2,579 research outputs found
The history of star formation in a LCDM universe
Employing hydrodynamic simulations of structure formation in a LCDM
cosmology, we study the history of cosmic star formation from the "dark ages"
at redshift z~20 to the present. In addition to gravity and ordinary
hydrodynamics, our model includes radiative heating and cooling of gas, star
formation, supernova feedback, and galactic winds. By making use of a
comprehensive set of simulations on interlocking scales and epochs, we
demonstrate numerical convergence of our results on all relevant halo mass
scales, ranging from 10^8 to 10^15 Msun/h. The predicted density of cosmic star
formation is broadly consistent with measurements, given observational
uncertainty. From the present epoch, it gradually rises by about a factor of
ten to a peak at z~5-6, which is beyond the redshift range where it has been
estimated observationally. 50% of the stars are predicted to have formed by
redshift z~2.1, and are thus older than 10.4 Gyr, while only 25% form at
redshifts lower than z~1. The mean age of all stars at the present is about 9
Gyr. Our model predicts a total stellar density at z=0 of Omega_*=0.004,
corresponding to about 10% of all baryons being locked up in long-lived stars,
in agreement with recent determinations of the luminosity density of the
Universe. We determine the "multiplicity function of cosmic star formation" as
a function of redshift; i.e. the distribution of star formation with respect to
halo mass. We also briefly examine possible implications of our predicted star
formation history for reionisation of hydrogen in the Universe. We find that
the star formation rate predicted by the simulations is sufficient to account
for hydrogen reionisation by z~6, but only if a high escape fraction close to
unity is assumed. (abridged)Comment: updated to match published version, minor plotting error in Fig.12
corrected, 25 pages, version with high-resolution figures available at
http://www.mpa-garching.mpg.de/~volker/paper_sfr
Orbital Parameters of Merging Dark Matter Halos
In order to specify cosmologically motivated initial conditions for major
galaxy mergers (mass ratios 4:1) that are supposed to explain the
formation of elliptical galaxies we study the orbital parameters of major
mergers of cold dark matter halos using a high-resolution cosmological
simulation. Almost half of all encounters are nearly parabolic with
eccentricities and no correlations between the halo spin planes
or the orbital planes. The pericentric argument shows no correlation
with the other orbital parameters and is distributed randomly. In addition we
find that 50 % of typical pericenter distances are larger than half the halo's
virial radii which is much larger than typically assumed in numerical
simulations of galaxy mergers. In contrast to the usual assumption made in
semi-analytic models of galaxy formation the circularities of major mergers are
found to be not randomly distributed but to peak around a value of . Additionally all results are independent of the minimum
progenitor mass and major merger definitions (i.e. mass ratios 4:1; 3:1;
2:1).Comment: 11 pages, 20 figures, replaced by version accepted to A&A, figure 1
low re
First Structure Formation: A Simulation of Small Scale Structure at High Redshift
We describe the results of a simulation of collisionless cold dark matter in
a LambdaCDM universe to examine the properties of objects collapsing at high
redshift (z=10). We analyze the halos that form at these early times in this
simulation and find that the results are similar to those of simulations of
large scale structure formation at low redshift. In particular, we consider
halo properties such as the mass function, density profile, halo shape, spin
parameter, and angular momentum alignment with the minor axis. By understanding
the properties of small scale structure formation at high redshift, we can
better understand the nature of the first structures in the universe, such as
Population III stars.Comment: 31 pages, 14 figures; accepted for publication in ApJ. Figure 1 can
also be viewed at http://cfa-www.harvard.edu/~hjang/research
What is the nature of RX J0720.4-3125?
