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

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 – the ‘starburst’ 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 ‘envelope’ made up of stars formed over more extended histories in separate objects, and violently assembled in mergers. Given high-resolution, detailed observations of these ‘burst relic’ components of ellipticals (specifically their stellar mass surface density profiles), together with the simple assumptions that some form of the Kennicutt–Schmidt 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 ‘burst relic’ 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) ∼ 10⁸ yr, the peak SFR of the burst is ∼M_(burst)/t_(burst) and bursts decay subsequently in power-law fashion as Ṁ_★ ∝ 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 size–mass relation, but are smaller than the full galaxy size by a factor of ∼10; the size grows in time as the central, most dense regions are more quickly depleted by star formation as R_(burst) ∝ t^(0.5). Combined with observational measurements of the nuclear stellar population ages of these systems – i.e. the distribution of times when these bursts occurred – 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∼ 0–2. 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 z≳ 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, ∼5–10 per cent, in good agreement with estimates of the contribution of merger-induced star formation

Global Properties of Multiple Merger Remnants

Merger 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, $v_r/\sigma \approx 1$ 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 [email protected]. Accepted to the Astrophysical Journa

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 $8 \times 10^5$ 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 $\sim 3 \times 10^5$ yr, yielding a polar field $\sim 10^{14}$ G for magnetic-dipole spin down and a value of ${\dot P}$ compatible with current observations.Comment: 4 pages, 1 figures, to appear in Monthly Notice