence of significant evolution in the inner density slopes suggest a collisional scenario where gas and dark matter strongly couple during galaxy formation, leading to a total mass distribution that rapidly converge to dynamical isothermality.

How well can cold dark matter substructures account for the observed radio flux-ratio anomalies

“…This has been supported by lensing observations from e.g. Koopmans et al (2006) and Sluse et al (2012).…”

“…It is also worth noting that although we draw shape parameters (e and a m ) from a galaxy sample at lower redshifts (z < 0.2), as addressed in Keeton et al (2003Keeton et al ( , 2005, these distributions are not expected to be significantly different from those of the observed lensing galaxies at intermediate redshifts; observations have shown no significant evolution in the mass assembly history of early-type galaxies since z ≈ 1 (Thomas et al 2005;Koopmans et al 2006).…”

The mass profile of early-type galaxies in overdense environments: the case of the double source-plane gravitational lens SL2SJ02176-0513

“…We note that the combination of stellar kinematics of the lens galaxy in a single aperture and strong lensing constraints coming from a single multiply-imaged system (e.g. Miralda-Escudé 1995; Sand et al 2004;Treu & Koopmans 2004;Koopmans 2006;Koopmans et al 2006;Czoske et al 2008) cannot break the degeneracy between both density slope and external convergence by itself, so additional information is necessary. This could include, for example, spatially resolved stellar kinematics, information on the external mass distribution, or multiple lens planes, together with assumptions on the form of the mass distribution as in the case presented here.…”

Results of optical monitoring of 5 SDSS double QSOs with the Nordic Optical Telescope

“…We used those models because of their simplicity and because they agree with the physical properties of many observed lenses (Kochanek 1993;Rusin et al 2003;Koopmans et al 2006).…”