arXiv subjects

Felix Stoehr

Publications and source records attributed to Felix Stoehr.

25 records · Page 2Linked to original sources

arXiv2005

Hierarchical Growth and Cosmic Star Formation: Enrichment, Outflows and Supernova Rates

The cosmic star formation histories are evaluated for different minimum masses of the initial halo structures, with allowance for realistic gas outflows. With a minimum halo mass of 10^{7} - 10^{8} M_odot and a moderate outflow efficiency, we reproduce both the current baryon fraction and the early chemical enrichment of the IGM. The intensity of the formation rate of ``normal'' stars is also well constrained by the observations: it has to be dominated by star formation in elliptical galaxies, except perhaps at very low redshift. The fraction of baryons in stars is predicted as are also the type Ia and II supernova event rates. Comparison with SN observations in the redshift range z=0-2 allows us to set strong constraints on the time delay of type Ia supernovae (a total delay of \sim 4 Gyr is required to fit the data), the lower end of the mass range of the progenitors (2 - 8 M_odot) and the fraction of white dwarfs that reproduce the type Ia supernova (about 1 per cent). The intensity in the initial starburst of zero metallicity stars below 270 M_\odot must be limited in order to avoid premature overenrichment of the IGM. Only about 10 - 20 % of the metals present in the IGM at z = 0 have been produced by population III stars at very high z. The remaining 80 - 90 % are ejected later by galaxies forming normal stars, with a maximum outflow efficiency occurring at a redshift of about 5. We conclude that 10^{-3} of the mass in baryons must lie in first massive stars in order to produce enough ionizing photons to allow early reionization of the IGM by z \sim 15.

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arXiv2005

Semi-analytic Simulations of Galactic Winds: Volume Filling Factor, Ejection of Metals and Parameter Study

We present a semi-analytic treatment of galactic winds within high resolution, large scale cosmological N-body simulations of a LCDM Universe. The evolution of winds is investigated by following the expansion of supernova driven superbubbles around the several hundred thousand galaxies that form in an approximately spherical region of space with diameter 52 Mpc/h and mean density close to the mean density of the Universe. We focus our attention on the impact of winds on the diffuse intergalactic medium. Initial conditions for mass loss at the base of winds are taken from Shu, Mo and Mao (2003). Results are presented for the volume filling factor and the mass fraction of the IGM affected by winds and their dependence on the model parameters is carefully investigated. The mass loading efficiency of bubbles is a key factor to determine the evolution of winds and their global impact on the IGM: the higher the mass loading, the later the IGM is enriched with metals. Galaxies with 10^9 < M_* < 10^10 M_sun are responsible for most of the metals ejected into the IGM at z=3, while galaxies with M_* < 10^9 M_sun give a non negligible contribution only at higher redshifts, when larger galaxies have not yet assembled. We find a higher mean IGM metallicity than Lyalpha forest observations suggest and we argue that the discrepancy may be explained by the high temperatures of a large fraction of the metals in winds, which may not leave detectable imprints in absorption in the Lyalpha forest.

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arXiv2004

The subhalo populations of LCDM dark haloes

We use a large cosmological simulation and a variety of high resolution resimulations of individual cluster and galaxy haloes to study the systematics of subhalo populations over ranges of 1000 in halo mass and 1000 in the ratio of subhalo to parent halo mass. The subhalo populations of different haloes are not scaled copies of each other, but vary systematically with halo properties. On average, the amount of substructure increases with halo mass. At fixed mass, it decreases with halo concentration and with halo formation redshift. These trends are comparable in size to the scatter in subhalo abundance between similar haloes. Averaged over all haloes of given mass, the abundance of low mass subhaloes per unit parent halo mass is independendent of parent mass.It is very similar to the abundance per unit mass of low mass haloes in the universe as a whole, once differing boundary definitions for subhaloes and haloes are accounted for. The radial distribution of subhaloes within their parent haloes is substantially less centrally concentrated than that of the dark matter. It varies at most weakly with the mass(or concentration) of the parent halo and not at all with subhalo mass. It does depend on the criteria used to define the subhalo population considered. About 90% of present-day subhaloes were accreted after z=1 and about 70% after z=0.5. Only about 8% the total mass of all haloes accreted at z=1 survives as bound subhaloes at z=0. For haloes accreted at z=2, the survival mass fraction is just 2%. Subhaloes seen near the centre of their parent typically were accreted earlier and retain less of their original mass than those seen near the edge. These strong systematics mean that comparison with galaxies in real clusters is only possible if the formation of the luminous component is modelled appropriately.

