No abstract

We develop and test a method for measuring the gravitational lensing induced distortion of faint background galaxies. We first describe how we locate the galaxies and measure a 2-component `polarisation' or ellipticity statistic $e_\alpha$ whose expectation value should be proportional to the gravitational shear $\gamma_\alpha$. We then show that an anisotropic instrumental psf perturbs the polarisation by $\delta e_\alpha = P^s_{\alpha\beta} p_\beta$, where $p_\alpha$ is a measure of the psf anisotropy and $P^s_{\alpha\beta}$ is the `linearised smear polarisability tensor'. By estimating $P^s_{\alpha\beta}$ for each object we can determine $p_\alpha$ from the foreground stars and apply a correction $-P^s_{\alpha\beta}p_\beta$ to the galaxies. We test this procedure using deep high-resolution images from HST which are smeared with an anisotropic psf and then have noise added to simulate ground-based observations. We find that the procedure works very well. A similar analysis yields a linear shear polarisability tensor $P^\gamma_{\alpha\beta}$ which describes the response to a gravitational shear. This calibrates the polarisation-shear relation, but only for galaxies which are well resolved. To empirically calibrate the effect of seeing on the smaller galaxies we artificially stretch HST images to simulate lensing and then degrade them as before. These experiments provide a rigorous and exacting test of the method under realistic conditions. They show that it is possible to remove the effect of instrumental psf anisotropy, and that the method provides an efficient and quantitative measurement of the gravitational shear.Comment: 39 pages, no figures, Latex, available by anonymous ftp inc figs from ftp://ftp.cita.utoronto.ca/cita/nick/methods.p

We report on the detection of weak gravitational lensing of faint, distant background objects by Cl 1358]62, a rich cluster of galaxies at a redshift of z \ 0.33. The observations consist of a large, multicolor mosaic of Hubble Space T elescope WFPC2 images. The number density of approximately 50 background objects arcmin~2 allows us to do a detailed weak lensing analysis of this cluster. We detect a weak lensing signal out toD1.5 Mpc from the cluster center. The observed distortion is consistent with a singular isothermal sphere model with a velocity dispersion of 780^50 km s~1. The total projected mass within a radius of 1 Mpc corresponding to this model is (4.4^0.6) ] 1014The errors given M _ . here represent the random error due to the ellipticities of the background galaxies. The uncertainty in the redshift distribution introduces an additional systematic error of D10% in the weak lensing mass. The weak lensing mass is slightly lower than dynamical estimates and agrees well with X-ray mass estimates. The mass distribution is elongated in a similar way as the light. The axis ratio of 0.30^0.15 and position angle of [21¡^7¡ were measured directly from the observations and agree very well with a previous strong lensing determination. A two-dimensional reconstruction of the cluster mass surface density shows that the peak of the mass distribution coincides with the peak of the light distribution. We Ðnd a value of for the mass-to-light ratio, consistent with being constant with (90^13)h 50 M _ L V_ 1 radius. The point-spread function of HST is highly anisotropic at the edges of the individual chips. This systematically perturbs the shapes of objects, and we present a method for applying the appropriate correction.

In the original paper, both axes of Figure 1a show units that are a factor of 1000 times too large. The corrected version is shown here. Fig. 1.-(a) 20 cm radio and 70 m IR luminosity correlation (see text) for the FLS long-wavelength sample and the distribution of monochromatic q 70 -values with redshift (b) uncorrected and (c) k-corrected using the DH01 SED-fitting method described in the text for the IR flux densities and assuming a (1 þ z) 0:7 k-corrected (boosting) of the 1.4 GHz values.

We analyze the ROSAT Deep Cluster Survey (RDCS) to derive cosmological constraints from the evolution of the cluster X-ray luminosity distribution. The sample contains 103 galaxy clusters out to z^0.85 and Ñux limit ergs s~1 cm~2 (RDCS-3) in the [0.5È2.0] keV energy band, F lim \ 3 ] 10~14 with a high-redshift extension containing four clusters at 0.90 ¹ z ¹ 1.26 and brighter than F lim \ 1 ] 10~14 ergs s~1 cm~2 (RDCS-1). We assume cosmological models to be speciÐed by the matter density parameter the rms Ñuctuation amplitude at the 8 h~1 Mpc scale and the shape parameter for the ) m , p 8 , cold dark matterÈlike power spectrum !. Model predictions for the cluster mass function are converted into the X-ray luminosity function in two steps. First, we convert mass into intracluster gas temperature by assuming hydrostatic equilibrium. Then, temperature is converted into X-ray luminosity by using the most recent data on the relation for nearby and distant clusters. These include the Chandra data L X -T X for six distant clusters at 0.57 ¹ z ¹ 1.27. From RDCS-3 we Ðnd and ) m \ 0.35~0 .10 0.13 p 8 \ for a spatially Ñat universe with a cosmological constant, with no signiÐcant constraint on ! 0.66~0 .05 0.06 (errors correspond to 1 p conÐdence levels for three Ðtting parameters). Even accounting for both theoretical and observational uncertainties in the massÈX-ray luminosity conversion, an EinsteinÈde Sitter model is always excluded at far more than the 3 p level. We also show that the number of X-rayÈbright clusters in RDCS-1 at z [ 0.9 is expected from the evolution inferred at z \ 0.9 data.

