The Sloan Lens ACS Survey. II. Stellar Populations and Internal Structure of Early‐Type Lens Galaxies (original) (raw)
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The Astrophysical Journal, 2006
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 parameter
THE SLOAN LENS ACS SURVEY. IX. COLORS, LENSING, AND STELLAR MASSES OF EARLY-TYPE GALAXIES
The Astrophysical Journal, 2009
We present the current photometric dataset for the Sloan Lens ACS (SLACS) Survey, including HST photometry from ACS, WFPC2, and NICMOS. These data have enabled the confirmation of an additional 15 grade 'A' (certain) lens systems, bringing the number of SLACS grade 'A' lenses to 85; including 13 grade 'B' (likely) systems, SLACS has identified nearly 100 lenses and lens candidates. Approximately 80% of the grade 'A' systems have elliptical morphologies while ∼10% show spiral structure; the remaining lenses have lenticular morphologies. Spectroscopic redshifts for the lens and source are available for every system, making SLACS the largest homogeneous dataset of galaxy-scale lenses to date. We have created lens models using singular isothermal ellipsoid mass distributions for the 11 new systems that are dominated by a single mass component and where the multiple images are detected with sufficient signal-to-noise; these models give a high precision measurement of the mass within the Einstein radius of each lens. We have developed a novel Bayesian stellar population analysis code to determine robust stellar masses with accurate error estimates. We apply this code to deep, high-resolution HST imaging and determine stellar masses with typical statistical errors of 0.1 dex; we find that these stellar masses are unbiased compared to estimates obtained using SDSS photometry, provided that informative priors are used. The stellar masses range from 10 10.5 to 10 11.8 M ⊙ and the typical stellar mass fraction within the Einstein radius is 0.4, assuming a Chabrier IMF. The ensemble properties of the SLACS lens galaxies, e.g. stellar masses and projected ellipticities, appear to be indistinguishable from other SDSS galaxies with similar stellar velocity dispersions. This further supports that SLACS lenses are representative of the overall population of massive early-type galaxies with M * 10 11 M ⊙ , and are therefore an ideal dataset to investigate the kpc-scale distribution of luminous and dark matter in galaxies out to z ∼ 0.5.
The Astrophysical Journal, 2013
We present optical and near infrared spectroscopy obtained at Keck, VLT, and Gemini for a sample of 36 secure strong gravitational lens systems and 17 candidates identified as part of the SL2S survey. The deflectors are massive early-type galaxies in the redshift range z d = 0.2 − 0.8, while the lensed sources are at z s = 1 − 3.5. We combine this data with photometric and lensing measurements presented in the companion paper III and with lenses from the SLACS and LSD surveys to investigate the cosmic evolution of the internal structure of massive early-type galaxies over half the age of the universe. We study the dependence of the slope of the total mass density profile γ ′ (ρ(r) ∝ r −γ ′) on stellar mass, size, and redshift. We find that two parameters are sufficent to determine γ ′ with less than 6% residual scatter. At fixed redshift, γ ′ depends solely on the surface stellar mass density ∂γ ′ /∂Σ * = 0.38 ± 0.07, i.e. galaxies with denser stars also have steeper slopes. At fixed M * and R eff , γ ′ depends on redshift, in the sense that galaxies at a lower redshift have steeper slopes (∂γ ′ /∂z = −0.31 ± 0.10). However, the mean redshift evolution of γ ′ for an individual galaxy is consistent with zero dγ ′ /dz = −0.10 ± 0.12. This result is obtained by combining our measured dependencies of γ ′ on z, M * ,R eff with the evolution of the R eff-M * taken from the literature, and is broadly consistent with current models of the formation and evolution of massive early-type galaxies. Detailed quantitative comparisons of our results with theory will provide qualitatively new information on the detailed physical processes at work.
The Astrophysical Journal, 2011
We present a joint gravitational lensing and stellar-dynamical analysis of 11 early-type galaxies (median deflector redshift z d = 0.5) from Strong Lenses in the Legacy Survey (SL2S). Using newly measured redshifts and stellar velocity dispersions from Keck spectroscopy with lens models from Paper I, we derive the total mass-density slope inside the Einstein radius for each of the 11 lenses. The average total density slope is found to be γ = 2.16 +0.09 −0.09 (ρ tot ∝ r −γ), with an intrinsic scatter of 0.25 +0.10 −0.07. We also determine the dark matter fraction for each lens within half the effective radius, R eff /2, and find the average-projected dark matter mass fraction to be 0.42 +0.08 −0.08 with a scatter of 0.20 +0.09 −0.07 for a Salpeter initial mass function. By combining the SL2S results with those from the Sloan Lens ACS Survey (median z d = 0.2) and the Lenses Structure and Dynamics Survey (median z d = 0.8), we investigate cosmic evolution of γ and find a mild trend ∂ γ /∂z d = −0.25 +0.10 −0.12. This suggests that the total density profile of massive galaxies has become slightly steeper over cosmic time. If this result is confirmed by larger samples, it would indicate that dissipative processes played some role in the growth of massive galaxies since z ∼ 1.
