4,613 research outputs found
A Second Shell in the Fornax dSph Galaxy
In the search for tidal structure in Galactic satellite systems, we have
conducted a photometric survey over a 10 square degree area centred on the
Fornax dSph galaxy. The survey was made in two colours, and the resulting
colour-magnitude data were used as a mask to select candidate Fornax RGB stars,
thereby increasing the contrast of Fornax stars to background sources in the
outer regions. Previously, we reported the presence of a shell (age 2 Gyr)
located towards the centre of Fornax. In this contribution we reveal a second
shell, significantly larger than the first, located 1.3 degrees NW from the
centre of Fornax, outside the nominal tidal radius. Moreover, the distribution
of Fornax RGB stars reveals two lobes extending to the spatial limit of our
survey, and aligned with the minor axis and with the two shells. These results
support the hypothesis of a merger between Fornax and a gas-rich companion
approximately 2 Gyr ago.Comment: Four pages,accepted for the Publications of the Astronomical Society
of Australia. Contribution the annual ASA meeting, Brisbane 200
Resonant helical deformations in nonhomogeneous Kirchhoff filaments
We study the three-dimensional static configurations of nonhomogeneous
Kirchhoff filaments with periodically varying Young's modulus. This type of
variation may occur in long tandemly repeated sequences of DNA. We analyse the
effects of the Young's modulus frequence and amplitude of oscillation in the
stroboscopic maps, and in the regular (non chaotic) spatial configurations of
the filaments. Our analysis shows that the tridimensional conformations of long
filaments may depend critically on the Young's modulus frequence in case of
resonance with other natural frequencies of the filament. As expected, far from
resonance the shape of the solutions remain very close to that of the
homogeneous case. In the case of biomolecules, it is well known that various
other elements, besides sequence-dependent effects, combine to determine their
conformation, like self-contact, salt concentration, thermal fluctuations,
anisotropy and interaction with proteins. Our results show that
sequence-dependent effects alone may have a significant influence on the shape
of these molecules, including DNA. This could, therefore, be a possible
mechanical function of the ``junk'' sequences.Comment: 18 pages (twocolumn), 5 figures Revised manuscrip
The Environmental Influence on the Evolution of Local Galaxies
The results of an Halpha photometric survey of 30 dwarf galaxies of various
morphologies in the Centaurus A and Sculptor groups are presented. Of these 30,
emission was detected in 13: eight are of late-type, two are early-type and
three are of mixed-morphology. The typical flux detection limit of 2e-16 erg
s-1 cm-2, translates into a Star Formation Rate (SFR) detection limit of 4e-6
M_sol yr-1 . In the light of these results, the morphology-density relation is
reexamined: It is shown that, despite a number of unaccounted parameters, there
are significant correlations between the factors determining the morphological
type of a galaxy and its environment. Dwarf galaxies in high density regions
have lower current SFR and lower neutral gas content than their low density
counterparts, confirming earlier results from the Local Group and other denser
environments. The effect of environment is also seen in the timescale formed
from the ratio of blue luminosity to current SFR - dwarfs in higher density
environments have larger values, indicating relatively higher past average SFR.
