Velocity Dispersion Estimates Research Articles

Evidence for Dynamically Driven Formation of the GW170817 Neutron Star Binary in NGC 4993

We present a study of NGC 4993, the host galaxy of the GW170817 gravitational wave event, the GRB170817A short gamma-ray burst (sGRB) and the AT2017gfo kilonova. We use Dark Energy Camera imaging, AAT spectra and publicly available data, relating our findings to binary neutron star (BNS) formation scenarios and merger delay timescales. NGC4993 is a nearby (40 Mpc) early-type galaxy, with $i$-band S\'ersic index $n=4.0$ and low asymmetry ($A=0.04\pm 0.01$). These properties are unusual for sGRB hosts. However, NGC4993 presents shell-like structures and dust lanes indicative of a recent galaxy merger, with the optical transient located close to a shell. We constrain the star formation history (SFH) of the galaxy assuming that the galaxy merger produced a star formation burst, but find little to no on-going star formation in either spatially-resolved broadband SED or spectral fitting. We use the best-fit SFH to estimate the BNS merger rate in this type of galaxy, as $R_{NSM}^{gal}= 5.7^{+0.57}_{-3.3} \times 10^{-6} {\rm yr}^{-1}$. If star formation is the only considered BNS formation scenario, the expected number of BNS mergers from early-type galaxies detectable with LIGO during its first two observing seasons is $0.038^{+0.004}_{-0.022}$, as opposed to $\sim 0.5$ from all galaxy types. Hypothesizing that the binary system formed due to dynamical interactions during the galaxy merger, the subsequent time elapsed can constrain the delay time of the BNS coalescence. By using velocity dispersion estimates and the position of the shells, we find that the galaxy merger occurred $t_{\rm mer}\lesssim 200~{\rm Myr}$ prior to the BNS coalescence.

Compact Groups analysis using weak gravitational lensing

We present a weak lensing analysis of a sample of SDSS Compact Groups (CGs). Using the measured radial density contrast profile, we derive the average masses under the assumption of spherical symmetry, obtaining a velocity dispersion for the Singular Isothermal Spherical model, $\sigma_V = 270 \pm 40 \rm ~km~s^{-1}$, and for the NFW model, $R_{200}=0.53\pm0.10\,h_{70}^{-1}\,\rm Mpc$. We test three different definitions of CGs centres to identify which best traces the true dark matter halo centre, concluding that a luminosity weighted centre is the most suitable choice. We also study the lensing signal dependence on CGs physical radius, group surface brightness, and morphological mixing. We find that groups with more concentrated galaxy members show steeper mass profiles and larger velocity dispersions. We argue that both, a possible lower fraction of interloper and a true steeper profile, could be playing a role in this effect. Straightforward velocity dispersion estimates from member spectroscopy yields $\sigma_V \approx 230 \rm ~km~s^{-1}$ in agreement with our lensing results.

VLT/MAGELLAN SPECTROSCOPY OF 29 STRONG LENSING SELECTED GALAXY CLUSTERS

ABSTRACT We present an extensive spectroscopic follow-up campaign of 29 strong lensing (SL) selected galaxy clusters discovered primarily in the Second Red-Sequence Cluster Survey (RCS-2). Our spectroscopic analysis yields redshifts for 52 gravitational arcs present in the core of our galaxy clusters, which correspond to 35 distinct background sources that are clearly distorted by the gravitational potential of these clusters. These lensed galaxies span a wide redshift range of 0.8 ≤ z ≤ 2.9, with a median redshift of z s = 1.8 ± 0.1. We also measure reliable redshifts for 1004 cluster members, allowing us to obtain robust velocity dispersion measurements for 23 of these clusters, which we then use to determine their dynamical masses by using a simulation-based σ DM − M 200 scaling relation. The redshift and mass ranges covered by our SL sample are 0.22 ≤ z ≤ 1.01 and , respectively. We analyze and quantify some possible effects that might bias our mass estimates, such as the presence of substructure, the region where cluster members are selected for spectroscopic follow-up, the final number of confirmed members, and line-of-sight effects. We find that 10 clusters of our sample with N mem ≳ 20 show signs of dynamical substructure. However, the velocity data of only one system is inconsistent with a uni-modal distribution. We therefore assume that the substructures are only marginal and not of comparable size to the clusters themselves. Consequently, our velocity dispersion and mass estimates can be used as priors for SL mass reconstruction studies and also represent an important step toward a better understanding of the properties of the SL galaxy cluster population.

