Tommaso Treu

A Wide-Field Hubble Space Telescope Study of the Cluster Cl 0024+16 at z = 0.4. I. Morphological Distributions to 5 Mpc Radius

We describe a new wide-field Hubble Space Telescope survey of the galaxy cluster Cl 0024+16 (z ≈ 0.4) consisting of a sparsely sampled mosaic of 39 Wide Field Planetary Camera 2 images that extends to a cluster radius of ~5 Mpc. Together with extensive ground-based spectroscopy taken from the literature, augmented with over a hundred newly determined redshifts, this unique data set enables us to examine environmental influences on the properties of cluster members from the inner core to well beyond the virial radius (~1.7 Mpc). We catalog photometric measures for 22,000 objects to I 25 and assign morphological types for 2181 to I = 22.5, of which 195 are spectroscopically confirmed cluster members. We examine both the morphology-radius (T-R) and morphology-density (T-Σ) relations and demonstrate sensitivities adequate for measures from the core to a radius of ~5 Mpc, spanning over 3 decades in local projected density. The fraction of early-type galaxies declines steeply from the cluster center to 1 Mpc radius and more gradually thereafter, asymptoting toward the field value at the periphery. We discuss our results in the context of three distinct cluster zones, defined according to different physical processes that may be effective in transforming galaxy morphology in each. By treating infalling galaxies as isolated test particles, we deduce that the most likely processes responsible for the mild gradient in the morphological mix outside the virial radius are harassment and starvation. Although more data are needed to pin down the exact mechanisms, starvation seems more promising in that it would naturally explain the stellar and dynamical homogeneity of cluster E/S0s. However, we find significant scatter in the local density at any given radius outside ~0.5 Mpc and that the same T-Σ relation holds in subregions of the cluster, independent of location. In this hitherto unprobed region, where the potential of the cluster is weak, galaxies apparently retain their identities as members of infalling subgroups whose characteristic morphological properties remain intact. Only upon arrival in the central regions is the substructure erased, as indicated by the tight correlation between cluster radius and Σ.

Constraints on the Equation of State of Dark Energy and the Hubble Constant from Stellar Ages and the Cosmic Microwave Background

We place constraints on the redshift-averaged, effective value of the equation of state of dark energy, w, using only the absolute ages of Galactic stars and the observed position of the first peak in the angular power spectrum of the cosmic microwave background (CMB). We find w 0.2 does not yield, currently, a precise determination of w(z), but shows that the age-redshift relation is consistent with a standard ?CDM universe with w = -1.

The Dark Matter Density Profile of the Lensing Cluster MS 2137–23: A Test of the Cold Dark Matter Paradigm*

We present new spectroscopic observations of the gravitational arcs and the brightest cluster galaxy (BCG) in the cluster MS 2137-23 (z = 0.313) obtained with the Echelle Spectrograph and Imager on the Keck II telescope. We find that the tangential and radial arcs arise from sources at almost identical redshifts (z = 1.501, 1.502). We combine the measured stellar velocity dispersion profile of the BCG with a lensing analysis to constrain the distribution of dark and stellar matter in the central 100 kpc of the cluster. Our data indicate a remarkably flat inner slope for the dark matter profile, ?d r-?, with ? < 0.9 at a 99% CL. Steep inner slopes obtained in cold dark matter cosmological simulations?such as Navarro, Frenk, & White (? = 1) or Moore (1.5) universal dark matter profiles?are ruled out at better than 99% CL. As baryon collapse is likely to have steepened the dark matter profile from its original form, our data provide a powerful test of the cold dark matter paradigm at the cluster mass scale.

The Evolution of Field Early-Type Galaxies to z ∼ 0.7*

We have measured the fundamental plane (FP) parameters for a sample of 30 field early-type galaxies (E/S0s) in the redshift range 0.1 < z < 0.66. We find that (i) the FP is defined and tight out to the highest redshift bin; (ii) the intercept γ evolves as dγ/dz = 0.58 (for Ω = 0.3, ΩΛ = 0.7) or, in terms of average effective mass-to-light ratio, as d log(M/LB)/dz = -0.72, i.e., faster than is observed for cluster E/S0s (-0.49 ± 0.05). In addition, we detect [O II] emission greater than 5 A in 22% of an enlarged sample of 42 massive E/S0s in the range 0.1 < z < 0.73, in contrast with the quiescent population observed in clusters at similar z. We interpret these findings as evidence that a significant fraction of massive field E/S0s experiences secondary episodes of star formation at z < 1.

The internal structure of the lens PG1115+080: breaking degeneracies in the value of the Hubble constant

We combine lensing, stellar kinematic and mass-to-light ratio constraints to build a two-component (luminous plus dark) mass model of the early-type lens galaxy in PG1115+080. We find a total mass density profile steeper than r^(−2), effectively ρ ∝r ^(−γ′) with γ′= 2.35 ± 0.1 ± 0.05 (random + systematic). The stellar mass fraction is f*= 0.67^(+0.20) − 0.25 ± 0.03 inside the Einstein radius (R_E ≈ 1.2 effective radii). The dynamical mass model breaks the degeneracies in the mass profile of the lens galaxy and allows us to obtain a value of the Hubble constant that is no longer dominated by systematic errors: H_0= 59_(+12) − 7 ± 3 km s^(−1) Mpc^(−1) (68 per cent confidence level; Ω_m= 0.3, Ω_Λ= 0.7). The offset of PG1115+080 from the Fundamental Plane might indicate deviations from homology of the mass profile of some early-type galaxies.

