1,607 research outputs found
Large Equivalent width Lyman-alpha line emission at z=4.5: young galaxies in a young universe
The Large Area Lyman Alpha survey has found ~ 150 Lyman-alpha emitters at
z=4.5. While stellar models predict a maximum Lyman-alpha equivalent width (EW)
of 240 angstrom, 60% of the Lyman-alpha emitters have EWs exceeding this value.
We attempt to model the observed EW distribution by combining stellar
population models with an extrapolation of Lyman break galaxy luminosity
function at z=4, incorporating observational selection effects and Malmquist
bias. To reproduce the high EWs seen in the sample we need to postulate a
stellar initial mass function (IMF) with extreme slope alpha = 0.5 (instead of
2.35); zero metallicity stars; or narrow-lined active galactic nuclei. The
models also reveal that only 7.5-15% of galaxies need show Lyman-alpha emission
to explain the observed number counts. This raises the possibility that either
star-formation in high redshift galaxies is episodic or the Lyman-alpha
galaxies we are seeing are the youngest 7.5-15% and that Lyman-alpha is
strongly quenched by dust at about 10 Mega-years of age.Comment: submitted to ApJ letter
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
First Results from the Large Area Lyman Alpha Survey
We report on a new survey for z=4.5 Lyman alpha sources, the Large Area Lyman
Alpha (LALA) survey. Our survey achieves an unprecedented combination of volume
and sensitivity by using narrow-band filters on the new 8192x8192 pixel CCD
Mosaic Camera at the 4 meter Mayall telescope of Kitt Peak National
Observatory.
Well-detected sources with flux and equivalent width matching known high
redshift Lyman alpha galaxies (i.e., observed equivalent width above 80
Angstroms and line+continuum flux between 2.6e-17 and 5.2e-17 erg/cm^2/sec in
an 80 Angstrom filter) have an observed surface density corresponding to 11000
+- 700 per square degree per unit redshift at z=4.5. Spatial variation in this
surface density is apparent on comparison between counts in 6561 and 6730
Angstrom filters.
Early spectroscopic followup results from the Keck telescope included three
sources meeting our criteria for good Lyman alpha candidates. Of these, one is
confirmed as a z=4.52 source, while another remains consistent with either
z=4.55 or z=0.81. We infer that 30 to 50% of our good candidates are bona fide
Lyman alpha emitters, implying a net density of about 4000 Lyman alpha galaxies
per square degree per unit redshift.Comment: 10 pages, 2 figures (3 .ps files), uses AASTeX 4. Submitted to The
Astrophysical Journal Letter
Effects of Dust Geometry in Lyman Alpha Galaxies at z = 4.4
Equivalent widths (EWs) observed in high-redshift Lyman alpha galaxies could
be stronger than the EW intrinsic to the stellar population if dust is present
residing in clumps in the inter-stellar medium (ISM). In this scenario,
continuum photons could be extinguished while the Lyman alpha photons would be
resonantly scattered by the clumps, eventually escaping the galaxy. We
investigate this radiative transfer scenario with a new sample of six Lyman
alpha galaxy candidates in the GOODS CDF-S, selected at z = 4.4 with
ground-based narrow-band imaging obtained at CTIO. Grism spectra from the HST
PEARS survey confirm that three objects are at z = 4.4, and that another object
contains an active galactic nuclei (AGN). If we assume the other five (non-AGN)
objects are at z = 4.4, they have rest-frame EWs from 47 -- 190 A. We present
results of stellar population studies of these objects, constraining their
rest-frame UV with HST and their rest-frame optical with Spitzer. Out of the
four objects which we analyzed, three objects were best-fit to contain stellar
populations with ages on the order of 1 Myr and stellar masses from 3 - 10 x
10^8 solar masses, with dust in the amount of A_1200 = 0.9 - 1.8 residing in a
quasi-homogeneous distribution. However, one object (with a rest EW ~ 150 A)
was best fit by an 800 Myr, 6.6 x 10^9 solar mass stellar population with a
smaller amount of dust (A_1200 = 0.4) attenuating the continuum only. In this
object, the EW was enhanced ~ 50% due to this dust. This suggests that large EW
Lyman alpha galaxies are a diverse population. Preferential extinction of the
continuum in a clumpy ISM deserves further investigation as a possible cause of
the overabundance of large-EW objects that have been seen in narrow-band