PECULIAR TRANSVERSE VELOCITIES OF GALAXIES FROM QUASAR MICROLENSING. TENTATIVE ESTIMATE OF THE PECULIAR VELOCITY DISPERSION AT Z ∼ 0.5

ABSTRACT We propose to use the flux variability of lensed quasar images induced by gravitational microlensing to measure the transverse peculiar velocity of lens galaxies over a wide range of redshift. Microlensing variability is caused by the motions of the observer, the lens galaxy (including the motion of the stars within the galaxy), and the source. Hence, its frequency is directly related to the galaxy’s transverse peculiar velocity. The idea is to count time-event rates (e.g., peak or caustic crossing rates) in the observed microlensing light curves of lensed quasars that can be compared with model predictions for different values of the transverse peculiar velocity. To compensate for the large timescale of microlensing variability, we propose to count and model the number of events in an ensemble of gravitational lenses. We develop the methodology to achieve this goal and apply it to an ensemble of 17 lensed quasar systems. In spite of the shortcomings of the available data, we have obtained tentative estimates of the peculiar velocity dispersion of lens galaxies at z ∼ 0.5, . Scaling at zero redshift, we derive , consistent with peculiar motions of nearby galaxies and with recent N-body nonlinear reconstructions of the Local Universe based on ΛCDM. We analyze the different sources of uncertainty of the method and find that for the present ensemble of 17 lensed systems the error is dominated by Poisson noise, but that for larger ensembles the impact of the uncertainty on the average stellar mass may be significant.

SPIDERS: the spectroscopic follow-up of X-ray-selected clusters of galaxies in SDSS-IV

SPIDERS (The SPectroscopic IDentification of eROSITA Sources) is a program dedicated to the homogeneous and complete spectroscopic follow-up of X-ray AGN and galaxy clusters over a large area ($\sim$7500 deg$^2$) of the extragalactic sky. SPIDERS is part of the SDSS-IV project, together with the Extended Baryon Oscillation Spectroscopic Survey (eBOSS) and the Time-Domain Spectroscopic Survey (TDSS). This paper describes the largest project within SPIDERS before the launch of eROSITA: an optical spectroscopic survey of X-ray selected, massive ($\sim 10^{14}$ to $10^{15}~M_{\odot}$) galaxy clusters discovered in ROSAT and XMM-Newton imaging. The immediate aim is to determine precise ($\Delta_z \sim 0.001$) redshifts for 4,000-5,000 of these systems out to $z \sim 0.6$. The scientific goal of the program is precision cosmology, using clusters as probes of large-scale structure in the expanding Universe. We present the cluster samples, target selection algorithms and observation strategies. We demonstrate the efficiency of selecting targets using a combination of SDSS imaging data, a robust red-sequence finder and a dedicated prioritization scheme. We describe a set of algorithms and work-flow developed to collate spectra and assign cluster membership, and to deliver catalogues of spectroscopically confirmed clusters. We discuss the relevance of line-of-sight velocity dispersion estimators for the richer systems. We illustrate our techniques by constructing a catalogue of 230 spectroscopically validated clusters ($0.031 < z < 0.658$), found in pilot observations. We discuss two potential science applications of the SPIDERS sample: the study of the X-ray luminosity-velocity dispersion ($L_X-\sigma$) relation and the building of stacked phase-space diagrams.

