Sloan Lens ACS Survey Research Articles

Photometric mass and mass decomposition in early-type lens galaxies

Context. The largest spectroscopically selected sample of strong gr avitational lens systems presented and analyzed to date is t hat of the Sloan Lens ACS (SLACS) survey. For the 57 massive early-type lens galaxies in the sample, photometric and spectroscopic measurements are available from the Sloan Digital Sky Survey (SDSS). Aims. By using the SDSS multicolor photometry and lens modeling, we study stellar-mass properties and the luminous and dark matter composition of the early-type lens galaxies in the sample. Methods. We fit the lens spectral energy distributions (SEDs) consist ing of ugriz magnitudes with a three-parameter grid (age, star-formati on timescale, and photometric mass) of Bruzual & Charlot’s and Maraston’s composite stellar-population models, compute d by adopting solar metallicity and various initial mass functions (IMFs). We also utilize the best-fit parameters derived from the lens mod els of the total projected mass enclosed within the disk defined by the Einstein radius o f each system. Results. We observe that early-type lens galaxies have the same physical properties as massive non-lens galaxies. In particular , we investigate the relationships between stellar mass and both the size and the surface stellar-mass density of the lens galaxies, whic h are consistent with those determined for non-lens galaxies in the local Univers e. We find statistically significant evidence that more lumin ous and massive lens galaxies tend to form in regions of higher galaxy density, as for all early-type galaxies. Interestingly, for the corres ponding stellar quantities we measure the same scaling law between effective mass-to-light ratio and mass used to explain the “til t” of the Fundamental Plane (FP), and the same evolution in the effective mass-to-light ratio with redshift as derived from th e FP. We conclude that the total (luminous+dark) mass of the lenses is linearly proportional to the luminous mass, at a co nfidence level of more than 99%. In addition, by assuming that the lens galaxies are homologous systems, we study their distribution of dark matter and estimate a value on the order of 30% for the dark over total projected mass fraction contained within the average Einstein ring of radi us of approximately 4 kpc. Conclusions. These results suggest that early-type lens galaxies are an u nbiased subsample representative of the complete sample of early-type galaxies. This allows us to generalize our findings on the amo unt and distribution of dark matter in lens galaxies to the po pulation of massive early-type galaxies. For the assumed metallicity, we note t hat a Salpeter IMF is better suited than either a Chabrier or K roupa IMF to describing the sample of lenses.

Lensing Probabilities for Spectroscopically Selected Galaxy‐Galaxy Strong Lenses

Spectroscopic galaxy-galaxy lens searches are presently the most prolific method of identifying strong lens systems in large data sets. We study the probabilities associated with these lens searches, namely the probability of identifying a candidate with rogue [O II] emission lines in a galaxy's spectrum, and the probability that the candidate will show features of strong lensing in follow-up photometric observations. We include selection effects unique to spectroscopic data, and apply them to the Sloan Lens ACS (SLACS) survey. The most significant selection effect is the finite size of the spectroscopic fiber which selects against large separation lenses and results in a nonmonotonic dependence of the rogue line probability on velocity dispersion. For example, with the 3'' diameter SDSS fiber and 2'' FWHM seeing, we find that the probability that a given LRG has a rogue [O II] line in its spectrum decreases with velocity dispersion from 150 to 300 km s−1 and then increases up to 400 km s−1 for a given source size. The total probability for observing a rogue line in a single survey spectrum is ~0.9%-3.0%, and the total lensing rate is ~0.5%-1.3%. The range is due to uncertainties in the physical size of [O II] emission regions, and in the evolution of the [O II] luminosity function. Our estimates are a factor of ~5 higher than the results of the SLACS survey, a discrepancy which we attribute to the SLACS requirement that multiple rogue lines be observed simultaneously.

