Search

Pan-STARRS1 has carried out a set of distinct synoptic imaging sky surveys including the $3\pi$ Steradian Survey and the Medium Deep Survey in 5 bands ($grizy_{P1}$). The mean 5$\sigma$ point source limiting sensitivities in the stacked 3$\pi$ Steradian Survey in $grizy_{P1}$ are (23.3, 23.2, 23.1, 22.3, 21.4) respectively. The upper bound on the systematic uncertainty in the photometric calibration across the sky is 7-12 millimag depending on the bandpass. The systematic uncertainty of the astrometric calibration using the Gaia frame comes from a comparison of the results with Gaia: the standard deviation of the mean and median residuals ($ \Delta ra, \Delta dec $) are (2.3, 1.7) milliarcsec, and (3.1, 4.8) milliarcsec respectively. The Pan-STARRS system and the design of the PS1 surveys are described and an overview of the resulting image and catalog data products and their basic characteristics are described together with a summary of important results. The images, reduced data products, and derived data products from the Pan-STARRS1 surveys are available to the community from the Mikulski Archive for Space Telescopes (MAST) at STScI., Comment: 38 pages, 29 figures, 12 tables

Membership identification is the first step to determine the properties of a star cluster. Low-mass members in particular could be used to trace the dynamical history, such as mass segregation, stellar evaporation, or tidal stripping, of a star cluster in its Galactic environment. We identified member candidates with stellar masses $\sim$0.11--2.4 Msun of the intermediate-age Praesepe cluster (M44), by using Pan-STARRS and 2MASS photometry, and PPMXL proper motions. Within a sky area of 3 deg radius, 1040 candidates are identified, of which 96 are new inclusions. Using the same set of selection criteria on field stars led to an estimate of a false positive rate 16%, suggesting 872 of the candidates being true members. This most complete and reliable membership list allows us to favor the BT-Settl model in comparison with other stellar models. The cluster shows a distinct binary track above the main sequence, with a binary frequency of 20--40%, and a high occurrence rate of similar mass pairs. The mass function is consistent with that of the disk population but shows a deficit of members below 0.3 solar masses. A clear mass segregation is evidenced, with the lowest-mass members in our sample being evaporated from this disintegrating cluster., Comment: ApJ accepted

We report on our serendipitous pre-discovery detection and detailed follow-up of the broad-lined Type Ic supernova (SN) 2010ay at z = 0.067 imaged by the Pan-STARRS1 3pi survey just ~4 days after explosion. The SN had a peak luminosity, M_R ~ -20.2 mag, significantly more luminous than known GRB-SNe and one of the most luminous SNe Ib/c ever discovered. The absorption velocity of SN 2010ay is v_Si ~ 19,000 km/s at ~40 days after explosion, 2-5 times higher than other broad-lined SNe and similar to the GRB-SN 2010bh at comparable epochs. Moreover, the velocity declines ~2 times slower than other SNe Ic-BL and GRB-SNe. Assuming that the optical emission is powered by radioactive decay, the peak magnitude implies the synthesis of an unusually large mass of 56 Ni, M_Ni = 0.9 M_solar. Modeling of the light-curve points to a total ejecta mass, M_ej ~ 4.7 M_sol, and total kinetic energy, E_K ~ 11x10^51 ergs. The ratio of M_Ni to M_ej is ~2 times as large for SN 2010ay as typical GRB-SNe and may suggest an additional energy reservoir. The metallicity (log(O/H)_PP04 + 12 = 8.19) of the explosion site within the host galaxy places SN 2010ay in the low-metallicity regime populated by GRB-SNe, and ~0.5(0.2) dex lower than that typically measured for the host environments of normal (broad-lined) Ic supernovae. We constrain any gamma-ray emission with E_gamma < 6x10^{48} erg (25-150 keV) and our deep radio follow-up observations with the Expanded Very Large Array rule out relativistic ejecta with energy, E > 10^48 erg. We therefore rule out the association of a relativistic outflow like those which accompanied SN 1998bw and traditional long-duration GRBs, but place less-stringent constraints on a weak afterglow like that seen from XRF 060218. These observations challenge the importance of progenitor metallicity for the production of a GRB, and suggest that other parameters also play a key role., Comment: 19 pages, 10 figures, V3 has revisions following referee's report; more information at http://www.cfa.harvard.edu/~nsanders/papers/2010ay/summary.html

