77 research outputs found
Short-lived star-forming giant clumps in cosmological simulations of z~2 disks
Many observed massive star-forming z\approx2 galaxies are large disks that
exhibit irregular morphologies, with \sim1kpc, \sim10^(8-10)Msun clumps. We
present the largest sample to date of high-resolution cosmological SPH
simulations that zoom-in on the formation of individual M*\sim10^(10.5)Msun
galaxies in \sim10^(12)Msun halos at z\approx2. Our code includes strong
stellar feedback parameterized as momentum-driven galactic winds. This model
reproduces many characteristic features of this observed class of galaxies,
such as their clumpy morphologies, smooth and monotonic velocity gradients,
high gas fractions (f_g\sim50%) and high specific star-formation rates
(\gtrsim1Gyr^(-1)). In accord with recent models, giant clumps
(Mclump\sim(5x10^8-10^9)Msun) form in-situ via gravitational instabilities.
However, the galactic winds are critical for their subsequent evolution. The
giant clumps we obtain are short-lived and are disrupted by wind-driven mass
loss. They do not virialise or migrate to the galaxy centers as suggested in
recent work neglecting strong winds. By phenomenologically implementing the
winds that are observed from high-redshift galaxies and in particular from
individual clumps, our simulations reproduce well new observational constraints
on clump kinematics and clump ages. In particular, the observation that older
clumps appear closer to their galaxy centers is reproduced in our simulations,
as a result of inside-out formation of the disks rather than inward clump
migration.Comment: 11 pages, 6 figures, 1 table. Accepted for publication in the
Astrophysical Journa
On the Use of Ly-alpha Emitters as Probes of Reionization
We use numerical simulations to study the effects of the patchiness of a
partly reionized intergalactic medium (IGM) on the observability of Ly-alpha
emitters (LAEs) at high redshifts (z ~ 6). We present a new model that divides
the Ly-alpha radiative transfer into a (circum-)galactic and an extragalactic
(IGM) part, and investigate how the choice of intrinsic line model affects the
IGM transmission results. We use our model to study the impact of neutral
hydrogen on statistical observables such as the Ly-alpha restframe equivalent
width (REW) distribution, the LAE luminosity function and the two-point
correlation function. We find that if the observed changes in LAE luminosity
functions and equivalent width distributions between z ~ 6 and z ~ 7 are to be
explained by an increased IGM neutral fraction alone, we require an extremely
late and rapid reionization scenario, where the Universe was ~ 40 % ionized at
z = 7, ~ 50 % ionized at z = 6.5 and ~ 100 % ionized at z = 6. This is in
conflict with other observations, suggesting that intrinsic LAE evolution at z
> 6 cannot be completely neglected. We show how the two-point correlation
function can provide more robust constraints once future observations obtain
larger LAE samples, and provide predictions for the sample sizes needed to tell
different reionization scenarios apart.Comment: Accepted for publication in MNRA
Magnetic field amplification and X-ray emission in galaxy minor mergers
We investigate the magnetic field evolution in a series of galaxy minor
mergers using the N-body/smoothed particle hydrodynamics (SPH) code
\textsc{Gadget}. The simulations include the effects of radiative cooling, star
formation and supernova feedback. Magnetohydrodynamics (MHD) is implemented
using the SPH method. We present 32 simulations of binary mergers of disc
galaxies with mass ratios of 2:1 up to 100:1, whereby we have additionally
varied the initial magnetic field strengths, disc orientations and resolutions.
We investigate the amplification of a given initial magnetic field within the
galaxies and an ambient intergalactic medium (IGM) during the interaction. We
find that the magnetic field strengths of merger remnants with mass ratios up
to 10:1 saturate at a common value of several G. For higher mass ratios,
the field strength saturates at lower values. The saturation values correspond
to the equipartition value of magnetic and turbulent energy density. The
initial magnetization, disc orientation and numerical resolution show only
minor effects on the saturation value of the magnetic field. We demonstrate
that a higher impact energy of the progenitor galaxies leads to a more
efficient magnetic field amplification. The magnetic and turbulent energy
densities are higher for larger companion galaxies, consistent with the higher
impact energy supplied to the system. We present a detailed study of the
evolution of the temperature and the bolometric X-ray luminosity within the
merging systems. Thereby we find that magnetic fields cause a more efficient
increase of the IGM temperature and the corresponding IGM X-ray luminosity
after the first encounter. However, the presence of magnetic fields does not
enhance the total X-ray luminosity. Generally, the final value of the X-ray
luminosity is even clearly lower for higher initial magnetic fields.Comment: 20 pages, 12 figures. Submitted to MNRA
Can Gas prevent the Destruction of Thin Stellar Discs by Minor Mergers?
