We describe a semi-analytic model to predict the triaxial shapes of dark matter haloes utilizing the sequences of random merging events captured in merger trees to follow the evolution of each halo's energy tensor. When coupled with a simple model for relaxation toward a spherical shape, we find that this model predicts distributions of halo axis length ratios that approximately agree with those measured from cosmological N-body simulations once constrained to match the median axial ratio at a single halo mass. We demonstrate the predictive and explanatory power of this model by considering conditioned distributions of axis length ratios, and the mass dependence of halo shapes, finding these to be in good agreement with N-body results. This model provides both insight into the physics driving the evolution of halo triaxial shapes, and rapid quantitative predictions for the statistics of triaxiality connected directly to the formation history of the halo.

We report BV(RI)(C) CCD photometric data for a group of seven Cepheid variables in the young, rich cluster NGC 1866 in the Large Magellanic Cloud. The photometry was obtained as part of a program to determine accurate distances to these Cepheids by means of the infrared surface brightness technique and to improve the LMC Cepheid database for constructing Cepheid P-L and P-L-C relations. Using the new data together with data from the literature, we have determined improved periods for all variables. For five fundamental mode pulsators, the light curves are now of excellent quality and will lead to accurate distance and radius determinations once complete infrared light curves and radial velocity curves for these variables become available.

We present the discovery and follow-up observations of the afterglow of the gamma-ray burst GRB 011121 and its associated supernova SN 2001ke. Images were obtained with the Optical Gravitational Lensing Experiment 1.3 m telescope in BVRI passbands, starting 10.3 hr after the burst. The temporal analysis of our early data indicates a steep decay, independent of wavelength, with F-nu proportional to t(-1.72+/-0.05). There is no evidence for a break in the light curve earlier than 2.5 days after the burst. The spectral energy distribution determined from the early broadband photometry is a power law with Fnu proportional to nu(-0.66+/-0.13) after correcting for a large reddening. Spectra obtained with the Magellan 6.5 m Baade telescope reveal narrow emission lines from the host galaxy that provide a redshift of z = 0.362 +/- 0.001 to the GRB. We also present late R - and J-band observations of the afterglow similar to7-17 days after the burst. The late-time photometry shows a large deviation from the initial decline, and our data combined with Hubble Space Telescope photometry provide strong evidence for a supernova peaking about 12 rest-frame days after the GRB. The first spectrum ever obtained of a GRB supernova at cosmological distance revealed a blue continuum. SN 2001ke was more blue near maximum than SN 1998bw and faded more quickly, which demonstrates that a range of properties are possible in supernovae that generate GRBs. The blue color is consistent with a supernova interacting with circumstellar gas, and this progenitor wind is also evident in the optical afterglow. This is the best evidence to date that classical, long GRBs are generated by core-collapse supernovae.

We present deep imaging and optical spectroscopy of the newly discovered Red-Sequence Cluster Survey cluster RCS 043938 - 2904.9. This cluster, drawn from an extensive preliminary list, was selected for detailed study on the basis of its apparent optical richness. Spectroscopy of 11 members places the cluster at z = 0.951 +/- 0.006 and confirms the photometric redshift estimate from the (R-z) color-magnitude diagram. Analysis of the infrared imaging data demonstrates that the cluster is extremely rich, with excess counts in the K-s band exceeding the expected background counts by 9 sigma. The properties of the galaxies in RCS 043938 - 2904.9 are consistent with those seen in other clusters at similar redshifts. Specifically, the red-sequence color, slope, and scatter and the size-magnitude relation of these galaxies are all consistent with that seen in the few other known high-redshift clusters and indeed are consistent with appropriately evolved properties of local cluster galaxies. The apparent consistency of these systems implies that the rich high-redshift RCS clusters are directly comparable to the few other systems known at, most of which have been selected on the basis of X-ray emission.

We present a first cosmological analysis of a refined cluster catalog from the Red-Sequence Cluster Survey ( RCS). The input cluster sample is derived from the deepest 72.07 deg(2) of the RCS images, which probe to the highest redshift and lowest mass limits. The catalog contains 956 clusters over 0.35 < z < 0.95, limited by cluster richness and richness error. The calibration of the survey images has been extensively cross-checked against publicly available Sloan Digital Sky Survey imaging, and the cluster redshifts and richness that result from this well-calibrated subset of data are robust. We analyze the cluster sample via a general self-calibration technique. We fit simultaneously for the matter density, Omega(m), the normalization of the power spectrum, sigma(8), and four parameters describing the calibration of cluster richness to mass, its evolution with redshift, and scatter in the mass-richness relation. The principal goal of this general analysis is to establish the consistency ( or lack thereof) between the fitted parameters ( both cosmological and cluster mass observables) and available results on both from independent measures. From an unconstrained analysis, the derived values of Omega(m) and sigma(8) are 0.31(-0.10)(+0. 11) and 0.67(-0.13)(+0: 18) , respectively. An analysis including Gaussian priors on the slope and zero point of the mass-richness relation gives very similar results: 0.30(-0.11)(+0.12) and 0.70(-0.15)(+0.27) . Both analyses are in acceptable agreement with the current literature. The derived parameters describing the mass- richness relation in the unconstrained fit are also eminently reasonable and in good agreement with existing follow- up data on both the RCS-1 and other cluster samples. Our results directly demonstrate that future surveys ( optical and otherwise), with much larger samples of clusters, can give constraints competitive with other probes of cosmology.

Context. Most sub-mm emission line studies of galaxies to date have targeted sources with known redshifts where the frequencies of the lines are well constrained. Recent blind line scans circumvent the spectroscopic redshift requirement, which could represent a selection bias.

We spectroscopically confirmed two narrowband-selected redshift 7.0 Ly alpha galaxies and studied their rest-frame UV spectra. The Ly alpha and other UV nebular lines are very useful to confirm the galactic redshifts and diagnose the different mechanisms driving the ionizing emission. We observed two narrowband-selected z = 7.0 Ly alpha candidates in the LAGER Chandra Deep Field South (CDFS) field with IMACS at the Magellan telescope and confirmed they are Ly alpha emitters at z = 6.924 and 6.931. In one galaxy, we also obtained deep near-infrared (NIR) spectroscopy, which yields non-detections of the high-ionization UV nebular lines. We measured the upper limits of the ratios of C IV lambda 1548/Ly alpha, He II lambda 1640/Ly alpha, O III]lambda 1660/Ly alpha, and C III]lambda 1909/Ly alpha from the NIR spectra. These upper limits imply that the ionizing emission in this galaxy is dominated by normal star formation instead of an active galactic nucleus.

Context. Characterizing the number counts of faint (i.e., sub-mJy and especially sub-100 mu Jy), dusty star-forming galaxies is currently a challenge even for deep, high-resolution observations in the FIR-to-mm regime. They are predicted to account for approximately half of the total extragalactic background light at those wavelengths. Searching for dusty star-forming galaxies behind massive galaxy clusters benefits from strong lensing, enhancing their measured emission while increasing spatial resolution. Derived number counts depend, however, on mass reconstruction models that properly constrain these clusters.