People's Interests

Dennis Zaritsky: I have worked on several related projects to this topic. On galaxy scales I have used satellite galaxies to explore the radial and azimuthal profiles of dark matter halos. Of particular current interest is the distribution of satellite galaxies around spiral galaxies. We are confirming the previous result that satellite galaxies prefer to lie along the poles of isolated spiral galaxies. This result runs counter to the simplest expectations of galaxy formation models, but those do not yet fully describe the formation of disks inside of halos. In the process of understanding why various studies have given conflicting results we've also learned alot about isolated galaxies and isolation criteria.

A second current interest is the packing of baryons in DM halos. We've found that with a slight modification to the Fundamental Plane formalism we can extend the range of spheroidal systems that lie on a tight relationship from the Local Group dwarf spheroidals to the intracluster stellar components of massive clusters. The mechanism for modulating M/L inside of R_e necessary to produce this relationship is unclear and must include various physical effects. The following figure shows this relationship, highlighting the Local Group dwarfs that were added to the existing relationship derived from more massive systems. With the exception of Ursa Minor and Draco, which are the two dwarfs most often hypothesized to be undergoing significant tidal effects, all of the spheroids lie on this projection with a scatter that is only mildly larger than that of the typical FP relationship just for E's. The relationship spans systems with 0.1 kpc < Re < 1000 kpc. Related papers here and here.

Questions/Interests: I'm interested in the state of lensing as a technique to measure the radial and azimuthal DM properties in the full range of systems.

Tommaso Treu: my previous work and current interests on this topic include:
i) work on the mass density profile of elliptical galaxies, based on lensing and stellar dynamics. The most puzzling result from this work is the "bulge-halo" conspiracy (analog to the disk-halo conspiracy for spiral galaxies), i.e. early-type galaxies seem to have a mass density profile that is very close to r^-2 (i.e. ~2.0+-0.2) on scales between a kpc and 10-20 kpc. This behaviour could possibly extend to 100kpc or so as suggested by ongoing weak lensing measurements by Raphael Gavazzi and the SLACS team. It is unclear to me what is the detalied physical process behind this "conspiracy". I don't know whether cosmological simulations are detailed enough to have predictive power in this regime, and if so whether they find this conspiracy as well.
ii) the inner mass density profile of clusters. Are clusters DM halo really as steep as predicted by simulations? With David Sand, Richard Ellis and Graham Smith we studied a few systems with radial arcs and claimed they are not. There has been some controversy over this result, but it seems to me there are a few cases where dark matter halos are clearly shallow in the center (David will talk about this at the conference in early October). Traditional baryonic effects, as described by the "adiabatic contraction" recipe, would only exacerbate the problem. But it is not clear to me that this is the case, as the effect of baryons on the underlying halo can be in the opposite sense as well (e.g. Nipoti et al. 2004). Are clusters really not "cuspy" as predicted by dark matter only cosmological N-body simulations? What is the effect of baryons on the cusp?

Maruša Bradac:
Recently I have been mainly interested in:
i) Mass estimates for clusters using strong and weak lensing
My two "favourites" are the "bullet" cluster 1E0657-56 and another merging and very X-ray luminous cluster RXJ1347-1145.
In both sistems the masses obtained from weak lensing, strong lensing, dynamics and X-ray all disagree. In order to resolve the issues I developed a combined strong and weak lensing "non"-parametric method to obtain reliable mass estimates. The related papers on the method can be found here, application to the RX J1347 is here, and to the bullet cluster here.
ii) Mass properties of galaxies using galaxy-galaxy lensing
This project is still at the very beginnings, currently I'm analysing the GEMS data (combined with COMBO-17 data to determine galaxy properties). I finished the analysis, have measured the galaxy-galaxy lensing signal in the data and am starting to form catalogs to investigate gg-lenisng signal as a function of galaxy color, redshift, morphology,.. Any input/suggestions are highly welcome
iii) Substructure in lens galaxies
I worked on the problem of missing satelites, specifically I was using N-body+hydro simulations to prtedict the signatures we expect to see in e.g. fluxes of a multiply imaged systems.

i) What is the mass distribution in clusters, how does density change with radius, can we measure the profile correctly, what is the origin of discrepant mass estimates from weak,strong lensing, xrays and dynamics?
ii) Scaling relations in galaxies as a function of redshift - e.g. how does concentration parameter change as a fct. of redshift? What is the shape of DM halo and how does it relate to the luminous one?