surveys in recent years.Comment: Submitted to the Astrophysical Journal. 35 pages, 7 figures and 4
table
Spectroscopic Properties of the z=4.5 Lyman-alpha Emitters
We present Keck/LRIS optical spectra of 17 Lya-emitting galaxies and one
Lyman break galaxy at z=4.5 discovered in the Large Area Lyman Alpha (LALA)
survey. The survey has identified a sample of ~350 candidate Lya-emitting
galaxies at z=4.5 in a search volume of 1.5 x 10^6 comoving Mpc^3. We targeted
25 candidates for spectroscopy; hence, the 18 confirmations presented herein
suggest a selection reliability of 72%. The large equivalent widths (median
W(rest)~80 A) but narrow physical widths (v < 500 km/s) of the Lya emission
lines, along with the lack of accompanying high-ionization state emission
lines, suggest that these galaxies are young systems powered by star formation
rather than by AGN activity. Theoretical models of galaxy formation in the
primordial Universe suggest that a small fraction of Lya-emitting galaxies at
z=4.5 may still be nascent, metal-free objects. Indeed, we find with 90%
confidence that 3 to 5 of the confirmed sources show W(rest) > 240 A, exceeding
the maximum Lya equivalent width predicted for normal stellar populations.
Nonetheless, we find no evidence for HeII 1640 emission in either individual or
composite spectra, indicating that though these galaxies are young, they are
not truly primitive, Population III objects.Comment: 12 pages, Accepted to Ap
How to Tell a Jet from a Balloon: A Proposed Test for Beaming in Gamma Ray Bursts
If gamma ray bursts are highly collimated, the energy requirements of each
event may be reduced by several (~ 4-6) orders of magnitude, and the event rate
increased correspondingly. Extreme conditions in gamma ray bursters lead to
highly relativistic motions (bulk Lorentz factors Gamma > 100). This results in
strong forward beaming of the emitted radiation in the observer's rest frame.
Thus, all information on gamma ray bursts comes from those ejecta emitted in a
narrow cone (opening angle 1/Gamma) pointing towards the observer. We are at
present ignorant of whether there are ejecta outside that cone or not.
The recent detection of longer wavelength transients following gamma ray
bursts allows an empirical test of whether gamma ray bursts are collimated jets
or spherical fireballs. The bulk Lorentz factor of the burst ejecta will
decrease with time after the event, as the ejecta sweep up the surrounding
medium. Thus, radiation from the ejecta is beamed into an ever increasing solid
angle as the burst remnant evolves. It follows that if gamma ray bursts are
highly collimated, many more optical and radio transients should be observed
without associated gamma rays than with them. Published supernova searches may
contain enough data to test the most extreme models of gamma ray beaming. We
close with a brief discussion of other possible consequences of beaming,
including its effect on the evolution of burst remnants.Comment: Original replaced with accepted refereed manuscript. 11 pages, uses
AASTeX 4.0 LaTeX macros. To be published in The Astrophysical Journal
Letters, vol. 487, p. L1 (20 September 1997
Spectroscopic Confirmation of Three Redshift 5.7 Lyman-alpha Emitters from the Large Area Lyman Alpha Survey
Narrow-band searches for Lyman alpha emission are an efficient way of
identifying star-forming galaxies at high redshifts. We present Keck telescope
spectra confirming redshifts z = 5.7 for three objects discovered in the Large
Area Lyman Alpha (LALA) survey at Kitt Peak National Observatory.
All three spectra show strong, narrow emission lines with the asymmetric
profile that is characteristically produced in high redshift Lyman alpha
emitters by preferential HI absorption in the blue wing of the line. These
objects are undetected in deep Bw, V, R, and 6600A narrow-band images from the
NOAO Deep Wide-Field Survey and from LALA, as expected from Lyman break and
Lyman alpha forest absorption at redshift z = 5.7. All three objects show large
equivalent widths (>= 150A in the rest-frame), suggesting at least one of the
following: a top-heavy initial mass function, very low stellar metallicity, or
the presence of an active nucleus. We consider the case for an active nucleus
to be weak in all three objects due to the limited width of the Lyman alpha
emission line (< 500 km/s) and the absence of any other indicator of quasar
activity.
