5,025 research outputs found
Dark Halo and Disk Galaxy Scaling Laws in Hierarchical Universes
We use cosmological N-body/gasdynamical simulations that include star
formation and feedback to examine the proposal that scaling laws between the
total luminosity, rotation speed, and angular momentum of disk galaxies reflect
analogous correlations between the structural parameters of their surrounding
dark matter halos. The numerical experiments follow the formation of
galaxy-sized halos in two Cold Dark Matter dominated universes: the standard
Omega=1 CDM scenario and the currently popular LCDM model. We find that the
slope and scatter of the I-band Tully-Fisher relation are well reproduced in
the simulations, although not, as proposed in recent work, as a result of the
cosmological equivalence between halo mass and circular velocity: large
systematic variations in the fraction of baryons that collapse to form galaxies
and in the ratio between halo and disk circular velocities are observed in our
numerical experiments. The Tully-Fisher slope and scatter are recovered in this
model as a direct result of the dynamical response of the halo to the assembly
of the luminous component of the galaxy. We conclude that models that neglect
the self-gravity of the disk and its influence on the detailed structure of the
halo cannot be used to derive meaningful estimates of the scatter or slope of
the Tully-Fisher relation. Our models fail, however, to match the zero-point of
the Tully-Fisher relation, as well as that of the relation linking disk
rotation speed and angular momentum. These failures can be traced,
respectively, to the excessive central concentration of dark halos formed in
the Cold Dark Matter cosmogonies we explore and to the formation of galaxy
disks as the final outcome of a sequence of merger events. (abridged)Comment: submitted to The Astrophysical Journa
The cosmological origin of the Tully-Fisher relation
We use high-resolution cosmological simulations that include the effects of
gasdynamics and star formation to investigate the origin of the Tully-Fisher
relation in the standard Cold Dark Matter cosmogony. Luminosities are computed
for each model galaxy using their full star formation histories and the latest
spectrophotometric models. We find that at z=0 the stellar mass of model
galaxies is proportional to the total baryonic mass within the virial radius of
their surrounding halos. Circular velocity then correlates tightly with the
total luminosity of the galaxy, reflecting the equivalence between mass and
circular velocity of systems identified in a cosmological context. The slope of
the relation steepens slightly from the red to the blue bandpasses, and is in
fairly good agreement with observations. Its scatter is small, decreasing from
\~0.45 mag in the U-band to ~0.34 mag in the K-band. The particular
cosmological model we explore here seems unable to account for the zero-point
of the correlation. Model galaxies are too faint at z=0 (by about two
magnitudes) if the circular velocity at the edge of the luminous galaxy is used
as an estimator of the rotation speed. The Tully-Fisher relation is brighter in
the past, by about ~0.7 magnitudes in the B-band at z=1, at odds with recent
observations of z~1 galaxies. We conclude that the slope and tightness of the
Tully-Fisher relation can be naturally explained in hierarchical models but
that its normalization and evolution depend strongly on the star formation
algorithm chosen and on the cosmological parameters that determine the
universal baryon fraction and the time of assembly of galaxies of different
mass.Comment: 5 pages, 4 figures included, submitted to ApJ (Letters
The Angular Momentum Distribution of Gas and Dark Matter in Galactic Halos
(Abridged) We report results of a series of non radiative N-body/SPH
simulations in a LCDM cosmology. We find that the spin of the baryonic
component is on average larger than that of the dark matter (DM) component and
we find this effect to be more pronounced at lower redshifts. A significant
fraction f of gas has negative angular momentum and this fraction is found to
increase with redshift. We describe a toy model in which the tangential
velocities of particles are smeared by Gaussian random motions. This model is
successful in explaining some of the angular momentum properties. We compare
and contrast various techniques to determine the angular momentum distributions
(AMDs). We show that broadening of velocity dispersions is unsuitable for
making comparisons between gas and DM. We smooth the angular momentum of the
particles over a fixed number of neighbors. We find that an analytical function
based on gamma distribution can be used to describe a wide variety of profiles,
with just one parameter \alpha. The distribution of the shape parameter
for both gas and DM follows roughly a log-normal distribution. The
mean and standard deviation of log(\alpha) for gas is -0.04 and 0.11
respectively. About 90-95% of halos have \alpha<1.3, while exponential disks in
NFW halos would require 1.3<\alpha<1.6. This implies that a typical halo in
simulations has an excess of low angular momentum material as compared to that
of observed exponential disks, a result which is consistent with the findings
of earlier works. \alpha for gas is correlated with that of DM but they have a
significant scatter =1.09 \pm 0.2. \alpha_Gas is also
biased towards slightly higher values compared to \alpha_DM.Comment: 19 pages, 32 figures (replaced to correct a typo in the authors field
in the above line, paper unchanged
Integration of altitude and airspeed information into a primary flight display via moving-tape formats
A ground-based aircraft simulation study was conducted to determine the effect on pilot performance of replacing the electromechanical altimeter and airspeed indicators with electronically generated representations integrated into the primary flight display via moving-tape (linear moving scale) formats. Several key factors relating to moving-tape formats were examined during the study: tape centering, secondary (trend) information, and tape orientation. The factor of centering refers to whether the tape was centered about the actual airspeed or altitude or about some defined reference value. Tape orientation refers to whether the values represented are arranged in either descending or ascending order. Six pilots participated in this study, with each subject performing 18 runs along a single, known flight profile. Subjective results indicated that the moving-tape formats were generally better than that of the conventional instruments. They also indicated that an actual-centered fixed pointer was preferred to a reference-centered pointer. Performance data for a visual secondary task showed that formats not containing trend information produced better performance; however, no difference was noted in airspeed tracking or altitude tracking performance. Regarding tape orientation, subjective comments indicated that there was lower work load and better performance when the airspeed tape had the high numbers at the top
Embrapa e Irga buscam validação do método de graus-dia em arroz irrigado.