Raphaël Gavazzi: I have been working on the density profile of galaxy clusters (my favorite being MS2137) using strong and weak lensing data. I have looked at concerns arising when combining lensing and eg X-rays or stellar kinematics within the oversimplistic assumption of spherical symmetry and/or a Gaussian velocity distribution function (here or there ). Because we are very good friends, I also studied the MS2137 mass profile in the context of MOND phenomology and confirmed the general failure of MOND at clusters scales (here).
On larger scales, I studied the possibility to use weak lensing to build "mass-selected" samples of galaxy clusters (arXiv) to avoid various possible biases in standard methods relying on baryons physics (X-rays, SZ, optical...).
More recently I joined the SLACS team (cf Tommaso's interests) and I am measuring the weak lensing shear profile around SLACS strong lenses in order to combine both constraints and break degeneracies between stellar and DM contributions to the overall mass budget.
In this research line, my present interests have to do with the processes at work to shape the 'tight' relation/conspiracy between stars and CDM in ETGs (do they form/grow through dissipative/dissipationless mechanisms?)...
I'm also interested in the effect of environment on lensing properties.

Chris Fassnacht: I am interested in the mass profile of moderate-redshift ellipticals, having come into this area from a Hubble Constant perspective (the steeper the mass profile, the larger the value of the Hubble Constant, given a measurement of the time delays in the system). Toward this end, I have started working with Tommaso and Leon on combining stellar dynamics with strong lensing measurements to determine effective mass slopes in these systems (e.g., this paper). I am also interested in the immediate environment of strong lenses, in particular in searching for galaxy groups associated with strong lenses. These groups can affect the lensing properties of the system, but can also be used to investigate the properties of moderate-redshift groups selected in a somewhat unusual manner. My work in this direction involves optical spectroscopy (e.g., this or that paper) and Chandra X-ray observations.

Lori Lubin:
Lately, my work has focused on how accurately we can measure/trace the total (dark matter) potential in clusters using the baryons, either through the member galaxies (velocity dispersion) or the intracluster gas (X-ray temperature). In nearby clusters, both the galaxy velocity dispersion and the temperature of the gas have been used to measure the total mass assuming hydrostatic equilibrium among other things. When a cluster is well-relaxed, these measures are accurate. For clusters at z < 0.5, one actually finds the expected correlation of sigma^2 proportional to T, albeit with a large statistical scatter (Mushotzky & Scharf 1997; Horner 2001, PhD Thesis, U of Maryland - very nice sample if you can find it). Since my work focuses on higher redshift clusters, I am interested in (1) if and how these relations break down at z > 0.5; (2) are there systematic differences between X-ray and optically-selected clusters at high redshift; and (3) what physical processes are responsible for these differences. Based on a small sample of clusters observed with XMM, we find that our optically-selected clusters at z > 0.7 tend to be underluminous and cooler than their X-ray--selected counterparts (see Lx-sigma relation below and Lubin et al. 2004). Others have found such effects as well (e.g., Holden et al. 1997 from the PDCS clusters; Hicks et al. 2005 from the RCS clusters). These results can be understood in light of the fact that the vast majority of clusters at high redshift (and perhaps the vast majority of clusters in general) are not relaxed and still in the process of forming. These signatures range from signficiant substructure to major mergers (e.g. MS1054, CL 0152-1357). On the optical side, these processes can result in an inflated velocity dispersion due to infall or surround large scale structure. In fact, we find for CL1604+4304 at z equals 0.9, which is a member of a very large scale structure, when we increase our number of measured redshifts from 20 to 90+, the velocity dispersion drops by over 25%. On the X-ray side, merging can cause the temperature and luminosity of the gas component to be significantly boosted, with T/Lx changing by several factors during the merger (see e.g. Randall et al. 2002). Of course, understanding and quantifying these dynamical effects will have a profound influence on whether clusters can be used for precision cosmology (for an attempt at doing just that, see Allen et al. 2004).lx-sig-NNG.jpg

Ann Zabludoff:
I am currently pursuing my interest in the relationship of baryons to dark matter with three projects: the first explores the efficiency with which baryons are packed in dark matter halos, the second the mass profiles of the cores of rich clusters and how baryons may have altered the profile shape, and the third the environments of gravitational lenses, their evolution, and their effect on lens models.