Recent searches by unbiased, wide-field surveys have uncovered a group of extremely luminous optical transients. The initial discoveries of SN 2005ap by the Texas Supernova Search and SCP-06F6 in a deep Hubble pencil beam survey were followed by the Palomar Transient Factory confirmation of host redshifts for other similar transients. The transients share the common properties of high optical luminosities (peak magnitudes ~ -21 to -23), blue colors, and a lack of H or He spectral features. The physical mechanism that produces the luminosity is uncertain, with suggestions ranging from jet-driven explosion to pulsational pair-instability. Here we report the most detailed photometric and spectral coverage of an ultra-bright transient (SN 2010gx) detected in the Pan-STARRS 1 sky survey. In common with other transients in this family, early-time spectra show a blue continuum, and prominent broad absorption lines of O II. However, about 25d after discovery, the spectra developed type Ic supernova features, showing the characteristic broad Fe II and Si II absorption lines. Detailed, post-maximum follow-up may show that all SN 2005ap and SCP-06F6 type transients are linked to supernovae Ic. This poses problems in understanding the physics of the explosions: there is no indication from late-time photometry that the luminosity is powered by 56Ni, the broad lightcurves suggest very large ejected masses, and the slow spectral evolution is quite different from typical Ic timescales. The nature of the progenitor stars and the origin of the luminosity are intriguing and open questions., Comment: 16 pages, 4 figures, 1 table. Accepted for publication in the Astrophysical Journal Letters on September 9

We present the early UV and optical light curve of Type IIP supernova (SN) 2010aq at z=0.0862, and compare it to analytical models for thermal emission following SN shock breakout in a red supergiant star. SN 2010aq was discovered in joint monitoring between the Galaxy Evolution Explorer (GALEX) Time Domain Survey (TDS) in the NUV and the Pan-STARRS1 Medium Deep Survey (PS1 MDS) in the g, r, i, and z bands. The GALEX and Pan-STARRS1 observations detect the SN less than 1 day after shock breakout, measure a diluted blackbody temperature of 31,000 +/- 6,000 K 1 day later, and follow the rise in the UV/optical light curve over the next 2 days caused by the expansion and cooling of the SN ejecta. The high signal-to-noise ratio of the simultaneous UV and optical photometry allows us to fit for a progenitor star radius of 700 +/- 200 R_sun, the size of a red supergiant star. An excess in UV emission two weeks after shock breakout compared to SNe well fitted by model atmosphere-code synthetic spectra with solar metallicity, is best explained by suppressed line blanketing due to a lower metallicity progenitor star in SN 2010aq. Continued monitoring of PS1 MDS fields by the GALEX TDS will increase the sample of early UV detections of Type II SNe by an order of magnitude, and probe the diversity of SN progenitor star properties., Comment: 4 pages, 3 color figures. Accepted for publication in ApJ Letters, corrections from proofs added

We present grism spectra of emission-line galaxies (ELGs) from 0.6-1.6 microns from the Wide Field Camera 3 on the Hubble Space Telescope. These new infrared grism data augment previous optical Advanced Camera for Surveys G800L 0.6-0.95 micron grism data in GOODS-South from the PEARS program, extending the wavelength covereage well past the G800L red cutoff. The ERS grism field was observed at a depth of 2 orbits per grism, yielding spectra of hundreds of faint objects, a subset of which are presented here. ELGs are studied via the Ha, [OIII], and [OII] emission lines detected in the redshift ranges 0.22., Comment: Accepted for publication in AJ. Updated to include referee comments. Updated sample using improved reduction contains 23 new galaxies (Table 1; Figures 2 & 3)

