D. Rusin

THE EVOLUTION AND STRUCTURE OF EARLY-TYPE FIELD GALAXIES: A COMBINED STATISTICAL ANALYSIS OF GRAVITATIONAL LENSES

We introduce a framework for simultaneously investigating the structure and luminosity evolution of early-type gravitational lens galaxies. The method is based on the fundamental plane, which we interpret using the aperture mass-radius relations derived from lensed image geometries. We apply this method to our previous sample of 22 lens galaxies with measured redshifts and excellent photometry. Modeling the population with a single mass profile and evolutionary history, we find that early-type galaxies are nearly isothermal (logarithmic density slope n = 2.06 ± 0.17, 68% C.L.) and that their stars evolve at a rate of d log(M/L)B/dz = -0.50 ± 0.19 (68% C.L.) in the rest-frame B band. For a Salpeter initial mass function and a concordance cosmology, this implies a mean star formation redshift of zf > 1.5 at 95% confidence. While this model can neatly describe the mean properties of early-type galaxies, it is clear that the scatter of the lens sample is too large to be explained by observational uncertainties alone. We therefore consider statistical models in which the galaxy population is described by a distribution of star formation redshifts. We find that stars must form over a significant range of redshifts (Δzf > 1.7, 68% C.L.), which can extend as low as zf ~ 1 for some acceptable models. However, the typical galaxy will still have an old stellar population (zf > 1.5). The lens sample therefore favors early star formation in field ellipticals, even if we make no a priori assumption regarding the shape of the mass distribution in lenses and include the range of possible deviations from homology in the uncertainties. Our evolution results call into question several recent claims that early-type galaxies in low-density environments have much younger stars than those in rich clusters.

Why Is the Fraction of Four-Image Radio Lens Systems So High?

We investigate the frequency of two- and four-image gravitational lens systems in the Jodrell-VLA Astrometric Survey (JVAS) and Cosmic Lens All-Sky Survey and the possible implications for dark matter halo properties. A simple lensing statistics model, which describes lens galaxies as singular isothermal ellipsoids with a projected axis ratio distribution derived from the surface brightness ellipticities of early-type galaxies in the Coma Cluster, is ruled out at the 98% level since it predicts too few four-image lenses (quads). We consider a range of factors that may be increasing the frequency of radio quads, including external shear fields, mass distributions flatter than the light, shallow lensing mass profiles, finite core radii, satellite galaxies, and alterations to the luminosity function for faint flat-spectrum radio sources. We find that none of these mechanisms provide a compelling solution to the quad problem on their own while remaining consistent with other observational constraints.

Constraints on the Inner Mass Profiles of Lensing Galaxies from Missing Odd Images

Most gravitational lens systems consist of two or four observable images. The absence of detectable odd images allows us to place a lower limit on the power-law slope of the inner mass profile of lensing galaxies. Using a sample of six two-image radio-loud lens systems and assuming a singular power-law surface density (Σ ∝ r-β) for the inner several kiloparsecs of the mass distribution, we find that there is less than a 10% probability that the data are consistent with profile slopes β 0.85 for B0739+366 and β > 0.91 for B1030+074. Modeling central black holes as additional point masses changes the constraints in these systems to β > 0.84 and β > 0.83, respectively. The inner mass profiles of lensing galaxies are therefore not much shallower than isothermal.

The Central image of a gravitationally lensed quasar

A galaxy can act as a gravitational lens, producing multiple images of a background object. Theory predicts that there should be an odd number of images produced by the lens, but hitherto almost all lensed objects have two or four images. The missing ‘central’ images, which should be faint and appear near the centre of the lensing galaxy, have long been sought as probes of galactic cores too distant to resolve with ordinary observations. There are five candidates for central images, but in one case the third image is not necessarily the central one, and in the others the putative central images might be foreground sources. Here we report a secure identification of a central image, based on radio observations of one of the candidates. Lens models using the central image reveal that the massive black hole at the centre of the lensing galaxy has a mass of <2 × 108 solar masses (M[circdot]), and the galaxys surface density at the location of the central image is > 20,000M[circdot] pc-2, which is in agreement with expections based on observations of galaxies that are much closer to the Earth.

CLASS B1152+199 and B1359+154: Two New Gravitational Lens Systems Discovered in the Cosmic Lens All-Sky Survey

The third phase of the Cosmic Lens All-Sky Survey (CLASS) has recently been completed, bringing the total number of sources imaged to over 15,000 in the CLASS and Jodrell-VLA Astrometric Survey combined survey. In the VLA observations carried out in 1998 March and April, two new candidate lensed systems were discovered: CLASS B1152+199 and B1359+154. B1152+199 is a 16 double, with a background quasar at z = 1.019 lensed by a foreground galaxy at z = 0.439. The relatively flat radio spectra of the lensed images (α = -0.32), combined with a previous ROSAT detection of the source, make B1152+199 a strong candidate for time delay studies at both radio and X-ray wavelengths. B1359+154 is a quadruply lensed quasar at z = 3.235, with a maximum image separation of 17. As yet, the redshift of the lensing object in this system is undetermined. The steep spectral index of the source (α = -0.9) suggests that B1359+154 will not exhibit strong variability and is therefore unlikely to be useful for determining H0 from measured time delays.