With Anthony Gonzalez and Dennis Zaritsky, I have explored the "fundamental manifold", an extension of the fundamental plane formalism that encompasses the structural and kinematic properties of spheroids ranging from dwarf spheroidals to the intracluster light component of rich clusters --- over four magnitudes in r_e. This manifold arises principally from a continuous change in the mass-to-light ratio within r_e with spheroid scale; M_e/L_e is large in dSphs, smaller in E's, and larger again in the intracluster stellar spheroidal component. This change reflects variations in the efficiency with which the baryons are packed relative to the dark matter within r_e. The continuity of the galaxy locus on the fundamental manifold and, more specifically, the overlap on the fundamental manifold of dwarf ellipticals like M32 and dwarf spheroidals like Leo II, imply that dwarf spheroidals belong to the same family of spheroids as their more massive counterparts. Related papers are here and here. I am particularly interested in understanding the mechanisms (dissipative vs. non-dissipative, inflows/outflows, etc) that may be responsible, to differing degrees, for the continuity and variation in the baryon packing efficiency for all spheroidals embedded in dark matter halos.

With Daniel Kelson and Anthony Gonzalez, I have obtained deep, long-slit spectroscopy along the major axis of the central, dominant elliptical in seven groups and clusters in order to measure the kinematics of the intracluster stars out to ~100 kpc. In the first system we studied, NGC 6166 in Abell 2199, the velocity dispersion initially decreases from the central value of 300 km/s to 200 km/s within a few kpc then steadily rises to to 660 km/s at a radius of 60 kpc, nearly reaching the velocity dispersion of the cluster (775 km/s). These data suggest that the stars in the halo of the cD trace the potential of the cluster and that the kinematics of these intracluster stars can be used to constrain the mass profile of the cluster. In addition, we find evidence for systematic rotation (V/sigma ~ 0.3) in the intracluster stars beyond 20 kpc. Such rotation is not seen in the kinematics of the cluster members. The observed major-axis kinematics can be reproduced only if the halo, parameterized by a generalized-NFW profile, has a soft core, i.e., alpha < 1 (a generalized-NFW halo with alpha=1 is excluded due to low implied stellar mass-to-light ratios). This result is inconsistent with the predictions of current N-body simulations for dark matter halos. Check out this paper. Analysis of the other six systems will reveal if they are discrepant from CDM predictions as well. At any rate, these observations should provide a more detailed understanding of the effects of baryons in altering the core mass profiles of rich clusters.

With Chuck Keeton, Iva Momcheva, and Kurtis Williams, I am conducting a major survey of the environments of strong gravitational lensing galaxies. Previously, Chuck and I (here) predicted that the effects of environment, particularly when a hard-to-detect group or cluster lies at the lens redshift, lead to significant biases in the lens models. These biases often exceed the quoted random errors on important quantities derived from lens models, such as the Hubble constant, dark energy density, and distribution of dark matter in lens galaxy halos. Using extensive photometric and spectroscopic data acquired from more than 35 nights of 6.5m telescope time and 25 nights of 4m telescope time, we have now found that the majority of lenses lie in overdense environments that can significantly affect lens models. In addition, we use theoretical arguments to show that, in principle, the lens potential may be affected by line-of-sight structures over a wide range of spatial and redshift offsets from the lens. We then quantify real line-of-sight effects using this survey and find that 10-50% of lenses have interloping structures that significantly perturb the lens potential. These papers are here and here. I am particularly interested in using our knowledge of the lens environment to obtain better lensing predictions for cosmological quantities such as H_0 and Lambda, to test the CDM paradigm with improved mass profile and substructure constraints, and to measure the evolution of lens environments and their galaxies over the large redshift range probed by our sample (0.3-0.8).