We present photometric and spectroscopic observations of a luminous type IIP Supernova 2009kf discovered by the Pan-STARRS 1 (PS1) survey and detected also by GALEX. The SN shows a plateau in its optical and bolometric light curves, lasting approximately 70 days in the rest frame, with absolute magnitude of M_V = -18.4 mag. The P-Cygni profiles of hydrogen indicate expansion velocities of 9000km/s at 61 days after discovery which is extremely high for a type IIP SN. SN 2009kf is also remarkably bright in the near-ultraviolet (NUV) and shows a slow evolution 10-20 days after optical discovery. The NUV and optical luminosity at these epochs can be modelled with a black-body with a hot effective temperature (T ~16,000 K) and a large radius (R ~1x10^{15} cm). The bright bolometric and NUV luminosity, the lightcurve peak and plateau duration, the high velocities and temperatures suggest that 2009kf is a type IIP SN powered by a larger than normal explosion energy. Recently discovered high-z SNe (0.7 < z < 2.3) have been assumed to be IIn SNe, with the bright UV luminosities due to the interaction of SN ejecta with a dense circumstellar medium (CSM). UV bright SNe similar to SN 2009kf could also account for these high-z events, and its absolute magnitude M_NUV = -21.5 +/- 0.5 mag suggests such SNe could be discovered out to z ~2.5 in the PS1 survey., Comment: Accepted for publication in APJL

The evolution of the B-band galaxy luminosity function is measured using a sample of more than 11,000 galaxies with spectroscopic redshifts from the DEEP2 Redshift Survey. The rest-frame M_B versus U-B color-magnitude diagram of DEEP2 galaxies shows that the color-magnitude bi-modality seen in galaxies locally is still present at redshifts z > 1. Dividing the sample at the trough of this color bimodality into predominantly red and blue galaxies, we find that the luminosity function of each galaxy color type evolves differently. Blue counts tend to shift to brighter magnitudes at constant number density, while the red counts remain largely constant at a fixed absolute magnitude. Using Schechter functions with fixed faint-end slopes we find that M*_B for blue galaxies brightens by ~ 1.3 magnitudes per unit redshift, with no significant evolution in number density. For red galaxies M*_B brightens somewhat less with redshift, while the formal value of phi* declines. When the population of blue galaxies is subdivided into two halves using the rest-frame color as the criterion, the measured evolution of both blue subpopulations is very similar., Comment: Accepted by ApJ, emulateapj format, 24 pages, 17 figures, 4 in JPEG, includes referee's comments

The DEEP2 and COMBO-17 surveys are used to study the evolution of the luminosity function of red and blue galaxies to $z \sim 1$. Schechter function fits show that, since $z = 1$, $M^*_B$ dims by $\sim$ 1.3 mag per unit redshift for both color classes, $\phi^*$ of blue galaxies shows little change, while $\phi^*$ for red galaxies has formally nearly quadrupled. At face value, the number density of blue galaxies has remained roughly constant since $ z = 1$, whereas that of red galaxies has been rising. Luminosity densities support both conclusions, but we note that most red-galaxy evolution occurs between our data and local surveys and in our highest redshift bin, where the data are weakest. We discuss the implications of having most red galaxies emerge after $z = 1$ from precursors among the blue population, taking into account the properties of local and distant E/S0s. We suggest a ``mixed'' scenario in which some blue galaxies have their star-formation quenched in gas-rich mergers, migrate to the red sequence with a variety of masses, and merge further on the red sequence in one or more purely stellar mergers. E/S0s of a given mass today will have formed via different routes, in a manner that may help to explain the fundamental plane and other local scaling laws., Comment: Submitted to ApJ. 73 pages, 12 figures. Part II of a two-paper series. The entire paper is available as a single postscript file at: http://www.ucolick.org/~cnaw/paper2_submitted.ps.gz

We use weak lensing shear measurements of six z>0.5 clusters of galaxies to derive the mean lensing redshift of the background galaxies used to measure the shear. Five of these clusters are compared to X-ray mass models and verify a mean lensing redshift for a 23