The mass-sheet degeneracy and time-delay cosmography: analysis of the strong lens RXJ1131-1231

We present extended modelling of the strong lens system RXJ1131-1231 with archival data in two HST bands in combination with existing line-of-sight contribution and velocity dispersion estimates. Our focus is on source size and its influence on time-delay cosmography. We therefore examine the impact of mass-sheet degeneracy and especially the degeneracy pointed out by Schneider & Sluse (2013) [1] using the source reconstruction scale. We also extend on previous work by further exploring the effects of priors on the kinematics of the lens and the external convergence in the environment of the lensing system. Our results coming from RXJ1131-1231 are given in a simple analytic form so that they can be easily combined with constraints coming from other cosmological probes. We find that the choice of priors on lens model parameters and source size are subdominant for the statistical errors for H0 measurements of this systems. The choice of prior for the source is sub-dominant at present (2% uncertainty on H0) but may be relevant for future studies. More importantly, we find that the priors on the kinematic anisotropy of the lens galaxy have a significant impact on our cosmological inference. When incorporating all the above modeling uncertainties, we find H0 = 86.6+6.8−6.9 km s−1 Mpc−1, when using kinematic priors similar to other studies. When we use a different kinematic prior motivated by Barnabè et al. (2012) [2] but covering the same anisotropic range, we find H0 = 74.5+8.0−7.8 km s−1 Mpc−1. This means that the choice of kinematic modeling and priors have a significant impact on cosmographic inferences. The way forward is either to get better velocity dispersion measures which would down weight the impact of the priors or to construct physically motivated priors for the velocity dispersion model.

Blue straggler star populations in globular clusters – II. Proper-motion cleaned HST catalogues of BSSs in 38 Galactic GCs

We present new Blue Straggler Star (BSS) catalogs in 38 Milky Way globular clusters (GCs) based on multi-passband and multi-epoch treasury survey data from the Hubble Space Telescope. We measure precise astrometry and relative proper motions of stars in all target clusters and performed a subsequent cluster membership selection. We study the accuracy of our proper motion measurements using estimates of central velocity dispersions and find very good agreement with previous studies in the literature. Finally, we present a homogeneous BSS selection method, that expands the classic BSS selection parameter space to more evolved BSS evolutionary stages. We apply this method to the proper-motion cleaned GC star catalogs in order to define proper-motion cleaned BSS catalogs in all 38 GCs, which we make publicly available to enable further study and follow-up observations.

Frequency dependent elastic impedance inversion for interstratified dispersive elastic parameters

The elastic impedance equation is extended to frequency dependent elastic impedance equation by taking partial derivative to frequency. With this equation as the forward solver, a practical frequency dependent elastic impedance inversion approach is presented to implement the estimation of the interstratified dispersive elastic parameters which makes full use of the frequency information of elastic impedances. Three main steps are included in this approach. Firstly, the elastic Bayesian inversion is implemented for the estimation of elastic impedances from different incident angle. Secondly, with those estimated elastic impedances, their variations are used to estimate P-wave velocity and S-wave velocity. Finally, with the prior elastic impedance and P-wave and S-wave velocity information, the frequency dependent elastic variation with incident angle inversion is presented for the estimation of the interstratified elastic parameters. With this approach, the interstratified elastic parameters rather than the interface information can be estimated, making easier the interpretation of frequency dependent seismic attributes. The model examples illustrate the feasibility and stability of the proposed method in P-wave velocity dispersion and S-wave velocity dispersion estimation. The field data example validates the possibility and efficiency in hydrocarbon indication of the estimated P-wave velocity dispersion and S-wave velocity dispersion.

Quantification of microcirculatory parameters by joint analysis of flow-compensated and non-flow-compensated intravoxel incoherent motion (IVIM) data.

The aim of this study was to improve the accuracy and precision of perfusion fraction and blood velocity dispersion estimates in intravoxel incoherent motion (IVIM) imaging, using joint analysis of flow‐compensated and non‐flow‐compensated motion‐encoded MRI data. A double diffusion encoding sequence capable of switching between flow‐compensated and non‐flow‐compensated encoding modes was implemented. In vivo brain data were collected in eight healthy volunteers and processed using the joint analysis. Simulations were used to compare the performance of the proposed analysis method with conventional IVIM analysis. With flow compensation, strong rephasing was observed for the in vivo data, approximately cancelling the IVIM effect. The joint analysis yielded physiologically reasonable perfusion fraction maps. Estimated perfusion fractions were 2.43 ± 0.81% in gray matter, 1.81 ± 0.90% in deep gray matter, and 1.64 ± 0.72% in white matter (mean ± SD, n = 8). Simulations showed improved accuracy and precision when using joint analysis of flow‐compensated and non‐flow‐compensated data, compared with conventional IVIM analysis. Double diffusion encoding with flow compensation was feasible for in vivo imaging of the perfusion fraction in the brain. The strong rephasing implied that blood flowing through the cerebral microvascular system was closer to the ballistic limit than the diffusive limit. © 2016 The Authors NMR in Biomedicine published by John Wiley & Sons Ltd.