Leon Koopmans:

My interests are in probing the inner to outer density profiles of galaxies through strong lensing, find evidence for DM around galaxies and quantify the DM haloes properties as function mass & redshift, and compare this the LCDM galaxy formation models. In particualr I'm intrigued by the fact that DM+baryons add to a dynamical isothermal profile. Why would the microphysics of star-formation care about the macro-physics of the DM potential in such a way that baryons stop cooling and form stars (collisionless) such that the galaxy becomes isothermal. What is special about isothermality? In addition, I'm interested in novel ways of quantifying CDM substructure lensing galaxies through non-parameteric reconstructions of strong lenses (e.g. Koopmans 2005) to test the low-mass end of the non-linear power-spectrum of the DM (or n(M)) as a test of DM properties and/or the effects of baryons in substructure survival. Both questions we are addressing through strong lenses, in particular (at present) those found in the Sloan Lens ACS Survey (SLACS), which currently composes a sample of ~50 early-type galaxies with HST imaging (snapshot and deeper multi-color imaging) and a subsample with Keck/VLT spectroscopy to obtain stellar kinematic information.

Giuseppe Bertin:
i) I have been working on a long-term project to construct physically justified self-consistent models of collisionless stellar systems able to match, at the same time, both the properties of quasi-equilibrium configurations produced by simulations of collisionless collapse and the properties of observed elliptical galaxies (e.g., see Trenti & Bertin 2005; Trenti et al. 2005). This type of modeling had led to the identification of dark halos around ellipticals (Bertin et al. 1994) with trends and regularities confirmed by more recent investigations. These studies appear to be relevant to high-luminosity ellipticals. Question: What about the formation and the structure of low-luminosity ellipticals?
ii) Studies of the process of dynamical friction. This study is carried out by means of dedicated simulations. With respect to many other investigations focusing on the capture by dynamical friction of a single satellite, we have considered the case of the capture of a shell of mini-satellites ("fragments"); the related simpler quasi-spherical environment allows us to study the relevant mechanisms in a better controlled environment. We find that the classical formulae of dynamical friction have only limited applicability and we demonstrate that the evolution induced on the collisionless underlying system, as a reaction to the effects of dynamical friction on the satellites, is a slow but non-adiabatic process (Bertin et al. 2003; Arena et al. 2006; Arena & Bertin 2006, submitted). Question: Can we find a reliable semi-empirical description of the process of dynamical friction beyond Chandrasekhar's formulation?
iii) The Fundamental Plane of early-type galaxies and the dark-luminous matter connection. In an earlier study, we had noted that an interpretation of the Fundamental Plane in terms of pure homology is not satisfactory (Bertin et al. 2002). Recently, taking advantage of the small scatter around the Fundamental Plane observed at relatively high redshift, we have suggested looking at the Fundamental Plane through a gravitational lens (Bertin & Lombardi 2006): the detection of the magnification effect of a lensing cluster will lead to simple and direct mass measurements, with interesting diagnostic characteristics. Question: Why do low and high luminosity ellipticals appear to stay within the same Plane, if the dynamical structure (and possibly the formation process) is likely to be different for the two classes of objects?

Matteo Barnabè:
I am interested in probing the mass density profile of early-type galaxies (with particular focus on the presence and shape of dark matter halos and their correlation with the total mass) and its evolution with time, and I am trying to achieve this goal by combining lensing and dynamics, working together with Leon Koopmans and the SLACS team.
I am currently working on developing a code to perform a joint and fully self-consistent analysis of lens early-type galaxies which makes full use of the information coming from both the gravitational lensing and the stellar dynamics (i.e the galaxy surface brightness distribution and the first and second velocity moments), under the assumption of axisymmetric potential. This code is going to be used in the next future to study the sample of lens galaxies from the SLACS survey.

Eric Agol
I am interested in the problem of using the flux ratios of strong gravitational lenses to place constraints on the dark matter potential. I have a program to observe strong gravitational lenses in the mid-infrared which is unaffected by microlensing, scintillation, or extinction, as well as having the advantage that 8-10 meter class telescopes are diffraction limited at mid-infrared wavelengths. I plan to spend my time at the KITP analyzing these data and carrying out simulations to place constraints on the clumpiness of the dark matter potential of the lensing galaxies.