The Galactic Exoplanet Survey Telescope (GEST) will observe a 2 square degree field in the Galactic bulge to search for extra-solar planets using a gravitational lensing technique. This gravitational lensing technique is the only method employing currently available technology that can detect Earth-mass planets at high signal-to-noise, and can measure the frequency of terrestrial planets as a function of Galactic position. GEST's sensitivity extends down to the mass of Mars, and it can detect hundreds of terrestrial planets with semi-major axes ranging from 0.7 AU to infinity. GEST will be the first truly comprehensive survey of the Galaxy for planets like those in our own Solar System., Comment: 17 pages with 13 figures, to be published in Proc. SPIE vol 4854, "Future EUV-UV and Visible Space Astrophysics Missions and Instrumentation"

The DEIMOS spectrograph has now been installed on the Keck-II telescope and commissioning is nearly complete. The DEEP2 Redshift Survey, which will take approximately 120 nights at the Keck Observatory over a three year period and has been designed to utilize the power of DEIMOS, began in the summer of 2002. The multiplexing power and high efficiency of DEIMOS enables us to target 1000 faint galaxies per clear night. Our goal is to gather high-quality spectra of \~60,000 galaxies with z>0.75 in order to study the properties and large scale clustering of galaxies at z ~ 1. The survey will be executed at high spectral resolution, $R=\lambda/\Delta \lambda \approx 5000$, allowing us to work between the bright OH sky emission lines and to infer linewidths for many of the target galaxies (for several thousand objects, we will obtain rotation curves as well). The linewidth data will facilitate the execution of the classical redshift-volume cosmological test, which can provide a precision measurement of the equation of state of the Universe. This talk reviews the project, summarizes our science goals and presents some early DEIMOS data., Comment: 12 pages, 5 figures, talk presented at SPIE conference, Aug. 2002

We present the results of a weak lensing survey of six high-redshift (z > 0.5), X-ray selected clusters of galaxies. We have obtained ultra-deep R-band images of each cluster with the Keck Telescope, and have measured a weak lensing signal from each cluster. From the background galaxy ellipticities we create two-dimensional maps of the surface mass density of each cluster. We find that the substructure seen in the mass reconstructions typically agree well with substructure in both the cluster galaxy distributions and X-ray images of the clusters. We also measure the one-dimensional radial profiles of the lensing signals and fit these with both isothermal spheres and "universal" CDM profiles. We find that the more massive clusters are less compact and not as well fit by isothermal spheres as the less massive clusters, possibly indicating that they are still in the process of collapse., Comment: 43 pages, 15 figures, uses aastex, submitted to ApJ 4 color plates produced here as jpg's, larger versions of the jpgs can be found at http://www.mpa-garching.mpg.de/~clowe

We propose a new strategy for obtaining enhanced resolution (FWHM = 0.12 arcsec) deep optical images over a wide field of view. As is well known, this type of image quality can be obtained in principle simply by fast guiding on a small (D = 1.5m) telescope at a good site, but only for target objects which lie within a limited angular distance of a suitably bright guide star. For high altitude turbulence this 'isokinetic angle' is approximately 1 arcminute. With a 1 degree field say one would need to track and correct the motions of thousands of isokinetic patches, yet there are typically too few sufficiently bright guide stars to provide the necessary guiding information. Our proposed solution to these problems has two novel features. The first is to use orthogonal transfer charge-coupled device (OTCCD) technology to effectively implement a wide field 'rubber focal plane' detector composed of an array of cells which can be guided independently. The second is to combine measured motions of a set of guide stars made with an array of telescopes to provide the extra information needed to fully determine the deflection field. We discuss the performance, feasibility and design constraints on a system which would provide the collecting area equivalent to a single 9m telescope, a 1 degree square field and 0.12 arcsec FWHM image quality., Comment: 46 pages, 22 figures, submitted to PASP, a version with higher resolution images and other supplementary material can be found at http://www.ifa.hawaii.edu/~kaiser/wfhri

