5,329 research outputs found
Do Quasars Lens Quasars?
If the unexpectedly high frequency of quasar pairs with very different
component redshifts is due to the lensing of a population of background quasars
by the foreground quasar, typical lens masses must be \sim10^{12}M_{\sun} and
the sum of all such quasar lenses would have to contain times the
closure density of the Universe. It then seems plausible that a very high
fraction of all \sim10^{12} M_{\sun} gravitational lenses with redshifts
contain quasars. Here I propose that these systems have evolved to
form the present population of massive galaxies with M and M
>5\times10^{11} M_{\sun}.Comment: 6 pages, aas style, ams symbols, ApJL (accepted
Pulsar timing arrays as imaging gravitational wave telescopes: angular resolution and source (de)confusion
Pulsar timing arrays (PTAs) will be sensitive to a finite number of
gravitational wave (GW) "point" sources (e.g. supermassive black hole
binaries). N quiet pulsars with accurately known distances d_{pulsar} can
characterize up to 2N/7 distant chirping sources per frequency bin \Delta
f_{gw}=1/T, and localize them with "diffraction limited" precision \delta\theta
\gtrsim (1/SNR)(\lambda_{gw}/d_{pulsar}). Even if the pulsar distances are
poorly known, a PTA with F frequency bins can still characterize up to
(2N/7)[1-(1/2F)] sources per bin, and the quasi-singular pattern of timing
residuals in the vicinity of a GW source still allows the source to be
localized quasi-topologically within roughly the smallest quadrilateral of
quiet pulsars that encircles it on the sky, down to a limiting resolution
\delta\theta \gtrsim (1/SNR) \sqrt{\lambda_{gw}/d_{pulsar}}. PTAs may be
unconfused, even at the lowest frequencies, with matched filtering always
appropriate.Comment: 7 pages, 1 figure, matches Phys.Rev.D versio
Proton lifetime bounds from chirally symmetric lattice QCD
We present results for the matrix elements relevant for proton decay in Grand
Unified Theories (GUTs). The calculation is performed at a fixed lattice
spacing a^{-1}=1.73(3) GeV using 2+1 flavors of domain wall fermions on
lattices of size 16^3\times32 and 24^3\times64 with a fifth dimension of length
16. We use the indirect method which relies on an effective field theory
description of proton decay, where we need to estimate the low energy
constants, \alpha = -0.0112(25) GeV^3 and \beta = 0.0120(26) GeV^3. We relate
these low energy constants to the proton decay matrix elements using leading
order chiral perturbation theory. These can then be combined with experimental
bounds on the proton lifetime to bound parameters of individual GUTs.Comment: 17 pages, 9 Figure
Nivat's conjecture holds for sums of two periodic configurations
Nivat's conjecture is a long-standing open combinatorial problem. It concerns
two-dimensional configurations, that is, maps where is a finite set of symbols. Such configurations are often
understood as colorings of a two-dimensional square grid. Let denote
the number of distinct block patterns occurring in a configuration
. Configurations satisfying for some
are said to have low rectangular complexity. Nivat conjectured that such
configurations are necessarily periodic.
Recently, Kari and the author showed that low complexity configurations can
be decomposed into a sum of periodic configurations. In this paper we show that
if there are at most two components, Nivat's conjecture holds. As a corollary
we obtain an alternative proof of a result of Cyr and Kra: If there exist such that , then is periodic. The
technique used in this paper combines the algebraic approach of Kari and the
author with balanced sets of Cyr and Kra.Comment: Accepted for SOFSEM 2018. This version includes an appendix with
proofs. 12 pages + references + appendi
The distance to the young cluster NGC 7129 and its age
The dust cloud TGU H645 P2 and embedded in it young open cluster NGC 7129 are
investigated using the results of medium-band photometry of 159 stars in the
Vilnius seven-colour system down to V = 18.8 mag. The photometric data were
used to classify about 50 percent of the measured stars in spectral and
luminosity classes. The extinction A_V vs. distance diagram for the 20x20
arcmin area is plotted for 155 stars with two-dimensional classification from
the present and the previous catalogues. The extinction values found range
between 0.6 and 3.4 mag. However, some red giants, located in the direction of
the dense parts of the cloud, exhibit the infrared extinction equivalent up to
A_V = 13 mag. The distance to the cloud (and the cluster) is found to be 1.15
kpc (the true distance modulus 10.30 mag). For determining the age of NGC 7129,
a luminosity vs. temperature diagram for six cluster members of spectral
classes B3 to A1 was compared with the Pisa pre-main-sequence evolution tracks
and the Palla birthlines. The cluster can be as old as about 3 Myr, but star
forming continues till now as witnessed by the presence in the cloud of many
younger pre-main-sequence objects identified with photometry from 2MASS,
Spitzer and WISE infrared surveys.Comment: 8 pages, 6 fugures, full Table 1 online. Accepted for publication in
MNRAS on 2013 November 3
Probing 3-D matter distribution at z~2 with QSO multiple lines of sight
We investigate the 3-D matter distribution at z~2 with high resolution (R ~
40000) spectra of QSO pairs and groups obtained with the UVES spectrograph at
ESO VLT. Our sample is unique for the number density of objects and the variety
of separations, between 0.5 and 7 proper Mpc. We compute the real space
cross-correlation function of the Lyman-alpha forest transmitted fluxes. There
is a significant clustering signal up to ~2 proper Mpc, which is still present
when absorption lines with high column density (log N > 13.8) are excluded.Comment: Poster paper presented at the IAU Colloquium #199 on "Probing
Galaxies through Quasar Absorption Lines" held in Shanghai, China from March
14th to 18th, 200
Tomography of the intergalactic medium with Ly-alpha forests in close QSO pairs
We study the three-dimensional distribution of non virialised matter at z~2
using high resolution spectra of QSO pairs and simulated spectra drawn from
cosmological hydrodynamical simulations. We have collected the largest sample
of QSO pairs ever observed with UVES at the ESO-VLT, with angular separations
between ~1 and 14 arcmin. The observed correlation functions of the transmitted
flux in the HI Lyman alpha forest along and transverse to the lines of sight
are in good agreement implying that the distortions in redshift space due to
peculiar velocities are small. The clustering signal is significant up to
velocity separations of ~200 km/s, or about 3 h^{-1} comoving Mpc. The regions
at lower overdensity (rho/ < 6.5) are still clustered but on smaller
scales (Delta v < 100 km/s). The observed and simulated correlation functions
are compatible at the 3 sigma level. A better concordance is obtained when only
the low overdensity regions are selected for the analysis or when the effective
optical depth of the simulated spectra is increased artificially, suggesting a
deficiency of strong lines in the simulated spectra. We found that also a lower
value of the power-law index of the temperature-density relation for the Lyman
alpha forest gas improves the agreement between observed and simulated results.
If confirmed, this would be consistent with other observations favouring a late
HeII reionization epoch (at z~3). We remark the detection of a significant
clustering signal in the cross correlation coefficient at a transverse velocity
separation Delta v_{\perp} ~500 km/s whose origin needs further investigation.Comment: Accepted for publication in MNRAS, revised version matching the
accepted on
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