380,834 research outputs found
An N-body/SPH Study of Isolated Galaxy Mass Density Profiles
We investigate the evolution of mass density profiles in secular disk galaxy
models, paying special attention to the development of a two-component profile
from a single initial exponential disk free of cosmological evolution (i.e., no
accretion or interactions). As the source of density profile variations, we
examine the parameter space of the spin parameter, halo concentration, virial
mass, disk mass and bulge mass, for a total of 162 simulations in the context
of a plausible model of star formation and feedback (GADGET-2). The evolution
of the galaxy mass density profile, including the development of a
two-component profile with an inner and outer segment, is controlled by the
ratio of the disk mass fraction, , to the halo spin parameter,
. The location of the break between the two components and speed at
which it develops is directly proportional to ; the amplitude of
the transition between the inner and outer regions is however controlled by the
ratio of halo concentration to virial velocity. The location of the divide
between the inner and outer profile does not change with time. (Abridged)Comment: 27 pages, 31 figures. Accepted for publication at MNRAS. A
high-resolution version of the paper with figures can be found here
http://www.mpia-hd.mpg.de/~foyle/papers/MN-07-1491-MJ.R1.pd
Cosmic Evolution of Stellar Disk Truncations: From z~1 to the Local Universe
We have conducted the largest systematic search so far for stellar disk
truncations in disk-like galaxies at intermediate redshift (z<1.1), using the
Great Observatories Origins Deep Survey South (GOODS-S) data from the Hubble
Space Telescope - ACS. Focusing on Type II galaxies (i.e. downbending profiles)
we explore whether the position of the break in the rest-frame B-band radial
surface brightness profile (a direct estimator of the extent of the disk where
most of the massive star formation is taking place), evolves with time. The
number of galaxies under analysis (238 of a total of 505) is an order of
magnitude larger than in previous studies. For the first time, we probe the
evolution of the break radius for a given stellar mass (a parameter well suited
to address evolutionary studies). Our results suggest that, for a given stellar
mass, the radial position of the break has increased with cosmic time by a
factor 1.3+/-0.1 between z~1 and z~0. This is in agreement with a moderate
inside-out growth of the disk galaxies in the last ~8 Gyr. In the same period
of time, the surface brightness level in the rest-frame B-band at which the
break takes place has increased by 3.3+/-0.2 mag/arcsec^2 (a decrease in
brightness by a factor of 20.9+/-4.2). We have explored the distribution of the
scale lengths of the disks in the region inside the break, and how this
parameter relates to the break radius. We also present results of the
statistical analysis of profiles of artificial galaxies, to assess the
reliability of our results.Comment: 22 pages, 14 figures, accepted for publication in ApJ. Figures 1, 3
and 6 have somehow downgraded resolution to match uploading requirement
The stars and gas in outer parts of galaxy disks: Extended or truncated -- flat or warped?