We present results of a weak gravitational lensing survey of six X-ray selected high-redshift clusters of galaxies. We find that the masses of the clusters derived from weak lensing are comparable to those derived from the X-ray observations. We show that many of the clusters have significant substructure not observed in the X-ray observations and that for the more massive clusters a singular isothermal sphere does not provide a good fit to the radial mass profile., Comment: 5 pages, 1 figure, to be published in ASP Conference Series: Proceedings of Clustering at High Redshift, Les Rencontres Internationales de l'IGRAP

We describe in detail the processing of a set of images of the z = 0.42 supercluster MS0302 taken with the UH8K camera at CFHT. The result of this is a pair of seamless combined V- and I-band images of the field, along with a characterization of the noise properties and of the point spread function (PSF), and catalogs of about 30,000 faint galaxies. The analysis involves the following steps: image preparation; detection of stars and registration to find the transformation from detector to sky coordinates; correction for extinction and/or gain variations; modeling of the PSF; generation of images with a circular PSF; image warping and averaging; modeling of the noise auto-correlation function; faint object detection, aperture photometry, and shape measurement. The shear analysis is described elsewhere., Comment: 26 pages, 5 figures, submitted to PASP

We present the results of a CCD imaging survey for gravitational lensing in a sample of 38 X-ray-selected clusters of galaxies. Our sample consists of the most X-ray luminous (Lx>= 2x10^{44} erg s^{-1}) clusters selected from the Einstein Observatory Extended Medium Sensitivity Survey (EMSS) that are observable from Mauna Kea (dec > -40deg). The sample spans a redshift range of 0.15 < z < 0.823 and includes 5 clusters with z > 0.5. CCD images of the clusters were obtained in excellent seeing. There is evidence of strong gravitational lensing in the form of giant arcs (length l > 8'', axis ratio l/w > 10) in 8 of the 38 clusters. Two additional clusters contain shorter arclets, and 6 more clusters contain candidate arcs that require follow-up observations to confirm their lensing origin. Since the survey does not have a uniform surface brightness limit we do not draw any conclusion based on the statistics of the arcs found. We note, however, that 60% (3 of 5) of the clusters with Lx > 10^{45} erg s^{-1}, and none of the 15 clusters with Lx < 4x10^{44} erg s^{-1} contain giant arcs, thereby confirming that high X-ray luminosity does identify the most massive systems, and thus X-ray selection is the preferred method for finding true, rich clusters at intermediate and high redshifts. The observed geometry of the arcs, most of which are thin, have large axis ratios (l/w > 10), and are aligned orthogonal to the optical major axes of the clusters, indicate the cluster core mass density profiles must be compact (steeper than isothermal). In several cases, however, there is also some evidence, in the form of possible radial arcs, for density profiles with finite core radii., Comment: Latex file, 17 pages, 7 jpeg figures, to be published in Astronomy and Astrophysics Supplements

We perform a weak lensing and photometric study of the z=0.42 supercluster MS0302+17 using deep I and V band images taken with the UH8K CCD mosaic camera at the CFHT. We use archival ROSAT HRI data to estimate fluxes, gas masses and, in one case, the binding mass of the three major clusters. We then use our CCD data to determine the optical richness and luminosities of the clusters and to map out the spatial distribution of the early type galaxies in the supercluster and in other foreground and background structures. We measure the gravitational shear from a sample of \simeq 30,000 faint background galaxies in the range 22 < m_I < 26 and find this correlates strongly with that predicted from the early type galaxies if they trace the mass with M/L_B \simeq 250 h. We make 2-dimensional reconstructions of the mass surface density. These recover all of the major concentrations of galaxies and indicate that most of the supercluster mass, like the early type galaxies, is concentrated in the three X-ray clusters, and we obtain mean mass-to-light ratios for the clusters of M/L_B \simeq 260 h. Cross-correlation of the measured mass surface density with that predicted from the early type galaxy distribution shows a strong peak at zero lag (significant at the \simeq 9-sigma level), and that at separations \gsim 200 h^{-1}$kpc the early galaxies trace the mass very accurately. This conclusion is supported by cross-correlation in Fourier space; we see little evidence for any variation of M/L or `bias' with scale, and from the longest wavelength modes with \lambda = 1.5-6 h^{-1}Mpc we find M/L \simeq (280 \pm 40)h, quite similar to that obtained for the cluster centers. We discuss the implication of these results for the cosmological density parameter., Comment: 46 pages, 21 figures, Latex, submitted to ApJ

