Do the low PN velocity dispersions around elliptical galaxies imply that these lack dark matter?

While kinematical modelling of the low PN velocity dispersions observed in the outer regions of elliptical galaxies suggest a lack of dark matter around these galaxies, we report on an analysis of a suite of $N$-body simulations (with gas) of major mergers of spiral galaxies embedded in dark matter halos, and find that the outer velocity dispersions are as low as observed for the PNe. The inconsistency between our dynamical modelling and previous kinematical modelling is caused by very radial stellar orbits and projection effects when viewing face-on oblate ellipticals. Our simulations (weakly) suggest the youth of PNe around ellipticals, and we propose that the universality of the PN luminosity function may be explained if the bright PNe in ellipticals are formed after the regular accretion of very low mass gas-rich galaxies.Comment: Contributed talk at meeting, "Planetary Nebulae as astronomical tools", Gdansk, Poland, June-July 2005, ed. R. Szczerba, G. Stasi\'nska, and S. K. G\'orny, AIP Conference Proceedings, Melville, New York, 2005. 4 or 5 pages, 6 figure

Lost and found dark matter in elliptical galaxies

The kinematical properties of elliptical galaxies formed during the mergers of equal mass, stars+gas+dark matter spiral galaxies are compared to the observed low velocity dispersions found for planetary nebulae on the outskirts of ellipticals, which have been interpreted as pointing to a lack of dark matter in ellipticals (which poses a problem for the standard model of galaxy formation). We find that the velocity dispersion profiles of the stars in the simulated ellipticals match well the observed ones. The low outer stellar velocity dispersions are mainly caused by the radial orbits of the outermost stars, which, for a given binding energy must have low angular momentum to reach their large radial distances, usually driven out along tidal tails.Comment: Talk presented at 21st IAP meeting, Mass Profiles andShapes of Cosmological Structures. Ed. G. A. Mamon, F. Combes, C. Deffayet & B. Fort (Paris: EDP), 4 pages, 3 figures (4 plots

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.Comment: Revised, Figure 7 added. To appear in MNRA

