876 research outputs found
The history of star formation in a LCDM universe
Full text linkEmploying 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
What is the nature of RX J0720.4-3125?
Get PDFRX 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
Full text linkPreviously, 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
Get PDFThe centres of ellipticals and bulges are formed dissipationally, via gas inflows over short time-scales ā the āstarburstā 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 āenvelopeā made up of stars formed over more extended histories in separate objects, and violently assembled in mergers. Given high-resolution, detailed observations of these āburst relicā components of ellipticals (specifically their stellar mass surface density profiles), together with the simple assumptions that some form of the KennicuttāSchmidt 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 āburst relicā 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 ā¼ 10 per cent of the total spheroid mass, the characteristic starburst time-scale implied is a nearly galaxy-mass-independent t_(burst) ā¼ 10āø yr, the peak SFR of the burst is ā¼M_(burst)/t_(burst) and bursts decay subsequently in power-law fashion as į¹_ā
ā 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 sizeāmass relation, but are smaller than the full galaxy size by a factor of ā¼10; the size grows in time as the central, most dense regions are more quickly depleted by star formation as R_(burst) ā t^(0.5). Combined with observational measurements of the nuclear stellar population ages of these systems ā i.e. the distribution of times when these bursts occurred ā 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 zā¼ 0ā2. At low luminosities, however, bursts are always unimportant; the transition luminosity between these regimes increases with redshift from the ultraluminous infrared galaxy threshold at zā¼ 0 to hyper-luminous infrared galaxy thresholds at zā¼ 2. At the highest redshifts zā³ 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, ā¼5ā10 per cent, in good agreement with estimates of the contribution of merger-induced star formation
Hotspot Emission from a Freely Precessing Neutron Star
Get PDFRecent observations of 1E~161348-5055, the neutron-star candidate at the
center of the supernova remnant RCW 103, show that a component of its emission
varies sinusoidally with a period of approximately six hours. We argue that
this period is what one would expect for a freely precessing neutron star with
a spin period of about one second. We produce light curves for a freely
precessing neutron star with a hotspot. By a suitable choice of parameters, we
obtain light curves which are constant with rotational phase when the flux from
the star reaches a maximum. At other phases of the precession, the flux varies
as the star rotates but the total flux decreases by a factor of several. These
models can explain the behavior observed from 1E~161348-5055 and predict that
the spin period should be detectable at minimum flux from sufficiently
sensitive measurements.Comment: 11 pages, 5 figures, submitted to Astrophysical Journal Letter
Global Properties of Multiple Merger Remnants
Get PDFMerger remnants of small groups of galaxies are contrasted with relics of
mergers of pairs of galaxies to determine which process produces objects that
best compare to real ellipticals. In both cases, the progenitors consist of
self-gravitating disks, halos, and, sometimes, bulges. Pairs of galaxies merge
from orbits that initially have zero--energy. The systems that produce multiple
merger remnants are dense, six--member groups in virial equilibrium with low
velocity dispersions. Multiple and pair mergers produce remnants which differ
in both their spatial and kinematic properties. Multiple merger remnants have
small triaxialities and are most likely to appear nearly round from most
viewing angles. They possess cores, with sizes of a few tenths of an effective
radius, that are more extended than pair remnant cores, even when bulges are
included in the progenitors. In multiple mergers, the spin of all components --
halo, disk, and bulge -- increases and, while velocity dispersion dominates in
the central regions, outside an effective radius in some
projections. The angular momentum and minor axis vectors are aligned for
multiple merger remnants. During merging of multiple progenitors, about half of
the orbital angular momentum in each luminous component is converted into
internal rotation in that component. Material is prevented from accumulating in
the center of multiple merger remnants as efficiently as it does in pair
mergers. In pair mergers that include gas, unrealistically steep surface
brightness profiles have been produced in center of the remnants; in multiple
mergers the formation of overdense nuclei may be impeded, thus allowing more
successful comparison with real elliptical galaxies.Comment: 43 pages of uuencoded compressed postscript with 17 figures.
Postscript figures 1,2,3,17, 3Mb total, available upon request from
weil@ucolick.org. Accepted to the Astrophysical Journa
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