63 research outputs found
Modified Newtonian Dynamics as an Alternative to Dark Matter
Modified Newtonian dynamics (MOND) is an empirically motivated modification
of Newtonian gravity or inertia suggested by Milgrom as an alternative to
cosmic dark matter. The basic idea is that at accelerations below a0 ~ 10^{-8}
cm/s^2 ~ cH0/6 the effective gravitational attraction approaches sqrt{gN*a0}
where gN is the usual Newtonian acceleration. This simple algorithm yields flat
rotation curves for spiral galaxies and a mass-rotation velocity relation of
the form M ~ V^4 that forms the basis for the observed luminosity-rotation
velocity relation-- the Tully-Fisher law. We review the phenomenological
success of MOND on scales ranging from dwarf spheroidal galaxies to
superclusters, and demonstrate that the evidence for dark matter can be equally
well interpreted as evidence for MOND. We discuss the possible physical basis
for an acceleration-based modification of Newtonian dynamics as well as the
extension of MOND to cosmology and structure formation.Comment: To be published in volume 40 of Annual Reviews of Astronomy &
Astrophysics. 36 pages plus 12 figures and 1 tabl
Cold gas accretion in galaxies
Evidence for the accretion of cold gas in galaxies has been rapidly
accumulating in the past years. HI observations of galaxies and their
environment have brought to light new facts and phenomena which are evidence of
ongoing or recent accretion:
1) A large number of galaxies are accompanied by gas-rich dwarfs or are
surrounded by HI cloud complexes, tails and filaments. It may be regarded as
direct evidence of cold gas accretion in the local universe. It is probably the
same kind of phenomenon of material infall as the stellar streams observed in
the halos of our galaxy and M31. 2) Considerable amounts of extra-planar HI
have been found in nearby spiral galaxies. While a large fraction of this gas
is produced by galactic fountains, it is likely that a part of it is of
extragalactic origin. 3) Spirals are known to have extended and warped outer
layers of HI. It is not clear how these have formed, and how and for how long
the warps can be sustained. Gas infall has been proposed as the origin. 4) The
majority of galactic disks are lopsided in their morphology as well as in their
kinematics. Also here recent accretion has been advocated as a possible cause.
In our view, accretion takes place both through the arrival and merging of
gas-rich satellites and through gas infall from the intergalactic medium (IGM).
The infall may have observable effects on the disk such as bursts of star
formation and lopsidedness. We infer a mean ``visible'' accretion rate of cold
gas in galaxies of at least 0.2 Msol/yr. In order to reach the accretion rates
needed to sustain the observed star formation (~1 Msol/yr), additional infall
of large amounts of gas from the IGM seems to be required.Comment: To appear in Astronomy & Astrophysics Reviews. 34 pages.
Full-resolution version available at
http://www.astron.nl/~oosterlo/accretionRevie
Galaxies appear simpler than expected
Galaxies are complex systems the evolution of which apparently results from
the interplay of dynamics, star formation, chemical enrichment, and feedback
from supernova explosions and supermassive black holes. The hierarchical theory
of galaxy formation holds that galaxies are assembled from smaller pieces,
through numerous mergers of cold dark matter. The properties of an individual
galaxy should be controlled by six independent parameters including mass,
angular-momentum, baryon-fraction, age and size, as well as by the accidents of
its recent haphazard merger history. Here we report that a sample of galaxies
that were first detected through their neutral hydrogen radio-frequency
emission, and are thus free of optical selection effects, shows five
independent correlations among six independent observables, despite having a
wide range of properties. This implies that the structure of these galaxies
must be controlled by a single parameter, although we cannot identify this
parameter from our dataset. Such a degree of organisation appears to be at odds
with hierarchical galaxy formation, a central tenet of the cold dark matter
paradigm in cosmology.Comment: 26 pages, 14 figure
Dwarf Galaxies of the Local Group
The Local Group (LG) dwarf galaxies offer a unique window to the detailed
properties of the most common type of galaxy in the Universe. In this review, I
update the census of LG dwarfs based on the most recent distance and radial
velocity determinations. I then discuss the detailed properties of this sample,
including (a) the integrated photometric parameters and optical structures of
these galaxies, (b) the content, nature and distribution of their ISM, (c)
their heavy-element abundances derived from both stars and nebulae, (d) the
complex and varied star-formation histories of these dwarfs, (e) their internal
kinematics, stressing the relevance of these galaxies to the dark-matter
problem and to alternative interpretations, and (f) evidence for past, ongoing
and future interactions of these dwarfs with other galaxies in the Local Group
and beyond. To complement the discussion and to serve as a foundation for
future work, I present an extensive set of basic observational data in tables
that summarize much of what we know, and what we still do not know, about these
nearby dwarfs. Our understanding of these galaxies has grown impressively in
the past decade, but fundamental puzzles remain that will keep the Local Group
at the forefront of galaxy evolution studies for some time.Comment: 66 pages; 9 figures; 8 table
HI Velocity Fields and Rotation Curves
Wetensch. publicati
Effects of circadian disruption on physiology and pathology: from bench to clinic (and back)
Nested within the hypothalamus, the suprachiasmatic nuclei (SCN) represent a central biological clock that regulates daily and circadian (i.e., close to 24 h) rhythms in mammals. Besides the SCN, a number of peripheral oscillators throughout the body control local rhythms and are usually kept in pace by the central clock. In order to represent an adaptive value, circadian rhythms must be entrained by environmental signals or zeitgebers, the main one being the daily light?dark (LD) cycle. The SCN adopt a stable phase relationship with the LD cycle that, when challenged, results in abrupt or chronic changes in overt rhythms and, in turn, in physiological, behavioral, and metabolic variables. Changes in entrainment, both acute and chronic, may have severe consequences in human performance and pathological outcome. Indeed, animal models of desynchronization have become a useful tool to understand such changes and to evaluate potential treatments in human subjects. Here we review a number of alterations in circadian entrainment, including jet lag, social jet lag (i.e., desynchronization between body rhythms and normal time schedules), shift work, and exposure to nocturnal light, both in human subjects and in laboratory animals. Finally, we focus on the health consequences related to circadian/entrainment disorders and propose a number of approaches for the management of circadian desynchronization.Fil: Chiesa, Juan JosĂ©. Universidad Nacional de Quilmes. Departamento de Ciencia y TecnologĂa; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; ArgentinaFil: Duhart, JosĂ© Manuel. Universidad Nacional de Quilmes. Departamento de Ciencia y TecnologĂa; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; ArgentinaFil: Casiraghi, Leandro Pablo. Universidad Nacional de Quilmes. Departamento de Ciencia y TecnologĂa; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; ArgentinaFil: Paladino, Natalia. Universidad Nacional de Quilmes. Departamento de Ciencia y TecnologĂa; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; ArgentinaFil: Bussi, Ivana Leda. Universidad Nacional de Quilmes. Departamento de Ciencia y TecnologĂa; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; ArgentinaFil: Golombek, Diego AndrĂ©s. Universidad Nacional de Quilmes. Departamento de Ciencia y TecnologĂa; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; Argentin
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