The prompt optical/near-infrared flare of GRB 050904: the most luminous transient ever detected

With a redshift of z=6.295, GRB 050904 is the most distant gamma-ray burst ever discovered. It was an energetic event at all wavelengths and the afterglow was observed in detail in the near-infrared bands. We gathered all available optical and NIR afterglow photometry of this GRB to construct a composite NIR light curve spanning several decades in time and flux density. Transforming the NIR light curve into the optical, we find that the afterglow of GRB 050904 was more luminous at early times than any other GRB afterglow in the pre-\emph{Swift} era, making it at these wavelengths the most luminous transient ever detected. Given the intrinsic properties of GRB 050904 and its afterglow, we discuss if this burst is markedly different from other GRBs at lower redshifts.Comment: The Astronomical Journal, in press; revised version, including the comments of the referee (one figure added, text restructured, all conclusions unchanged), 7 pages, 3 figure

Selection of high-z supernovae candidates

Deep, ground based, optical wide-field supernova searches are capable of detecting a large number of supernovae over a broad redshift range up to z~1.5. While it is practically unfeasible to obtain spectroscopic redshifts of all the supernova candidates right after the discovery, we show that the magnitudes and colors of the host galaxies, as well as the supernovae, can be used to select high-z supernova candidates, for subsequent spectroscopic and photometric follow-up. Using Monte-Carlo simulations we construct criteria for selecting galaxies in well-defined redshift bands. For example, with a selection criteria using B-R and R-I colors we are able to pick out potential host galaxies for which z>0.85 with 80% confidence level and with a selection efficiency of 64-86%. The method was successfully tested using real observations from the HDF. Similarly, we show that that the magnitude and colors of the supernova discovery data can be used to constrain the redshift. With a set of cuts based on V-R and R-I in a search to m_I~25, supernovae at z~1 can be selected in a redshift interval sigma_z <0.15.Comment: 33 pages, 13 figures, accepted for publication in PASP (March 2002 issue

Evidence for extended, obscured starbursts in submm galaxies

We compare high-resolution optical and radio imaging of 12 luminous submm galaxies at z=2.2+/-0.2 observed with HST and the MERLIN and VLA at comparable spatial resolution, 0.3" (2kpc). The radio emission traces the likely far-infrared morphology of these dusty, luminous galaxies. In ~30% of the sample the radio appears unresolved, suggesting that the emission is compact: either an obscured AGN or nuclear starburst. However, in the majority, ~70% (8/12), the radio emission is resolved by MERLIN/VLA on scales of ~1" (10 kpc). For these galaxies the radio morphologies are broadly similar to their restframe UV emission seen by HST. We discuss the probable mechanisms for the extended emission and conclude that their luminous radio and submm emission arises from a large, spatially-extended starburst. The median SFRs are 1700Mo/yr occuring within a ~40kpc^2 region, giving a star formation density of 45Mo/yr/kpc^2. Such vigorous and extended starbursts appear to be uniquely associated with the submm population. A more detailed comparison of the distribution of UV and radio emission shows that the broad similarities on large scales are not carried through to smaller scales, where there is rarely a one-to-one correspondance. We interpret this as resulting from highly structured internal obscuration, suggesting that the vigorous activity is producing wind-blown channels through the obscuration in these galaxies. If correct this underlines the difficulty of using UV morphologies to understand structural properties of this population and also may explain the surprising frequency of Ly-alpha emission in their spectra. [Abridged]Comment: 7 pages, 3 figure

Lens or Binary? Chandra Observations of the Wide Separation Broad Absorption Line Quasar Pair UM425

We have obtained a 110 ksec Chandra ACIS-S exposure of UM425, a pair of QSOs at z=1.47 separated by 6.5 arcsec, which show remarkably similar emission and broad absorption line (BAL) profiles in the optical/UV. Our 5000 count X-ray spectrum of UM425A (the brighter component) is well-fit with a power law (photon spectral index Gamma=2.0) partially covered by a hydrogen column of 3.8x10^22 cm^-2. The underlying power-law slope for this object and for other recent samples of BALQSOs is typical of radio-quiet quasars, lending credence to the hypothesis that BALs exist in every quasar. Assuming the same Gamma for the much fainter image of UM425B, we detect an obscuring column 5 times larger. We search for evidence of an appropriately large lensing mass in our Chandra image and find weak diffuse emission near the quasar pair, with an X-ray flux typical of a group of galaxies at redshift z ~ 0.6. From our analysis of archival HST WFPC2 and NICMOS images, we find no evidence for a luminous lensing galaxy, but note a 3-sigma excess of galaxies in the UM425 field with plausible magnitudes for a z=0.6 galaxy group. However, the associated X-ray emission does not imply sufficient mass to produce the observed image splitting. The lens scenario thus requires a dark (high M/L ratio) lens, or a fortuitous configuration of masses along the line of sight. UM425 may instead be a close binary pair of BALQSOs, which would boost arguments that interactions and mergers increase nuclear activity and outflows.Comment: 13 pages, 9 figures, Accepted for publication in the Astrophysical Journa