HeCS-SZ: THE HECTOSPEC SURVEY OF SUNYAEV–ZELDOVICH-SELECTED CLUSTERS

ABSTRACT We estimate cluster masses and velocity dispersions for 123 clusters from optical spectroscopy to compare the Sunyaev–Zeldovich (SZ) mass proxy and dynamical masses. Our new survey, HeCS-SZ (Hectospec Cluster Survey of SZ-selected clusters), includes 7721 new or remeasured redshifts from MMT/Hectospec observations of 21 SZ-selected clusters at redshifts z = 0.05–0.20. We supplement the Hectospec data with spectra from the Sloan Digital Sky Survey (SDSS) and cluster data from the Cluster Infall Regions in SDSS project and the Hectospec Cluster Survey, our Hectospec survey of clusters selected by X-ray flux. We measure the scaling relation between velocity dispersion and SZ mass estimates from the integrated Compton parameter for an SZ-complete sample of 83 clusters. The observed relation agrees very well with a simple virial scaling from mass (based on SZ) to velocity dispersion. The SZ mass estimates (calibrated with hydrostatic X-ray mass estimates) are not significantly biased compared to dynamical mass estimates under the assumption of small velocity bias of galaxies compared to dark matter particles. Significant mass bias in SZ mass estimates could relieve tension between cosmological results from Planck SZ cluster counts and Planck CMB data. In principle, SZ mass bias and velocity bias of galaxies could conspire to yield good agreement, but the required velocity bias is &sgr; galaxy &approx; 0.77 &sgr; DM ?> , outside the range of recent models of velocity bias in the literature. More likely, SZ mass bias and velocity bias are both small, and the tension between SZ cluster counts and CMB data requires another explanation.

Galaxy luminosity functions in WINGS clusters

Using V band photometry of the WINGS survey, we derive galaxy luminosity functions (LF) in nearby clusters. This sample is complete down to Mv=-15.15, and it is homogeneous, thus allowing the study of an unbiased sample of clusters with different characteristics. We constructed the photometric LF for 72 out of the original 76 WINGS clusters, excluding only those without a velocity dispersion estimate. For each cluster we obtained the LF for galaxies in a region of radius=0.5 x r200, and fitted them with single and double Schechter's functions. We also derive the composite LF for the entire sample, and those pertaining to different morphological classes. Finally we derive the spectroscopic cumulative LF for 2009 galaxies that are cluster members. The double Schechter fit parameters are neither correlated with the cluster velocity dispersion, nor with the X-ray luminosity. Our median values of the Schechter's fit slope are, on average, in agreement with measurements of nearby clusters, but are less steep that those derived from large surveys, such as the SDSS. Early--type galaxies outnumber late-types at all magnitudes, but both early and late types contribute equally to the faint end of the LF. Finally, the spectroscopic LF is in excellent agreement with the ones derived for A2199, A85 and Virgo, and with the photometric one at the bright magnitudes (where both are available). There is a large spread in the LF of different clusters. However, this spread is not caused by correlation of the LF shape with cluster characteristics such as X--ray luminosity or velocity dispersions. The faint end is flatter than what previously derived (alpha_f=-1.7) at odds with what predicted from numerical simulations.

