3,306 research outputs found
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
Star Formation, Supernovae Feedback and the Angular Momentum Problem in Numerical CDM Cosmogony: Half Way There?
We present a smoothed particle hydrodynamic (SPH) simulation that reproduces
a galaxy that is a moderate facsimile of those observed. The primary failing
point of previous simulations of disk formation, namely excessive transport of
angular momentum from gas to dark matter, is ameliorated by the inclusion of a
supernova feedback algorithm that allows energy to persist in the model ISM for
a period corresponding to the lifetime of stellar associations. The inclusion
of feedback leads to a disk at a redshift , with a specific angular
momentum content within 10% of the value required to fit observations. An
exponential fit to the disk baryon surface density gives a scale length within
17% of the theoretical value. Runs without feedback, with or without star
formation, exhibit the drastic angular momentum transport observed elsewhere.Comment: 4 pages, 3 figures, accepted for publication in ApJ Letter
AstroGrid-D: Enhancing Astronomic Science with Grid Technology
We present AstroGrid-D, a project bringing together astronomers and experts in Grid technology to enhance astronomic science in many aspects. First, by sharing currently dispersed resources, scientists can calculate their models in more detail. Second, by developing new mechanisms to efficiently access and process existing datasets, scientific problems can be investigated that were until now impossible to solve. Third, by adopting Grid technology large instruments such as robotic telescopes and complex scientific workflows from data aquisition to analysis can be managed in an integrated manner. In this paper, we present prominent astronomic use cases, discuss requirements on a Grid middleware and present our approach to extend/augment existing middleware to facilitate the improvements mentioned above
Accuracy of Mesh Based Cosmological Hydrocodes: Tests and Corrections
We perform a variety of tests to determine the numerical resolution of the
cosmological TVD eulerian code developed by Ryu et al (1993). Tests include
512^3 and 256^3 simulations of a Pk=k^{-1} spectrum to check for
self-similarity and comparison of results with those from higher resolution SPH
and grid-based calculations (Frenk et al 1998). We conclude that in regions
where density gradients are not produced by shocks the code degrades resolution
with a Gaussian smoothing (radius) length of 1.7 cells. At shock caused
gradients (for which the code was designed) the smoothing length is 1.1 cells.
Finally, for \beta model fit clusters, we can approximately correct numerical
resolution by the transformation R^2_{core}\to R^2_{core}-(C\Delta l)^2, where
\Delta l is the cell size and C=1.1-1.7. When we use these corrections on our
previously published computations for the SCDM and \Lambda CDM models we find
luminosity weighted, zero redshift, X-ray cluster core radii of (210\pm 86,
280\pm 67)h^{-1}kpc, respectively, which are marginally consistent with
observed (Jones & Forman 1992) values of 50-200h^{-1}kpc. Using the corrected
core radii, the COBE normalized SCDM model predicts the number of bright
L_x>10^{43}erg/s clusters too high by a factor of \sim 20 and the \Lambda CDM
model is consistent with observations.Comment: ApJ in press (1999
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
CIV Absorption From Galaxies in the Process of Formation
We investigate the heavy element QSO absorption systems caused by gas
condensations at high redshift which evolve into galaxies with circular
velocity of 100 to 200 km/s at the present epoch. Artificial QSO spectra were
generated for a variety of lines-of-sight through regions of the universe
simulated with a hydrodynamics code. The CIV and HI absorption features in
these spectra closely resemble observed CIV and HI absorption systems over a
wide range in column density. CIV absorption complexes with multiple-component
structure and velocity spreads up to about 600 km/s are found. The broadest
systems are caused by lines-of-sight passing through groups of protogalactic
clumps with individual velocity dispersions of less than 150 km/s aligned along
filamentary structures. The temperature of most of the gas does not take the
photoionization equilibrium value. This invalidates density and size estimates
derived from thermal equilibrium models. Consequences for metal abundance
determinations are briefly discussed. We predict occasional exceptionally large
ratios of CIV to HI column density (up to a third) for lines-of-sight passing
through compact halos of hot gas with temperature close to 3 10^5 K. Our model
may be able to explain both high-ionization multi-component heavy-element
absorbers and damped Lyman alpha systems as groups of small protogalactic
clumps.Comment: 13 pages, uuencoded postscript file, 4 figures included submitted to
ApJ (Letters); complete version also available at
http://www.mpa-garching.mpg.de/Galaxien/prep.htm
Hysteresis multicycles in nanomagnet arrays
We predict two new physical effects in arrays of single-domain nanomagnets by
performing simulations using a realistic model Hamiltonian and physical
parameters. First, we find hysteretic multicycles for such nanomagnets. The
simulation uses continuous spin dynamics through the Landau-Lifshitz-Gilbert
(LLG) equation. In some regions of parameter space, the probability of finding
a multicycle is as high as ~0.6. We find that systems with larger and more
anisotropic nanomagnets tend to display more multicycles. This result
demonstrates the importance of disorder and frustration for multicycle
behavior. We also show that there is a fundamental difference between the more
realistic vector LLG equation and scalar models of hysteresis, such as Ising
models. In the latter case, spin and external field inversion symmetry is
obeyed but in the former it is destroyed by the dynamics, with important
experimental implications.Comment: 7 pages, 2 figure
The Evolved Red Stellar Contents of the Sculptor Group Galaxies NGC55, NGC300, and NGC7793
Deep J, H, and K images are used to probe the evolved stellar contents in the
central regions of the Sculptor group galaxies NGC55, NGC300, and NGC7793. The
brightest stars are massive red supergiants (RSGs) with K ~ 15 - 15.5. The peak
RSG brightness is constant to within ~0.5 mag in K, suggesting that NGC55,
NGC300, and NGC7793 are at comparable distances. Comparisons with bright RSGs
in the Magellanic Clouds indicate that the difference in distance modulus with
respect to the LMC is = 7.5. A rich population of asymptotic giant branch (AGB)
stars, which isochrones indicate have ages between 0.1 and 10 Gyr, dominates
the (K, J-K) color-magnitude diagram (CMD) of each galaxy. The detection of
significant numbers of AGB stars with ages near 10 Gyr indicates that the disks
of these galaxies contain an underlying old population. The CMDs and luminosity
functions reveal significant galaxy-to-galaxy variations in stellar content.
