44 research outputs found
The Long-Term Future of Extragalactic Astronomy
If the current energy density of the universe is indeed dominated by a
cosmological constant, then high-redshift sources will remain visible to us
only until they reach some finite age in their rest-frame. The radiation
emitted beyond that age will never reach us due to the acceleration of the
cosmic expansion rate, and so we will never know what these sources look like
as they become older. As a source image freezes on a particular time frame
along its evolution, its luminosity distance and redshift continue to increase
exponentially with observation time. The higher the current redshift of a
source is, the younger it will appear as it fades out of sight. For the popular
set of cosmological parameters, I show that a source at a redshift z=5-10 will
only be visible up to an age of 4-6 billion years. Arguments relating the
properties of high-redshift sources to present-day counterparts will remain
indirect even if we continue to monitor these sources for an infinite amount of
time. These sources will not be visible to us when they reach the current age
of the universe.Comment: Phys. Rev. D, in press (2001
Disparate MgII Absorption Statistics towards Quasars and Gamma-Ray Bursts : A Possible Explanation
We examine the recent report by Prochter et al. (2006) that gamma-ray burst
(GRB) sight lines have a much higher incidence of strong MgII absorption than
quasar sight lines. We propose that the discrepancy is due to the different
beam sizes of GRBs and quasars, and that the intervening MgII systems are
clumpy with the dense part of each cloudlet of a similar size as the quasars,
i.e. < 10^16 cm, but bigger than GRBs. We also discuss observational
predictions of our proposed model. Most notably, in some cases the intervening
MgII absorbers in GRB spectra should be seen varying, and quasars with smaller
sizes should show an increased rate of strong MgII absorbers. In fact, our
prediction of variable MgII lines in the GRB spectra has been now confirmed by
Hao et al. (2007), who observed intervening FeII and MgII lines at z=1.48 to be
strongly variable in the multi-epoch spectra of z=4.05 GRB060206.Comment: 12 pages, 2 figures; substantially revised model calculation;
accepted for publication in Astrophysics & Space Science as a Lette
The influence of magnetic fields on the Sunyaev Zel'dovich effect in clusters of galaxies
We study the influence of intracluster large scale magnetic fields on the
thermal Sunyaev-Zel'dovich (SZ) effect. In a macroscopic approach we complete
the hydrostatic equilibrium equation with the magnetic field pressure
component. Comparing the resulting mass distribution with a standard one, we
derive a new electron density profile. For a spherically symmetric cluster
model, this new profile can be written as the product of a standard (-)
profile and a radius dependent function, close to unity, which takes into
account the magnetic field strength. For non-cooling flow clusters we find that
the observed magnetic field values can reduce the SZ signal by with
respect to the value estimated from X-ray observations and the -model.
If a cluster harbours a cooling flow, magnetic fields tend to weaken the
cooling flow influence on the SZ-effect.Comment: Accepted for publication in New Astronom
Robust evidence for reversal in the aerosol effective climate forcing trend
Anthropogenic aerosols exert a cooling inïŹuence that offsets part of the greenhouse gas warming. Due to their short tropospheric lifetime of only up to several days, the aerosol forcing responds quickly to emissions. Here we present and discuss the evolution of the aerosol forcing since 2000. There are multiple lines of evidence that allow to robustly conclude that the anthropogenic aerosol effective radiative forcing â both aerosol-radiation and aerosol-cloud interactions â has become globally less negative, i.e. that the trend in aerosol effective radiative forcing changed sign from negative to positive. Bottom-up inventories show that anthropogenic primary aerosol and aerosol precursor emissions declined in most regions of the world; observations related to aerosol burden show declining trends, in particular of the ïŹne-mode particles that make up most of the anthropogenic aerosols; satellite retrievals of cloud droplet numbers show trends consistent in sign, as do observations of top-of-atmosphere radiation. Climate model results, including a revised set that is constrained by observations of the ocean heat content evolution show a consistent sign and magnitude for a positive forcing relative to 2000 due to reduced aerosol effects. This reduction leads to an acceleration of the forcing of climate change, i.e. an increase in forcing by 0.1 to 0.3 W m-2, up to 12 % of the total climate forcing in 2019 compared to 1750 according to IPCC
Salinization lowers nutrient availability in formerly brackish freshwater wetlands; unexpected results from a long-term field experiment
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Synthesis of C60-attached SCS Pincer Palladium(II) Complexes
The synthesis of C60-attached SCS ligands ([C6H2(CH2SPh)2-2,6-R-4]â) is described. Starting from 4-formyl-SCS-H (2), 1,2-methanofullerene and fulleropyrrolidine SCS ligands were obtained. Subsequent palladation with [Pd(MeCN)4](BF4)2 afforded the corresponding palladium(II) complexes