The three confirmed high redshift objects were among four spectroscopically
observed targets drawn from the sample of 18 candidates presented by Rhoads and
Malhotra (2001). Thus, these spectra support the Lyman alpha emitter population
statistics from our earlier photometric study, which imply little evolution in
number density from z=5.7 to z=4.5 and provide strong evidence that the
reionization redshift is greater than 5.7.Comment: Submitted to AJ, June 2002. 15 pages, AASTe
Survey of bacterial diversity in chronic wounds using Pyrosequencing, DGGE, and full ribosome shotgun sequencing
<p>Abstract</p> <p>Background</p> <p>Chronic wound pathogenic biofilms are host-pathogen environments that colonize and exist as a cohabitation of many bacterial species. These bacterial populations cooperate to promote their own survival and the chronic nature of the infection. Few studies have performed extensive surveys of the bacterial populations that occur within different types of chronic wound biofilms. The use of 3 separate16S-based molecular amplifications followed by pyrosequencing, shotgun Sanger sequencing, and denaturing gradient gel electrophoresis were utilized to survey the major populations of bacteria that occur in the pathogenic biofilms of three types of chronic wound types: diabetic foot ulcers (D), venous leg ulcers (V), and pressure ulcers (P).</p> <p>Results</p> <p>There are specific major populations of bacteria that were evident in the biofilms of all chronic wound types, including <it>Staphylococcus, Pseudomonas, Peptoniphilus, Enterobacter, Stenotrophomonas, Finegoldia</it>, and <it>Serratia </it>spp. Each of the wound types reveals marked differences in bacterial populations, such as pressure ulcers in which 62% of the populations were identified as obligate anaerobes. There were also populations of bacteria that were identified but not recognized as wound pathogens, such as <it>Abiotrophia para-adiacens </it>and <it>Rhodopseudomonas </it>spp. Results of molecular analyses were also compared to those obtained using traditional culture-based diagnostics. Only in one wound type did culture methods correctly identify the primary bacterial population indicating the need for improved diagnostic methods.</p> <p>Conclusion</p> <p>If clinicians can gain a better understanding of the wound's microbiota, it will give them a greater understanding of the wound's ecology and will allow them to better manage healing of the wound improving the prognosis of patients. This research highlights the necessity to begin evaluating, studying, and treating chronic wound pathogenic biofilms as multi-species entities in order to improve the outcomes of patients. This survey will also foster the pioneering and development of new molecular diagnostic tools, which can be used to identify the community compositions of chronic wound pathogenic biofilms and other medical biofilm infections.</p
Constraining the Lyα escape fraction with far-infrared observations of Lyα emitters
We study the far-infrared properties of 498 Lyα emitters (LAEs) at z = 2.8, 3.1, and 4.5 in the Extended Chandra Deep Field-South, using 250, 350, and 500μm data from the Herschel Multi-tiered Extragalactic Survey and 870μm data from the LABOCA ECDFS Submillimeter Survey. None of the 126, 280, or 92 LAEs at z = 2.8, 3.1, and 4.5, respectively, are individually detected in the far-infrared data. We use stacking to probe the average emission to deeper flux limits, reaching 1σ depths of ∼0.1 to 0.4 mJy. The LAEs are also undetected at ?3σ in the stacks, although a 2.5σ signal is observed at 870μm for the z = 2.8 sources. We consider a wide range of far-infrared spectral energy distributions (SEDs), including an M82 and an Sd galaxy template, to determine upper limits on the far-infrared luminosities and far-infrared-derived star formation rates of the LAEs. These star formation rates are then combined with those inferred from the Lyα and UV emission to determine lower limits on the LAEs’ Lyα escape fraction (f esc (Lyα)). For the Sd SED template, the inferred LAEs f esc (Lyα) are ?30% (1σ) at z = 2.8, 3.1, and 4.5, which are all significantly higher than the global f esc (Lyα) at these redshifts. Thus, if the LAEs f esc (Lyα) follows the global evolution, then they have warmer far-infrared SEDs than the Sd galaxy template. The average and M82 SEDs produce lower limits on the LAE f esc (Lyα) of ∼10%–20% (1σ), all of which are slightly higher than the global evolution of f esc (Lyα), but consistent with it at the 2σ–3σ level
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