An Overdensity of Lyman-alpha Emitters at Redshift z=5.7 near the Hubble Ultra Deep Field

We have identified an obvious and strong large scale structure at redshift z=5.75 in a wide (31 by 33 arcminute) field, narrowband survey of the Chandra Deep Field South region. This structure is traced by 17 candidate Lyman alpha emitters, among which 12 are found in an 823nm filter (corresponding to Lyman alpha at z=5.77 +- 0.03) and 5 in an 815nm image (z=5.70 +- 0.03). The Lyman alpha emitters in both redshift bins are concentrated in one quadrant of the field. The Hubble Ultra Deep Field, Chandra Deep Field South, and GOODS-South fields all lie near the edge of this overdense region. Our results are consistent with reports of an overdensity in the UDF region at z=5.9. This structure is the highest redshift overdensity found so far.Comment: 12 pages, AASTeX. Submitted to ApJ Letters, and revised in response to referee's comment

The Dynamics and Light Curves of Beamed Gamma Ray Burst Afterglows

The energy requirements of gamma ray bursts have in past been poorly constrained because of three major uncertainties: The distances to bursts, the degree of burst beaming, and the efficiency of gamma ray production. The first of these has been resolved, with both indirect evidence (the distribution of bursts in flux and position) and direct evidence (redshifted absorption features in the afterglow spectrum of GRB 970508) pointing to cosmological distances. We now wish to address the second uncertainty. Afterglows allow a statistical test of beaming, described in an earlier paper. In this paper, we modify a standard fireball afterglow model to explore the effects of beaming on burst remnant dynamics and afterglow emission. If the burst ejecta are beamed into angle zeta, the burst remnant's evolution changes qualitatively once its bulk Lorentz factor Gamma < 1/zeta: Before this, Gamma declines as a power law of radius, while afterwards, it declines exponentially. This change results in a broken power law light curve whose late-time decay is faster than expected for a purely spherical geometry. These predictions disagree with afterglow observations of GRB 970508. We explored several variations on our model, but none seems able to change this result. We therefore suggest that this burst is unlikely to have been highly beamed, and that its energy requirements were near those of isotropic models. More recent afterglows may offer the first practical applications for our beamed models.Comment: 18 pages, uses emulateapj.sty, four embedded postscript figures. Submitted to The Astrophysical Journal, 199

GRB 990123: Reverse and Internal Shock Flashes and Late Afterglow

The prompt (t \siml 0.16 days) light curve and initial 9-th magnitude optical flash from GRB 990123 can be attributed to a reverse external shock, or possibly to internal shocks. We discuss the time decay laws and spectral slopes expected under various dynamical regimes, and discuss the constraints imposed on the model by the observations, arguing that they provide strongly suggestive evidence for features beyond those in the simple standard model. The longer term afterglow behavior is discussed in the context of the forward shock, and it is argued that, if the steepening after three days is due to a jet geometry, this is likely to be due to jet-edge effects, rather than sideways expansion.Comment: M.N.R.A.S., subm. 2/26/99; (preprint uses aaspp4.sty), 9 page

Young red supergiants and the near infrared light appearance of disk galaxies

Disk galaxies often show prominent nonaxisymmetric features at near-infrared wavelengths. Such features may indicate variations in the surface density of stellar mass, contributions from young red supergiants in star forming regions, or substantial dust obscuration. To distinguish among these possibilities, we have searched for spatial variations in the 2.3 micron photometric CO index within the disks of three nearby galaxies (NGC 278, NGC 2649, & NGC 5713). This index measures the strength of the absorption bands of molecular CO in stellar atmospheres, and is strong in cool, low surface-gravity stars, reaching the largest values for red supergiants. We observe significant spatial CO index variations in two galaxies (NGC 278 & NGC 5713), indicating that the dominant stellar population in the near-infrared is not everywhere the same. Central CO index peaks are present in two galaxies; these could be due to either metallicity gradients or recent star formation activity. In addition, significant azimuthal CO index variations are seen in NGC 278. Because strong azimuthal metallicity gradients are physically implausible in disk galaxies, these features are most naturally explained by the presence of a young stellar population. The fraction of 2 micron light due to young stellar populations in star forming regions can be calculated from our data. Overall, young stellar populations can contribute ~3% of a (normal) galaxy's near infrared flux. Locally, this fraction may rise to ~33%. Thus, young stars do not dominate the total near infrared flux, but can be locally dominant in star forming regions, and can bias estimates of spiral arm amplitude or other nonaxisymmetric structures in galaxies' mass distributions.Comment: 28 pages including 3 postscript figures. A fourth figure is in jpeg format. Uses AASTeX. Accepted for publication in The Astronomical Journa

