Jesper Sommer-Larsen

Formation of Disk Galaxies: Warm Dark Matter and the Angular Momentum Problem

We have performed TreeSPH simulations of disk galaxy formation in various warm dark matter (WDM) cosmologies. Our results indicate that for a range of WDM free-streaming masses, the disk galaxy formation angular momentum problem can be considerably alleviated (and, we speculate, perhaps even completely resolved) by going to the WDM structure formation scenario, without having to invoke stellar feedback processes. They also strongly suggest that part of the angular momentum problem is due to numerical effects, most likely related to the shock-capturing artificial viscosity used in smoothed particle hydrodynamics (SPH). Furthermore, we find that we can match the observed I-band Tully-Fisher (TF) relation, provided that the mass-to-light ratio of disk galaxies is M/LI 0.6-0.7. We argue that this is a fairly reasonable value compared to various dynamical and spectrophotometric estimates, including two given in this paper. Finally, we extensively discuss possible physical candidates for WDM particles. We find that the most promising are neutrinos with weaker or stronger interactions than normal, majorons (light pseudo-Goldstone bosons), or mirror- or shadow-world neutrinos.

X-ray emission from haloes of simulated disc galaxies

ABSTRACT Bolometric and 0.2-2 keV X-ray luminosities of the hot gas haloes of simulated discgalaxies have been calculated at redshift z=0. The TreeSPH simulations are fullycosmological and the sample of 44 disc galaxies span a range in characteristic circularspeeds of V c = 130-325 km s −1 . The galaxies have been obtained in simulations witha considerable range of physical parameters, varying the baryonic fraction, the gasmetallicity, the meta-galactic UV field, the cosmology, the dark matter type, and alsothe numerical resolution. The models are found to be in agreement with the (few)relevant X-ray observations available at present. The amount of hot gas in the haloesis also consistent with constraints from pulsar dispersion measures in the Milky Way.Forthcoming XMM and Chandra observations should enable much more stringenttests and provide constraints on the physical parameters. We find that simple coolingflow models over-predict X-ray luminosities by up to two orders of magnitude forhigh (but still realistic) cooling efficiencies relative to the models presented here. Ourresults display a clear trend that increasing cooling efficiency leads to decreasing X-ray luminosities at z=0. The reason is found to be that increased cooling efficiencyleads to a decreased fraction of hot gas relative to total baryonic mass inside of thevirial radius at present. At gas metal abundances of a third solar this hot gas fractionbecomes as low as just a few percent. We also find that most of the X-ray emissioncomes from the inner parts (r<∼20 kpc) of the hot galactic haloes. Finally, we find forrealistic choices of the physical parameters that disc galaxy haloes possibly were morethan one order of magnitude brighter in soft X-ray emission at z∼1, than at present.Key words: methods: N-body simulations –cooling flows –galaxies: evolution –galaxies: formation –galaxies: halos –galaxies: spiral –X-rays: galaxies

ON DDO 154 AND COLD DARK MATTER HALO PROFILES

We investigate the claim by Burkert & Silk that the observed rotation curve of the dwarf irregular galaxy DDO 154 cannot be reconciled with the universal CDM halo profile of Navarro, Frenk & White, even when allowance is made for the effect of violent gas outflow events on the structure of the galaxy. By means of N-body simulations we show that under certain conditions it is possible to obtain a reasonable fit to the observed rotation curve without invoking Burkert & Silks proposed spheroidal MACHO component. We are able to reproduce the observed decline in the rotation curve best by postulating additional hidden disc mass, in an amount that is compatible with disc stability requirements. In the process we improve upon the results of Navarro, Eke & Frenk on the formation of halo cores by mass-loss by using actual haloes from cold dark matter simulations instead of Hernquist distributions.

The Density Profile of Cluster-Scale Dark Matter Halos

We measure the average gravitational shear profile of six massive clusters (Mvir ~ 1015 M☉) at z = 0.3 out to a radius of ~2 h-1 Mpc. The measurements are fitted to a generalized Navarro-Frenk-White-like halo model ρ(r) with an arbitrary r → 0 slope, α. The data are well fitted by such a model with a central cusp with α ~ 0.9-1.6 (68% confidence interval). For the standard Navarro-Frenk-White case α = 1.0, we find a concentration parameter cvir that is consistent with recent predictions from high-resolution cold dark matter N-body simulations. Our data are also well fitted by an isothermal sphere model with a softened core. For this model, our 1 σ upper limit for the core radius corresponds to a limit σ ≤ 0.1 cm2 g-1 on the elastic collision cross section in a self-interacting dark matter model.

The chemical evolution of gas‐rich dwarf galaxies

In an eort to understand the evolution of N, O, and He abundances in gas-rich dwarf galaxies, we investigate the dispersion and mixing of supernova ejecta in relation to H ii region evolution and develop a numerical model of chemical evolution based on a double-bursting mode of star formation (with an interval of the order of 3 10 7 years between bursts of a pair) which has been designed to account for the existence of signican t scatter in the N/O{O/H relation. The dependence of the abundances on gas fraction is explored on the basis of this and similar models, in combination with various hypotheses concerning ino w and selective and non-selective outo w. The gas fractions are uncertain within wide limits for blue compact galaxies, but more well dened for some dwarf irregulars. Selective winds do not give a good t to N/O, while closed models and models with non-selective winds with or without ino w are all found to be viable.

