153 research outputs found
The protogalactic origin for cosmic magnetic fields
It is demonstrated that strong magnetic fields are produced from a zero initial magnetic field during the pregalactic era, when the galaxy is first forming. Their development proceeds in three phases. In the first phase, weak magnetic fields are created by the Biermann battery mechanism. During the second phase, results from a numerical simulation make it appear likely that homogenous isotropic Kolmogorov turbulence develops that is associated with gravitational structure formation of galaxies. Assuming that this turbulence is real, then these weak magnetic fields will be amplified to strong magnetic fields by this Kolmogorov turbulence. During this second phase, the magnetic fields reach saturation with the turbulent power, but they are coherent only on the scale of the smallest eddy. During the third phase, which follows this saturation, it is expected that the magnetic field strength will increase to equipartition with the turbulent energy and that the coherence length of the magnetic fields will increase to the scale of the largest turbulent eddy, comparable to the scale of the entire galaxy. The resulting magnetic field represents a galactic magnetic field of primordial origin. No further dynamo action after the galaxy forms is necessary to explain the origin of magnetic fields. However, the magnetic field will certainly be altered by dynamo action once the galaxy and the galactic disk have formed. It is first shown by direct numerical simulations that thermoelectric currents associated with the Biermann battery build the field up from zero to 10(-21) G in the regions about to collapse into galaxies, by z similar to 3. For weak fields, in the absence of dissipation, the cyclotron frequency -omega(cyc) = eB/m(H)c and omega/(1 + chi), where omega = del x upsilon is the vorticity and chi is the degree of ionization, satisfy the same equations, and initial conditions omega(cyc) = omega = 0, so that, globally, -omega(cyc)(r, t) = omega(r, t)/(1 + chi). The vorticity grows rapidly after caustics (extreme nonlinearities) develop in the cosmic fluid. At this time, it is made plausible that turbulence has developed into Kolmogorov turbulence. Numerical simulations do not yet have the resolution to demonstrate that, during the second phase, the magnetic fields are amplified by the dynamo action of the turbulence. Instead, an analytic theory of the turbulent amplification of magnetic fields is employed to explore this phase of the magnetic field development. From this theory, it is shown that, assuming the turbulence is really Kolmogorov turbulence, the dynamo action of this protogalactic turbulence is able to amplify the magnetic fields by such a large factor during the collapse of the protogalaxy that the power into the magnetic field must reach saturation with the turbulent power. For the third phase, there is as yet no analytic theory capable of describing this phase. However, preliminary turbulence calculations currently in progress seem to confirm that the magnetic fields may proceed to equipartition with the turbulent energy, and that the coherence length may increase to the largest scales. Simple physical arguments are presented that show that this may be the case. Such an equipartition field is actually too strong to allow immediate collapse to a disk. Possible ways around this difficulty are discussedopen2553
Star Formation in Violent and Normal Evolutionary Phases
Mergers of massive gas-rich galaxies trigger violent starbursts that - over
timescales of Myr and regions kpc - form massive and compact
star clusters comparable in mass and radii to Galactic globular clusters. The
star formation efficiency is higher by 1 - 2 orders of magnitude in these
bursts than in undisturbed spirals, irregulars or even BCDs. We ask the
question if star formation in these extreme regimes is just a scaled-up version
of the normal star formation mode of if the formation of globular clusters
reveals fundamentally different conditions.Comment: 4 pages To appear in The Evolution of Galaxies. II. Basic building
blocks, eds. M. Sauvage, G. Stasinska, L. Vigroux, D. Schaerer, S. Madde
Baryons in the warm-hot intergalactic medium
Approximately 30%-40% of all baryons in the present-day universe reside in a warm-hot intergalactic medium (WHIM), with temperatures in the range 105 \u3c T \u3c 107 K. This is a generic prediction from six hydrodynamic simulations of currently favored structure formation models having a wide variety of numerical methods, input physics, volumes, and spatial resolutions. Most of these warm-hot baryons reside in diffuse large-scale structures with a median overdensity around 10-30, not in virialized objects such as galaxy groups or galactic halos. The evolution of the WHIM is primarily driven by shock heating from gravitational perturbations breaking on mildly nonlinear, nonequilibrium structures such as filaments. Supernova feedback energy and radiative cooling play lesser roles in its evolution. WHIM gas may be consistent with observations of the 0.25 keV X-ray background without being significantly heated by nongravitational processes because the emitting gas is very diffuse. Our results confirm and extend previous work by Cen & Ostriker and Davé et al
A Search for Gravitational MilliâLenses
We have searched for gravitational milliâlens systems by examining VLBI maps of ~ 300 flatâspectrum radio sources. So far we have followed up 7 candidates, with separations in the range 2â20 mas. None have been confirmed as lenses but several of them can not yet be definitively ruled out. If there are no milli-lenses in this sample then uniformlyâdistributed black holes of 10^6 to 10^8 M_â cannot contribute more than ~ 1% of the closure density
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.
