1,154 research outputs found
Strong Clustering of Lyman Break Galaxies around Luminous Quasars at z~4
In the standard picture of structure formation, the first massive galaxies
are expected to form at the highest peaks of the density field, which
constitute the cores of massive proto-clusters. Luminous quasars (QSOs) at z~4
are the most strongly clustered population known, and should thus reside in
massive dark matter halos surrounded by large overdensities of galaxies,
implying a strong QSO-galaxy cross-correlation function. We observed six z~4
QSO fields with VLT/FORS exploiting a novel set of narrow band filters custom
designed to select Lyman Break Galaxies (LBGs) in a thin redshift slice of
Delta_z~0.3, mitigating the projection effects that have limited the
sensitivity of previous searches for galaxies around z>~4 QSOs. We find that
LBGs are strongly clustered around QSOs, and present the first measurement of
the QSO-LBG cross-correlation function at z~4, on scales of 0.1<~R<~9 Mpc/h
(comoving). Assuming a power law form for the cross-correlation function
xi=(r/r0_QG)^gamma, we measure r0_QG=8.83^{+1.39}_{-1.51} Mpc/h for a fixed
slope of gamma=2.0. This result is in agreement with the expected
cross-correlation length deduced from measurements of the QSO and LBG
auto-correlation function, and assuming a linear bias model. We also measure a
strong auto-correlation of LBGs in our QSO fields finding
r0_GG=21.59^{+1.72}_{-1.69} Mpc/h for a fixed slope of gamma=1.5, which is ~4
times larger than the LBG auto-correlation length in random fields, providing
further evidence that QSOs reside in overdensities of LBGs. Our results
qualitatively support a picture where luminous QSOs inhabit exceptionally
massive (M_halo>10^12 M_sun) dark matter halos at z~4.Comment: 25 pages, 22 figures, submitted to the Ap
Clustering of Lyman-alpha Emitters Around Quasars at
The strong observed clustering of quasars indicates they are hosted
by massive () dark matter
halos. Assuming quasars and galaxies trace the same large-scale structures,
this should also manifest as strong clustering of galaxies around quasars.
Previous work on high-redshift quasar environments, mostly focused at ,
have failed to find convincing evidence for these overdensities. Here we
conduct a survey for Lyman alpha emitters (LAEs) in the environs of 17 quasars
at probing scales of . We measure an
average LAE overdensity around quasars of 1.4 for our full sample, which we
quantify by fitting the quasar-LAE cross-correlation function. We find
consistency with a power-law shape with correlation length of
for a fixed slope of
. We also measure the LAE auto-correlation length and find
\,cMpc (), which is
times higher than the value measured in blank fields. Taken together our
results clearly indicate that LAEs are significantly clustered around
quasars. We compare the observed clustering with the expectation from a
deterministic bias model, whereby LAEs and quasars probe the same underlying
dark matter overdensities, and find that our measurements fall short of the
predicted overdensities by a factor of 2.1. We discuss possible explanations
for this discrepancy including large-scale quenching or the presence of excess
dust in galaxies near quasars. Finally, the large cosmic variance from
field-to-field observed in our sample (10/17 fields are actually underdense)
cautions one from over-interpreting studies of quasar environments
based on a single or handful of quasar fields.Comment: 19 pages, 12 figures, submitted to the Ap
Spectroscopy of moderately high-redshift RCS-1 clusters
We present spectroscopic observations of 11 moderately high-redshift (z~0.7-
1.0) clusters from the first Red-Sequence Cluster Survey (RCS-1). We find
excellent agreement between the red-sequence estimated redshift and the
spectroscopic redshift, with a scatter of 10% at z>0.7. At the high-redshift
end (z>~0.9) of the sample, we find two of the systems selected are projections
of pairs of comparably rich systems, with red-sequences too close to
discriminate in (R-z') colour. In one of these systems, the two components are
close enough to be physically associated. For a subsample of clusters with
sufficient spectroscopic members, we examine the correlation between B_gcR
(optical richness) and the dynamical mass inferred from the velocity
dispersion. We find these measurements to be compatible, within the relatively
large uncertainties, with the correlation established at lower redshift for the
X-ray selected CNOC1 clusters and also for a lower redshift sample of RCS-1
clusters. Confirmation of this and calibration of the scatter in the relation
will require larger samples of clusters at these and higher redshifts.