We report the detection of weak gravitational lensing of distant background galaxies by two rich, X-ray luminous clusters of galaxies, MS 1137+66 at z=0.783 and RXJ 1716+67 at z=0.813. We detect a shear signal in each cluster over the radial range 100~0.8., Comment: 11 pages, 2 figures, figures 1 and 2 can be found at http://www.ifa.hawaii.edu/~clowe contains corrected Navarro CDM profiles

We present an analysis of the properties of the lensing cluster MS0440+0204 at z=0.1965. MS0440+0204 has been observed with a variety of telescopes at diverse wavelengths: from the ground with CFHT, MMT and KECK and from Earth orbit with HST, ROSAT and ASCA. Mass determinations are separately obtained from galaxy virial motions and X-ray profile fitting. A simple beta-model fit to the X-ray data yields a mass of (1.3+/-0.2) x10^{14} M_sun within 583 kpc of the cluster center, but more general models fit all of our data better and allow a wider range of masses that are consistent with the lensing data. In addition, the X-ray data yield a mass distribution profile that is well described by a beta model with a core radius of 26.7 kpc. The velocity dispersion of galaxies yields a mass of 4.8^{+1.5}_{-0.94} x 10^{14} M_sun within 900 kpc. In the inner 24.5arcsec there are 24 arcs that appear to be strong gravitationally lensed images of background sources. Models of the cluster mass distribution and its lensing properties reveal 5 background sources at various redshifts each forming 2 or more arcs. At 100 kpc, the lower limit mass from lensing is about a factor of 2 greater than the X-ray determined mass. lens model than the X-ray determination. To reconcile the mass estimates from the X-rays and the lensing and to try to understand the steep slope of the gravitational lens mass, we tentatively explore a model with a supercluster surrounding the cluster and with a mass profile that increases more rapidly than a beta model at large radii., Comment: 37 pages, LaTex

We present the results of the deepest optically identified X-ray survey yet made. The X-ray survey was made with the ROSAT PSPC and reaches a flux limit of 1.6x10^-15 erg cm^-2 s^-1 (0.5--2.0 keV). Above a flux limit of 2x10^-15 erg cm^-2 s^-1 we define a complete sample of 70 sources of which 59 are identified. Some (5) other sources have tentative identifications and in a further 4 the X-ray error-boxes are blank to R=23 mag. At the brighter flux levels (>= 10^-14 erg cm^-2 s^-1) we confirm the results of previous less deep X-ray surveys with 84% of the sources begin QSOs. At fainter fluxes, however, the survey is dominated by a population of galaxies with narrow optical emission lines (NELGs). In addition, a number of groups and clusters of galaxies are found at intermediate fluxes. Most of these are poor systems of low X-ray luminosity and are generally found at redshifts of > 0.3. Their numbers are consistent with a zero evolutionary scenario, in contrast to the situation for high luminosity clusters at the same redshift. We discuss the significance of these results to the determination of the cosmic soft X-ray background (XRB) and show that at 2x10^-15 erg cm^-2 s^-1, we have resolved more than 50% of the background. We also briefly consider the probable importance of NELG objects to the residual background and look at some of the properties of these unusual objects., Comment: 28 pages, LaTeX, 17 figures from 32 postscript files. Uses mn.sty and epsf.sty. Accepted by MNRAS. Revised version now contains data table. For more information see http://www.astro.soton.ac.uk/~amn/UKdeep

