101,615 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
SAWdoubler: a program for counting self-avoiding walks
This article presents SAWdoubler, a package for counting the total number
Z(N) of self-avoiding walks (SAWs) on a regular lattice by the length-doubling
method, of which the basic concept has been published previously by us. We
discuss an algorithm for the creation of all SAWs of length N, efficient
storage of these SAWs in a tree data structure, and an algorithm for the
computation of correction terms to the count Z(2N) for SAWs of double length,
removing all combinations of two intersecting single-length SAWs.
We present an efficient numbering of the lattice sites that enables
exploitation of symmetry and leads to a smaller tree data structure; this
numbering is by increasing Euclidean distance from the origin of the lattice.
Furthermore, we show how the computation can be parallelised by distributing
the iterations of the main loop of the algorithm over the cores of a multicore
architecture. Experimental results on the 3D cubic lattice demonstrate that
Z(28) can be computed on a dual-core PC in only 1 hour and 40 minutes, with a
speedup of 1.56 compared to the single-core computation and with a gain by
using symmetry of a factor of 26. We present results for memory use and show
how the computation is made to fit in 4 Gbyte RAM. It is easy to extend the
SAWdoubler software to other lattices; it is publicly available under the GNU
LGPL license.Comment: 29 pages, 3 figure
VLBI Polarimetry of 177 Sources from the Caltech-Jodrell Bank Flat-spectrum Survey
We present VLBA observations and a statistical analysis of 5 GHz VLBI
polarimetry data from 177 sources in the Caltech-Jodrell Bank flat-spectrum
(CJF) survey. The CJF survey, a complete, flux-density-limited sample of 293
extragalactic radio sources, gives us the unique opportunity to compare a broad
range of source properties for quasars, galaxies and BL Lacertae objects. We
focus primarily on jet properties, specifically the correlation between the jet
axis angle and the polarization angle in the core and jet. A strong correlation
is found for the electric vector polarization angle in the cores of quasars to
be perpendicular to the jet axis. Contrary to previous claims, no correlation
is found between the jet polarization angle and the jet axis in either quasars
or BL Lac objects. With this large, homogeneous sample we are also able to
investigate cosmological issues and AGN evolution.Comment: Accepted to the Astrophysical Journal: 37 pages, 14 figure
Submillimeter and Far-Infrared Polarimetric Observations of Magnetic Fields in Star-Forming Regions
Observations of star-forming regions by the current and upcoming generation
of submillimeter polarimeters will shed new light on the evolution of magnetic
fields over the cloud-to-core size scales involved in the early stages of the
star formation process. Recent wide-area and high-sensitivity polarization
observations have drawn attention to the challenges of modeling magnetic field
structure of star forming regions, due to variations in dust polarization
properties in the interstellar medium. However, these observations also for the
first time provide sufficient information to begin to break the degeneracy
between polarization efficiency variations and depolarization due to magnetic
field sub-beam structure, and thus to accurately infer magnetic field
properties in the star-forming interstellar medium. In this article we discuss
submillimeter and far-infrared polarization observations of star-forming
regions made with single-dish instruments. We summarize past, present and
forthcoming single-dish instrumentation, and discuss techniques which have been
developed or proposed to interpret polarization observations, both in order to
infer the morphology and strength of the magnetic field, and in order to
determine the environments in which dust polarization observations reliably
trace the magnetic field. We review recent polarimetric observations of
molecular clouds, filaments, and starless and protostellar cores, and discuss
how the application of the full range of modern analysis techniques to recent
observations will advance our understanding of the role played by the magnetic
field in the early stages of star formation.Comment: 29 pages, 12 figures, 1 table, published in Frontiers in Astronomy
and Space Sciences. Open-access, available here:
https://www.frontiersin.org/articles/10.3389/fspas.2019.00015/ful
Prestellar Core Formation, Evolution, and Accretion from Gravitational Fragmentation in Turbulent Converging Flows
We investigate prestellar core formation and accretion based on
three-dimensional hydrodynamic simulations. Our simulations represent local
pc regions within giant molecular clouds where a supersonic turbulent
flow converges, triggering star formation in the post-shock layer. We include
turbulence and self-gravity, applying sink particle techniques, and explore a
range of inflow Mach number . Two sets of cores are identified
and compared: -cores are identified of a time snapshot in each simulation,
representing dense structures in a single cloud map; -cores
are identified at their individual time of collapse, representing the initial
mass reservoir for accretion. We find that cores and filaments form and evolve
at the same time. At the stage of core collapse, there is a well-defined,
converged characteristic mass for isothermal fragmentation that is comparable
to the critical Bonner-Ebert mass at the post-shock pressure. The core mass
functions (CMFs) of -cores show a deficit of high-mass cores
() compared to the observed stellar initial mass function
(IMF). However, the CMFs of -cores are similar to the observed CMFs and
include many low-mass cores that are gravitationally stable. The difference
between -cores and -cores suggests that the full sample
from observed CMFs may not evolve into protostars. Individual sink particles
accrete at a roughly constant rate throughout the simulations, gaining one
-core mass per free-fall time even after the initial mass
reservoir is accreted. High-mass sinks gain proportionally more mass at late
times than low-mass sinks. There are outbursts in accretion rates, resulting
from clumpy density structures falling into the sinks
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