The Astrophysical Journal, 2007
We present a weak gravitational lensing analysis of 22 early-type (strong) lens galaxies, based on deep Hubble Space Telescope images obtained as part of the Sloan Lens ACS Survey. Using advanced techniques to control systematic uncertainties related to the variable point spread function and charge transfer efficiency of the Advanced Camera for Surveys (ACS), we show that weak lensing signal is detected out to the edge of the Wide Field Camera ( 300 h −1 kpc at the mean lens redshift z = 0.2). We analyze blank control fields from the COSMOS survey in the same manner, inferring that the residual systematic uncertainty in the tangential shear is less than 0.3%. A joint strong and weak lensing analysis shows that the average total mass density profile is consistent with isothermal (i.e. ρ ∝ r −2 ) over two decades in radius (3-300 h −1 kpc, approximately 1-100 effective radii). 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 twocomponent mass model (de Vaucouleurs + Navarro Frenk & White) 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 hM ⊙ /L ⊙ (at z = 0.2), 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 hM ⊙ /L ⊙ . Taking into account the scatter in the mass-luminosity relation, this latter result is in good agreement with semi-analytical models of massive galaxies formation. The dark matter fraction inside the sphere of radius the effective radius is found to be 27±4%. Our results are consistent with galaxy-galaxy lensing studies of early-type galaxies that are not strong lenses, in the region of overlap (30-300 h −1 kpc). Thus, within the uncertainties, our results are representative of early-type galaxies in general.
Monthly Notices of the Royal Astronomical Society, 2014
We present an investigation about the shape of the initial mass function (IMF) of early-type galaxies (ETGs), based on a joint lensing and dynamical analysis, and on stellar population synthesis models, for a sample of 55 lens ETGs identified by the Sloan Lens ACS (SLACS) Survey. We construct axisymmetric dynamical models based on the Jeans equations which allow for orbital anisotropy and include a dark matter halo. The models reproduce in detail the observed HST photometry and are constrained by the total projected mass within the Einstein radius and the stellar velocity dispersion (σ) within the SDSS fibers. Comparing the dynamically-derived stellar mass-to-light ratios (M * /L) dyn , obtained for an assumed halo slope ρ h ∝ r −1 , to the stellar population ones (M * /L) Salp , derived from full-spectrum fitting and assuming a Salpeter IMF, we infer the mass normalization of the IMF. Our results confirm the previous analysis by the SLACS team that the mass normalization of the IMF of high σ galaxies is consistent on average with a Salpeter slope. Our study allows for a fully consistent study of the trend between IMF and σ for both the SLACS and ATLAS 3D samples, which explore quite different σ ranges. The two samples are highly complementary, the first being essentially σ selected, and the latter volume-limited and nearly mass selected. We find that the two samples merge smoothly into a single trend of the form log α = (0.38 ± 0.04) × log(σ e /200 km s −1) + (−0.06 ± 0.01), where α = (M * /L) dyn /(M * /L) Salp and σ e is the luminosity averaged σ within one effective radius R e. This is consistent with a systematic variation of the IMF normalization from Kroupa to Salpeter in the interval σ e ≈ 90 − 270 km s −1 .
The Astrophysical Journal, 2000
Most gravitational lenses are early-type galaxies in relatively low density environments -a "field" rather than a "cluster" population. Their average properties are the mass-averaged properties of all early-type galaxies. We show that field early-type galaxies with 0 < z < 1, as represented by the lens galaxies, lie on the same fundamental plane as those in rich clusters at similar redshifts. We then use the fundamental plane to measure the combined evolutionary and K-corrections for early-type galaxies in the V, I and H bands. Only for passively evolving stellar populations formed at z f > ∼ 2 (H 0 = 65 km s −1 Mpc −1 , Ω 0 = 0.3, λ 0 = 0.7) can the lens galaxies be matched to the local fundamental plane. The high formation epoch and the lack of significant differences between the field and cluster populations contradict many current models of the formation history of early-type galaxies. Lens galaxy colors and the fundamental plane provide good photometric redshift estimates with an empirical accuracy of z F P − z l = −0.04 ± 0.09 for the 20 lenses with known redshifts. A mass model dominated by dark matter is more consistent with the data than either an isotropic or radially anisotropic constant M/L mass model, and a radially anisotropic model is better than an isotropic model.