We study the effect of dissipational gas physics on the vertical heating and
thickening of disc galaxies during minor mergers. We produce a suite of minor
merger simulations for Milky Way-like galaxies. This suite consists of
collisionless simulations as well as hydrodynamical runs including a gaseous
component in the galactic disc. We find that in dissipationless simulations
minor mergers cause the scale height of the disc to increase by up to a factor
of ~2. When the presence of gas in the disc is taken into account this
thickening is reduced by 25% (50%) for an initial disc gas fraction of 20%
(40%), leading to a final scale height z0 between 0.6 and 0.7 kpc, regardless
of the initial scale height. We argue that the presence of gas reduces disc
heating via two mechanisms: absorption of kinetic impact energy by the gas
and/or formation of a new thin stellar disc that can cause heated stars to
recontract towards the disc plane. We show that in our simulations most of the
gas is consumed during the merger and thus the regrowth of a new thin disc has
a negligible impact on the z0 of the post merger galaxy. Final disc scale
heights found in our simulations are in good agreement with studies of the
vertical structure of spiral galaxies where the majority of the systems are
found to have scale heights of 0.4 kpc < z0 < 0.8 kpc. We also found no tension
between recent measurements of the scale height of the Milky Way thin disc and
results coming from our hydrodynamical simulations. We conclude that the
existence of a thin disc in the Milky Way and in external galaxies is not in
obvious conflict with the predictions of the CDM model.Comment: 12 pages, 13 figures, two additional simulations, discussion
enlarged, two figures added, updated references, accepted for publication in
MNRA
The Lyman-alpha forest in optically-thin hydrodynamical simulations
We study the statistics of the Lyman- forest in a flat LCDM cosmology
with the N-body + Eulerian hydrodynamics code Nyx. We produce a suite of
simulations, covering the observationally relevant redshift range . We find that a grid resolution of 20 kpc/h is required to produce one
percent convergence of Lyman- flux statistics, up to k = 10 h/Mpc. In
addition to establishing resolution requirements, we study the effects of
missing modes in these simulations, and find that box sizes of L > 40 Mpc/h are
needed to suppress numerical errors to a sub-percent level. Our optically-thin
simulations with the ionizing background prescription of Haardt & Madau (2012)
reproduce an IGM equation of state with and at z=2, with a mean transmitted flux close to the observed
values. When using the ionizing background prescription of Faucher-Giguere et
al. (2009), the mean flux is 10-15 per cent below observed values at z=2, and a
factor of 2 too small at z = 4. We show the effects of the common practice of
rescaling optical depths to the observed mean flux and how it affects
convergence rates. We also investigate the common practice of `splicing'
results from a number of different simulations to estimate the 1D flux power
spectrum and show it is accurate at the 10 percent level. Finally, we find that
collisional heating of the gas from dark matter particles is negligible in
modern cosmological simulations.Comment: 28 pages, 31 figures; version as accepted for MNRAS publication on
November 7, 201
Astrophysics with the Laser Interferometer Space Antenna
Laser Interferometer Space Antenna (LISA) will be a transformative experiment for gravitational wave astronomy as it will offer unique opportunities to address many key astrophysical questions in a completely novel way. The synergy with ground-based and other space-based instruments in the electromagnetic domain, by enabling multi-messenger observations, will add further to the discovery potential of LISA. The next decade is crucial to prepare the astrophysical community for LISA's first observations. This review outlines the extensive landscape of astrophysical theory, numerical simulations, and astronomical observations that are instrumental for modeling and interpreting the upcoming LISA datastream. To this aim, the current