Árdís Elíasdóttir
I am interested in studying mass density profiles of both galaxies and cluster, in particular with respect to the dark matter distribution. I'm working on modelling of clusters, trying to use strong lensing to constrain the mass profile.

Sherry Suyu
I am interested in probing the dark matter distribution using strong lensing. In particular, I will be helping the Strong Lensing Legacy Survey team to model the mass distribution of galaxy groups. This new population of lenses will hopefully tell us something about galaxy assembly.

Claudio Grillo:
I have been working mainly on mass estimates from strong gravitational lensing in high-redshift galaxies and clusters of galaxies. I am particularly interested in understanding:
1. the so-called “bulge-halo” conspiracy in elliptical galaxies. What is its origin? What is the intrinsic scatter around isothermality for the total (dark and luminous) density distribution in ellipticals?
2. the limits of the different total mass diagnostics (virial, X-ray, strong lensing, and weak lensing) in clusters of galaxies. At which level do the mass measurements agree in the “best sample” of clusters? What can we do in order to find more accurate mass estimates?

Magda Arnaboldi:
I have studied the connection between dark matter and baryons in "hot" stellar systems, i.e. elliptical galaxies in low and high density environments. My goals are to develop observational techniques to measure projected two-dimensional velocity fields around elliptical galaxies and constraint their total mass distribution. These data can provide information of the 1) shape of the gravitational potential, and 2) the mass profile as function of distance from the center of the light distribution. I have done this for Polar Ring Galaxies, using both stars and the extended HI disks around them (see Iodice et al. 2006 and Arnaboldi et al. 1997), and for the halos of elliptical galaxies, using the [OIII] 5007 AA emission from planetary nebulae. In the case of the giant ellipticals and cD galaxies the study of their halos led to the discovery of intracluster planetary nebulae (Arnaboldi et al. 2006) and their connection with the diffuse light in clusters. With dedicated observational techniques, we can now observed stellar motions out to 70 - 100 kpc distances as in the case of M87 in the Virgo cluster (Arnaboldi et al. 2004), or in Coma (Gerhard et al. 2005)

Ortwin Gerhard:
I have been involved in studies of the mass distributions of galaxies and the contributions of dark and baryonic matter. One of my main interests has focussed on elliptical galaxies, where I have been involved in an early comprehensive program based on extended slit stellar kinematics (see summaries in Gerhard et al. 2001 and 2005), and am currently participating in the PN.S project using planetary nebulae as mass tracers. A second main interest has been to construct a mass model of the Milky Way galaxy from NIR photometry, gas kinematics and stellar kinematics; see papers starting with Bissantz & Gerhard 2002 . In the Milky Way, we can use the optical depth for microlensing of bulge stars to break the degeneracy between dark and luminous matter. The current data imply that most of the mass in the inner bulge and disk of the Milky Way is baryonic.

Paul Schechter:
I am especially interested in using quasar flux ratio anomalies (observed at X-ray wavelengths) to determine the dark matter fraction at 1.5 R_e. Joachim Wambsganss and I described the method our contribution to IAU 220 arxiv but the results, using optical flux ratio anomalies, were disappointing. We now suspect this was due to the relatively large size of the optical emitting regions when compared to the Einstein rings. By contrast we think that the X-ray emitting regions are considerably smaller. The X-ray sample has now grown sufficiently large that a new analysis is warranted.

Neal Jackson:

I am involved in studies to try and discover more gravitational lenses suitable for substructure investigations, in particular 4-image radio lenses. Such studies are currently difficult in finite amounts of telescope time, but we are working on more efficient algorithms to do this using current and future telescopes. We are also currently obtaining VLBI images of radio lenses in order to assess whether substructure is required, including more observations of CLASS B0128+437 for which Biggs et al. (2004) published some early results. I'm also interested in observations of (or limits on) central images in galaxy lens systems, and with Ming Zhang, Ian Browne and Richard Porcas am working on a limit for CLASS B1030+074 from a deep VLBI observation (see also the ELVIS collaboration - Boyce et al. 2006 - who have more such observations).