2014
We present an investigation about the shape of the initial mass function (IMF) of early-type galaxies (ETGs), based on a joint lensing and dynamical analysis, and on stellar population synthesis models, for a sample of 55 lens ETGs identified by the Sloan Lens ACS (SLACS) Survey. We construct axisymmetric dynamical models based on the Jeans equations which allow for orbital anisotropy and include a dark matter halo. The models reproduce in detail the observed HST photometry and are constrained by the total projected mass within the Einstein radius and the stellar velocity dispersion (σ) within the SDSS fibers. Comparing the dynamically-derived stellar mass-to-light ratios (M * /L) dyn , obtained for an assumed halo slope ρ h ∝ r −1 , to the stellar population ones (M * /L) Salp , derived from full-spectrum fitting and assuming a Salpeter IMF, we infer the mass normalization of the IMF. Our results confirm the previous analysis by the SLACS team that the mass normalization of the IMF of high σ galaxies is consistent on average with a Salpeter slope. Our study allows for a fully consistent study of the trend between IMF and σ for both the SLACS and ATLAS 3D samples, which explore quite different σ ranges. The two samples are highly complementary, the first being essentially σ selected, and the latter volume-limited and nearly mass selected. We find that the two samples merge smoothly into a single trend of the form log α = (0.38 ± 0.04) × log(σ e /200 km s −1) + (−0.06 ± 0.01), where α = (M * /L) dyn /(M * /L) Salp and σ e is the luminosity averaged σ within one effective radius R e. This is consistent with a systematic variation of the IMF normalization from Kroupa to Salpeter in the interval σ e ≈ 90 − 270 km s −1 .
The Astrophysical Journal, 2011
We exploit the strong lensing effect to explore the properties of intrinsically faint and compact galaxies at intermediate redshift (z s 0.4-0.8) at the highest possible resolution at optical wavelengths. Our sample consists of 46 strongly lensed emission line galaxies (ELGs) discovered by the Sloan Lens ACS Survey (SLACS). The galaxies have been imaged at high resolution with the Hubble Space Telescope (HST) in three bands (V HST , I 814 , and H 160), allowing us to infer their size, luminosity, and stellar mass using stellar population synthesis models. Lens modeling is performed using a new fast and robust code, klens, which we test extensively on real and synthetic non-lensed galaxies, and also on simulated galaxies multiply imaged by SLACS-like galaxy-scale lenses. Our tests show that our measurements of galaxy size, flux, and Sérsic index are robust and accurate, even for objects intrinsically smaller than the HST point-spread function. The median magnification is 8.8, with a long tail that extends to magnifications above 40. Modeling the SLACS sources reveals a population of galaxies with colors and Sérsic indices (median n ∼ 1) consistent with the galaxies detected with HST in the Galaxy Evolution from Morphology and SEDs (GEMS) and Hubble Ultra Deep Field (HUDF) surveys, but that are (typically) ∼2 mag fainter and ∼5 times smaller in apparent size than GEMS and ∼4 mag brighter than but similar in size to HUDF. The size-stellar-mass and size-luminosity relations for the SLACS sources are offset to smaller sizes with respect to both comparison samples. The closest analog are ultracompact ELGs identified by HST grism surveys. The lowest mass galaxies in our sample are comparable to the brightest Milky Way satellites in stellar mass (10 7 M) and have well-determined half-light radii of 0. 05 (≈0.3 kpc).
On the evolution of environmental and mass properties of strong lens galaxies in COSMOS
Astronomy & Astrophysics, 2011
Context. Nearly 100 new strong lens candidates have been discovered in the COSMOS field. Among these, 20 lens candidates with 0.34 z lens 1.13, feature multiple images of background sources. Aims. Using the multi-wavelength coverage of the field and its spectroscopic follow-up, we characterize the evolution with redshift of the environment and of the dark-matter (DM) fraction of the lens galaxies. Methods. We present spectroscopic and new photometric redshifts of the strong lens candidates. The lens environment is characterized in the following way: we account for the projected 10 closest galaxies around each lens and for galaxies with a projected distance less than 1 Mpc at the lens galaxy redshift. In both cases, we perform similar measurements on a control sample of "twin" non-lens early type galaxies (ETGs). In addition, we identify group members and field galaxies in the X-ray and optical catalogs of galaxy groups and clusters. From those catalogs, we measure the external shear contribution of the groups/clusters surrounding the lens galaxies. The systems are then modeled using a SIE for the lens galaxies plus the external shear due to the groups/clusters. Results. We observe that the average stellar mass of lens galaxies increases with redshift. In addition, we measure that the environment of lens galaxies is compatible with that of the twins over the whole redshift range tested here. During the lens modeling, we notice that, when let free, the external shear points in a direction which is the mean direction of the external shear due to groups/clusters and of the closest galaxy to the lens. We also notice that the DM fraction of the lens galaxies measured within the Einstein radius significantly decreases as the redshift increases. Conclusions. Given these, we conclude that, while the environment of lens galaxies is compatible with that of non-lens ETGS over a wide range of redshifts, their mass properties evolves significantly with redshift: it is still not clear whether this advocates in favor of a stronger lensing bias toward massive objects at high redshift or is simply representative of the high proportion of massive and high stellar density galaxies at high redshift.