24,749 research outputs found
Radio Observations of the Magnetic Fields in Galaxies
After a short introduction on how we get information of the magnetic fields
from radio observations I discuss the results concerning the magnetic field
structure in galaxies: Large-scale regular magnetic field pattern of spiral
structure exist in grand-design spirals, flocculent and even irregular
galaxies. The regular field in spirals is aligned along the optical spiral arms
but strongest in the interarm region, sometimes forming 'magnetic arms'. The
strongest total field is found in the optical arms, but mainly irregular. The
large-scale regular field is best explained by some kind of dynamo action. Only
a few galaxies show a dominant axisymmetric field pattern, most field
structures seem to be a superposition of different dynamo modes or rather
reveal more local effects related to density waves, bars or shocks.
Observations of edge-on galaxies show that the magnetic fields are mainly
parallel to the disk except in some galaxies with strong star formation and
strong galactic winds as e.g. NGC 4631.Comment: 9 pages with 4 figures. To be published in Acta Astronomica Sinica
Vol. 44, 2003 (Conf. Proc. "Radio Studies of Galactic Objects, Galaxies and
AGNs", eds. J.L. Han et al.). Final published version also available at
http://www.bao.ac.cn/bao/hjl/xian/proceedings
Extragalactic Radio Sources as a Piece of the Cosmological Jigsaw
Early on, extragalactic radio sources have pointed to a cosmologically evolv- ing Universe. They were also an important piece of evidence for the existence of supermassive black holes, now thought to be a key component of galaxies. The observation that the power of radio sources increases with redshift, whereas the cosmological assembly of mass proceeds vice versa means that radio sources have their strongest impact in the early Universe. Our simulations suggest that radio sources heat hot halo gas, boost star formation in disc galaxies and other cold gas in the vicinity, possibly filaments, by a surround and squash mechanism. They might cause gaseous outflows in connection with stellar feedback. This might be an important mode of star formation for forming massive galaxies. Analysis of the jet-environment interaction may provide insights into black-hole physics and jet formation, e.g., rotational energy extraction (Blandford-Znajek) or how frequent black-hole binaries or multiple systems are. The former relates to fundamental questions about the nature of black holes. The latter is expected from hierarchical cosmology. Extragalactic radio sources thus continue to corroborate the cosmolog- ical picture and lead the way towards new, exciting discoveries.Non peer reviewedFinal Published versio
Impact of Electroweak Corrections on Neutral Higgs Boson Decays in Extended Higgs Sectors
Precision predictions play an important role in the search for indirect New
Physics effects in the Higgs sector itself. For the electroweak (EW)
corrections of the Higgs bosons in extended Higgs sectors several
renormalization schemes have been worked out that provide
gauge-parameter-independent relations between the input parameters and the
computed observables. Our recently published program codes 2HDECAY and
ewN2HDECAY allow for the computation of the EW corrections to the Higgs decay
widths and branching ratios of the Two-Higgs-Doublet Model (2HDM) and the
Next-to-Minimal-2HDM (N2HDM) for different renormalization schemes of the
scalar mixing angles. In this paper, we present a comprehensive and complete
overview over the relative size of the EW corrections to the branching ratios
of the 2HDM and N2HDM neutral Higgs bosons for different applied
renormalization schemes. We quantify the size of the EW corrections of Standard
Model(SM)- and non-SM-like Higgs bosons and moreover also identify
renormalization schemes that are well-behaved and do not induce unnaturally
large corrections. We furthermore pin down decays and parameter regions that
feature large EW corrections and need further treatment in order to improve the
predictions. Our study sets the scene for future work in the computation of
higher-order corrections to the decays of non-minimal Higgs sectors
Optimal redundancy against disjoint vulnerabilities in networks
Redundancy is commonly used to guarantee continued functionality in networked
systems. However, often many nodes are vulnerable to the same failure or
adversary. A "backup" path is not sufficient if both paths depend on nodes
which share a vulnerability.For example, if two nodes of the Internet cannot be
connected without using routers belonging to a given untrusted entity, then all
of their communication-regardless of the specific paths utilized-will be
intercepted by the controlling entity.In this and many other cases, the
vulnerabilities affecting the network are disjoint: each node has exactly one
vulnerability but the same vulnerability can affect many nodes. To discover
optimal redundancy in this scenario, we describe each vulnerability as a color
and develop a "color-avoiding percolation" which uncovers a hidden
color-avoiding connectivity. We present algorithms for color-avoiding
percolation of general networks and an analytic theory for random graphs with
uniformly distributed colors including critical phenomena. We demonstrate our
theory by uncovering the hidden color-avoiding connectivity of the Internet. We
find that less well-connected countries are more likely able to communicate
securely through optimally redundant paths than highly connected countries like
the US. Our results reveal a new layer of hidden structure in complex systems
and can enhance security and robustness through optimal redundancy in a wide
range of systems including biological, economic and communications networks.Comment: 15 page
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