RX J0720.4-3125 has recently been identified as a pulsating soft X-ray source
in the ROSAT all-sky survey with a period of 8.391 s. Its spectrum is well
characterized by a black-body with a temperature of K. We
propose that the radiation from this object is thermal emission from a cooling
neutron star. For this black-body temperature we can obtain a robust estimate
of the object's age of yr, yielding a polar field G for magnetic-dipole spin down and a value of compatible
with current observations.Comment: 4 pages, 1 figures, to appear in Monthly Notice
A QED Model for Non-thermal Emission from SGRs and AXPs
Previously, we showed that, owing to effects arising from quantum
electrodynamics (QED), magnetohydrodynamic fast modes of sufficient strength
will break down to form electron-positron pairs while traversing the
magnetospheres of strongly magnetised neutron stars. The bulk of the energy of
the fast mode fuels the development of an electron-positron fireball. However,
a small, but potentially observable, fraction of the energy (
ergs) can generate a non-thermal distribution of electrons and positrons far
from the star. In this paper, we examine the cooling and radiative output of
these particles. We also investigate the properties of non-thermal emission in
the absence of a fireball to understand the breakdown of fast modes that do not
yield an optically thick pair plasma. This quiescent, non-thermal radiation
associated with fast mode breakdown may account for the recently observed
non-thermal emission from several anomalous X-ray pulsars and soft-gamma
repeaters.Comment: 14 pages, 2 figures, submitted to MNRA
A new empirical method to infer the starburst history of the Universe from local galaxy properties
The centres of ellipticals and bulges are formed dissipationally, via gas inflows over short time-scales â the âstarburstâ mode of star formation. Recent work has shown that the surface brightness profiles, kinematics and stellar populations of spheroids can be used to separate the dissipational component from the dissipationless âenvelopeâ made up of stars formed over more extended histories in separate objects, and violently assembled in mergers. Given high-resolution, detailed observations of these âburst relicâ components of ellipticals (specifically their stellar mass surface density profiles), together with the simple assumptions that some form of the KennicuttâSchmidt law holds and that the burst was indeed a dissipational, gas-rich event, we show that it is possible to invert the observed profiles and obtain the time- and space-dependent star formation history of each burst. We perform this exercise using a large sample of well-studied spheroids, which have also been used to calibrate estimates of the âburst relicâ populations. We show that the implied bursts scale in magnitude, mass and peak star formation rate (SFR) with galaxy mass in a simple manner, and provide fits for these correlations. The typical burst mass M_(burst) is ⌠10 per cent of the total spheroid mass, the characteristic starburst time-scale implied is a nearly galaxy-mass-independent t_(burst) ⌠10âž yr, the peak SFR of the burst is âŒM_(burst)/t_(burst) and bursts decay subsequently in power-law fashion as áč_â
â t^(-2.4). As a function of time, we obtain the spatial size of the starburst; burst sizes at peak activity scale with burst mass in a manner similar to the observed spheroid sizeâmass relation, but are smaller than the full galaxy size by a factor of âŒ10; the size grows in time as the central, most dense regions are more quickly depleted by star formation as R_(burst) â t^(0.5). Combined with observational measurements of the nuclear stellar population ages of these systems â i.e. the distribution of times when these bursts occurred â it is possible to re-construct the dissipational burst contribution to the distribution of SFRs and infrared (IR) luminosity functions (LFs) and luminosity density of the Universe. We do so and show that these burst LFs agree well with the observed IR LFs at the brightest luminosities, at redshifts z⌠0â2. At low luminosities, however, bursts are always unimportant; the transition luminosity between these regimes increases with redshift from the ultraluminous infrared galaxy threshold at z⌠0 to hyper-luminous infrared galaxy thresholds at z⌠2. At the highest redshifts zâł 2, we can set strict upper limits on starburst magnitudes, based on the maximum stellar mass remaining at high densities at z= 0, and find tension between these and estimated number counts of sub-millimetre galaxies, implying that some change in bolometric corrections, the number counts themselves or the stellar initial mass function may be necessary. At all redshifts, bursts are a small fraction of the total SFR or luminosity density, âŒ5â10 per cent, in good agreement with estimates of the contribution of merger-induced star formation
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