612 research outputs found
A Size of ~10 Mpc for the Ionized Bubbles at the End of Cosmic Reionization
The first galaxies to appear in the universe at redshifts z>20 created
ionized bubbles in the intergalactic medium of neutral hydrogen left over from
the Big-Bang. It is thought that the ionized bubbles grew with time, surrounded
clusters of dwarf galaxies and eventually overlapped quickly throughout the
universe over a narrow redshift interval near z~6. This event signaled the end
of the reionization epoch when the universe was a billion years old. Measuring
the hitherto unknown size distribution of the bubbles at their final overlap
phase is a focus of forthcoming observational programs aimed at highly
redshifted 21cm emission from atomic hydrogen. Here we show that the combined
constraints of cosmic variance and causality imply an observed bubble size at
the end of the overlap epoch of ~10 physical Mpc, and a scatter in the observed
redshift of overlap along different lines-of-sight of ~0.15. This scatter is
consistent with observational constraints from recent spectroscopic data on the
farthest known quasars. Our novel result implies that future radio experiments
should be tuned to a characteristic angular scale of ~0.5 degrees and have a
minimum frequency band-width of ~8 MHz for an optimal detection of 21cm flux
fluctuations near the end of reionization.Comment: Accepted for publication in Nature. Press embargo until publishe
Supermassive black holes do not correlate with dark matter halos of galaxies
Supermassive black holes have been detected in all galaxies that contain
bulge components when the galaxies observed were close enough so that the
searches were feasible. Together with the observation that bigger black holes
live in bigger bulges, this has led to the belief that black hole growth and
bulge formation regulate each other. That is, black holes and bulges
"coevolve". Therefore, reports of a similar correlation between black holes and
the dark matter halos in which visible galaxies are embedded have profound
implications. Dark matter is likely to be nonbaryonic, so these reports suggest
that unknown, exotic physics controls black hole growth. Here we show - based
in part on recent measurements of bulgeless galaxies - that there is almost no
correlation between dark matter and parameters that measure black holes unless
the galaxy also contains a bulge. We conclude that black holes do not correlate
directly with dark matter. They do not correlate with galaxy disks, either.
Therefore black holes coevolve only with bulges. This simplifies the puzzle of
their coevolution by focusing attention on purely baryonic processes in the
galaxy mergers that make bulges.Comment: 12 pages, 9 Postscript figures, 1 table; published in Nature (20
January 2011
Challenges in monitoring and managing engineered slopes in a changing climate
Geotechnical asset owners need to know which parts of their asset network are vulnerable to climate
change induced failure in order to optimise future investment. Protecting these vulnerable slopes requires monitoring
systems capable of identifying and alerting to asset operators changes in the internal conditions that precede failure.
Current monitoring systems are heavily reliant on point sensors which can be difficult to interpret across slope scale.
This paper presents challenges to producing such a system and research being carried out to address some of these
using electrical resistance tomography (ERT). Experimental results show that whilst it is possible to measure soil
water content indirectly via resistivity the relationship between resistivity and water content will change over time for
a given slope. If geotechnical parameters such as pore water pressure are to be estimated using this method then ERT
systems will require integrating with more conventional geotechnical instrumentation to ensure correct representative
information is provided. The paper also presents examples of how such data can be processed and communicated to
asset owners for the purposes of asset management
A direct image of the obscuring disk surrounding an active galactic nucleus
Active galactic nuclei (AGN) are generally accepted to be powered by the
release of gravitational energy in a compact accretion disk surrounding a
massive black hole. Such disks are also necessary to collimate powerful radio
jets seen in some AGN. The unifying classification schemes for AGN further
propose that differences in their appearance can be attributed to the opacity
of the accreting material, which may obstruct our view of the central region of
some systems. The popular model for the obscuring medium is a parsec-scale disk
of dense molecular gas, although evidence for such disks has been mostly
indirect, as their angular size is much smaller than the resolution of
conventional telescopes. Here we report the first direct images of a pc-scale
disk of ionised gas within the nucleus of NGC 1068, the archetype of obscured
AGN. The disk is viewed nearly edge-on, and individual clouds within the
ionised disk are opaque to high-energy radiation, consistent with the unifying
classification scheme. In projection, the disk and AGN axes align, from which
we infer that the ionised gas disk traces the outer regions of the long-sought
inner accretion disk.Comment: 14 pages, LaTeX, PSfig, to appear in Nature. also available at
http://hethp.mpe-garching.mpg.de/Preprint
Persistence of magnetic field driven by relativistic electrons in a plasma
The onset and evolution of magnetic fields in laboratory and astrophysical
plasmas is determined by several mechanisms, including instabilities, dynamo
effects and ultra-high energy particle flows through gas, plasma and
interstellar-media. These processes are relevant over a wide range of
conditions, from cosmic ray acceleration and gamma ray bursts to nuclear fusion
in stars. The disparate temporal and spatial scales where each operates can be
reconciled by scaling parameters that enable to recreate astrophysical
conditions in the laboratory. Here we unveil a new mechanism by which the flow
of ultra-energetic particles can strongly magnetize the boundary between the
plasma and the non-ionized gas to magnetic fields up to 10-100 Tesla (micro
Tesla in astrophysical conditions). The physics is observed from the first
time-resolved large scale magnetic field measurements obtained in a laser
wakefield accelerator. Particle-in-cell simulations capturing the global plasma
and field dynamics over the full plasma length confirm the experimental
measurements. These results open new paths for the exploration and modelling of
ultra high energy particle driven magnetic field generation in the laboratory
Detection of weak gravitational lensing distortions of distant galaxies by cosmic dark matter at large scales
Most of the matter in the universe is not luminous and can be observed
directly only through its gravitational effect. An emerging technique called
weak gravitational lensing uses background galaxies to reveal the foreground
dark matter distribution on large scales. Light from very distant galaxies
travels to us through many intervening overdensities which gravitationally
distort their apparent shapes. The observed ellipticity pattern of these
distant galaxies thus encodes information about the large-scale structure of
the universe, but attempts to measure this effect have been inconclusive due to
systematic errors. We report the first detection of this ``cosmic shear'' using
145,000 background galaxies to reveal the dark matter distribution on angular
scales up to half a degree in three separate lines of sight. The observed
angular dependence of this effect is consistent with that predicted by two
leading cosmological models, providing new and independent support for these
models.Comment: 18 pages, 5 figures: To appear in Nature. (This replacement fixes tex
errors and typos.