THE GIANT MOLECULAR CLOUD ENVIRONMENTS OF INFRARED DARK CLOUDS

We study Giant Molecular Cloud (GMC) environments surrounding 10 Infrared Dark Clouds (IRDCs), using $^{13}$CO(1-0) emission from the Galactic Ring Survey. We measure physical properties of these IRDCs/GMCs on a range of scales extending to radii, R, of 30 pc. By comparing different methods for defining cloud boundaries and for deriving mass surface densities and velocity dispersions, we settle on a preferred "CE,$\tau$,G" method of "Connected Extraction" in position-velocity space plus Gaussian fitting to opacity-corrected line profiles for velocity dispersion and mass estimation. We examine how cloud definition affects measurements of the magnitude and direction of line-of-sight velocity gradients and velocity dispersions, including associated dependencies on size scale. CE,$\tau$,G-defined GMCs show velocity dispersion versus size relations $\sigma\propto{s}^{1/2}$, which are consistent with the large-scale gradients being caused by turbulence. However, IRDCs have velocity dispersions that are moderately enhanced above those predicted by this scaling relation. We examine the dynamical state of the clouds finding mean virial parameters $\bar{\alpha}_{\rm{vir}}\simeq 1.0$ for GMCs and 1.6 for IRDCs, broadly consistent with models of magnetized virialized pressure-confined polytropic clouds, but potentially indicating that IRDCs have more disturbed kinematics. CE,$\tau$,G-defined clouds exhibit a tight correlation of $\sigma/R^{1/2}\propto\Sigma^n$, with $n\simeq0.7$ for GMCs and 1.3 for IRDCs (c.f., a value of 0.5 expected for a population of virialized clouds). We conclude that while GMCs show evidence for virialization over a range of scales, IRDCs may be moderately super virial. Alternatively, IRDCs could be virialized but have systematically different $^{13}$CO gas phase abundances, i.e., due to freeze-out, affecting mass estimations.

HERSCHELDUST EMISSION AS A PROBE OF STARLESS CORES MASS: MCLD 123.5+24.9 OF THE POLARIS FLARE

We present newly processed archival Herschel images of molecular cloud MCLD 123.5+24.9 in the Polaris Flare. This cloud contains five starless cores. Using the spectral synthesis code Cloudy, we explore uncertainties in the derivation of column densities, hence, masses of molecular cores from Herschel data. We first consider several detailed grain models that predict far-IR grain opacities. Opacities predicted by the models differ by more than a factor of two, leading to uncertainties in derived column densities by the same factor. Then we consider uncertainties associated with the modified blackbody fitting process used by observers to estimate column densities. For high column density clouds (N(H) $\gg$ 10$^{22}$ cm$^{-2}$), this fitting technique can underestimate column densities by about a factor of three. Finally, we consider the virial stability of the five starless cores in MCLD 123.5+24.9. All of these cores appear to have strongly sub-virial masses, assuming, as we argue, that $^{13}$CO line data provide reliable estimates of velocity dispersions. Evidently, they are not self-gravitating, so it is no surprise that they are starless.

Kinematics and stellar disk modeling of lenticular galaxies

We present the results of spectroscopic observations of three S0-Sa galaxies: NGC 338, NGC 3245, and NGC 5440 at the SAO RAS 6-m BTA telescope. The radial distributions of the line-ofsight velocities and radial velocity dispersions of stars and ionized gas were obtained, and rotation curves of galaxies were computed. We construct the numerical dynamic N-body galaxy models with N ≥ 106 points. The models include three components: a “live” bulge, a collisionless disk, dynamically evolving to the marginally stable state, and a pseudo-isothermal dark halo. The estimates of radial velocities and velocity dispersions of stars obtained from observations are compared with model estimates, projected onto the line of sight. We show that the disks of NGC 5440 and the outer regions of NGC 338 are dynamically overheated. Taking into account the previously obtained observations, we conclude that the dynamic heating of the disk is present in a large number of early-type disk galaxies, and it seems to ensue from the external effects. The estimates of the disk mass and relative mass of the dark halo are given, as well as the disk mass-to-luminosity ratio for seven galaxies, observed at the BTA.