A deep HST image of the MS 2137-2353 core has revealed detailed morphological structures in two arc systems, which are modelled and well reproduced after a complete analysis of the lensing properties of the dark matter component. Latter could have a simple elliptical mass distribution with ellipticity and angular orientation similar to those of the visible and X-ray light, which suggests that the MS 2137-2353 is a relaxed cluster at z=0.313. The predicted density profile ($\rho$ $\sim$ $r^{-1.56\pm0.1}$ with $r_{c}$ $\le$ 22.5$h_{50}^{-1}$ kpc) within 150$h_{50}^{-1}$ kpc implies increasing M/L ratio with the radius, and could be in agreement with predictions from standard CDM simulations. At least two faint sources (unlensed magnitude, R=23.9 and 26, respectively) are aligned with the cluster core and are responsible of the arc systems. They have been reconstructed with details as small as 0".02 (or 160$h_{50}^{-1}$ pc in the source assumed at z= 1), one could be a nearly edge-on barred spiral galaxy, and the other has a more complex morphology, which could be related to a close interacting pair and/or to dust. They show strong signs of star formation indicated by compact HII regions well off their center. The observation of giant luminous arcs by HST could even resolve the size of giant HII regions at z$\sim$ 1., Comment: 12 pages,Latex, available at ftp://gin.obspm.fr/depot/hammer , Submitted to ApJ Letter

In the deepest optically identified X-ray survey yet performed, we have identified 32 X-ray selected QSOs to a flux limit of 2x10^{-15} erg cm^{-2} s^{-1} (0.5-2 keV). The survey, performed with the ROSAT PSPC, has 89% spectroscopic completeness. The QSO log(N)-log(S) relation is found to have a break to a flat slope at faint fluxes. The surface density of QSOs at the survey limit is 230+/-40 per square degree, the largest so far of any QSO survey. We have used this survey to measure the QSO X-ray luminosity function at low luminosities (Lx<10^{44.5} erg s^{-1}) and high redshifts (1

We describe a new 4096x4096 pixel CCD mosaic camera (MOCAM) available at the prime focus of the Canada-France-Hawaii Telescope (CFHT). The camera is a mosaic of four 2048x2048$ Loral frontside-illuminated CCDs with 15 $\mu$m pixels, providing a field of view of 14'x14' at a scale of 0.21''/pixel. MOCAM is equipped with B, V, R and I filters and has demonstrated image quality of 0.5''-0.6'' FWHM over the entire field. MOCAM will also be used with the CFHT adaptive optic bonnette and will provide a field of view of 90'' at a scale of 0.02 ''/pixel. MOCAM works within the CFHT Pegasus software environment and observers familiar with this system require no additional training to use this camera effectively. The technical details, the performance and the first images obtained on the telescope with MOCAM are presented. In particular, we discuss some important improvements with respect to the standard single-CCD FOCAM camera, such as multi-output parallel readout and dynamic anti-blooming. We also discuss critical technical issues concerning future wide-field imaging facilities at the CFHT prime focus in light of our experience with MOCAM and our recent experience with the even larger UH 8192x8192 pixel CCD mosaic camera., Comment: Accepted for publication in Publication of the Astronomical Society of the Pacific. Latex with aas2pp4.sty and epsf.sty. 12 pages, 10 figures

We report the detection of weak gravitational lensing of faint, distant background galaxies by the rich, X-ray luminous cluster of galaxies MS1054-03 at z=0.83. This is the first measurement of weak lensing by a bona fide cluster at such a high redshift. We detect tangential shear at the 5% - 10% level over a range of radii 50'' < r < 250'' centered on the optical position of the cluster. Two-dimensional mass reconstruction using galaxies with 21.5 < I < 25.5 shows a strong peak which coincides with the peak of the smoothed cluster light distribution. Splitting this sample by magnitude (at I = 23.5) and color (at R-I = 0.7), we find that the brighter and redder subsamples are only very weakly distorted, indicating that the faint blue galaxies (FBG's), which dominate the shear signal, are relatively more distant. The derived cluster mass is quite sensitive to the N(z) for the FBG's. At one extreme, if all the FBG's are at z_s = 3, then the mass within a $0.5h^{-1}$Mpc aperture is $(5.9 \pm 1.24)\times 10^{14}$\h1 $M_\odot$, and the mass-to-light ratio is $M/L_V = 350 \pm 70 h$ in solar units. For $z_s = 1.5$ the derived mass is $\sim$70\% higher and $M/L \simeq 580 h$. If $N(z)$ follows the no evolution model (in shape) then $M/L \simeq 800h$, and if all the FBG's lie at $z_s\la 1$ the required $M/L$ exceeds $1600h$. These data provide clear evidence that large, dense mass concentrations existed at early epochs; that they can be weighed efficiently by weak lensing observations; and that most of the FBG's are at high redshift., Comment: Submitted to ApJ, 15 pages (incl 8 figs, 3 of which are plates). Plate images not included, but are available from ftp://hubble.ifa.hawaii.edu/pub/ger/ms1054/ms1054_fig[1,3,5].ps.Z