Breakdown of the adiabatic limit in low dimensional gapless systems
It is generally believed that a generic system can be reversibly transformed
from one state into another by sufficiently slow change of parameters. A
standard argument favoring this assertion is based on a possibility to expand
the energy or the entropy of the system into the Taylor series in the ramp
speed. Here we show that this argumentation is only valid in high enough
dimensions and can break down in low-dimensional gapless systems. We identify
three generic regimes of a system response to a slow ramp: (A) mean-field, (B)
non-analytic, and (C) non-adiabatic. In the last regime the limits of the ramp
speed going to zero and the system size going to infinity do not commute and
the adiabatic process does not exist in the thermodynamic limit. We support our
results by numerical simulations. Our findings can be relevant to
condensed-matter, atomic physics, quantum computing, quantum optics, cosmology
and others.Comment: 11 pages, 5 figures, to appear in Nature Physics (originally
submitted version
Small Scale Structure and High Redshift HI
Cosmological simulations with gas dynamics suggest that the Lyman-alpha
forest is produced mainly by "small scale structure" --- filaments and sheets
that are the high redshift analog of today's galaxy superclusters. There is no
sharp distinction between Lyman-alpha clouds and "Gunn-Peterson" absorption
produced by the fluctuating IGM -- the Lyman-alpha forest {\it is} the
Gunn-Peterson effect. Lyman limit and damped Lyman-alpha absorption arises in
the radiatively cooled gas of forming galaxies. At , most of the gas is
in the photoionized, diffuse medium associated with the Lyman-alpha forest, but
most of the {\it neutral} gas is in damped Lyman-alpha systems. We discuss
generic evolution of cosmic gas in a hierarchical scenario of structure
formation, with particular attention to the prospects for detecting 21cm
emission from high redshift HI. A scaling argument based on the present-day
cluster mass function suggests that objects with M_{HI} >~ 5e11 h^{-1} \msun
should be extremely rare at , so detections with existing instruments will
be difficult. An instrument like the proposed Square Kilometer Array could
detect individual damped Lyman-alpha systems at high redshift, making it
possible to map structure in the high redshift universe in much the same way
that today's galaxy redshift surveys map the local large scale structure.Comment: 15 pages, latex w/ crckapb & epsf macros, ps figures; get ps version
with all figures from ftp://bessel.mps.ohio-state.edu/pub/dhw/Preprints To
appear in Cold Gas at High Redshift, eds. M. Bremer et al. (Kluwer, 1996
Ram pressure feeding super-massive black holes
When supermassive black holes at the center of galaxies accrete matter
(usually gas), they give rise to highly energetic phenomena named Active
Galactic Nuclei (AGN). A number of physical processes have been proposed to
account for the funneling of gas towards the galaxy centers to feed the AGN.
There are also several physical processes that can strip gas from a galaxy, and
one of them is ram pressure stripping in galaxy clusters due to the hot and
dense gas filling the space between galaxies. We report the discovery of a
strong connection between severe ram pressure stripping and the presence of AGN
activity. Searching in galaxy clusters at low redshift, we have selected the
most extreme examples of jellyfish galaxies, which are galaxies with long
tentacles of material extending for dozens of kpc beyond the galaxy disk. Using
the MUSE spectrograph on the ESO Very Large Telescope, we find that 6 out of
the 7 galaxies of this sample host a central AGN, and two of them also have
galactic-scale AGN ionization cones. The high incidence of AGN among the most
striking jellyfishes may be due to ram pressure causing gas to flow towards the
center and triggering the AGN activity, or to an enhancement of the stripping
caused by AGN energy injection, or both. Our analysis of the galaxy position
and velocity relative to the cluster strongly supports the first hypothesis,
and puts forward ram pressure as another, yet unforeseen, possible mechanism
for feeding the central supermassive black hole with gas.Comment: published in Nature, Vol.548, Number 7667, pag.30
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