[en] We study the spatial distribution of faint satellites of intermediate redshift (0.1 < z < 0.8), early-type galaxies, selected from the GOODS fields. We combine high-resolution Hubble Space Telescope images and state-of-the-art host subtraction techniques to detect satellites of unprecedented faintness and proximity to intermediate redshift host galaxies (up to 5.5 mag fainter and as close as 0.''5/2.5 kpc to the host centers). We model the spatial distribution of objects near the hosts as a combination of an isotropic, homogeneous background/foreground population and a satellite population with a power-law radial profile and an elliptical angular distribution. We detect a significant population of satellites (N_s = 1.7^+0.9_-0.8) that is comparable to the number of Milky Way satellites with similar host-satellite contrast. The average projected radial profile of the satellite distribution is isothermal (γ_p = -1.0^+0.3_-0.4), which is consistent with the observed central mass density profile of massive early-type galaxies. Furthermore, the satellite distribution is highly anisotropic (isotropy is ruled out at a >99.99% confidence level). Defining φ to be the offset between the major axis of the satellite spatial distribution and the major axis of the host light profile, we find a maximum posterior probability of φ = 0 and |φ| less than 42⁰ at the 68% confidence level. The alignment of the satellite distribution with the light of the host is consistent with simulations, assuming that light traces mass for the host galaxy as observed for lens galaxies. The anisotropy of the satellite population enhances its ability to produce the flux ratio anomalies observed in gravitationally lensed quasars.

[en] Modern galaxy surveys produce redshift probability density functions (PDFs) in addition to traditional photometric redshift (photo-z) point estimates. However, the storage of photo-z PDFs may present a challenge with increasingly large catalogs, as we face a trade-off between the accuracy of subsequent science measurements and the limitation of finite storage resources. This work presents qp, a Python package for manipulating parameterizations of one-dimensional PDFs, as suitable for photo-z PDF compression. We use qp to investigate the performance of three simple PDF storage formats (quantiles, samples, and step functions) as a function of the number of stored parameters on two realistic mock data sets, representative of upcoming surveys with different data qualities. We propose some best practices for choosing a photo-z PDF approximation scheme and demonstrate the approach on a science case using performance metrics on both ensembles of individual photo-z PDFs and an estimator of the overall redshift distribution function. We show that both the properties of the set of PDFs we wish to approximate and the fidelity metric(s) chosen affect the optimal parameterization. Additionally, we find that quantiles and samples outperform step functions, and we encourage further consideration of these formats for PDF approximation.

[en] We infer the normalization and the radial and angular distributions of the number density of satellites of massive galaxies (log₁₀[M*_h/M_☉] > 10.5) between redshifts 0.1 and 0.8 as a function of host stellar mass, redshift, morphology, and satellite luminosity. Exploiting the depth and resolution of the COSMOS Hubble Space Telescope images, we detect satellites up to 8 mag fainter than the host galaxies and as close as 0.3 (1.4) arcsec (kpc). Describing the number density profile of satellite galaxies to be a projected power law such that P(R)∝R^γ_p, we find γ_p = –1.1 ± 0.3. We find no dependency of γ_p on host stellar mass, redshift, morphology, or satellite luminosity. Satellites of early-type hosts have angular distributions that are more flattened than the host light profile and are aligned with its major axis. No significant average alignment is detected for satellites of late-type hosts. The number of satellites within a fixed magnitude contrast from a host galaxy is dependent on its stellar mass, with more massive galaxies hosting significantly more satellites. Furthermore, high-mass late-type hosts have significantly fewer satellites than early-type galaxies of the same stellar mass, possibly indicating that they reside in more massive halos. No significant evolution in the number of satellites per host is detected. The cumulative luminosity function of satellites is qualitatively in good agreement with that predicted using SubHalo Abundance Matching techniques. However, there are significant residual discrepancies in the absolute normalization, suggesting that properties other than the host galaxy luminosity or stellar mass determine the number of satellites.