Properties of Faint Distant Galaxies as seen through Gravitational Telescopes

This paper reviews the most recent developments related to the use of lensing clusters of galaxies as Gravitational Telescopes in deep Universe studies. We summarize the state of the art and the most recent results aiming at studying the physical properties of distant galaxies beyond the limits of conventional spectroscopy. The application of photometric redshift techniques in the context of gravitational lensing is emphasized for the study of both lensing structures and the background population of lensed galaxies. A presently ongoing search for the first building blocks of galaxies behind lensing clusters is presented and discussed.Comment: Review lecture given at "Gravitational Lensing: a unique tool for cosmology",Aussois, France, January 2003. To appear in ASP Conf. S., eds. D. Valls-Gabaud & J.-P. Kneib, 26 pages, 8 figure

Discovery of a Galaxy Cluster via Weak Lensing

We report the discovery of a cluster of galaxies via its weak gravitational lensing effect on background galaxies, the first spectroscopically confirmed cluster to be discovered through its gravitational effects rather than by its electromagnetic radiation. This fundamentally different selection mechanism promises to yield mass-selected, rather than baryon or photon-selected, samples of these important cosmological probes. We have confirmed this cluster with spectroscopic redshifts of fifteen members at z=0.276, with a velocity dispersion of 615 km/s. We use the tangential shear as a function of source photometric redshift to estimate the lens redshift independently and find z_l = 0.30 +- 0.08. The good agreement with the spectroscopy indicates that the redshift evolution of the mass function may be measurable from the imaging data alone in shear-selected surveys.Comment: revised version with minor changes, to appear in ApJ

Finding Galaxy Groups In Photometric Redshift Space: the Probability Friends-of-Friends (pFoF) Algorithm

We present a structure finding algorithm designed to identify galaxy groups in photometric redshift data sets: the probability friends-of-friends (pFoF) algorithm. This algorithm is derived by combining the friends-of-friends algorithm in the transverse direction and the photometric redshift probability densities in the radial dimension. The innovative characteristic of our group-finding algorithm is the improvement of redshift estimation via the constraints given by the transversely connected galaxies in a group, based on the assumption that all galaxies in a group have the same redshift. Tests using the Virgo Consortium Millennium Simulation mock catalogs allow us to show that the recovery rate of the pFoF algorithm is larger than 80% for mock groups of at least 2\times10^{13}M_{\sun}, while the false detection rate is about 10% for pFoF groups containing at least $\sim8$ net members. Applying the algorithm to the CNOC2 group catalogs gives results which are consistent with the mock catalog tests. From all these results, we conclude that our group-finding algorithm offers an effective yet simple way to identify galaxy groups in photometric redshift catalogs.Comment: AJ accepte

The Chandra Large Area Synoptic X-ray Survey (CLASXS) of the Lockman Hole-Northwest: The X-ray Catalog

We present the X-ray catalog and basic results from our Chandra Large Area Synoptic X-ray Survey (CLASXS) of the Lockman Hole-Northwest field. Our 9 ACIS-I fields cover a contiguous solid angle of ~0.4 sq. deg. and reach fluxes of 5E-16 cgs (0.4-2 keV) and 3E-15 cgs (2-8keV). Our survey bridges the gap between ultradeep pencil-beam surveys, such as the Chandra Deep Fields (CDFs), and shallower, large area surveys, allowing a better probe of the X-ray sources that contribute most of the 2-10 keV cosmic X-ray background (CXB). We find a total of 525 X-ray point sources and 4 extended sources. At ~10E-14 cgs 2-8 keV, our number counts are significantly higher than those of several non-contiguous, large area surveys. On the other hand, the integrated flux from the CLASXS field, combined with ASCA and Chandra ultradeep surveys, is consistent with results from other large area surveys, within the variance of the CXB. Spectral evolution is seen in the hardening of the sources at fluxes below 1E-14 cgs Above 4E1-14 cgs(0.4-8 keV), ~60 of the sources are variable. Four extended sources in CLASXS is consistent with the previously measured LogN-LogS of galaxy clusters. We report the discovery of a gravitational lensing arc associated with one of these sources. (Abridged)Comment: 67 pages, 26 figures, accepted for publication in the Astronomical Journa