This Letter presents the serendipitous discovery of a large arc in an X-ray selected cluster detected in the Rosat North Ecliptic Pole (NEP) survey. The cluster, associated with Abell 2280, is identified as the optical counterpart of the X-ray source RXJ 1743.5+6341. This object is a medium--distant z=0.326 and luminous (L_(0.5-2 kev) = 5.06x10**44 erg/s) cluster dominated by a central bright galaxy. The arc is located ~ 14 arcsec to the North-West of the cD and it is detected in the R and I bands but not in the B band. Photometric and spectroscopic observations of the cluster and of the arc are presented. Other similar discoveries in the course of imaging and spectroscopic surveys of Rosat are expected in the future., Comment: 4 pages

During the course of a gravitational lensing survey of distant, X-ray selected, EMSS clusters of galaxies, we have studied 6 X-ray luminous (Lx > 5x10^44 h_{50}^{-2} erg s^{-1}) clusters at redshifts exceeding z=0.5. All of these clusters are apparently massive. In addition to their high X-ray luminosity, two of the clusters at z~0.6 exhibit gravitationally lensed arcs. Furthermore, the highest redshift cluster in our sample, MS1054-0321 at z=0.826, is both extremely X-ray luminous (L[0.3-3.5 keV]=9.3x10^{44} h_{50}^{-2} erg s^{-1}), and exceedingly rich with an optical richness comparable to an Abell Richness Class 4 cluster. In this {\it Letter}, we discuss the cosmological implications of the very existence of these clusters for hierarchical stucture formation theories such as standard \Omega=1 CDM, hybrid \Omega=1 C+HDM, and flat, low-density \Lambda+CDM models., Comment: 11 pages, compressed and uuencoded postscript. 3 postscript images can be found in ftp://hubble.ifa.hawaii.edu/pub/ger/*.ps.Z To appear in ApJ Letters.

An atlas of deep, wide-field R-band CCD images of a complete sample of distant, X-ray-selected clusters of galaxies is presented. These clusters are the 41 most distant (z>= 0.15) and most X-ray luminous (Lx >= 2x10**44 erg\s-1) clusters in the Einstein Observatory Extended Medium Sensitivity Survey (EMSS) catalog that are observable from Mauna Kea (delta > -40 deg). The sample spans a redshift range of 0.15 <= z < 0.81 and includes at least 2 and possibly as many as 6 rich clusters with z>0.5. For the most part, the data are of superior quality, with a median seeing of 0''.8 FWHM and coverage of at least 1 Mpc X 1 Mpc in the cluster frame (Ho=50; q0=1/2). In addition, we update the available optical, X-ray and radio data on the entire EMSS sample of 104 clusters. We outline the cluster selection criteria in detail, and emphasize that X-ray-selected cluster samples may prove to be more useful for cosmological studies than optically selected samples. The EMSS cluster sample in particular can be exploited for diverse cosmological investigations, as demonstrated by the detection of evolution in the X-ray luminosity function previously reported, and more recently by the discovery of a large number of gravitationally lensed images in these clusters., Comment: 20 pages, LaTeX Version 2.09, IfA-94/6