The Stellar Velocity Dispersion of the Lens Galaxy in MG 2016+112 at z = 1.004

We present a direct measurement of the stellar velocity dispersion of the early-type lens galaxy D in the system MG 2016+112 (z = 1.004), determined from a spectrum obtained with the Echelle Spectrograph and Imager on the W. M. Keck II Telescope, as part of the Lenses Structure and Dynamics Survey. We find a velocity dispersion of σap = 304 ± 27 km s-1 inside an effective circular aperture with a radius of 065, corresponding to a central velocity dispersion of σ = 328 ± 32 km s-1. From a Hubble Space Telescope F160W-band image, we measure the effective radius and effective surface brightness in order to determine the offset of the lens galaxy with respect to the local fundamental plane (FP). The offset corresponds to an evolution of the rest-frame effective mass-to-light ratio of Δ log(M/LB) = -0.62 ± 0.08 from z = 0 to z = 1.004. By interpreting colors and offset of the FP with two independent stellar population synthesis models, we obtain a single-burst equivalent age of 2.8 ± 0.8 Gyr (i.e., zf > 1.9) and a supersolar metallicity of log(Z/Z☉) = 0.25 ± 0.25. The lens galaxy is therefore a massive elliptical dominated by an old and metal-rich stellar population at z > 1. The excellent agreement of the stellar velocity dispersion with that predicted from recent lens models confirms that the angular separation of the multiple images of the background QSO is predominantly due to the lens galaxy and not to a massive dark cluster, in agreement with recent weak-lensing and X-ray observations. However, the significant overdensity of galaxies in the field might indicate that this system is a protocluster, in formation around galaxy D, responsible for the ~10% external shear inferred from the strong-lens models.

The redshift of the Einstein ring in MG 1549+305

A deep spectrum taken with the Echelle Spectrograph and Imager (ESI) at the Keck II Telescope as part of the Lenses Structure and Dynamics (LSD) Survey reveals the redshifts of the extremely red source of the radio Einstein Ring in the gravitational lens system MG1549+305 (zs = 1.170 ± 0.001) and an intermediate redshift lensed spiral galaxy (zG2 = 0.604 ± 0.001). The source redshift allows us to determine the mass of the SB0 lens galaxy enclosed by the Einstein Radius (RE = 1. 15 ± 0. 05) ME≡M(< RE) = 8.4±0.7×10 10 h 65 M⊙. This corresponds to a Singular Isothermal Ellipsoid (SIE) velocity dispersion σSIE = 214 ± 5 km s , in good agreement with the measured stellar velocity dispersion σ = 227 ± 18 km s (Lehár et al. 1996). The mass-to-light ratio within the Einstein Radius (∼1.4 effective radii) is 10 ± 1 h65 M⊙/LB,⊙. This is only marginally larger than typical stellar mass-to-light ratios of local early-type galaxies, indicating that dark matter is not likely to be dominant inside the Einstein Radius.

The Lenses Structure & Dynamics Survey

The Lenses Structure & Dynamics (LSD) Survey aims at studying the internal structure of luminous and dark matter - as well as their evolution - of field earlytype (E/SO) galaxies to z ∼ 1. In particular, E/S0 lens galaxies are studied by combining gravitational lensing, photometric and kinematic data obtained with ground-based (VLA/Keck/VLT) and space-based telescopes (HST). Here, we report on preliminary results from the LSD Survey, in particular on (i) the constraints set on the luminous and dark-matter distributions in the inner several R eff of E/S0 galaxies, (ii) the evolution of their stellar component and (iii) the constraints set on the value of H0 from time-delay systems by combining lensing and kinematic data to break degeneracies in gravitational-lens models.

Measuring the mass of high-z galaxies with NGST

We discuss dynamical mass measurements of high-z galaxies with the Next Generation Space Telescope (NGST). In particular, we review some of the observational limits with the current instrument/telescope generation, we discuss the redshift limits and caveats for absorption and emission lines studies with NGST, and the existence of suitable targets at high redshift. We also briefly summarize strengths and weaknesses of proposed NGST instruments for dynamical studies.

Lyman alpha emitters with red colors at z~=2.4

We have carried out a search for Lyman alpha emission from galaxies at z~=2.4 over a field of 1200 sq. arcmin using the CFH12K camera at the CFHT and a custom medium band filter. The search has uncovered 58 candidates, corresponding to a completeness-corrected source density of ~0.07 sq. arcmin^-2. Our sources have red colors (B-I~=1.8) which imply either that a large fraction of the light is highly reddened and we are detecting Lyman alpha through special lines of sight, or that these objects contain an underlying older stellar population. While for each individual object we cannot discriminate between these alternatives, we conclude that most of the objects actually contain an older component because the star formation rates inferred from the picture based on reddening, applied to all candidates, would imply an exceedingly high star formation rate, i.e. more than two orders of magnitude above the peak cosmic star formation rate (e.g. Lilly et al. 1996).