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arXiv2004

Galactic winds and the Lya forest

We study the effect of galactic outflows on the Lya forest. The winds are modelled as fully ionised spherical bubbles centered around galactic haloes. The observed flux probability distribution and flux power spectrum limit the volume filling factor of bubbles to be less than 10%. We have compared the mean flux as a function of distance from haloes with the Adelberger etal (ASSP) measurement. A model with bubbles of constant radius of 1.5Mpc/h surrounding the most massive haloes matches the high transmissivity at separations <0.5Mpc/h, but the increase of the transmissivity at small scales is more gradual than observed. At separations <0.5Mpc/h, our estimate of the cosmic variance error is $ΔF\sim 0.3$, 30% higher than that of ASSP. The difficulty in matching the rise in the transmissivity at small separations is caused by residual absorption of neutral hydrogen lying physically outside the bubbles but having a redshift position similar to the haloes. The flux level is thus sensitive to the amplitude of the coherent velocity shear near haloes which increases with halo mass in the simulation. A model where LBGs are starbursts in small mass haloes matches the observations with smaller bubble size than one where more massive haloes host the LBGs. A starburst model with a bubble radius of 1Mpc/h and a volume filling factor of 2% is consistent with the ASSP measurements at the 1-1.5$σ$ level. If this model is correct the sharp rise of the transmissivity at small separation in the ASSP sample is due to cosmic variance and is expected to become more moderate for a larger sample.

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arXiv2004

Dark matter annihilation in the halo of the Milky Way

If the dark matter in the Universe is made of weakly self-interacting particles, they may self-annihilate and emit gamma-rays. We use high resolution numerical simulations to estimate directly the annihilation flux from the central regions of the Milky Way and from dark matter substructures in its halo. Although such estimates remain uncertain because of their strong dependence on the structure of the densest regions, our numerical experiments suggest that less direct calculations have overestimated the emission both from the centre and from halo substructure. We estimate a maximal enhancement of at most a factor of a few with respect to a smooth spherical halo of standard Navarro-Frenk-White (NFW) structure. We discuss detection strategies for the next generation of gamma-ray detectors and find that the annihilation flux may be detectable, regardless of uncertainties about the densest regions, for the annihilation cross-sections predicted by currently popular elementary particle models for the dark matter.

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arXiv2002

The Milky Way's satellite population in a LambdaCDM universe

We compare the structure and kinematics of the 11 known satellites of the Milky Way with high resolution simulations of the formation of its dark halo in a LambdaCDM universe. In contrast to earlier work, we find excellent agreement. The observed kinematics are exactly those predicted for stellar populations with the observed spatial structure orbiting within the most massive ``satellite'' substructures in our simulations. Less massive substructures have weaker potential wells than those hosting the observed satellites. If there is a halo substructure ``problem'', it consists in understanding why halo substructures have been so inefficient in making stars. Suggested modifications of dark matter properties (for example, self-interacting or warm dark matter) may well spoil the good agreement found for standard Cold Dark Matter.

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arXiv2002

Gas cooling in simulations of the formation of the galaxy population

We compare two techniques for following the cooling of gas and its condensation into galaxies within high resolution simulations of cosmologically representative regions. Both techniques treat the dark matter using N-body methods. One follows the gas using smoothed particle hydrodynamics (SPH) while the other uses simplified recipes from semi-analytic (SA) models. We compare the masses and locations predicted for dense knots of cold gas (the `galaxies') when the two techniques are applied to evolution from the same initial conditions and when the additional complications of star formation and feedback are ignored. We find that above the effective resolution limit of the two techniques, they give very similar results both for global quantities such as the total amount of cooled gas and for the properties of individual `galaxies'. The SA technique has systematic uncertainties arising from the simplified cooling model adopted, while details of the SPH implementation can produce substantial systematic variations in the galaxy masses it predicts. Nevertheless, for the best current SPH methods and the standard assumptions of the SA model, systematic differences between the two techniques are remarkably small. The SA technique gives adequate predictions for the condensation of gas into `galaxies' at less than one percent of the computational cost of obtaining similar results at comparable resolution using SPH.

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