The influence of environment extends very far and no dwarfs from our sample can
be identified as 'field' objects.Comment: 23 pages, 9 figures, accepted in A
Instanton picture of the spin tunneling in the Lipkin model
A consistent theory of the ground state energy and its splitting due to the
process of tunneling for the Lipkin model is presented. For the functional
integral in terms of the spin coherent states for the partition function of the
model we accurately calculate the trivial and the instanton saddle point
contributions. We show that such calculation has to be perfomed very accurately
taking into account the discrete nature of the functional integral. Such
accurate consideration leads to finite corrections to a naive continous
consideration. We present comparison with numerical calculation of the ground
state energy and the tunneling splitting and with the results obtained by the
quasiclassical method and get excellent agreement.Comment: REVTEX, 32 pages, 3 figure
Electron self-trapping at quantum and classical critical points
Using Feynman path integral technique estimations of the ground state energy
have been found for a conduction electron interacting with order parameter
fluctuations near quantum critical points. In some cases only \textit{singular}
perturbation theory in the coupling constant emerges for the electron ground
state energy. It is shown that an autolocalized state (quantum fluctuon) can be
formed and its characteristics have been calculated depending on critical
exponents for both weak and strong coupling regimes. The concept of fluctuon is
considered also for the classical critical point (at finite temperatures) and
the difference between quantum and classical cases has been investigated. It is
shown that, whereas the quantum fluctuon energy is connected with a true
boundary of the energy spectrum, for classical fluctuon it is just a
saddle-point solution for the chemical potential in the exponential density of
states fluctuation tail.Comment: 45 pages, 1 eps figure, elsart, submitted to Annals of Physic
Dark Energy in an Axion Model with Explicit Z(N) Symmetry Breaking
We point out that a well known axion model with an explicit Z(N) symmetry
breaking term predicts both dark energy and cold dark matter. We estimate the
parameters of this model which fit the observed densities of the dark
components of the universe. We find that the parameters do not conflict with
any observations.Comment: 5 pages, minor change
The Nature of the Density Clump in the Fornax Dwarf Spheroidal Galaxy
We have imaged the recently discovered stellar overdensity located
approximately one core radius from the center of the Fornax dwarf spheroidal
galaxy using the Magellan Clay 6.5m telescope with the Magellan Instant Camera
(MagIC). Superb seeing conditions allowed us to probe the stellar populations
of this overdensity and of a control field within Fornax to a limiting
magnitude of R=26. The color-magnitude diagram of the overdensity field is
virtually identical to that of the control field with the exception of the
presence of a population arising from a very short (less than 300 Myr in
duration) burst of star formation 1.4 Gyr ago. Coleman et al. have argued that
this overdensity might be related to a shell structure in Fornax that was
created when Fornax captured a smaller galaxy. Our results are consistent with
this model, but we argue that the metallicity of this young component favors a
scenario in which the gas was part of Fornax itself.Comment: 24 pages including 8 figures and 3 tables. Accepted by Astronomical
Journa
The Absence of Extra-Tidal Structure in the Sculptor Dwarf Spheroidal Galaxy
The results of a wide-field survey of the Sculptor dwarf spheroidal galaxy
are presented. Our aims were to obtain an accurate map of the outer structure
of Sculptor, and to determine the level of interaction between this system and
the Galaxy. Photometry was obtained in two colours down to the magnitude limits
of V=20 and I=19, covering a 3.1 times 3.1 square deg area centred on Sculptor.
The resulting colour-magnitude data were used as a mask to select candidate
horizontal branch and red giant branch stars for this system. Previous work has
shown that the red horizontal branch (HB) stars are more concentrated than the
blue HB stars. We have determined the radial distributions of these two
populations and show that the overall Sculptor density profile is well
described by a two component model based on a combination of these radial
distributions. Additionally, spectra of the Ca ii triplet region were obtained
for over 700 candidate red giant stars over the 10 square deg region using the
2dF instrument on the Anglo-Australian Telescope. These spectra were used to
remove foreground Galactic stars based on radial velocity and Ca ii triplet
strength. The final list of Sculptor members contained 148 stars, seven of
which are located beyond the nominal tidal radius. Both the photometric and
spectroscopic datasets indicate no significant extra-tidal structure. These
results support at most a mild level of interaction between this system and the
Galaxy, and we have measured an upper mass limit for extra-tidal material to be
2.3 +/- 0.6% of the Sculptor luminous mass. This lack of tidal interaction
indicates that previous velocity dispersion measurements (and hence the amount
of dark matter detected) in this system are not strongly influenced by the
Galactic tidal field.Comment: 53 pages, 23 figures. Accepted for publication in the Astronomical
Journal. Some figures are reduced in size, and a full version is available
at: ftp://ftp.mso.anu.edu.au/pub/coleman/sculptor.pd
Fixed points and vacuum energy of dynamically broken gauge theories
We show that if a gauge theory with dynamical symmetry breaking has
non-trivial fixed points, they will correspond to extrema of the vacuum energy.
This relationship provides a different method to determine fixed points.Comment: 17 pages, uuencoded latex file, 3 figures, uses epsf and epsfig.
Submitted to Mod. Phys. Lett.
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