Articles by Latin American Authors in Prestigious Journals Have Fewer Citations
Background: the journal Impact factor (IF) is generally accepted to be a good measurement of the relevance/quality of articles that a journal publishes. in spite of an, apparently, homogenous peer-review process for a given journal, we hypothesize that the country affiliation of authors from developing Latin American (LA) countries affects the IF of a journal detrimentally.Methodology/Principal Findings: Seven prestigious international journals, one multidisciplinary journal and six serving specific branches of science, were examined in terms of their IF in the Web of Science. Two subsets of each journal were then selected to evaluate the influence of author's affiliation on the IF. They comprised contributions (i) with authorship from four Latin American (LA) countries (Argentina, Brazil, Chile and Mexico) and (ii) with authorship from five developed countries (England, France, Germany, Japan and USA). Both subsets were further subdivided into two groups: articles with authorship from one country only and collaborative articles with authorship from other countries. Articles from the five developed countries had IF close to the overall IF of the journals and the influence of collaboration on this value was minor. in the case of LA articles the effect of collaboration (virtually all with developed countries) was significant. the IFs for non-collaborative articles averaged 66% of the overall IF of the journals whereas the articles in collaboration raised the IFs to values close to the overall IF.Conclusion/Significance: the study shows a significantly lower IF in the group of the subsets of non-collaborative LA articles and thus that country affiliation of authors from non-developed LA countries does affect the IF of a journal detrimentally. There are no data to indicate whether the lower IFs of LA articles were due to their inherent inferior quality/relevance or psycho-social trend towards under-citation of articles from these countries. However, further study is required since there are foreseeable consequences of this trend as it may stimulate strategies by editors to turn down articles that tend to be under-cited.Fundação de Amparo Ă Pesquisa do Estado de SĂŁo Paulo (FAPESP)Conselho Nacional de Desenvolvimento CientĂfico e TecnolĂłgico (CNPq)Latin Amer & Caribbean Ctr Hlth Sci Informat, BIREME PAHO WHO, SĂŁo Paulo, BrazilUniversidade Federal de SĂŁo Paulo, DIS Dept Informat Med, SĂŁo Paulo, BrazilUniversidade Federal de SĂŁo Paulo, DIS Dept Informat Med, SĂŁo Paulo, BrazilFAPESP: 05/57665-8CNPq: 2006-0919Web of Scienc
Theory of Star Formation
We review current understanding of star formation, outlining an overall
theoretical framework and the observations that motivate it. A conception of
star formation has emerged in which turbulence plays a dual role, both creating
overdensities to initiate gravitational contraction or collapse, and countering
the effects of gravity in these overdense regions. The key dynamical processes
involved in star formation -- turbulence, magnetic fields, and self-gravity --
are highly nonlinear and multidimensional. Physical arguments are used to
identify and explain the features and scalings involved in star formation, and
results from numerical simulations are used to quantify these effects. We
divide star formation into large-scale and small-scale regimes and review each
in turn. Large scales range from galaxies to giant molecular clouds (GMCs) and
their substructures. Important problems include how GMCs form and evolve, what
determines the star formation rate (SFR), and what determines the initial mass
function (IMF). Small scales range from dense cores to the protostellar systems
they beget. We discuss formation of both low- and high-mass stars, including
ongoing accretion. The development of winds and outflows is increasingly well
understood, as are the mechanisms governing angular momentum transport in
disks. Although outstanding questions remain, the framework is now in place to
build a comprehensive theory of star formation that will be tested by the next
generation of telescopes.Comment: 120 pages, to appear in ARAA. No changes from v1 text; permission
statement adde
A Unified Picture of Disk Galaxies where Bars, Spirals and Warps Result from the Same Fundamental Causes
Bars and spiral arms have played an important role as constraints on the
dynamics and on the distribution of dark matter in the optical parts of disk
galaxies. Dynamics linked to the dissipative nature of gas, and its
transformation into stars provide clues that spiral galaxies are driven by
dissipation close to a state of \textit{marginal stability} with respect to the
dynamics in the galaxy plane. Here we present numerical evidences that warps
play a similar role but in the transverse direction. N-body simulations show
that typical galactic disks are also marginally stable with respect to a
bending instability leading to typical observed warps. The frequent occurrence
of warps and asymmetries in the outer galactic disks give therefore, like bars
in the inner disks, new constraints on the dark matter, but this time in the
outer disks. If disks are marginally stable with respect to bending
instabilities, our models suggest that the mass within the HI disks must be a
multiple of the detected HI and stars, i.e., disks must be heavier than seen.
But the models do not rule out a traditional thick halo with a mass within the
HI disk radius similar to the total disk mass.Comment: 10 pages, 2 postscript figures, to appear in the proceedings of the
conference "Penetrating Bars through Masks of Cosmic Dust: The Hubble Tuning
Fork strikes a New Note", South Africa, June 200
Clues from nearby galaxies to a better theory of cosmic evolution
The great advances in the network of cosmological tests show that the
relativistic Big Bang theory is a good description of our expanding universe.
But the properties of nearby galaxies that can be observed in greatest detail
suggest a still better theory would more rapidly gather matter into galaxies
and groups of galaxies. This happens in theoretical ideas now under discussion.Comment: published in Natur
TOPOLOGICAL DEFECTS AND HIGHEST ENERGY COSMIC AND GAMMA RAYS
In this paper we review the hypothesis that a considerable part of the cosmic
ray flux observed above about 10^{19}\eV may be produced by decaying or
annihilating topological defects left over from phase transitions in the early
universe at grand unification energy scales (\approx10^{16}\GeV). Possible
signatures of cosmic ray producing defect models are discussed which could be
tested experimentally in the near future. We thereby focus on model independent
universal spectral properties of the predicted particle fluxes.Comment: 11 pages of uuencoded compressed postscript, including 3 figures, to
be published in Space Science Reviews
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