A New Measurement of the Baryonic Fraction Using the Sparse NGC 3258 Group of Galaxies

New X-ray observations of the sparse NGC 3258 group of galaxies made by the ASCA satellite with good spectral and spatial resolution has revealed that this group has a gravitational potential deep enough to prohibit significant mass removal from the system. The baryonic fraction within 240 h−150 kpc is found to be 0.065+ 0.051−0.020 for h50 = 1, where h50 ≡ H0/50 km s-1 Mpc-1, in good agreement with the universal value of 0.05 ± 0.01 predicted by standard big bang nucleosynthesis for a universe with Ω0 = 1 and h50 = 1. Since the deep potential of the NGC 3258 group ensures that all pristine intragroup gas has been retained, the baryonic fraction of the NGC 3258 group is indicative of the universal value. Consequently, it seems premature to rule out a critical universe.

Galaxy Formation: Warm Dark Matter, Missing Satellites, and the Angular Momentum Problem

We present warm dark matter (WDM) as a possible solution to the missing satellites and angular momentum problem in galaxy formation and introduce improved initial conditions for numerical simulations of WDM models, which avoid the formation of unphysical haloes found in earlier simulations. There is a hint, that because of that the mass function of satellite haloes has been overestimated so far, pointing to higher values for the WDM particle mass.

High-redshift X-ray properties of the haloes of simulated disc galaxies

X-ray luminosities and surface brightness profiles of the hot gas haloes of simulated disc galaxies at redshifts z = 0 2 are presented. The galaxies are extracted from fully cosmological simulations and correspond in mass to the Milky Way. We find that the bolometric X-ray luminosities of the haloes decrease by a factor 4 10 from z � 1 to z � 0, reflecting the decrease in the rate at which hot halo gas cools out on to the disc. At all redshifts, most of the emission is found to originate within 10–15 kpc of the disc. When combined with models in which the evolution of disc X-ray luminosity is dominated by X-ray binaries, the predicted halo luminosities at z � 1 show good agreement with constraints from spiral galaxies in Chandra Deep Field data. There is an indication that haloes with a metal abundance of 0.3Z⊙ overpredict observed Xray luminosities at z � 1, suggesting that halo metallicities are lower than this value. Prospects for direct detection of the haloes of Milky Way–sized galaxies with current and future X-ray instrumentation are discussed. It is found that XEUS should be able to single out the halo emission of highly inclined Milky Way–sized disc galaxies out to z � 0.3. For such galaxies in this redshift interval, we estimate a lower limit to the surface density of detectable haloes on the sky of � 10 deg −2 . More generally, owing to their luminosity evolution, the optimum redshifts at which to observe such haloes could be 0.5 < z < 1, depending on their assembly history.

The Structure of Isothermal, Self-Gravitating, Stationary Gas Spheres for Softened Gravity

A theory for the structure of isothermal, self-gravitating gas spheres in pressure equilibrium is developed for softened gravity, assuming an ideal gas equation of state. The one-parameter spline softening proposed by Hernquist & Katz is used. We show that the addition of this extra scale parameter implies that the set of equilibrium solutions constitute a one-parameter family, rather than the one and only one isothermal sphere solution for Newtonian gravity, and we develop a number of approximate, analytical or semianalytical solutions. For softened gravity, the structure of isothermal spheres is, in general, very different from the Newtonian isothermal sphere. For example, for any finite choice of softening length and temperature T, it is possible to deposit an arbitrarily large mass of gas in pressure equilibrium and with a nonsingular density distribution inside of r0 for any r0 > 0. Furthermore, it is sometimes claimed that the size of the small-scale, self-gravitating gas structures formed in dissipative Tree-SPH simulations is simply of the order the gravitational softening length. We demonstrate, that this claim, in general, is not correct. The main purpose of the paper is to compare the theoretical predictions of our models with the properties of the small, massive, quasi-isothermal gas clumps (r ~ 1 kpc, M ~ 1010 M?, and T 104 K) which form in numerical Tree-SPH simulations of the formation of Milky Way-sized galaxies when effects of stellar feedback processes are not included. We find reasonable agreement, despite the neglect of effects of rotational support in the models presented in this paper. We comment on whether the hydrodynamical resolution is sufficient in our numerical simulations of galaxy formation involving highly supersonic, radiative shocks, and we give a necessary condition, in the form of a simple test, that the hydrodynamical resolution in any such simulations is sufficient. Finally, we conclude that one should be cautious, when comparing results of numerical simulations, involving gratitational softening and hydrodynamical smoothing, with reality.

On the global structure of self-gravitating discs for softened gravity

Effects of gravitational softening on the global structure of self-gravitating disks in centrifugal equillibrium are examined in relation to hydrodynamical/gravitational simulations. The one-parameter spline softening proposed by Hernquist&Katz is used. It is found that if the characteristic size of a disk, r, is comparable or less than the gravitational softening length, epsilon, then the cross section of the simulated disk is significantly larger than that of a no-softening (Newtonian) disk with the same mass and angular momentum. We furthermore demonstrate that if r is less than or about epsilon/2 then the scaling relation r proportional to epsilon^3/4 holds for a given mass and specific angular momentum distribution with mass. Finally we compare some of the theoretical results obtained in this and a previous paper with the results of numerical Tree-SPH simulations and find qualitative agreement.