79 research outputs found
Galaxy Formation and Large-Scale Bias
We outline a simple approach to understanding the physical origin of bias in
the distribution of galaxies relative to that of dark matter. The first step is
to specify how collapsed, virialized halos of dark matter trace the overall
matter distribution. The next step is to make a connection between halos and
the luminous galaxies we observe. We appeal to the results of semi-analytic
models of galaxy formation that are tuned to fit the observed luminosity
functions of local groups and clusters. We have also used a high-resolution
N-body simulation of a cold dark matter (CDM) universe to study the bias
relation in more detail. The differences between the galaxy and dark matter
distributions are quantified using a number of different clustering statistics.
We arrive at the following general conclusions: 1) A comparison of the galaxy
and dark matter density fields shows that linear biasing is a good description
on large scales. 2) The bias factor b depends on the shape and normalization of
the power spectrum. The lower the normalization, the larger the bias. More bias
is obtained for spectra with more power on large scales. For "realistic"
models, b ranges from 1 to 2.5. 3) Galaxies of different luminosity or
morphology have different bias factors. 4) The scale dependence of the bias
factor is weak.Comment: 29 pages, uuencoded postscript file, 15 figures, 12 figures included,
submitted to MNRAS, complete version available at
http://www.mpa-garching.mpg.de/Galaxien/prep.htm
On the chemical and microstructural requirements for the pesting-resistance of Mo-Si-Ti alloys
In recent publications [Schliephake et al. in Intermetallics 104 (2019) 133-142 and Obert, Kauffmann & Heilmaier in Acta Materialia, 184 (2020) 132-142], an unexpected pesting-stability of fully eutectic and specific eutectic-eutectoid Mo-Si-Ti alloys was found. Several potential reasons were proposed: microstructural length scale being typically very fine due to the eutectic and eutectoid reactions, the phase distribution including the fraction of the eutectic and eutectoid regions, as well as the chemical composition of the individual phases. In the present study, we prove the Ti content to be decisive for the pesting-resistance in air at 800 °C by investigating the microstructure and oxidation behaviour of Mo-Si-Ti alloys with systematically varying nominal Ti content. A critical Ti content of 43 at% was identified. Due to the phase equilibrium, this corresponds to a local Ti content in Mo solid solution of 35 at%. Other microstructural properties such as (i) lamellar size within eutectic and eutectoid regions and (ii) volume fraction of the eutectic and eutectoid regions were demonstrated to have an insignificant influence on the pesting-resistance of the alloys. Rather, the assessment of the oxidation behaviour of the monolithic phase MoSS, which was identified to be crucial for the oxidation behaviour of the Mo-Si-Ti alloys, confirmed an improvement in oxidation behaviour with increasing Ti content
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
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
Simulations of galaxy formation in a Λ cold dark matter universe : I : dynamical and photometric properties of a simulated disk galaxy.
We present a detailed analysis of the dynamical and photometric properties of a disk galaxy simulated in the cold dark matter (CDM) cosmogony. The galaxy is assembled through a number of high-redshift mergers followed by a period of quiescent accretion after z1 that lead to the formation of two distinct dynamical components: a spheroid of mostly old stars and a rotationally supported disk of younger stars. The surface brightness profile is very well approximated by the superposition of an R1/4 spheroid and an exponential disk. Each photometric component contributes a similar fraction of the total luminosity of the system, although less than a quarter of the stars form after the last merger episode at z1. In the optical bands the surface brightness profile is remarkably similar to that of Sab galaxy UGC 615, but the simulated galaxy rotates significantly faster and has a declining rotation curve dominated by the spheroid near the center. The decline in circular velocity is at odds with observation and results from the high concentration of the dark matter and baryonic components, as well as from the relatively high mass-to-light ratio of the stars in the simulation. The simulated galaxy lies 1 mag off the I-band Tully-Fisher relation of late-type spirals but seems to be in reasonable agreement with Tully-Fisher data on S0 galaxies. In agreement with previous simulation work, the angular momentum of the luminous component is an order of magnitude lower than that of late-type spirals of similar rotation speed. This again reflects the dominance of the slowly rotating, dense spheroidal component, to which most discrepancies with observation may be traced. On its own, the disk component has properties rather similar to those of late-type spirals: its luminosity, its exponential scale length, and its colors are all comparable to those of galaxy disks of similar rotation speed. This suggests that a different form of feedback than adopted here is required to inhibit the efficient collapse and cooling of gas at high redshift that leads to the formation of the spheroid. Reconciling, without fine-tuning, the properties of disk galaxies with the early collapse and high merging rates characteristic of hierarchical scenarios such as CDM remains a challenging, yet so far elusive, proposition
A Maturity Model for Assessing the Digitalization of Public Health Agencies : Development and Evaluation
Damped Lyman alpha absorbers at high redshift -- large disks or galactic building blocks?