The Sloan Lens ACS Survey. VII. Elliptical Galaxy Scaling Laws from Direct Observational Mass Measurements

We use a sample of 53 massive early-type strong gravitational lens galaxies with well-measured redshifts (ranging from z = 0.06 to 0.36) and stellar velocity dispersions (between 175 and 400 km s−1) from the Sloan Lens ACS (SLACS) Survey to derive numerous empirical scaling relations. The ratio between central stellar velocity dispersion and isothermal lens-model velocity dispersion is nearly unity within errors. The SLACS lenses define a fundamental plane (FP) that is consistent with the FP of the general population of early-type galaxies. We measure the relationship between strong-lensing mass Mlens within one-half effective radius (Re/2) and the dimensional mass variable Mdim ≡ G−1σe22(Re/2) to be log (Mlens/1011 M☉) = (1.03 ± 0.04) log (Mdim/1011 M☉) + (0.54 ± 0.02) (where σe2 is the projected stellar velocity dispersion within Re/2). The near-unity slope indicates that the mass-dynamical structure of massive elliptical galaxies is independent of mass and that the "tilt" of the SLACS FP is due entirely to variation in total (luminous plus dark) mass-to-light ratio with mass. Our results imply that dynamical masses serve as a good proxies for true masses in massive elliptical galaxies. Regarding the SLACS lenses as a homologous population, we find that the average enclosed two-dimensional (2D) mass profile goes as log [M(< R)/Mdim] = (1.10 ± 0.09) log (R/Re) + (0.85 ± 0.03) , consistent with an isothermal (flat rotation curve) model when deprojected into three dimensions (3D). This measurement is inconsistent with the slope of the average projected aperture luminosity profile at a confidence level greater than 99.9%, implying a minimum dark matter fraction of fDM = 0.38 ± 0.07 within 1 effective radius. We also present an analysis of the angular mass structure of the lens galaxies, which further supports the need for dark matter inside one effective radius.

The Sloan Lens ACS Survey. VI. Discovery and Analysis of a Double Einstein Ring1

We report the discovery of two concentric partial Einstein rings around the gravitational lens SDSSJ0946+1006, as part of the Sloan Lens ACS Survey. The main lens is at redshift zl = 0.222, while the inner ring (1) is at redshift zs1 = 0.609 and Einstein radius REin1 = 1.43 ± 0.01 00 . The wider image separation (REin2 = 2.07±0.02 00 ) of the outer ring (2) implies that it is at higher redshift than Ring 1. Although no spectroscopic feature was detected in » 9 hours of spectroscopy at the Keck I Telescope, the detection of Ring 2 in the F814W ACS filter implies an upper limit on the redshift of zs2 . 6.9. The lens configuration can be well described by a power law total mass density profile for the main lens ‰tot / r i∞ 0 with logarithmic slope ∞ 0 = 2.00±0.03 (i.e. close to isothermal), velocity dispersion aeSIE = 287 ± 5kms i1 (in good agreement with the stellar velocity dispersion aev,⁄ = 284 ± 24kms i1 ) with little dependence upon cosmological parameters or the redshift of Ring 2. Using strong lensing constraints only we show that the enclosed mass to light ratio increases as a function of radius, inconsistent with mass following light. Adopting a prior on the stellar mass to light ratio from previous SLACS work we infer that 73 ± 9% of the mass is in form of dark matter within the cylinder of radius equal to the eective radius of the lens. We consider whether the double source plane configuration can be used to constrain cosmological parameters exploiting the ratios of angular distance ratios entering the set of lens equations. We find that constraints for SDSSJ0946+1006 are uninteresting due to the sub-optimal lens and source redshifts for this application. We then consider the perturbing eect of the mass associated with Ring 1 (modeled as a singular isothermal sphere) building a double lens plane compound lens model. This introduces minor changes to the mass of the main lens, allows to estimate the redshift of the Ring 2 (zs2 = 3.1 +2.0 i1.0 ), and the mass of the source responsible for Ring 1 (aeSIE,s1 = 94 +2747 kms i1 ). We conclude by examining the prospects of doing cosmography with a sample of 50 double source plane gravitational lenses, expected from future space based surveys such as DUNE or JDEM. Taking full account of the uncertainty in the mass density profile of the main lens, and of the eect of the perturber, and assuming known redshifts for both sources, we find that such a sample could be used to measure ›m and w with 10% accuracy, assuming a flat cosmological model. Subject headings: Gravitational lensing ‐ galaxies : Ellipticals and lenticulars, cD ‐ galaxies: structure ‐ galaxies: halos ‐ cosmology: dark matter ‐ cosmology: cosmological parameters