PROPER MOTIONS OF THE ARCHES CLUSTER WITH KECK LASER GUIDE STAR ADAPTIVE OPTICS: THE FIRST KINEMATIC MASS MEASUREMENT OF THE ARCHES

We report the first detection of the intrinsic velocity dispersion of the Arches cluster - a young (~2 Myr), massive (~10,000 Solar Mass) starburst cluster located near the Galactic center. This was accomplished using proper motion measurements within the central region of the cluster, obtained with the laser guide star adaptive optics system at Keck Observatory over a 3 year time baseline (2006-2009). This uniform dataset results in proper motion measurements that are improved by a factor ~5 over previous measurements from heterogeneous instruments, yielding internal velocity dispersion estimates 0.15 +/- 0.01 mas/yr, which corresponds to 5.4 +/- 0.4 km/s at a distance of 8.4 kpc. Projecting a simple model for the cluster onto the sky to compare with our proper motion dataset, in conjunction with surface density data, we estimate the total present-day mass of the cluster to be 15,000 (+7400 -6000) Solar masses. The mass in stars observed within a cylinder of radius R=0.4 pc is found to be 9000 (+4000 -3500) Solar Masses at formal 3-sigma confidence. This mass measurement is free from assumptions about the mass function of the cluster, and thus may be used to check mass estimates from photometry and simulation. When we conduct this check, we find that the present-day mass function of the Arches cluster is likely either top-heavy or truncated at low-mass, or both. Collateral benefits of our data and analysis include: 1. cluster membership probabilities, which may be used to extract a clean cluster sample for future photometric work; 2. a refined estimate of the bulk motion of the Arches cluster with respect to the field, which we find to be 172 +/- 15 km/s, which is slightly slower than suggested by previous VLT-Keck measurements; and 3. a velocity dispersion estimate for the field itself, which is likely dominated by the inner galactic bulge and the nuclear disk.

On the spectral resolution of the MILES stellar library

Empirical stellar libraries are extensively used to extract stellar kinematics in galaxies and to build stellar population models. An accurate knowledge of the spectral resolution of these libraries is critical to avoid propagation errors and uncertain estimates of the intrinsic stellar velocity dispersion of galaxies. In this research note we re-assess the spectral resolution of the MILES stellar library and of the stellar population models based on it. This exercise was performed, because of a recent controversy over the exact MILES resolution. We perform our test through the comparison of MILES stellar spectra with three different sets of higher-resolution templates, one fully theoretical - the MARCS library - and two empirical ones, namely the Indo-U.S. and ELODIE v3.1 libraries. The theoretical template has a well-defined very high (R=20000) resolution. Hence errors on this theoretical value do not affect our conclusions. Our approach based on the MARCS library was crucial to constrain the values of the resolution also for the other two empirical templates. We find that the MILES resolution has previously been slightly overestimated. We derive a new spectral resolution of 2.54 A FWHM, instead of the nominal 2.3 A. The reason for this difference is due to an overestimation of the resolution for the Indo-U.S. library that was previously used for estimates of the MILES resolution. For the Indo-U.S. we obtain a new value of 1.35 A FWHM. Most importantly, the results derived from the MARCS and ELODIE libraries are in very good agreement. These results are important for users of the MILES spectra library and for further development of stellar population models aimed to obtain accurate stellar kinematics in galaxies.

Acoustic log simulation in a viscoelastic formation

Abstract It is known that sedimentary rocks demonstrate velocity dispersion in the acoustic log frequency range. In this paper we have calculated the waveforms of sonic log for a borehole located in a viscoelastic medium. The acoustic field in the borehole has been obtained for acoustic multipole sources. To describe the viscoelastic properties of a rock we used the Cole–Cole model. This model describes the dispersion of acoustic wave velocities and quality factors in a wide frequency range. To solve the acoustic log direct problem we have applied the double integral Fourier transform (RAI method). The results obtained have shown the feasibility of S-wave velocity dispersion estimation from acoustic dipole waveform processing.