[abridged]Comment: AJ accepted. 30 pages, 7 figures (figure 5 reduced quality
Squark Loop Correction to W^{+-} H^{-+} Associated Hadroproduction
We study the squark loop correction to W^{+-} H^{-+} associated
hadroproduction via gluon-gluon fusion within the minimal supersymmetric
extension of the standard model. We list full analytic results and
quantitatively analyze the resulting shift in the cross section at the CERN
Large Hadron Collider assuming a supergravity-inspired scenario.Comment: 13 pages (Latex), 5 figures (Postscript
The Optical Afterglow of GRB 011211
We present early-time optical photometry and spectroscopy of the optical
afterglow of the gamma-ray burst GRB 011211. The spectrum of the optical
afterglow contains several narrow metal lines which are consistent with the
burst occurring at a redshift of 2.140 +/- 0.001. The optical afterglow decays
as a power law with a slope of alpha = 0.83 +/- 0.04 for the first
approximately two days after the burst at which time there is evidence for a
break. The slope after the break is at least 1.4. There is evidence for rapid
variations in the R-band light approximately 0.5 days after the burst. These
variations suggest that there are density fluctuations near the gamma-ray burst
on spatial scales of approximately 40--125 AU. The magnitude of the break in
the light curve, the spectral slope, and the rate of decay in the optical,
suggest that the burst expanded into an ambient medium that is homogeneous on
large scales. We estimate that the local particle density is between
approximately 0.1 and 10 cm^{-3} and that the total gamma-ray energy in the
burst was 1.2--1.9 x 10^{50} erg. This energy is smaller than, but consistent
with, the ``standard'' value of (5 +/- 2) x 10^{50} erg. Comparing the observed
color of the optical afterglow with predictions of the standard beaming model
suggests that the rest-frame V-band extinction in the host galaxy is less than
approximately 0.03 mag.Comment: 17 pages, 4 figures, AASTeX 5.02, to appear in AJ Referee's report
incorporated, minor changes in the tex
Galaxy Clusters in the Line of Sight to Background Quasars: I. Survey Design and Incidence of MgII Absorbers at Cluster Redshifts
We describe the first optical survey of absorption systems associated with
galaxy clusters at z= 0.3-0.9. We have cross-correlated SDSS DR3 quasars with
high-redshift cluster/group candidates from the Red-Sequence Cluster Survey. We
have found 442 quasar-cluster pairs for which the MgII doublet might be
detected at a transverse (physical) distance d<2 Mpc from the cluster centers.
To investigate the incidence (dN/dz) and equivalent-width distribution n(W) of
MgII systems at cluster redshifts, two statistical samples were drawn out of
these pairs: one made of high-resolution spectroscopic quasar observations (46
pairs), and one made of quasars used in MgII searches found in the literature
(375 pairs). The results are: (1) the population of strong MgII systems
(W_0>2.0 Ang.) near cluster redshifts shows a significant (>3 sigma)
overabundance (up to a factor of 15) when compared with the 'field' population;
(2) the overabundance is more evident at smaller distances (d<1 Mpc) than
larger distances (d<2 Mpc) from the cluster center; and, (3) the population of
weak MgII systems (W_0<0.3 Ang.) near cluster redshifts conform to the field
statistics. Unlike in the field, this dichotomy makes n(W) in clusters appear
flat and well fitted by a power-law in the entire W-range. A sub-sample of the
most massive clusters yields a stronger and still significant signal. Since
either the absorber number density or filling-factor/cross-section affects the
absorber statistics, an interesting possibility is that we have detected the
signature of truncated halos due to environmental effects. Thus, we argue that
the excess of strong systems is due to a population of absorbers in an
overdense galaxy environment, and the lack of weak systems to a different
population, that got destroyed in the cluster environment. (Abridged)Comment: Accepted for publication in the Astrophysical Journa
La identificación del uso de fuentes secundarias de materiales líticos asistida por SIG: el Arroyo Chasicó (Argentina) como caso de estudio
The aim of this paper is to present the methodology deployed in the Chasicó basin (southern Pampas, Argentina) in order to discuss the differential contribution of primary and secondary lithic sources in the spatial and technological organization of late Holocene hunter-gatherers societies. Such methodology is based on the comparative analysis of curves constructed with data obtained from virtual transects traced over continuous spatial models representing different properties of regional lithic landscapes. Those models were created by interpolation using a technique called Inverse Distance Weighted (IDW) from scattered information recovered at point locations (i.e. archaeological sites and localities; n = 12). Investigated raw materials were rhyolites and two varieties of quartzites, and the monitored variables were relative frequency (F%), relative weight (P%) and cortex index (IC). Our results suggest that both varieties of quartzites were mainly procured from secondary sources distributed along the basin and that rhyolites were mostly obtained from primary sources located at the headwaters.El objetivo de este trabajo es presentar la metodología implementada en la cuenca del Arroyo Chasicó (sur de la Región Pampeana Argentina), orientada a discutir el uso diferencial de fuentes de materias primas líticas primarias y secundarias en relación con la organización espacial y tecnológica en sociedades cazadoras-recolectoras del Holoceno tardío. La metodología se basa en el análisis comparativo de curvas construidas con datos procedentes de transectos virtuales, trazados sobre modelos espaciales continuos representando diferentes propiedades de los “paisajes líticos” del área de estudio. Tales modelos se generaron por interpolación mediante la técnica denominada Inverse Distance Weighted (IDW) a partir de información procedente de localizaciones puntuales (i.e. sitios y localidades arqueológicos; n = 12). Las materias primas investigadas fueron riolitas y dos variedades de cuarcita y las variables consideradas fueron frecuencia relativa y peso relativo, expresadas ambas en términos porcentuales (F% y P%, respectivamente) e índice de corteza (IC). Los resultados obtenidos indican que, para ambas variedades de cuarcitas, la explotación de fuentes secundarias —distribuidas a lo largo de la cuenca— fue mayor que la de las primarias, mientras que las riolitas fueron obtenidas, principalmente, de las fuentes primarias situadas en las cabeceras
Cosmic Evolution of a Sample of Infrared Luminous Galaxies
A sample of faint, southern-hemisphere 60 μm sources
(f_(60) > 100mJy) detected as part of the IRAS Additional Observations has been used, in conjunction with CCD imaging, to produce a list of faint infrared galaxies. Redshifts of this sample of galaxies can be compared
with model predictions for several evolutionary scenarios; the comparisons show an excess of galaxies at higher redshifts (z > 0.1) compared with the predictions of models which assume no evolution of the infrared-galaxy luminosity function
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