Two-dimensional kinematics of SLACS lenses - I. Phase-space analysis of the early-type galaxy SDSS J2321−097 atz≈ 0.1★

We present the first results of a combined VLT VIMOS integral-field unit and Hubble Space Telescope (HST)/ACS study of the early-type lens galaxy SDSS J2321-097 at z = 0.0819, extending kinematic studies to a look-back time of 1 Gyr. This system, discovered in the Sloan Lens ACS Survey, has been observed as part of a VLT Large Programme with the goal of obtaining two-dimensional stellar kinematics of 17 early-type galaxies to z approximate to 0.35 and Keck spectroscopy of an additional dozen lens systems. Bayesian modelling of both the surface brightness distribution of the lensed source and the two-dimensional measurements of velocity and velocity dispersion has allowed us, under the only assumptions of axisymmetry and a two-integral stellar distribution function (DF) for the lens galaxy, to dissect this galaxy in three dimensions and break the classical mass-anisotropy, mass-sheet and inclination-oblateness degeneracies. Our main results are that the galaxy (i) has a total density profile well described by a single power law p alpha r(-gamma') with gamma' = 2.06(-0.06)(+0.03); (ii) is a very slow rotator (specific stellar angular momentum parameter lambda(R) = 0.075); (iii) shows only mild anisotropy (delta approximate to 0.15); and (iv) has a dark matter contribution of similar to 30 per cent inside the effective radius. Our first results from this large combined imaging and spectroscopic effort with the VLT, Keck and HST show that the structure of massive early-type galaxies beyond the local Universe can now be studied in great detail using the combination of stellar kinematics and gravitational lensing. Extending these studies to look-back times where evolutionary effects become measurable holds great promise for the understanding of formation and evolution of early-type galaxies.

The environments of SLACS gravitational lenses

Abstract We report on an investigation of the environments of the Sloan Lens ACS Survey (SLACS) sample of gravitational lenses. The local and global environments of the lenses are characterized using Sloan Digital Sky Survey (SDSS) photometry and, when available, spectroscopy. We find that the lens systems that are best modelled with steeper than isothermal density profiles are more likely to have close companions than lenses with shallower than isothermal profiles. This suggests that the profile steepening may be caused by interactions with a companion galaxy as indicated by N-body simulations of group galaxies. The global environments of the SLACS lenses are typical of non-lensing SDSS galaxies with comparable properties to the lenses, and the richnesses of the lens groups are not as strongly correlated with the lens density profiles as are the local environments. Furthermore, we investigate the possibility of line-of-sight contamination affecting the lens models, but do not find a significant over-density of sources compared to lines of sight without lenses.

Unveiling dark haloes in lensing galaxies

We present a spatially resolved comparison of the stellar mass and total mass surface distributions of nine early-type galaxies. The galaxies are a subset of the Sloan Lens ACS Survey. The total mass distributions are obtained by exploring pixelated mass models that reproduce the lensed images. The stellar mass distributions are derived from population-synthesis models fit to the photometry of the lensing galaxies. Uncertainties – mainly model degeneracies – are also computed. Stars can account for all the mass in the inner regions. A Salpeter initial mass function actually gives too much stellar mass in the inner regions and hence appears ruled out. Dark matter becomes significant by the half-light radius and becomes increasingly dominant at larger radii. The stellar and dark components are closely aligned, but the actual ellipticities are not correlated. Finally, we attempt to intuitively summarize the results by rendering the density, stellar versus dark ratio, and uncertainties as false-colour maps.

