Mass and light in the supercluster of galaxies MS0302+17 (original) (raw)
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Stellar mass estimates in early-type galaxies from lensing+dynamical and photometric measurements
Astronomy & Astrophysics, 2007
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 twocomponent 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.
Massive Dark Matter Halos and Evolution of Early‐Type Galaxies to z ≈ 1
The Astrophysical Journal, 2004
The combination of gravitational lensing and stellar dynamics breaks the mass-anisotropy degeneracy and provides stringent constraints on the distribution of luminous and dark matter in early-type (E/S0) galaxies out to z ≈ 1. We present new observations and models of three lens systems (CFRS03.1077, HST14176+5226, HST15433+5352) and the combined results from the five field E/S0 lens galaxies at z ≈ 0.5−1.0 analyzed as part of the Lenses Structure & Dynamics (LSD) Survey. Our main results are: (i) Constant mass-to-light ratio models are ruled out at > 99% CL for all five E/S0 galaxies, consistent with the presence of massive and extended dark-matter halos. The range of projected darkmatter mass fractions inside the Einstein radius is f DM =0.37-0.72, or 0.15-0.65 inside the effective radius R e for isotropic models. (ii) The average effective power-law slope of the total (luminous+dark; ρ tot ∝ r −γ ′ ) mass distribution is γ ′ =1.75±0.10 (1.57±0.16) for Osipkov-Merritt models with anisotropy radius r i = ∞(R e ) with an rms scatter of 0.2 (0.35), i.e. marginally flatter than isothermal (γ ′ = 2). The ratio between the observed central stellar velocity dispersion and that from the best-fit singular isothermal ellipsoid (SIE) lens model is f SIE = σ/σ SIE = 0.87 ± 0.04 with 0.08 rms, consistent with flatter-than-isothermal density profiles. Considering that γ ′ > 2 and f SIE > 1 have been reported for other systems (i.e. B1608+656 and PG1115+080), we conclude that there is a significant intrinsic scatter in the slope of the mass-density profile of lens galaxies (rms ∼15%), similar to what is found for local E/S0 galaxies. Hence, the isothermal approximation is not sufficiently accurate for applications that depend critically on the slope of the mass-density profile, such as the measurement of the Hubble Constant from time-delays. (iii) The average inner power-law slope γ of the dark-matter halo is constrained to be γ = 1.3 +0.2 −0.4 (68% C.L.), if the stellar velocity ellipsoid is isotropic (r i = ∞) or an upper limit of γ < 0.6, if the galaxies are radially anisotropic (r i = R e ). The observed range of slopes of the inner dark-matter distribution is consistent with the results from numerical simulations only for an isotropic velocity ellipsoid and if baryonic collapse and star-formation do not steepen dark-matter density profiles. (iv) The average stellar mass-to-light ratio evolves as d log(M * /L B )/dz=−0.72 ± 0.10, obtained via a Fundamental Plane analysis. An independent analysis based on lensing and dynamics gives an average d log(M * /L B )/dz =−0.75 ± 0.17. Both values indicate that the mass-to-light ratio evolution for our sample of field E/S0 galaxies is slighly faster than those in clusters, consistent with the hypothesis that field E/S0 galaxies experience secondary bursts (∼10% in mass) of star formation at z < 1. These findings are consistent with pure luminosity evolution of E/S0 galaxies in the past 8 Gyrs, and would be hard to reconcile with scenarios involving significant structural and dynamical evolution.
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.
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, 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 Initial Mass Function of Early-Type Galaxies
The Astrophysical Journal, 2010
We determine an absolute calibration of the initial mass function (IMF) of early-type galaxies, by studying a sample of 56 gravitational lenses identified by the Sloan Lenses ACS Survey. Under the assumption of standard Navarro, Frenk, and White dark matter halos, a combination of lensing, dynamical, and stellar population synthesis models is used to disentangle the stellar and dark matter contribution for each lens. We define an "IMF mismatch" parameter α ≡M LD * ,Ein /M SPS * ,Ein as the ratio of stellar mass inferred by a joint lensing and dynamical model (M LD * ,Ein) to the current stellar mass inferred from stellar populations synthesis models (M SPS * ,Ein). We find that a Salpeter IMF provides stellar masses in agreement with those inferred by lensing and dynamical models (log α = −0.00 ± 0.03 ± 0.02), while a Chabrier IMF underestimates them (log α = 0.25 ± 0.03 ± 0.02). A tentative trend is found, in the sense that α appears to increase with galaxy velocity dispersion. Taken at face value, this result would imply a nonuniversal IMF, perhaps dependent on metallicity, age, or abundance ratios of the stellar populations. Alternatively, the observed trend may imply non-universal dark matter halos with inner density slope increasing with velocity dispersion. While the degeneracy between the two interpretations cannot be broken without additional information, the data imply that massive early-type galaxies cannot have both a universal IMF and universal dark matter halos.