We investigate the nature of the physical structures giving rise to damped
Lyman alpha absorption systems (DLAS) at high redshift. In particular, we
examine the suggestion that rapidly rotating large disks are the only viable
explanation for the characteristic observed asymmetric profiles of low
ionization absorption lines. We demonstrate using hydrodynamic simulations of
galaxy formation in a cosmological context that irregular protogalactic clumps
can reproduce the observed velocity width distribution and asymmetries of the
absorption profiles equally well. The velocity broadening in the simulated
clumps is due to a mixture of rotation, random motions, infall and merging. The
observed velocity width correlates with the virial velocity of the dark matter
halo of the forming protogalactic clump (v_{wid} ~ 0.6 times v_{vir} for the
median values with a large scatter of order a factor two between different
lines-of-sight). The typical virial velocity of the halos required to give rise
to the DLAS population is about 100 km/s and most standard hierarchical
structure formation scenarios can easily account even for the largest observed
velocity widths. We conclude that the evidence that DLAS at high redshift are
related to large rapidly rotating disks with v_circ >= 200 km/s is not
compelling.Comment: 24 pages, LaTeX , 10 postscript figures included; submitted to ApJ.
The paper can also be retrieved at http://www.mpa-garching/~haehnel
Effects of Disks on Gravitational Lensing by Spiral Galaxies
Gravitational lensing of a quasar by a spiral galaxy should often be
accompanied by damped Lyman-alpha absorption and dust extinction due to the
intervening gaseous disk. In nearly edge-on configurations, the surface mass
density of the gas and stars in the disk could by itself split the quasar image
and contribute significantly to the overall lensing cross section. We calculate
the lensing probability of a disk+halo mass model for spiral galaxies,
including cosmic evolution of the lens parameters. A considerable fraction of
the lens systems contains two images with sub-arcsecond separation, straddling
a nearly edge-on disk. Because of that, extinction by dust together with
observational selection effects (involving a minimum separation and a maximum
flux ratio for the lensed images), suppress the detection efficiency of spiral
lenses in optical wavebands by at least an order of magnitude. The missing
lenses could be recovered in radio surveys. In modifying the statistics of
damped Lyman-alpha absorbers, the effect of extinction dominates over the
magnification bias due to lensing.Comment: 19 pages, 12 figures; submitted to Ap
Halo model description of the non-linear dark matter power spectrum at Mpc
Accurate knowledge of the non-linear dark-matter power spectrum is important
for understanding the large-scale structure of the Universe, the statistics of
dark-matter haloes and their evolution, and cosmological gravitational lensing.
We analytically model the dark-matter power spectrum and its cross-power
spectrum with dark-matter haloes. Our model extends the halo-model formalism,
including realistic substructure population within individual dark-matter
haloes and the scatter of the concentration parameter at fixed halo mass. We
consider three prescriptions for the mass-concentration relation and two for
the substructure distribution in dark-matter haloes. We show that this
extension of the halo model mainly increases the predicted power on the small
scales, and is crucial for proper modeling the cosmological weak-lensing signal
due to low-mass haloes. Our extended formalism shows how the halo model
approach can be improved in accuracy as one increases the number of ingredients
that are calibrated from n-body simulations.Comment: Accepted for publication in MNRAS - minor change
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