Stellar mass estimates in early-type galaxies from lensing+dynamical and photometric measurements

Aims. Our purpose is to compare two different diagnostics for estimating stellar masses in early-type galaxies and to establish their level of reliability. In particular, we consider the well-studied sample of 15 field elliptical galaxies selected from the Sloan Lens ACS (SLACS) Survey (z = 0.06-0.33). We examine here the correlation between the stellar mass values, enclosed inside the Einstein radius (R Ein ) of each lens, based on analyses of lensing and stellar dynamics combined and based on multiwavelength photometry spectral template fitting. Methods. The lensing+dynamics stellar mass M* len+dYn (≤R Ein ) is obtained from the published SLACS Survey results, assuming a two-component density distribution model and a prior from the fundamental plane on the mass-to-light ratio for the lens galaxies. The photometric stellar mass M* phot (≤R Ein ) is measured by fitting the observed spectral energy distribution of the galaxies (from the SDSS multi-band photometry over 354-913 nm) with composite stellar population templates, under the assumption that light traces stellar mass. Results. The two methods are completely independent. They rely on several different assumptions, and so, in principle, both can have significant biases. Based on our sample of massive galaxies (log M * phot (≤R Ein ) ≃[10.3,11.5]), we find consistency between the values of M * len+dyn (≤R Ein ) and M * phot (≤R Ein ). We obtain a Pearson linear correlation coefficient of 0.94 and a median value of the ratio between the former and the latter mass measurements of 1.1 ± 0.1. This suggests that both methods can separately yield reliable stellar masses of early-type galaxies, and confirms that photometric mass estimates are accurate, as long as optical/near-IR rest frame photometry is available.

The Sloan Lens ACS Survey. IV. The Mass Density Profile of Early‐Type Galaxies out to 100 Effective Radii

We present a weak gravitational lensing analysis of 22 early-type strong lens galaxies, based on deep HST images obtained as part of the Sloan Lens ACS Survey. Using the most advanced techniques to control systematic uncertainties related to the variable PSF and charge transfer efficiency of the ACS, we detect weak lensing signal out to 300 kpc/h. We analyze blank control fields from the COSMOS survey in the same manner, inferring that the residual systematic uncertainty in the tangential shear is <0.3%. A joint strong and weak lensing analysis shows that the average total mass density profile is consistent with isothermal over two decades in radius (3-300 kpc/h, approximately 1-100 Reff). This finding extends by over an order of magnitude in radius previous results, based on strong lensing and/or stellar dynamics, that luminous and dark component ``conspire'' to form an isothermal mass distribution. In order to disentangle the contributions of luminous and dark matter, we fit a two-component mass model (R^1/4 + NFW) to the weak and strong lensing constraints. It provides a good fit to the data with only two free parameters; i) the average stellar mass-to-light ratio M_*/L_V=4.48 +- 0.46 hMo/Lo, in agreement with that expected for an old stellar population; ii) the average virial mass-to-light ratio M_vir/L_V = 246+101-87 hMo/Lo. [abridged]

A More Fundamental Plane

We combine strong-lensing masses with SDSS stellar velocity dispersions and HST ACS effective (half-light) radii for 36 lens galaxies from the Sloan Lens ACS (SLACS) Survey to study the mass dependence of mass-dynamical structure in early-type galaxies. We find that over a 180-390 km s(-1) range in velocity dispersion, structure is independent of lensing mass to within 5%. This result suggests a systematic variation in the total (i.e., luminous plus dark matter) mass-to-light ratio as the origin of the tilt of the fundamental plane ( FP) scaling relationship between galaxy size, velocity dispersion, and surface brightness. We construct the FP of the lens sample, which we find to be consistent with the FP of the parent SDSS early-type galaxy population. We also present the first observational correlation between the mass-to-light ratio and residuals about the FP. Finally, we reformulate the FP in terms of surface mass density rather than surface brightness. By removing the complexities of stellar-population effects, this mass-plane formulation will facilitate comparison to numerical simulations and possible use as a cosmological distance indicator.