B1359+154: A Six-Image Lens Produced by a z 1 Compact Group of Galaxies

Hubble Space Telescope (HST) V- and I-band observations show that the gravitational lens B1359+154 consists of six images of a single zs = 3.235 radio source and its star-forming host galaxy, produced by a compact group of galaxies at zl 1. Very Long Baseline Array (VLBA) observations at 1.7 GHz strongly support this conclusion, showing six compact cores with similar low-frequency radio spectra. B1359+154 is the first example of galaxy-scale gravitational lensing in which more than four images are observed of the same background source. The configuration is due to the unique lensing mass distribution: three primary lens galaxies lying on the vertices of a triangle separated by 07 4 h-1 kpc, inside the 17 diameter Einstein ring defined by the radio images. The gravitational potential has additional extrema within this triangle, creating a pair of central images that supplement the standard four-image geometry of the outer components. Simple mass models, consisting of three lens galaxies constrained by HST and VLBA astrometry, naturally reproduce the observed image positions but must be finely tuned to fit the flux densities.

High-resolution observations and mass modelling of the CLASS gravitational lens B1152+199

We present a series of high-resolution radio and optical observations of the CLASS gravitational lens system B1152+199 obtained with the Multi-Element Radio-Linked Interferometer Network, Very Long Baseline Array and Hubble Space Telescope. Based on the milliarcsecond-scale substructure of the lensed radio components and precise optical astrometry for the lensing galaxy, we construct models for the system and place constraints on the galaxy mass profile. For a single galaxy model with surface mass density Σ(r)∝r−β, we find that 0.95β1.21 at 2σ confidence. Including a second deflector to represent a possible satellite galaxy of the primary lens leads to slightly steeper mass profiles.

Redshifts of CLASS Radio Sources

Spectroscopic observations of a sample of 42 flat-spectrum radio sources from the Cosmic Lens All-Sky Survey (CLASS) have yielded a mean redshift of z = 1.27 with an rms spread of 0.95, at a completeness level of 64%. The sample consists of sources with a 5 GHz flux density of 25–50 mJy, making it the faintest flat-spectrum radio sample for which the redshift distribution has been studied. The spectra, obtained with the William Herschel Telescope, consist mainly of broad-line quasars at z > 1 and narrow-line galaxies at z < 0.5. Though the mean redshift of flat-spectrum radio sources exhibits little variation over more than 2 orders of magnitude in radio flux density, there is evidence of a decreasing fraction of quasars at weaker flux levels. We present the results of our spectroscopic observations and discuss the implications for constraining cosmological parameters with statistical analyses of CLASS.

CLASS B2108+213: a new wide-separation gravitational lens system

We present observations of CLASS B2108+213, the widest separation gravitational lens system discovered by the Cosmic Lens All-Sky Survey. Radio imaging using the VLA at 8.46 GHz and MERLIN at 5 GHz shows two compact components separated by 4.56 arcsec with a faint third component in between which we believe is emission from a lensing galaxy. 5-GHz VLBA observations reveal milliarcsecond-scale structure in the two lensed images that is consistent with gravitational lensing. Optical emission from the two lensed images and two lensing galaxies within the Einstein radius is detected in Hubble Space Telescope imaging. Furthermore, an optical gravitational arc, associated with the strongest lensed component, has been detected. Surrounding the system are a number of faint galaxies which may help explain the wide image separation. A plausible mass distribution model for CLASS B2108+213 is also presented.

Galaxy Groups Associated with Gravitational Lenses and

Compact groups of galaxies recently have been discovered in association with several strong gravitational lens systems. These groups provide additional convergence to the lensing potential and thus affect the value of H_0 derived from the systems. Lens system time delays are now being measured with uncertainties of only a few percent or better. Additionally, vast improvements are being made in incorporating observational constraints such as Einstein ring structures and stellar velocity dispersions into the lens models. These advances are reducing the uncertainties on H_0 to levels at which the the effects of associated galaxy groups may contribute significantly to the overall error budget. We describe a dedicated multiwavelength program, using Keck, HST, and Chandra, to find such groups and measure their properties. We present, as a case study, results obtained from observations of the CLASS lens system B1608+656 and discuss the implications for the value of H_0 derived from this system.