Star-forming activity in the central arcmin of NGC300 has been suppressed for
the past Gyr with respect to disk fields at larger radii. Nevertheless,
comparisons between fields within each galaxy indicate that star-forming
activity during intermediate epochs was coherent on spatial scales of a kpc or
more. A large cluster of stars, which isochrones suggest has an age near 100
Myr, is seen in one of the NGC55 fields. The luminosity function of the
brightest stars in this cluster is flat, as expected if a linear
luminosity-core mass relation is present.Comment: 30 pages, including 13 figure
Differential expression of skeletal muscle genes following administration of clenbuterol to exercised horses.
BackgroundClenbuterol, a beta2-adrenergic receptor agonist, is used therapeutically to treat respiratory conditions in the horse. However, by virtue of its mechanism of action it has been suggested that clenbuterol may also have repartitioning affects in horses and as such the potential to affect performance. Clenbuterol decreases the percent fat and increases fat-free mass following high dose administration in combination with intense exercise in horses. In the current study, microarray analysis and real-time PCR were used to study the temporal effects of low and high dose chronic clenbuterol administration on differential gene expression of several skeletal muscle myosin heavy chains, genes involved in lipid metabolism and the ÎČ2-adrenergic receptor. The effect of clenbuterol administration on differential gene expression has not been previously reported in the horse, therefore the primary objective of the current study was to describe clenbuterol-induced temporal changes in gene expression following chronic oral administration of clenbuterol at both high and low doses.ResultsSteady state clenbuterol concentrations were achieved at approximately 50 h post administration of the first dose for the low dose regimen and at approximately 18-19 days (10 days post administration of 3.2 ÎŒg/kg) for the escalating dosing regimen. Following chronic administration of the low dose (0.8 ÎŒg/kg BID) of clenbuterol, a total of 114 genes were differentially expressed, however, none of these changes were found to be significant following FDR adjustment of the p-values. A total of 7,093 genes were differentially expressed with 3,623 genes up regulated and 3,470 genes down regulated following chronic high dose administration. Of the genes selected for further study by real-time PCR, down-regulation of genes encoding myosin heavy chains 2 and 7, steroyl CoA desaturase and the ÎČ2-adrenergic receptor were noted. For most genes, expression levels returned towards baseline levels following cessation of drug administration.ConclusionThis study showed no evidence of modified gene expression following chronic low dose administration of clenbuterol to horses. However, following chronic administration of high doses of clenbuterol alterations were noted in transcripts encoding various myosin heavy chains, lipid metabolizing enzymes and the ÎČ2-adrenergic receptor
The Galactic globular cluster system as a fossil record of reionization
We propose that structural, kinematical, and chemical properties of the
Galactic globular clusters (GCs) can contain fossil information of the cosmic
reionization history. We first summarize possible observational evidences for
the influence of reionization on the Galactic GC formation. We then show how
structural properties of the GC system (GCS) in the Galaxy can be influenced by
suppression of GC formation due to reionization during the Galaxy formation
through hierarchical merging of subgalactic clumps, by using numerical
simulations with and without suppression of GC formation by reionization. In
particular, we show that if GC formation in dwarf galaxies that are building
blocks of the Galaxy and virialized after reionization era () are
completely suppressed, the present-day radial distribution of the Galactic GCs
depends strongly on . Our numerical results imply that if GC
formation after 15 is strongly suppressed, the origin of the
observed structural properties of the Galactic GCS can be more naturally
explained in the framework of the hierarchical clustering scenario.Comment: 8 pages 3 figures (1 color jpg) ApJL in press (626, L93
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