Strong Gravitational Lensing: Bright Galaxies & Lost Dark-Matter

AbstractGravitational lensing and stellar dynamics provide two complementary methods to probe the smooth and clumpy stellar and dark-matter mass distribution in early-type galaxies, currently already over a range of two orders of magnitude in virial mass, more than ten orders of magnidude in dynamic mass range in each galaxy (i.e. from stars, CDM substructure to massive dark-matter halos), over 0–1 in redshift, and a range of 0–100 effective radii. This makes their unification a powerful new tool in the study of the formation, structure and evolution of these massive systems. I review recent results that we obtained, based on gravitational lens systems from the Sloan Lens ACS Survey (SLACS), and outline some ongoing and future work.

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

We present a joint gravitational lensing and stellar-dynamical analysis of 15 massive field early-type galaxies selected from the Sloan Lens ACS (SLACS) Survey. The following numerical results are found: (1) A joint likelihood gives an average logarithmic density slope for the total mass density of ⟨γ'⟩ = 2.01 (68% CL; ρtot ∝ r) inside ⟨REinst⟩ = 4.2 ± 0.4 kpc (rms of 1.6 kpc) for isotropic models. The inferred intrinsic rms spread in logarithmic density slopes is σ = 0.12. (2) The average position-angle difference between the light distribution and the total mass distribution is found to be ⟨Δθ⟩ = 0° ± 3° (rms of 10°), setting an upper limit of ⟨γext⟩ ≲ 0.035 on the average external shear. The total mass has an average ellipticity ⟨qSIE⟩ = 0.78 ± 0.03 (rms of 0.12), which correlates extremely well with the stellar ellipticity, q*, resulting in ⟨qSIE/q*⟩ = 0.99 ± 0.03 (rms of 0.11) for σ ≳ 225 km s-1 omitting three S0 lens galaxies. (3) The average projected dark matter mass fraction is inferred to be ⟨fDM⟩ = 0.25 ± 0.06 (rms of 0.22) inside ⟨REinst⟩, using the stellar mass-to-light ratios derived from the fundamental plane as priors. (4) Combined with results from the Lenses Structure and Dynamics (LSD) Survey at z ≳ 0.3, we find no significant evolution of the total density slope inside one effective radius for galaxies with σap ≥ 200 km s-1: a linear fit gives α ≡ d⟨γ'⟩/dz = 0.23 ± 0.16 (1 σ) for the range z = 0.08-1.01. The small scatter and absence of significant evolution in the inner density slopes suggest a collisional scenario in which gas and dark matter strongly couple during galaxy formation, leading to a total mass distribution that rapidly converges to dynamical isothermality.

Constraint on the post-Newtonian parameterγon galactic size scales

We constrain the post-Newtonian gravity parameter $\ensuremath{\gamma}$ on kiloparsec scales by comparing the masses of 15 elliptical lensing galaxies from the Sloan Lens ACS Survey as determined in two independent ways. The first method assumes only that Newtonian gravity is correct and is independent of $\ensuremath{\gamma}$, while the second uses gravitational lensing which depends on $\ensuremath{\gamma}$. More specifically, we combine Einstein radii and radial surface-brightness gradient measurements of the lens galaxies with empirical distributions for the mass concentration and velocity anisotropy of elliptical galaxies in the local universe to predict $\ensuremath{\gamma}$-dependent probability distributions for the lens-galaxy velocity dispersions. By comparing with observed velocity dispersions, we derive a maximum-likelihood value of $\ensuremath{\gamma}=0.98\ifmmode\pm\else\textpm\fi{}0.07$ (68% confidence). This result is in excellent agreement with the prediction of general relativity that $\ensuremath{\gamma}=1$, which has previously been verified to this accuracy only on solar-system length scales.