[en] Stars and dark matter account for most of the mass of early-type galaxies, but uncertainties in the stellar population and the dark matter profile make it challenging to distinguish between the two components. Nevertheless, precise observations of stellar and dark matter are extremely valuable for testing the many models of structure formation and evolution. We present a measurement of the stellar mass and inner slope of the dark matter halo of a massive early-type galaxy at z = 0.222. The galaxy is the foreground deflector of the double Einstein ring gravitational lens system SDSSJ0946+1006, also known as the 'Jackpot'. By combining the tools of lensing and dynamics we first constrain the mean slope of the total mass density profile (ρ_tot∝r^-γ') within the radius of the outer ring to be γ' = 1.98 ± 0.02 ± 0.01. Then we obtain a bulge-halo decomposition, assuming a power-law form for the dark matter halo. Our analysis yields γ_DM = 1.7 ± 0.2 for the inner slope of the dark matter profile, in agreement with theoretical findings on the distribution of dark matter in ellipticals, and a stellar mass from lensing and dynamics M^LD_* = 5.5_–1.3^+0.4 × 10¹¹ M_☉. By comparing this measurement with stellar masses inferred from stellar population synthesis fitting we find that a Salpeter initial mass function (IMF) provides a good description of the stellar population of the lens while the probability of the IMF being heavier than Chabrier is 95%. Our data suggest that growth by accretion of small systems from a compact red nugget is a plausible formation scenario for this object.

[en] In this paper, we describe a procedure for modelling strong lensing galaxy clusters with parametric methods, and to rank models quantitatively using the Bayesian evidence. We use a publicly available Markov chain Monte-Carlo (MCMC) sampler ('bayesys'), allowing us to avoid local minima in the likelihood functions. To illustrate the power of the MCMC technique, we simulate three clusters of galaxies, each composed of a cluster-scale halo and a set of perturbing galaxy-scale subhalos. We ray-trace three light beams through each model to produce a catalogue of multiple images, and then use the MCMC sampler to recover the model parameters in the three different lensing configurations. We find that, for typical Hubble Space Telescope (HST)-quality imaging data, the total mass in the Einstein radius is recovered with ∼1-5% error according to the considered lensing configuration. However, we find that the mass of the galaxies is strongly degenerated with the cluster mass when no multiple images appear in the cluster centre. The mass of the galaxies is generally recovered with a 20% error, largely due to the poorly constrained cut-off radius. Finally, we describe how to rank models quantitatively using the Bayesian evidence. We confirm the ability of strong lensing to constrain the mass profile in the central region of galaxy clusters in this way. Ultimately, such a method applied to strong lensing clusters with a very large number of multiple images may provide unique geometrical constraints on cosmology. The implementation of the MCMC sampler used in this paper has been done within the framework of the lenstool software package, which is publicly available

[en] We use stellar dynamics, strong lensing, stellar population synthesis models, and weak lensing shear measurements to constrain the dark matter (DM) profile and stellar mass in a sample of 53 massive early-type galaxies. We explore three DM halo models (unperturbed Navarro, Frenk, and White (NFW) halos and the adiabatic contraction models of Blumenthal and Gnedin) and impose a model for the relationship between the stellar and virial mass (i.e., a relationship for the star formation efficiency as a function of halo mass). We show that, given our model assumptions, the data clearly prefer a Salpeter-like initial mass function (IMF) over a lighter IMF (e.g., Chabrier or Kroupa), irrespective of the choice of DM halo. In addition, we find that the data prefer at most a moderate amount of adiabatic contraction (Blumenthal adiabatic contraction is strongly disfavored) and are only consistent with no adiabatic contraction (i.e., an NFW halo) if a mass-dependent IMF is assumed, in the sense of a more massive normalization of the IMF for more massive halos.

[en] Strong gravitational lens systems with measured time delays between the multiple images provide a method for measuring the 'time-delay distance' to the lens, and thus the Hubble constant. We present a Bayesian analysis of the strong gravitational lens system B1608+656, incorporating (1) new, deep Hubble Space Telescope (HST) observations, (2) a new velocity-dispersion measurement of 260 ± 15 km s^-1 for the primary lens galaxy, and (3) an updated study of the lens' environment. Our analysis of the HST images takes into account the extended source surface brightness, and the dust extinction and optical emission by the interacting lens galaxies. When modeling the stellar dynamics of the primary lens galaxy, the lensing effect, and the environment of the lens, we explicitly include the total mass distribution profile logarithmic slope γ' and the external convergence κ_ext; we marginalize over these parameters, assigning well-motivated priors for them, and so turn the major systematic errors into statistical ones. The HST images provide one such prior, constraining the lens mass density profile logarithmic slope to be γ' = 2.08 ± 0.03; a combination of numerical simulations and photometric observations of the B1608+656 field provides an estimate of the prior for κ_ext: 0.10^+0.08_-0.05. This latter distribution dominates the final uncertainty on H₀. Fixing the cosmological parameters at Ω_m = 0.3, Ω_Λ = 0.7, and w = -1 in order to compare with previous work on this system, we find H₀ = 70.6^+3.1_-3.1 km s^-1 Mpc^-1. The new data provide an increase in precision of more than a factor of 2, even including the marginalization over κ_ext. Relaxing the prior probability density function for the cosmological parameters to that derived from the Wilkinson Microwave Anisotropy Probe (WMAP) five-year data set, we find that the B1608+656 data set breaks the degeneracy between Ω_m and Ω_Λ at w = -1 and constrains the curvature parameter to be -0.031 < Ω_k < 0.009 (95% CL), a level of precision comparable to that afforded by the current Type Ia SNe sample. Asserting a flat spatial geometry, we find that, in combination with WMAP, H₀ = 69.7^+4.9_-5.0 km s^-1 Mpc^-1 and w = -0.94^+0.17_-0.19 (68% CL), suggesting that the observations of B1608+656 constrain w as tightly as the current Baryon Acoustic Oscillation data do.