Masamune Oguri:
One thing I am interested in is what kind of objects strong lensing select. Sometime the population of lens objects are biased very much, therefore it is important to take the bias into account in order to make a fair comparison between theory and observations. An example of this is lens galaxy environment: The environmental convergence increases the lensing probability, and as a result it enhances the fraction of lens galaxies that lie in groups and clusters (here). I pushed forward the research to construct a new model that takes account of both halo and subhalo populations (here). It allows us to predict the fraction of lenses caused by 'central' or 'satellite' galaxies, and the fraction of lenses that belong to groups or clusters. One interesting question related with this is how different properties/environments of lens galaxies are between double and quad lenses, because people tend to prefer quad lenses in doing detailed mass modeling or discussing flux ratio anomalies.

Ole Moeller:

My main interest at the moment lies in the statistics of lensing. Very recently I have written a paper (here) that combines semi-analytic models of galaxy formation with statistical lensing calculations to look at possible biases in gravitational lens systems. Even though the models we have for galaxy formation and evolution are far from perfect, I think that this approach, of taking the best self-consisten models we have so far in the LCDM paradigm and work out what the predicted lensing cross sections, image configurations etc. are. So far, it seems quite clear that lensing selects out a very specific population of galaxies. It selects out systems with high L, high M/L high velocity dispersion, higher concentration parameters for the DM . It is very important to keep these selection biases into account: strong lensing does not tell us something about the entire galaxy population as a whole directly, but only if a very specific subsample.
It also seems that 4-image systems trace a different population of galaxies than 2-image systems.

In the past I also worked on complex lens models and their predicted lensing profiles, mainly spiral lenses and early-type galaxies that contain a disk, systems with offsets between baryonic and DM component and so on. My conclusion from all that is that lens systems are "maximally complicated". In many cases, relatively small changes to the mass model can make a huge impact in terms of e.g. the inferred Hubble constant from time delay measurements. This also affects studies of lens statistics: one needs to take into account a multitude of effects if the correct biases etc. are to be determined -- just taking the most simple profiles for everything is not a good idea.

Something that I also find quite fascinating is the prospect of source reconstructions of high redshift extended sources using strong lensing -- especially using the strong lensing magnification effect to determine the kinematics of high-z galaxies.

Glenn van de Ven

The main focus of my research is on the dynamical structure and evolution of galaxies, globular clusters and other stellar systems
through detailed modeling of their observed photometry and two-dimensional kinematics.

Recently, we have developed a triaxial Schwarschild method that allows us to numerically build realistic dynamical models of early-type galaxies, and recover their full phase-space distribution. By applying these modeling techniques within the SAURON team, to a large set of nearby galaxies with detailed intergral-field spectroscopy, allows us to constrain their (differences in) formation history (see SAURON website).

A very different application to proper motion and radial velocity measurements of individual stars in the globular clusters omega Centauri and M15 (van de Ven et al. 2006, van den Bosch et al. 2006), show that these modeling techniques are general and flexible. Henceforth, I am interested in applying them to nearby strong gravitational lens galaxies with two-dimensional kinematics, where the combination of dynamics and lensing can provide an unique constraint on the (total) mass distribution.

The resulting detailed 'inside' view of nearby stellar systems provides a crucial 'calibration' for the studies at higher redshift, were we have to rely on simpler models and scaling relations such the Fundamental Plane.

Oleg Gnedin

I have checked the effect of halo contraction due to the cooling and condensation of baryons (loosely called "adiabatic contraction"). The deeper central potential well, created by stars and cool gas, changes the orbits of collisionless dark matter particles. However, the original formalism of Blumenthal et al. ("standard adiabatic contraction") assumed slow contraction of spherical halos, and the applicability of both of these assumptions in a hierarchical galaxy formation is questionable. Cosmological halos are triaxial and the potential typically changes on the dynamical time. To test the effect, we have used a sample of cluster-sized and galaxy-sized cosmological hydrodynamic simulations, run twice (with and without gas cooling) beginning with the same initial conditions. The overall effect is indeed contraction of the halo, although not as strong as follows from the Blumenthal et al. formula. We have derived a modification of the model that maintains the simplicity of the original method and reproduces the mass profiles from the simulations to 10-20%.

This modified contraction model is included in a publicly-available code contra.
This code also calculates a line-of-sight velocity dispersion of a tracer population in the combined potential of baryons and the contracted dark matter. It has many applications and I welcome suggestions for improving it.