The localization of the short-duration, hard-spectrum gamma-ray burst GRB 050509b by the Swift satellite was a watershed event. Never before had a member of this mysterious subclass of classic GRBs been rapidly and precisely positioned in a sky accessible to the bevy of ground-based follow-up facilities. Thanks to the nearly immediate relay of the GRB position by Swift, we began imaging the GRB field 8 minutes after the burst and continued for the following 8 days. Though the Swift X-ray Telescope (XRT) discovered an X-ray afterglow of GRB 050509b, the first ever of a short-hard burst, no convincing optical/infrared candidate afterglow or supernova was found for the object. We present a re-analysis of the XRT afterglow and find an absolute position of R.A. = 12 h 36 m 13. s 59, Decl. = +28 • 59 ′ 04. ′′ 9 (J2000), with a 1σ uncertainty of 3. ′′ 68 in R.A., 3. ′′ 52 in Decl.; this is about 4 ′′ to the west of the XRT position reported previously. Close to this position is a bright elliptical galaxy with redshift z = 0.2248 ± 0.0002, about 1 ′ from the center of a rich cluster of galaxies. This cluster has detectable diffuse emission, with a temperature of kT = 5.25 +3.36 −1.68 keV. We also find several (∼11) much fainter galaxies consistent with the XRT position from deep Keck imaging and have obtained Gemini spectra of several of these sources. Nevertheless we argue, based on positional coincidences, that the GRB and the bright elliptical are likely to be physically related. We thus have discovered evidence that supports the notion that at least some short-duration, hard-spectra GRBs are at cosmological distances.We also explore the connection of the properties of the burst and the afterglow, finding that GRB 050509b was underluminous in both of these relative to long-duration GRBs. However, we also demonstrate that the ratio of the blast-wave energy to the γ-ray energy is consistent with that of long-duration GRBs. This suggests a comparably high efficiency of γ-ray conversion as in long GRBs as might be expected if the same emission mechanism is at work in short and long GRBs. Based on this analysis, on the location of the GRB (40 ± 13 kpc from a bright galaxy), on the galaxy type (elliptical), and the lack of a coincident supernova, we suggest that there is now observational consistency with the hypothesis that short-hard bursts arise during the merger of a compact binary (two neutron stars, or a neutron star and a black hole). In this context, we limit the properties of a Li-Paczyński "mini-supernova" that is predicted to arise on ∼day timescales. Other progenitor models are still viable, and additional rapidly localized bursts from the Swift mission will undoubtedly help to further clarify the progenitor picture.

We explore the dark matter distribution in ms1224.7+2007 using the gravitational distortion of the images of faint background galaxies. Projected mass image reconstruction reveals a highly significant concentration coincident with the X-ray and optical location. The concentration is seen repeatably in reconstructions from independent subsamples and the azimuthally averaged tangential shear pattern is also clearly seen in the data. The projected mass within a 2.76 ′ radius aperture is ≃ 3.5 × 10 14 h −1 M ⊙ . This is ≃ 3 times larger than that predicted if mass traces light with M/L = 275h as derived from virial analysis. It is very hard to attribute the discrepancy to a statistical fluctuation, and a further indication of a significant difference between the mass and the light comes from a second mass concentration which is again seen in independent subsamples but which is not seen at all in the cluster light. We find a mass per galaxy visible to I = 22 of ≃ 8 × 10 12 h −1 M ⊙ which, if representative of the universe, implies a density parameter Ω ∼ 2. We find a null detection of any net shear from large-scale structure with a precision of 0.9% per component. This is much smaller than the possible detection in a recent comparable study, and the precision here is comparable to to the minimum level of rms shear fluctuations implied by observed large-scale structure.

We present photometric and spectroscopic observations of supernova (SN) 2002cx, which reveal it to be unique among all observed type Ia supernovae (SNe Ia). SN 2002cx exhibits a SN 1991T-like premaximum spectrum, a SN 1991bg-like luminosity, and expansion velocities roughly half those of normal SNe Ia. Photometrically, SN 2002cx has a broad peak in the $R$ band and a plateau phase in the $I$ band, and slow late-time decline. The $(B - V)$ color evolution is nearly normal, but the $(V - R)$ and $(V - I)$ colors are very red. Early-time spectra of SN 2002cx evolve very quickly and are dominated by lines from Fe-group elements; features from intermediate-mass elements (Ca, S, Si) are weak or absent. Mysterious emission lines are observed around 7000 \AA\ at about 3 weeks after maximum brightness. The nebular spectrum of SN 2002cx is also unique, consisting of narrow iron and cobalt lines. The observations of SN 2002cx are inconsistent with the observed spectral/photometric sequence, and provide a major challenge to our understanding of SNe Ia. No existing theoretical model can successfully explain all observed aspects of SN 2002cx.Comment: 60 pages, 12 figures. A high resolution PostScript version is available at http://astro.berkeley.edu/~weidong/sn2002cx.p