I review observations of truncations of stellar disks and models for their
origin, compare observations of truncations in moderately inclined galaxies to
those in edge-on systems and discuss the relation between truncations and
HI-warps and their systematics and origin. Truncations are a common feature in
edge-on stellar disks, but the relation of truncations in face-on to those in
edge-on galaxies needs further clarification. The origin of truncations is most
likely related to a maximum in the specific angular momentum in the material
that formed the stellar disks, but this model does probably require some
redistribution of angular momentum. HI-warps start just beyond the truncation
radius and disks and warps appear distinct components. This suggests that inner
disks form initially and settle as rigid, very flat structures, while HI-warps
result from later infall of gas with a different orientation of the angular
momentum.Comment: Invited Review at the Vatican Symposium: Formation and Evolution of
Galaxy Disks, October 2007, proceeding editors Jose G. Funes, SJ and Enrico
M. Corsin
Lessons from the Milky Way: the Kapteyn Universe
Jacobus Cornelius Kapteyn (1851-1922) presented a model for the distribution
of stars in space together with a dynamical interpretation in terms of an
equilibrium between the gravitational field of the stars and their random
motion and rotation. In the vertical direction Kapteyn's results are
substantially correct. Usually the Kapteyn Universe is described as being
flawed due to neglect of interstellar absorption. Kapteyn was led to adopt this
on the basis of widely accepted evidence by Shapley on an absence of reddening
of stars in globular clusters. But another, equally important misconception was
Kapteyn's interpretation of the two Star Streams as manifestations of two
groups of stars rotating around a center in opposite directions. This was
supported by the observation of very different mixes in stellar types in the
two streams. Had Kapteyn adopted the absorption as he himself had determined it
he would not have been able to arrive at a consistent picture.Comment: To appear in LESSONS FROM THE LOCAL GROUP: A Conference in Honour of
David Block and Bruce Elmegreen, eds. Freeman, K.C., Elmegreen, B.G., Block,
D.L.and Woolway, M., Springer: New Yor
The rise, development and internationalization of Japan's motor industry : a thesis presented in partial fulfilment of the requirements for the degree of Master of Philosophy in Geography at Massey University
The rise of Japan's motor industry to world prominence has been one of the most spectacular developments of modern times and has come to epitomize Japan's 'economic miracle' in the postwar era. The industry's phenomenal growth was the result of various factors. During its infant and developing phases these included: a legacy in terms of industrial expertise from before the War; a much improved labour relations climate; and the favourable economic conditions that prevailed both at home and abroad that provided the framework for an era of high-speed growth. The most significant factor though, was the support given by the government which considered the industry as economically 'strategic' and 'nurtured' it to growth — along with many of its supporting industries — with a wide range of industrial policies. Notwithstanding the contribution of these factors, however, the industry showed a remarkable resourcefulness and creativity of its own. Borrowing engineering and management techniques from abroad such as Quality Control, and experimenting with new indigenous concepts such as Just-in-Time, the industry was able to dramatically raise the quality of its products and its levels of productivity. Based on these strengths the Japanese automakers became, during the 1970s, major exporters of motor vehicles and were able to capture ever larger shares of foreign markets. In recent years, however, confronted with increased international trade friction, the erection of trade barriers in its major foreign markets, and the rising value of the yen, the industry has inceasingly sought to move operations abroad. These moves to overseas locations are on such an unprecedented scale that they are contributing to create a new international geography of motor vehicle production
Flaring and self-shadowed disks around Herbig Ae stars: simulations for 10 micron interferometers
We present simulations of the interferometric visibilities of Herbig Ae star
disks. We investigate whether interferometric measurements in the 10 micrometer
atmospheric window are sensitive to the presence of an increased scale height
at the inner disk edge, predicted by recent models. Furthermore, we investigate
whether such measurements can discriminate between disks with a ``flaring''
geometry and disks with a ``flat'' geometry. We show that both these questions
can be addressed, using measurements at a small number of appropriately chosen
baselines. The classification of Herbig Ae stars in two groups, based on the
appearance of the spectral energy distribution (SED), has been attributed to a
difference in disk geometry. Sources with a group I SED would have a flaring
outer disk geometry, whereas the disk of group II sources is proposed to be
flat (or ``self-shadowed''). We show that this hypothesis can be tested using
long-baseline interferometric measurements in the micrometer atmospheric
window.Comment: 10 pages, 7 figures, accepted for publiction in Astronomy and
Astrophysic
Non-collinearity in di-jet fragmentation in electron-positron scattering
We study fragmentation in electron-positron annihilation assuming a di-jet
situation, using variables defined independent of any frame. In a collinear
situation some of the variables are centered around zero with the small
deviations attributed to intrinsic transverse momenta and large deviations
attributed to additional hard subprocesses. Of course there is a gradual
transition. Our modest goal is to show that covariantly defined variables are
well suited to get a feeling for the magnitude of intrinsic transverse momenta.Comment: 10 pages, 12 figures, several minor correction
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