knowledge in three main source classes for LISA is reviewed: ultra-compact stellar-mass binaries, massive black hole binaries, and extreme or intermediate mass ratio inspirals. The relevant astrophysical processes and the established modeling techniques are summarized. Likewise, open issues and gaps in our understanding of these sources are highlighted, along with an indication of how LISA could help make progress in the different areas. New research avenues that LISA itself, or its joint exploitation with studies in the electromagnetic domain, will enable, are also illustrated. Improvements in modeling and analysis approaches, such as the combination of numerical simulations and modern data science techniques, are discussed. This review is intended to be a starting point for using LISA as a new discovery tool for understanding our Universe
Konzeption eines Managementsystems zur Erhaltung von Brücken- und Ingenieurbauwerken
Die Bundesanstalt für Straßenwesen (BASt) wurde vom Bundesministerium für Verkehr, Bau- und Wohnungswesen (BMVBW) mit der Konzeption eines Managementsystems der Erhaltungsplanung für Brücken- und Ingenieurbauwerke des deutschen Fernstraßennetzes beauftragt. Dabei sollte ein System konzipiert werden, welches zum einen den Bund in die Lage versetzt, neben einem Überblick über den aktuellen Zustand der Bauwerke auf Netzebene auch Aussagen zum Finanzbedarf zu erlangen und Strategien, langfristige Ziele sowie Rahmenbedingungen in der Erhaltungspraxis zu verwirklichen. Zum anderen sollten Ländern und Behörden Empfehlungen zur Durchführung von Verbesserungen auf Objektebene geliefert werden, die mit den Strategien, langfristigen Zielen, Rahmenbedingungen und Haushaltszwängen vereinbar sind. In einem ersten Schritt wurden bestehende Bauwerks-Managementsysteme (BMS) analysiert sowie aktuelle Entwicklungstendenzen aufgezeigt. Das aktuelle Vorgehen von Bund und Ländern bei der Erhaltungsplanung wurde im Rahmen einer Länderbefragung im Detail bestimmt. Dabei wurden auch die Zielvorstellungen der Betreiber eines zukünftigen Managementsystems der Bauwerkserhaltung identifiziert. Aufbauend auf diesen Erkenntnissen sowie einer bereits entwickelten Grobkonzeption des BMS wurden die Strukturen des Managementsystems erarbeitet, einzelne Module festgelegt und die Abläufe der Erhaltungsplanung unter Berücksichtigung von Randbedingungen und Zielkriterien erarbeitet. Ein wesentliches Ergebnis neben der Konzeption ist die Erarbeitung eines Stufenplans mit Zeitachse für die Entwicklung des Managementsystems sowie die Formulierung von Teilprojekten zur Fertigstellung des Systems mit Zeitplan. Aufbauend auf diesen Planungen ist die Realisierung eines kompletten BMS für Bund und Länder bis zum Jahr 2005 möglich. Voraussetzung ist jedoch die Bereitstellung ausreichender Finanzmittel beim BMVBW oder der BASt sowie die Bereitstellung aller erforderlichen Daten (ASB, Teilsystem Bauwerksdaten, Verkehrsdaten, Unfalldaten, Netzdaten und so weiter) durch die Länder. Hierbei wäre die Unterstützung des Bundes hilfreich
Bauwerks-Management-System (BMS) - Stand der Entwicklung
Zur ordnungsgemäßen Erhaltung der Bauwerke der Bundesfernstraßen (über 35.000 Brücken sowie über 150 Tunnelbauwerke und eine große Anzahl sonstiger Ingenieurbauwerke) wurde es notwendig, ein Bauwerks-Management-System (BMS) zu entwickeln, mit dem die Steuerung der erforderlichen Instandsetzungsmaßnahmen sowie der dafür notwendigen Haushaltsmittel durchgeführt werden kann. Grundlagen für die Erfassung der Bauwerksdaten und die durchzuführenden Instandsetzungen sind die bei den Straßenbauverwaltungen der Länder vorliegenden einheitlichen Datenbanken der Bauwerke und die regelmäßig durchzuführenden Bauwerksprüfungen nach DIN 1076. Aufgrund der bei den Prüfungen festgestellten Befunde sollen über die Instandsetzungsart und deren Zeitpunkt entschieden und die erforderlichen Mittel eingeplant werden. Die Prozesse des Erhaltungsmanagements - Planung, Realisierung und Controlling - werden im Einzelnen beschrieben und der vorgesehene Zeitplan für die Realisierung des Projektes wird angegeben
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