The Sloan Lens ACS Survey. II. Stellar Populations and Internal Structure of Early‐Type Lens Galaxies

We use HST images to derive effective radii and effective surface brightnesses of 15 early-type (E+S0) lens galaxies identified by the SLACS Survey. Our measurements are combined with stellar velocity dispersions from the SDSS database to investigate for the first time the distribution of lens galaxies in the fundamental plane (FP) space. Accounting for selection effects (top priority to the largest Einstein radii and thus approximately to the largest velocity dispersions, σ ≳ 240 km s-1) and for passive evolution, the distribution of the lens galaxies inside the FP is indistinguishable from that of the parent sample of SDSS galaxies. We conclude that SLACS lenses are a fair sample of high velocity dispersion E+S0s. By comparing the central stellar velocity dispersion (σ) with the velocity dispersion that best fits the lensing models (σSIE) we find ⟨fSIE⟩ ≡ ⟨σ/σSIE⟩ = 1.01 ± 0.02 with 0.065 rms scatter. We infer that within the Einstein radii (typically Re/2) the SLACS lenses are very well approximated by isothermal ellipsoids, requiring a fine tuning of the stellar and dark matter distribution (the bulge-halo ``conspiracy''). Interpreting the offset from the local FP in terms of evolution of the stellar mass-to-light ratio, we find d log(M/LB)/dz = -0.69 ± 0.08 (rms 0.11) consistent with the rate found for field E+S0s and with most of the stars being old (zf > 2) and less than ~10% of the stellar mass having formed below z = 1. We discuss our results in the context of formation mechanisms such as collisionless (``dry'') mergers.

The Sloan Lens ACS Survey. I. A Large Spectroscopically Selected Sample of Massive Early‐Type Lens Galaxies

The Sloan Lens ACS (SLACS) Survey is an efficient Hubble Space Telescope Snapshot imaging survey for new galaxy-scale strong gravitational lenses. The targeted lens candidates are selected spectroscopically from within the Sloan Digital Sky Survey (SDSS) database of galaxy spectra for having multiple nebular emission lines at a redshift significantly higher than that of the SDSS target galaxy. In this paper, we present a catalog of 19 newly discovered gravitational lenses, along with 9 other observed candidate systems that are either possible lenses, non-lenses, or non-detections. The survey efficiency is thus >=68%. We also present Gemini and Magellan IFU data for 9 of the SLACS targets, which further support the lensing interpretation. A new method for the effective subtraction of foreground galaxy images to reveal faint background features is presented. We show that the SLACS lens galaxies have colors and ellipticities typical of the spectroscopic parent sample from which they are drawn (SDSS luminous red galaxies and quiescent main-sample galaxies), but are somewhat brighter and more centrally concentrated. Several explanations for the latter bias are suggested. The SLACS survey provides the first statistically significant and homogeneously selected sample of bright early-type lens galaxies, furnishing a powerful probe of the structure of early-type galaxies within the half-light radius. The high confirmation rate of lenses in the SLACS survey suggests consideration of spectroscopic lens discovery as an explicit science goal of future spectroscopic galaxy surveys (abridged).

Gravitational Lensing &amp; Stellar Dynamics

Strong gravitational lensing and stellar dynamics provide two complementary and orthogonal constraints on the density profiles of galaxies. Based on spherically symmetric, scale-free, mass models, it is shown that the combination of both techniques is powerful in breaking the mass-sheet and mass-anisotropy degeneracies. Second, observational results are presented from the Lenses Structure & Dynamics (LSD) Survey and the Sloan Lens ACS (SLACS) Survey collaborations to illustrate this new methodology in constraining the dark and stellar density profiles, and mass structure, of early-type galaxies to redshifts of unity.