Central mass-to-light ratios and dark matter fractions in early-type galaxies
MONTHLY NOTICES OF THE ROYAL …, 2009
Dynamical studies of local elliptical galaxies and the Fundamental Plane point to a strong dependence of the total mass-to-light ratio on luminosity with a relation of the form M/L ∝ L γ . The "tilt" γ may be caused by various factors, including stellar population properties (metallicity, age and star formation history), IMF, rotational support, luminosity profile non-homology and dark matter (DM) fraction. We evaluate the impact of all these factors using a large uniform dataset of local early-type galaxies from . We take particular care in estimating the stellar masses, using a general star formation history, and comparing different population synthesis models. We find that the stellar M/L contributes little to the tilt. We estimate the total M/L using simple Jeans dynamical models, and find that adopting accurate luminosity profiles is important but does not remove the need for an additional tilt component, which we ascribe to DM. We survey trends of the DM fraction within one effective radius, finding it to be roughly constant for galaxies fainter than M B ∼ −20.5, and increasing with luminosity for the brighter galaxies; we detect no significant differences among S0s and fast-and slow-rotating ellipticals. We construct simplified cosmological mass models and find general consistency, where the DM transition point is caused by a change in the relation between luminosity and effective radius. A more refined model with varying galaxy star formation efficiency suggests a transition from total mass profiles (including DM) of faint galaxies distributed similarly to the light, to near-isothermal profiles for the bright galaxies. These conclusions are sensitive to various systematic uncertainties which we investigate in detail, but are consistent with the results of dynamics studies at larger radii.
The Astrophysical …, 2009
We present a new wide field image of the distant cluster AC114 (z = 0.31) obtained with Wide Field and Planetary Camera II onboard the Hubble Space Telescope. This image considerably extends our knowledge of the lensing properties of the cluster beyond that derived by Smail et al. (1995a) from a single WF/PC-1 pointing. In conjunction with published ground-based spectroscopy, we utilise several newly-discovered multiple images to construct an improved mass model for the central regions of the cluster. Using this model, we apply the methodology introduced by Natarajan & Kneib (1997) to interpret local perturbations to the cluster shear field on small scales resulting from mass associated with individual cluster galaxies. We use the lensing signal to place new constraints on the average mass-to-light ratio and spatial extents of the dark matter halos associated with morphologically-classified early-type cluster members.
AN INVENTORY OF THE STELLAR INITIAL MASS FUNCTION IN EARLY-TYPE GALAXIES
The Astrophysical Journal, 2013
Given a flurry of recent claims for systematic variations in the stellar initial mass function (IMF), we carry out the first inventory of the observational evidence using different approaches. This includes literature results, as well as our own new findings from combined stellar-populations synthesis (SPS) and Jeans dynamical analyses of data on ∼ 4500 early-type galaxies (ETGs) from the SPIDER project. We focus on the mass-to-light ratio mismatch relative to the Milky Way IMF, δ IMF , correlated against the central stellar velocity dispersion, σ ⋆ . We find a strong correlation between δ IMF and σ ⋆ , for a wide set of dark matter (DM) model profiles. These results are robust if a uniform halo response to baryons is adopted across the sample. The overall normalization of δ IMF , and the detailed DM profile, are less certain, but the data are consistent with standard cold-DM halos, and a central DM fraction that is roughly constant with σ ⋆ . For a variety of related studies in the literature, using SPS, dynamics, and gravitational lensing, similar results are found. Studies based solely on spectroscopic line diagnostics agree on a Salpeter-like IMF at high σ ⋆ , but differ at low σ ⋆ . Overall, we find that multiple independent lines of evidence appear to be converging on a systematic variation in the IMF, such that high-σ ⋆ ETGs have an excess of low-mass stars relative to spirals and low-σ ⋆ ETGs. Robust verification of super-Salpeter IMFs in the highest-σ ⋆ galaxies will require additional scrutiny of scatter and systematic uncertainties. The implications for the distribution of DM are still inconclusive.
Photometric estimates of stellar masses in high-redshift galaxies
Astronomy and Astrophysics, 2004
We present a new tool for the photometric estimate of stellar masses in distant galaxies. The observed source spectral energy distributions are fitted by combining sets of various simple stellar populations, with different normalizations and different amounts of dust extinction, for a given (Salpeter) IMF. This treatment gives us the best flexibility and robustness when dealing with the widest variety of physical situation for the target galaxies, including inactive spheroidal and active starburst systems. We tested the code on three classes of sources: complete samples of dusty ISO-selected starbursts and of K-band selected ellipticals and S0s in the HDF South, and a representative sample of z ∼ 2 to 3 Lyman-break galaxies in the HDF North. We pay particular attention in evaluating the uncertainties in the stellar mass estimate, due to degeneracies in the physical parameters, different star formation histories and different metallicities. Based on optical-NIR photometric data, the stellar masses are found to have overall uncertainties of a factor of ∼2 for E/S0s, while for the starburst population these rise to factors 2−5 (even including ISO/15 µm photometric data), and up to ≥10 for Ly-break galaxies. Our analysis reveals in any case the latter to correspond to a galaxy population significantly less massive (M < a few 10 10 M) than those observed at lower redshifts (for which typically M > several 10 10 M), possibly indicating substantial stellar build-up happening at z ∼ 1 to 2 in the field galaxy population. Using simulated deep SIRTF/IRAC observations of starbursts and Lyman-break galaxies, we investigate how an extension of the wavelength dynamic range will decrease the uncertainties in the stellar mass estimate, and find that they will reduce for both classes to factors of 2−3, comparable to what found for E/S0s and good enough for statistically reliable determinations of the galaxy evolutionary mass functions.