[en] We infer gravitational lensing shear and convergence fields from galaxy ellipticity catalogs under a spatial process prior for the lensing potential. We demonstrate the performance of our algorithm with simulated Gaussian-distributed cosmological lensing shear maps and a reconstruction of the mass distribution of the merging galaxy cluster Abell 781 using galaxy ellipticities measured with the Deep Lens Survey. Given interim posterior samples of lensing shear or convergence fields on the sky, we describe an algorithm to infer cosmological parameters via lens field marginalization. In the most general formulation of our algorithm we make no assumptions about weak shear or Gaussian-distributed shape noise or shears. Because we require solutions and matrix determinants of a linear system of dimension that scales with the number of galaxies, we expect our algorithm to require parallel high-performance computing resources for application to ongoing wide field lensing surveys.

[en] We present a joint gravitational lensing and near-infrared study of the galaxy cluster Abell 3192 (A3192) that has been associated both with galaxies at z = 0.168 and with the X-ray luminous cluster RXC J0358.8–2955 (RXC J0358) at z = 0.425. Weak-lensing analysis of our Hubble Space Telescope snapshot observation with the Advanced Camera for Surveys detects two mass over-densities separated by ∼2 arcmin, one adjacent to the optical position of A3192 (4.4σ significance) and the other adjacent to the X-ray position of RXC J0358 (6.2σ significance). These mass peaks coincide with peaks in the K-band luminosity density of galaxies with near-infrared colors consistent with the red sequence at z = 0.168 and z 0.425, respectively. Moreover, the Bayesian evidence of parameterized mass models that include two cluster/group-scale halos centered on the respective mass peaks exceeds that of single-halo models by a factor of ≥10. The total projected mass of each galaxy system within 250 kpc of the respective peaks is M_WL(z = 0.168) ≅ 3 × 10¹³ M_☉ and M_WL(z = 0.425) ≅ 1.2 × 10¹⁴ M_☉, both with total mass-to-light ratios of M_WL/L_K ≅ 20 M_☉/L_☉. The original Abell cluster therefore comprises two independent galaxy systems—a foreground group at z = 0.168 and RXC J0358 at z = 0.425. Our results demonstrate the power of combining X-ray, near-infrared, and weak-lensing observations to select massive clusters, place those clusters and interloper galaxy systems along the line of sight, and measure their masses. This combination will be invaluable to robust interpretation of future high-redshift cluster surveys, including eROSITA.

[en] We measure quasar variability using the Panoramic Survey Telescope and Rapid Response System 1 Survey (Pan-STARRS1 or PS1) and the Sloan Digital Sky Survey (SDSS) and establish a method of selecting quasars via their variability in 10⁴ deg² surveys. We use 10⁵ spectroscopically confirmed quasars that have been well measured in both PS1 and SDSS and take advantage of the decadal timescales that separate SDSS measurements and PS1 measurements. A power law model fits the data well over the entire time range tested, 0.01-10 yr. Variability in the current PS1-SDSS data set can efficiently distinguish between quasars and nonvarying objects. It improves the purity of a griz quasar color cut from 4.1% to 48% while maintaining 67% completeness. Variability will be very effective at finding quasars in data sets with no u band and in redshift ranges where exclusively photometric selection is not efficient. We show that quasars' rest-frame ensemble variability, measured as a root mean squared in Δ magnitudes, is consistent with V(z, L, t) = A ₀(1 + z)^0.37(L/L ₀)^–0.16(t/1 yr)^0.246, where L ₀ = 10⁴⁶ erg s^–1 and A ₀ = 0.190, 0.162, 0.147, or 0.141 in the g _P1, r _P1, i _P1, or z _P1filter, respectively. We also fit across all four filters and obtain median variability as a function of z, L, and λ as V(z, L, λ, t) = 0.079(1 + z)^0.15(L/L ₀)^–0.2(λ/1000 nm)^–0.44(t/1 yr)^0.246.