Simulations of Galaxy Formation and Large Scale Structure

Galaxy formation is one of the most fascinating topics of modern cosmology. Since time immemorial, people have desired to understand the origin, motion and evolution of planets, stars and, more recently, galaxies and the Universe as a whole. Great advances in astronomy always have had impact on philosophy and redefined the self-understanding of mankind within the Universe. The first milestone on the long road of discoveries was undoubtedly the formulation of the laws of gravity and mechanics in 1687 by Newton. Einstein's extension of these laws in the years 1905 and 1913 led to a revolutionised understanding of space and time. In 1929 Hubble established the expanding Universe which subsequently led to the postulation of the hot Big Bang by Lemaitre (1934). Zwicky (1933) found that most of matter in the Universe is dark. The nature of this matter, interacting only through gravity and perhaps through the weak interaction, is still a mystery. Finally, Penzias and Wilson (1965) discovered the cosmic microwave background radiation, not only confirming the theory of the Big Bang, but also - as was observed later - revealing the origin of structure in the Universe. Today, cosmology and especially galaxy formation are fast paced exciting scientific fields. Surveys like the Sloan Digital Sky Survey will soon provide a catalogue of about 500 million galaxies with an unprecedent wealth of data. Deep observations with 8 or 10 m telescopes or with the Hubble Space telescope allow to observe objects in their very early evolutionary stages. In addition to this, the dramatic increase in computer power now allows us to carry out numerical experiments on galaxies and even on the large scale structure of the Universe. The latter is possible because of the extraordinary fact that as a result of microwave background observations the properties of the Universe some 300,000 years after the Big Bang are well known. As ordinary matter makes up only about ten percent of the total matter in the Universe, it can be neglected in simulations in a first approximation. An initial density field can then be evolved under the sole influence of gravity. The result of such simulations may be combined with semi-analytic models for the baryonic physics associated with galaxy formation. Gravity is a long range force, and it turns out that length scales of 100 Mpc or more have to be included in large scale structure simulations in order to obtain results that are representative for the Universe as a whole. The sizes of galaxies, however, are three to four orders of magnitude smaller than this so that numerical resolution has always been a concern in simulations which try to include galaxy formation. A clever and powerful trick alleviates this problem. After a low-resolution simulation has been performed, a small region of interest is selected and the simulation is run again, this time concentrating most of the computational effort on the small region, allowing the resolution to be increase dramatically without losing tidal influences from the large cosmological volume. This technique - called resimulation - is the driving force behind all the simulations that were performed for this thesis. After having run about 1500 supercomputer jobs it is clear that this technique is extremely powerful and allows the faithful simulation of objects that are far into the regime of non-linear evolution while taking into account the full cosmological context. In the first chapter of this work we briefly introduce aspects of the observable Universe and discuss the relevant theoretical background for this thesis. In the second chapter we use high-resolution simulations of structure formation to investigate the influence of the local environment of dark matter haloes on their properties. We run a series of four re-simulations of a typical, carefully selected representative region of the Universe so that we can explicitly check for convergence of the numerical results. In our highest resolution simulation we are able to resolve dark matter haloes as small as the one of the large Magellanic cloud. We propose a new method to estimate the density in the environment of a collapsed object and find weak correlations of the spin parameter and the concentration parameter with the local halo density. We find no such correlation for the halo shapes, the formation time and the last major merging event. In a second step we produce catalogues of model galaxies using a semi-analytic model of galaxy formation. We find correlations between the bulge-to-disk luminosity and the B-V colour index with the local environment. In chapter three we compare observations of the internal structure and kinematics of the eleven known satellites of the Milky Way with simulations of the formation of its dark halo in a LambdaCDM universe. Earlier work by Moore et al. 1999 and Klypin et al. 1999 claimed the cosmological concordance model of the Universe, the LambdaCDM model, to disagree with observations. The so-called "substructure-problem" is one of the two major challenges for this model and has attracted much attention. In order to remove the discrepancy, changes of the cosmological model have been proposed. We reinvestigate the substructure-problem using our ultra-high resolution simulations. For a galaxy-sized dark matter halo, our mass resolution is the highest resolution ever achieved. In contrast to the work of Moore et al. 1999 and Klypin et al. 1999, 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. We find that suggested modifications of dark matter properties (e.g. self-interacting or warm dark matter) may well spoil the good agreement found for standard Cold Dark Matter. If the dark matter in the Universe is made of weakly self-interacting particles, they may self-annihilate and emit gamma-rays. The detection of the gamma-ray signal would finally, after seventy years since its discovery, shed light on the nature of the dark matter. In chapter four we use our ultra-high resolution numerical simulations to estimate directly the annihilation flux from the central region of the Milky Way and from dark matter substructures in its halo. Such estimates remain uncertain because of their strong dependence on the structure of the densest regions of the halo. Our numerical experiments suggest, however, that less direct calculations have typically overestimated the emission from the centre of the Milky Way and from its halo's substructure. We find an overall enhancement of at most a factor of a few with respect to a smooth halo of standard NFW structure. For an observation outside the region around the galactic centre where the diffuse galactic gamma-ray background is dominant, GLAST can probe a large region of possible MSSM models. This result is independent of the exact structure of the innermost region of the Galaxy. Our analysis shows that the flux from the inner galaxy exceeds the expected contribution from the brightest substructure by a large factor. Nevertheless, for certain MSSM models substructure halos might be detectable with GLAST.Das Studium der Entstehung und Entwicklung von Galaxien ist eines der interessantesten Gebiete der Kosmologie. In dieser Arbeit verwenden wir die sogenannte Resimulationstechnik und berechnen ultrahochauflösende Computersimulationen der Strukturentstehung im Universum. Zunächst simulieren wir eine sorgfältig ausgewählte große Region mittlerer Dichte mit vier verschiedenen Auflösungen und untersuchen die Abhängigkeit der Eigenschaften der Halos aus dunkler Materie von der Materiedichte in ihrer lokalen Umgebung. Wir finden eine schwache Abhängigkeit der Spin- und Konzentrationsparameter. Die Form der Halos, ihre Entstehungszeitpunkte sowie der Zeitpunkt des letzten Verschmelzens mit einer etwa gleich großen Galaxie sind dagegen umgebungsunabhängig. In einem weiteren Schritt wenden wir ein semi-analytisches Modell der Galaxienentstehung auf die Simulationen an. Unsere Modellgalaxien zeigen starke Umgebungsabhängigkeiten. Im zweiten Teil dieser Arbeit vergleichen wir die beobachtete Struktur und Kinematik der bekannten Satellitengalaxien der Milchstraße mit den Eigenschaften simulierter Subhalos. Es war behauptet worden, diese beobachteten Eigenschaften würden nicht mit den simulierten übereinstimmen, was das kosmologische LambdaCDM Modell in Frage gestellt hat. Wir berechnen die Entstehung des Halos einer Galaxie von der Größe der Milchstraße mit bisher unerreichter Auflösung und untersuchen dieses sogenannte "Substruktur-Problem" erneut. Wir finden, daß die von unserer Simulation vorhergesagten Geschwindigkeitsdispersionen beobachteter Sternverteilungen exakt mit den beobachteten Dispersionen übereinstimmen. Dies ist ein Triumph für das LambdaCDM Modell. Andere Formen dunkler Materie, die vorgeschlagen wurden, um das Substruktur-Problem zu lösen, würden die gefundene übereinstimmung zerstören. Im dritten Teil dieser Arbeit untersuchen wir, ob die Gammastrahlung, die durch Selbst-Annihilation entstehen kann, falls die dunkle Materie im Universum aus schwach wechselwirkenden massiven Teilchen besteht, mit der nächsten Generation von Gammastrahlungsteleskopen beobachtet werden kann. Wir verwenden unsere hochauflösende Simulation des Galaxienhalos und finden, daß frühere Untersuchungen die Bedeutung der Gammastrahlung von Subhalos überschätzt haben. Wir schlagen eine neue Beobachtungsstrategie vor und zeigen, daß bei derzeit diskutierten Modellen für die Teilchen der dunklen Materie das galaktische Zentrum mit GLAST beobachtet werden kann

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.Comment: 10 pages, 8 figures, replaced with version accepted by MNRAS (very minor changes

Circular velocity profiles of dark matter haloes

We use a high-resolution simulation of a galaxy-sized dark matter halo, published simulated data as well as four cluster-sized haloes from Fukushige, Kawai & Makino to study the inner halo structure in a Lambda cold dark matter cosmology. We find that the circular velocity curves are substantially better described by Stoehr et al. (SWTS) profiles than by Navarro, Frenk & White (NFW) or Moore et al. profiles. Our findings confirm that no asymptotic slope is reached and that the profiles are nearly universal, but not perfectly. The velocity profiles curve at a_constant_ rate in log(r) over the full converged range in radii and the corresponding extrapolated density profiles reach a finite maximum density. We find, that the claim of a strong discrepancy between the observed inner slopes of the density profiles of low surface brightness galaxies (LSB) and their simulated counterparts on the grounds of currently available observations and simulations is unfounded. In addition, if the SWTS profile turns out to be a good description of the halo profile for the regions that cannot be probed with simulations of today, then even in these regions the agreeement between simulations and observations is very reasonable.Comment: 7 pages, 3 figures, replaced with version accepted to MNRA