bitstream/item/76293/1/silvio-1.pdfDisponível em: http://www.paginarural.com.br/artigos_detalhes.php?id=2378&im
The Effects of a Photoionizing UV Background on the Formation of Disk Galaxies
We use high resolution N-body/gasdynamical simulations to investigate the
effects of a photoionizing UV background on the assembly of disk galaxies in
hierarchically clustering universes. We focus on the mass and rotational
properties of gas that can cool to form centrifugally supported disks in dark
matter halos of different mass. Photoheating can significantly reduce the
amount of gas that can cool in galactic halos. Depending on the strength of the
UV background field, the amount of cooled gas can be reduced by up to in
systems with circular speeds in the range - \kms. The magnitude of the
effect, however, is not enough to solve the ``overcooling'' problem that
plagues hierarchical models of galaxy formation if the UV background is chosen
to be consistent with estimates based on recent observations of QSO absorption
systems. Photoionization has little effect on the collapse of gas at high
redshift and affects preferentially gas that is accreted at late times. Since
disks form inside-out, accreting higher angular momentum gas at later times,
disks formed in the presence of a UV background have spins that are even
smaller than those formed in simulations that do not include the effects of
photoionization. This exacerbates the angular momentum problem that afflicts
hierarchical models of disk formation. We conclude that photoionization cannot
provide the heating mechanism required to reconcile hierarchically clustering
models with observations. Energy feedback and enrichment processes from the
formation and evolution of stars must therefore be indispensable ingredients
for any successful model of the formation of disk galaxies.Comment: 36 pages, w/ embedded figures, submitted to ApJ. Also available at
http://penedes.as.arizona.edu/~jfn/preprints/dskform.ps.g
A Unified Scaling Law in Spiral Galaxies
We investigate the origin of a unified scaling relation in spiral galaxies.
Observed spiral galaxies are spread on a plane in the
three-dimensionallogarithmic space of luminosity L, radius R and rotation
velocity V. The plane is expressed as in I-passband,
where is a constant. On the plane, observed galaxies are distributed
in an elongated region which looks like the shape of a surfboard. The
well-known scaling relations, L-V (Tully-Fisher relation), V-R (also the
Tully-Fisher relation) and R-L (Freeman's law), can be understood as oblique
projections of the surfboard-like plane into 2-D spaces. This unified
interpretation of the known scaling relations should be a clue to understand
the physical origin of all the relations consistently. Furthermore, this
interpretation can also explain why previous studies could not find any
correlation between TF residuals and radius.
In order to clarify the origin of this plane, we simulate formation and
evolution of spiral galaxies with the N-body/SPH method, including cooling,
star formation and stellar feedback. Initial conditions are set to isolated 14
spheres with two free parameters, such as mass and angular momentum. The CDM
(h=0.5, ) cosmology is considered as a test case. The simulations
provide the following two conclusions: (a) The slope of the plane is well
reproduced but the zero-point is not. This zero-point discrepancy could be
solved in a low density ($\Omega_00.5) cosmology.
(b) The surfboard-shaped plane can be explained by the control of galactic mass
and angular momentum.Comment: Accepted for publication in ApJ Letters. 6 pages including 2 figure
Integration of altitude and airspeed information into a primary flight display via moving-tape formats: Evaluation during random tracking task
A ground-based aircraft simulation study was conducted to determine the effects on pilot preference and performance of integrating airspeed and altitude information into an advanced electronic primary flight display via moving-tape (linear moving scale) formats. Several key issues relating to the implementation of moving-tape formats were examined in this study: tape centering, tape orientation, and trend information. The factor of centering refers to whether the tape was centered about the actual airspeed or altitude or about some other defined reference value. Tape orientation refers to whether the represented values are arranged in descending or ascending order. Two pilots participated in this study, with each performing 32 runs along seemingly random, previously unknown flight profiles. The data taken, analyzed, and presented consisted of path performance parameters, pilot-control inputs, and electrical brain response measurements
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