2,170 research outputs found
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
Starburst Intensity Limit of Galaxies at z~5-6
The peak star formation intensity in starburst galaxies does not vary
significantly from the local universe to redshift z~6. We arrive at this
conclusion through new surface brightness measurements of 47 starburst galaxies
at z~5-6, doubling the redshift range for such observations. These galaxies are
spectroscopically confirmed in the Hubble Ultra Deep Field (HUDF) through the
GRism ACS program for Extragalactic Science (GRAPES) project. The starburst
intensity limit for galaxies at z~5-6 agree with those at z~3-4 and z~0 to
within a factor of a few, after correcting for cosmological surface brightness
dimming and for dust. The most natural interpretation of this constancy over
cosmic time is that the same physical mechanisms limit starburst intensity at
all redshifts up to z~6 (be they galactic winds, gravitational instability, or
something else). We do see two trends with redshift: First, the UV spectral
slope of galaxies at z~5-6 is bluer than that of z~3 galaxies, suggesting an
increase in dust content over time. Second, the galaxy sizes from z~3 to z~6
scale approximately as the Hubble parameter 1/H(z). Thus, galaxies at z~6 are
high redshift starbursts, much like their local analogs except for slightly
bluer colors, smaller physical sizes, and correspondingly lower overall
luminosities. If we now assume a constant maximum star formation intensity, the
differences in observed surface brightness between z~0 and z~6 are consistent
with standard expanding cosmology and strongly inconsistent with tired light
model.Comment: Accepted for publication in ApJ (23 pages, 5 figures). Minor changes
to tex
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
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
Searching for GRB remnants in nearby galaxies
Gamma Ray Bursts (GRBs) are expected to leave behind GRB remnants, similar to
how ``standard'' supernovae (SN) leave behind SN remnants. The identification
of these remnants in our own and in nearby galaxies would allow a much closer
look at GRB birth sites, and possibly lead to the discovery of the compact
object left behind. It would also provide independent constraints on GRB rates
and energetics. We have initiated an observational program to search for GRB
remnants in nearby galaxies. The identification is based on specific line
ratios, such as OIII/ and HeII/, which are expected to be
unusually high in case of GRB remnants according to the theoretical predictions
of Perna et al. (2000). The observing strategies and preliminary studies from a
test run at 2.34 m VBT as well as archival data from planetary nebulae surveys
of spiral galaxies are discussed.Comment: 4 pages, uses aipproc.cls To appear in AIP, proceeding of GRB
conference '30 years after the discovery' held at Santa Fe, NM from 8-12
Sept, 200
Enhanced Acoustic Transmission Into Dissipative Solid Materials Through The Use Of Inhomogeneous Plane Waves
A number of applications, for instance ultrasonic imaging and nondestructive testing, involve the transmission of acoustic energy across fluid–solid interfaces into dissipative solids. However, such transmission is generally hindered by the large impedance mismatch at the interface. In order to address this problem, inhomogeneous plane waves were investigated in this work for the purpose of improving the acoustic energy transmission. To this end, under the assumption of linear hysteretic damping, models for fluid–structure interaction were developed that allow for both homogeneous and inhomogeneous incident waves. For low-loss solids, the results reveal that, at the Rayleigh angle, a unique value of the wave inhomogeneity can be found which minimizes the reflection coefficient, and consequently maximizes the transmission. The results also reveal that with sufficient dissipation levels in the solid material, homogeneous incident waves yield lower reflection values than inhomogeneous waves, due to the large degrees of inhomogeneity inherent in the transmitted waves. Analytical conditions have also been derived which predict the dependence of the optimal incident wave type on the dissipation level and wave speeds in the solid medium. Finally, implications related to the use of acoustic beams of limited spatial extent are discussed
On the energy of gamma-ray bursts
We show that gamma-ray burst (GRB) afterglow observations strongly suggest,
within the fireball model framework, that radiating electrons are shock
accelerated to a power-law energy distribution, with universal index p \approx
2.2, and that the fraction of shock energy carried by electrons, \xi_e, is
universal and close to equipartition, \xi_e ~ 1/3. For universal p and \xi_e, a
single measurement of the X-ray afterglow flux on the time scale of a day
provides a robust estimate of the fireball energy per unit solid angle,
\epsilon, averaged over a conical section of the fireball of opening angle
\theta ~ 0.1.
Applying our analysis to BeppoSAX afterglow data we find that: (i) Fireball
energies are in the range of 4\pi\epsilon=10^{51.5} to 10^{53.5} erg; (ii) The
ratio of observed -ray to total fireball energy per unit solid angle,
\epsilon_\gamma / \epsilon, is of order unity, satisfying
abs[log10(\epsilon_\gamma/\epsilon)]<0.5; (iii) If fireballs are jet like,
their opening angle should satisfy \theta>=0.1.
Our results imply that if typical opening angles are \theta ~ 0.1, a value
consistent with our analysis, the total energy associated with a GRB event is
in the range of 10^{50} erg to 10^{51.5} erg.Comment: 16 pages; Submitted to Ap
Lyman-alpha emission galaxies at a redshift of z = 5.7 in the FORS Deep Field
We present the results of a search for Lyman-alpha emission galaxies at z~
5.7 in the FORS Deep Field. The objective of this study is to improve the faint
end of the luminosity function of high-redshift Lyman-alpha emitting galaxies
and to derive properties of intrinsically faint Lyman-alpha emission galaxies
in the young universe. Using FORS2 at the ESO VLT and a set of special
interference filters, we identified candidates for high-redshift Lyman-alpha
galaxies. We then used FORS2 in spectroscopic mode to verify the
identifications and to study their spectral properties. The narrow-band
photometry resulted in the detection of 15 likely Lyman-alpha emission
galaxies. Spectra with an adequate exposure time could be obtained for eight
galaxies. In all these cases the presence of Lyman-alpha emission at z = 5.7
was confirmed spectroscopically. The line fluxes of the 15 candidates range
between 3 and 16 * 10^-21 Wm^-2, which corresponds to star-formation rates not
corrected for dust between 1 and 5 Msun/yr. The luminosity function derived for
our photometrically identified objects extends the published luminosity
functions of intrinsically brighter Lyman-alpha galaxies. With this technique
the study of high-redshift Lyman-alpha emission galaxies can be extended to low
intrinsic luminosities.Comment: 9 pages, 17 figures. Accepted by A&A. PDF version with higher
resolution figures here:
http://www.lsw.uni-heidelberg.de/users/jheidt/fdf/pubs/fdflae5_7_110406.pd
Detecting Primordial Stars
We discuss the expected properties of the first stellar generations in the
Universe. We find that it is possible to discern truly primordial populations
from the next generation of stars by measuring the metallicity of high-z star
forming objects. The very low background of the future James Webb Space
Telescope (JWST) will enable it to image and study first-light sources at very
high redshifts, whereas its relatively small collecting area limits its
capability in obtaining spectra of z~10-15 first-light sources to either the
bright end of their luminosity function or to strongly lensed sources. With a
suitable investment of observing time JWST will be able to detect individual
Population III supernovae, thus identifying the very first stars that formed in
the Universe.Comment: [8 pages, 5 figures] Invited Talk, to appear in IMF@50: The Stellar
Initial Mass Function Fifty Years Later, eds E. Corbelli, F. Palla, and H.
Zinnecker (Dordrecht: Kluwer
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