GRB Energetics and the GRB Hubble Diagram: Promises and Limitations

We present a complete sample of 29 GRBs for which it has been possible to determine temporal breaks (or limits) from their afterglow light curves. We interpret these breaks within the framework of the uniform conical jet model, incorporating realistic estimates of the ambient density and propagating error estimates on the measured quantities. In agreement with our previous analysis of a smaller sample, the derived jet opening angles of those 16 bursts with redshifts result in a narrow clustering of geometrically-corrected gamma-ray energies about E_gamma = 1.33e51 erg; the burst-to-burst variance about this value is a factor of 2.2. Despite this rather small scatter, we demonstrate in a series of GRB Hubble diagrams, that the current sample cannot place meaningful constraints upon the fundamental parameters of the Universe. Indeed for GRBs to ever be useful in cosmographic measurements we argue the necessity of two directions. First, GRB Hubble diagrams should be based upon fundamental physical quantities such as energy, rather than empirically-derived and physically ill-understood distance indicators. Second, a more homogeneous set should be constructed by culling sub-classes from the larger sample. These sub-classes, though now first recognizable by deviant energies, ultimately must be identifiable by properties other than those directly related to energy. We identify a new sub-class of GRBs (``f-GRBs'') which appear both underluminous by factors of at least 10 and exhibit a rapid fading at early times. About 10-20% of observed long-duration bursts appear to be f-GRBs.Comment: Accepted to the Astrophysical Journal (20 May 2003). 19 pages, 3 Postscript figure

Three-dimensional flow structure and bed morphology in large elongate meander loops with different outer bank roughness characteristics

© 2016. American Geophysical Union. All Rights Reserved. Few studies have examined the three-dimensional flow structure and bed morphology within elongate loops of large meandering channels. The present study focuses on the spatial patterns of three-dimensional flow structure and bed morphology within two elongate meander loops and examines how differences in outer bank roughness influence near-bank flow characteristics. Three-dimensional velocities were measured during two different events—a near-bankfull flow and an overbank event. Detailed data on channel bathymetry and bed form geometry were obtained during a near-bankfull event. Flow structure within the loops is characterized by strong topographic steering by the point bar, by the development of helical motion associated with flow curvature, and by acceleration of flow where bedrock is exposed along the outer bank. Near-bank velocities during the overbank event are less than those for the near-bankfull flow, highlighting the strong influence of the point bar on redistribution of mass and momentum of the flow at subbankfull stages. Multiple outer bank pools are evident within the elongate meander loop with low outer bank roughness, but are not present in the loop with high outer bank roughness, which may reflect the influence of abundant large woody debris on near-bank velocity characteristics. The positions of pools within both loops can be linked to spatial variations in planform curvature. The findings indicate that flow structure and bed morphology in these large elongate loops is similar to that in small elongate loops, but differs somewhat from flow structure and bed morphology reported for experimental elongate loops

Probing the Reionization History of the Universe using the Cosmic Microwave Background Polarization

The recent discovery of a Gunn--Peterson (GP) trough in the spectrum of the redshift 6.28 SDSS quasar has raised the tantalizing possibility that we have detected the reionization of the universe. However, a neutral fraction (of hydrogen) as small as 0.1% is sufficient to cause the GP trough, hence its detection alone cannot rule out reionization at a much earlier epoch. The Cosmic Microwave Background (CMB) polarization anisotropy offers an alternative way to explore the dark age of the universe. We show that for most models constrained by the current CMB data and by the discovery of a GP trough (showing that reionization occurred at z > 6.3), MAP can detect the reionization signature in the polarization power spectrum. The expected 1-sigma error on the measurement of the electron optical depth is around 0.03 with a weak dependence on the value of that optical depth. Such a constraint on the optical depth will allow MAP to achieve a 1-sigma error on the amplitude of the primordial power spectrum of 6%. MAP with two years (Planck with one year) of observation can distinguish a model with 50% (6%) partial ionization between redshifts of 6.3 and 20 from a model in which hydrogen was completely neutral at redshifts greater than 6.3. Planck will be able to distinguish between different reionization histories even when they imply the same optical depth to electron scattering for the CMB photons.Comment: ApJ version. Added Figure 2 and reference

Chandra X-ray Sources in the LALA Cetus Field

The 174 ks Chandra Advanced CCD Imaging Spectrometer exposure of the Large Area Lyman Alpha Survey (LALA) Cetus field is the second of the two deep Chandra images on LALA fields. In this paper we present the Chandra X-ray sources detected in the Cetus field, along with an analysis of X-ray source counts, stacked X-ray spectrum, and optical identifications. A total of 188 X-ray sources were detected: 174 in the 0.5-7.0 keV band, 154 in the 0.5-2.0 keV band, and 113 in the 2.0-7.0 keV band. The X-ray source counts were derived and compared with LALA Bootes field (172 ks exposure). Interestingly, we find consistent hard band X-ray source density, but 36+-12% higher soft band X-ray source density in Cetus field. The weighted stacked spectrum of the detected X-ray sources can be fitted by a powerlaw with photon index Gamma = 1.55. Based on the weighted stacked spectrum, we find that the resolved fraction of the X-ray background drops from 72+-1% at 0.5-1.0 keV to 63+-4% at 6.0-8.0 keV. The unresolved spectrum can be fitted by a powerlaw over the range 0.5-7 keV, with a photon index Gamma = 1.22. We also present optical counterparts for 154 of the X-ray sources, down to a limiting magnitude of r' = 25.9 (Vega), using a deep r' band image obtained with the MMT.Comment: 21 pages, including 6 figures, 1 table, ApJ accepte