1,773 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
The Near Infrared and Multiwavelength Afterglow of GRB 000301c
We present near-infrared observations of the counterpart of GRB 000301c. The
K' filter (2.1 micron) light curve shows a well-sampled break in the decay
slope at t=3.5 days post-burst. The early time slope is very shallow (~ -0.1),
while the late time slope is steep (-2.2). Comparison with the optical (R band)
light curve shows marginally significant differences, especially in the early
time decay slope (which is steeper in the optical) and the break time (which
occurs later in the optical). This is contrary to the general expectation that
light curve breaks should either be achromatic (e.g., for breaks due to
collimation effects) or should occur later at longer wavelengths (for most
other breaks). The observed color variations might be intrinsic to the
afterglow, or might indicate systematic errors of > 0.08 magnitude in all
fluxes. Even if the break is achromatic, we argue that its sharpness poses
difficulties for explanations that depend on collimated ejecta. The R light
curve shows further signs of fairly rapid variability (a bump, steep drop, and
plateau) that are not apparent in the K' light curve. In addition, by combining
the IR-optical-UV data with millimeter and radio fluxes, we are able to
constrain the locations of the self-absorption break and cooling break and to
infer the location of the spectral peak at t=3 days: f_nu = 3.4 mJy at nu=1e12
Hz. Using the multiwavelength spectral energy distribution, we are able to
constrain the blast wave energy, which was E > 3e53 erg if the explosion was
isotropic. This implies a maximum gamma ray production efficiency of ~ 0.15 for
GRB 000301C.Comment: Accepted to The Astrophysical Journal. 24 pages, 4 figures, 3 tables;
uses AASTeX 5 macros. This version includes a new figure (R-K' color vs.
time), a better sampled R band light curve, and more extensive discussion of
the optical data and error analysi
Luminosity functions of Lyman-alpha emitters at z=6.5, and z=5.7: evidence against reionization at z=6
Lyman-alpha emission from galaxies should be suppressed completely or
partially at redshifts beyond reionization. Without knowing the instrinsic
properties of galaxies at z = 6.5, this attenuation is hard to infer in any one
source, but can be infered from a comparison of luminosity functions of
lyman-alpha emitters at redshifts just before and after reionization. We
combine published surveys of widely varying depths and areas to construct
luminosity functions at z=6.5 and 5.7, where the characteristic luminosity
L_star and density phi_star are well constrained while the faint-end slope of
the luminosity function is essentially unconstrained. Excellent consistency is
seen in all but one published result. We then calculate the likelihood of
obtaining the z=6.5 observations given the z=5.7 luminosity function with (A)
no evolution and (B) an attenuation of a factor of three. Hypothesis (A) gives
an acceptable likelihood while (B) does not. This indicates that the z=6.5
lyman-alpha lines are not strongly suppressed by a neutral intergalactic medium
and that reionization was largely complete at z = 6.5.Comment: Submitted to Astrophysical Journal Letter
Dynamics of ``Small Galaxies'' in the Hubble Deep Field
We have previously found in the Hubble Deep Field a significant angular
correlation of faint, high color-redshift objects on scales below one
arcsecond, or several kiloparsecs in metric size. We examine the correlation
and nearest neighbor statistics to conclude that 38% of these objects in the
HDF have a companion within one arcsecond, three times the number expected in a
random distribution with the same number of objects. We examine three dynamical
scenarios for these object multiplets: 1) the objects are star-forming regions
within normal galaxies, whose disks have been relatively dimmed by K-correction
and surface brightness dimming; 2) they are fragments merging into large
galaxies; 3) they are satellites accreting onto normal L_* galaxies. We find
that hypothesis 1 is most tenable. First, large galaxies in the process of a
merger formation would have accumulated too much mass in their centers (5e12
M_sun inside 2 kpc) to correspond to present day objects. Second, accretion by
dynamical friction occurs with a predictable density vs. radius slope, not seen
among the faint HDF objects. Since the dynamical friction time is roughly (1
Gyr), a steady-state should have been reached by redshift z < 5. Star-forming
regions within galaxies clearly present no dynamical problems. Since large
spirals would still appear as such in the HDF, we favor a scenario in which the
faint compact sources in the HDF are giant starforming regions within small
normal galaxies, such as Magellanic irregulars. Finally we checked that
reduction in mass-to-light from induced star-formation cannot alone explain the
luminosity overdensity.Comment: AASTeX 4.0 (preprint), 4 PostScript figure
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
Clustering of Lyman alpha emitters at z ~ 4.5
We present the clustering properties of 151 Lyman alpha emitting galaxies at
z ~ 4.5 selected from the Large Area Lyman Alpha (LALA) survey. Our catalog
covers an area of 36' x 36' observed with five narrowband filters. We assume
that the angular correlation function w(theta) is well represented by a power
law A_w = Theta^(-beta) with slope beta = 0.8, and we find A_w = 6.73 +/- 1.80.
We then calculate the correlation length r_0 of the real-space two-point
correlation function xi(r) = (r/r_0)^(-1.8) from A_w through the Limber
transformation, assuming a flat, Lambda-dominated universe. Neglecting
contamination, we find r_0 = 3.20 +/- 0.42 Mpc/h. Taking into account a
possible 28% contamination by randomly distributed sources, we find r_0 = 4.61
+/- 0.6 Mpc/h. We compare these results with the expectations for the
clustering of dark matter halos at this redshift in a Cold Dark Matter model,
and find that the measured clustering strength can be reproduced if these
objects reside in halos with a minimum mass of 1-2 times 10^11 Solar masses/h.
Our estimated correlation length implies a bias of b ~ 3.7, similar to that of
Lyman-break galaxies (LBG) at z ~ 3.8-4.9. However, Lyman alpha emitters are a
factor of ~ 2-16 rarer than LBGs with a similar bias value and implied host
halo mass. Therefore, one plausible scenario seems to be that Lyman alpha
emitters occupy host halos of roughly the same mass as LBGs, but shine with a
relatively low duty cycle of 6-50%.Comment: 23 pages in preprint format, 4 figures, ApJ accepte
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
Galactic winds and stellar populations in Lyman emitting galaxies at z ~ 3.1
We present a sample of 33 spectroscopically confirmed z ~ 3.1
Ly-emitting galaxies (LAEs) in the Cosmological Evolution Survey
(COSMOS) field. This paper details the narrow-band survey we conducted to
detect the LAE sample, the optical spectroscopy we performed to confirm the
nature of these LAEs, and a new near-infrared spectroscopic detection of the [O
III] 5007 \AA\ line in one of these LAEs. This detection is in addition to two
[O III] detections in two z ~ 3.1 LAEs we have reported on previously (McLinden
et al 2011). The bulk of the paper then presents detailed constraints on the
physical characteristics of the entire LAE sample from spectral energy
distribution (SED) fitting. These characteristics include mass, age,
star-formation history, dust content, and metallicity. We also detail an
approach to account for nebular emission lines in the SED fitting process -
wherein our models predict the strength of the [O III] line in an LAE spectrum.
We are able to study the success of this prediction because we can compare the
model predictions to our actual near-infrared observations both in galaxies
that have [O III] detections and those that yielded non-detections. We find a
median stellar mass of 6.9 10 M and a median star
formation rate weighted stellar population age of 4.5 10 yr. In
addition to SED fitting, we quantify the velocity offset between the [O III]
and Ly lines in the galaxy with the new [O III] detection, finding that
the Ly line is shifted 52 km s redwards of the [O III] line,
which defines the systemic velocity of the galaxy.Comment: 38 pages, 27 figures, 4 tables, Accepted for publication in MNRA
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