PRESSURE SUPPORT IN GALAXY DISKS: IMPACT ON ROTATION CURVES AND DARK MATTER DENSITY PROFILES

Rotation curves constrain a galaxy's underlying mass density profile, under the assumption that the observed rotation produces a centripetal force that exactly balances the inward force of gravity. However, most rotation curves are measured using emission lines from gas, which can experience additional forces due to pressure. In realistic galaxy disks, the gas pressure declines with radius, providing additional radial support to the disk. The measured tangential rotation speed will therefore tend to lag the true circular velocity of a test particle. The gas pressure is dominated by turbulence, and we evaluate its likely amplitude from recent estimates of the gas velocity dispersion and surface density. We show that where the amplitude of the rotation curve is comparable to the characteristic velocities of the interstellar turbulence, pressure support may lead to underestimates of the mass density of the underlying dark matter halo and the inner slope of its density profile. These effects may be significant for galaxies with rotation speeds <75km/s, but are unlikely to be significant in higher mass galaxies. We find that pressure support can be sustained over long timescales, because any reduction in support due to the conversion of gas into stars is compensated for by an inward flow of gas. However, we point to many uncertainties in assessing the importance of pressure support in galaxies. Thus, while pressure support may alleviate possible tensions between rotation curve observations and LambdaCDM on kiloparsec scales, it should not be viewed as a definitive solution at this time.

Cluster galaxy dynamics and the effects of large-scale environment

We use a high-resolution N-body simulation to study how the influence of large-scale structure in and around clusters causes correlated signals in different physical probes and discuss some implications this has for multi-physics probes of clusters. We pay particular attention to velocity dispersions, matching galaxies to subhalos which are explicitly tracked in the simulation. We find that not only do halos persist as subhalos when they fall into a larger host, groups of subhalos retain their identity for long periods within larger host halos. The highly anisotropic nature of infall into massive clusters, and their triaxiality, translates into an anisotropic velocity ellipsoid: line-of-sight galaxy velocity dispersions for any individual halo show large variance depending on viewing angle. The orientation of the velocity ellipsoid is correlated with the large-scale structure, and thus velocity outliers correlate with outliers caused by projection in other probes. We quantify this orientation uncertainty and give illustrative examples. Such a large variance suggests that velocity dispersion estimators will work better in an ensemble sense than for any individual cluster, which may inform strategies for obtaining redshifts of cluster members. We similarly find that the ability of substructure indicators to find kinematic substructures is highly viewing angle dependent. While groups of subhalos which merge with a larger host halo can retain their identity for many Gyr, they are only sporadically picked up by substructure indicators. (Abridged)

DARK MATTER DISTRIBUTION IN GALAXY GROUPS FROM COMBINED STRONG LENSING AND DYNAMICS ANALYSIS

Using a combined analysis of strong lensing and galaxy dynamics, we characterize the mass distributions and M/L ratios of galaxy groups, which form an important transition regime in Lambda-CDM cosmology. By mapping the underlying mass distribution, we test whether groups are dark matter dominated as hypothesized by the standard cosmogony, or isothermal as observed in baryon rich field galaxies. We present our lensing + galaxy dynamics formalism built around the dark matter dominant NFW and Hernquist distributions, compared against the Isothermal Sphere observed in galaxy scale objects. We show that mass measurement in the core of the group (r ~ 0.2 r_{vir}), determined jointly from a lens model and from differential velocity dispersion estimates, may effectively distinguish between these density distributions. We apply our method to MOS observations of two groups, SL2SJ1430+5546 and SL2SJ1431+5533, drawn from our CFHTLS lens catalog. With the measured lensing and dynamical masses, combined with a maximum likelihood estimator built around our model, we estimate the concentration index characterizing each density distribution and the corresponding virial mass of each group. Our results indicate that both groups are dark matter dominant, and reject the Isothermal distribution at >>3 sigma level. For both groups, the estimated i-band M/L ratios of ~260 Msun/Lsun, are similar to other published values for groups. The Gaussian distributions of the velocities of their member galaxies support a high degree of virialization. The differences in their virial masses, 2.8 and 1.6 x 10^14 Msun, and velocity dispersions, 720 and 560 km/s respectively, may indicate however that each group is at a different stage of transition to a cluster. We aim to populate this important transition regime with additional results from ongoing observations of the remaining lensing groups in our catalog.