Modelling the formation of today's massive ellipticals

The discovery of a population of massive, compact and quiescent early-type galaxies has changed the view on plausible formation scenarios for the present day population of elliptical galaxies. Traditionally assumed formation histories dominated by 'single events' like early collapse or major mergers appear to be incomplete and have to be embedded in the context of hierarchical cosmological models with continuous gas accretion and the merging of small stellar systems (minor mergers). Once these processes are consistently taken into account the hierarchical models favor a two-phase assembly process and are in much better shape to capture the observed trends. We review some aspects of recent progress in the field.Comment: To appear in the Proceedings of IAU Symposium No. 295: "The intriguing life of massive galaxies", D. Thomas, A. Pasquali and I. Ferreras, ed

Probing the kinematics of early-type galaxy halos using planetary nebulae

We present first results of a study of the halo kinematics for a sample of early type galaxies using planetary nebulae (PNe) as kinematical tracers. PNe allow to extend up to several effective radii (Re) the information from absorption line kinematics (confined to within 1 or 2 Re), providing valuable information and constraints for merger simulations and galaxy formation models. We find that the specific angular momentum per unit mass has a more complex radial dependence when the halo region is taken into account and that the halo velocity dispersion is related to the total galaxy luminosity, isophotal shape, and number of PNe per unit of luminosityComment: 4 pages, 3 figures. refereed proceeding of the "Galactic and Stellar Dynamics 2008" conference, Ed. C.M. Boily. To be published in AN, year: 2008, vol: 329, Issue: 9-10, pages: 912-91

Decenntralization and Democratic Local Governance in Ghana: Assessing the Performance of District Assemblies and Exploring the Scope of Partnerships

Matthias Zana Naab, PhDUniversity of Pittsburgh, 2005This study examines decentralization and democratic local governance in Ghana by assessing the effectiveness of the performance of District Assemblies (DAs) in order to better understand how DAs plan, implement, and manage development activities in close partnership with communities. It applied the proposition that decentralization and democratic local governance are expected to result in more efficient, effective, sustainable, and equitable outcomes through the hypotheses that decentralization results in more effective local government; more responsive local government; local government that is democratic, more accountable, and more participatory; local people having more positive perceptions of government; and local governments providing high quality services that respond to local demands.Engaging in both exploratory and explanatory research, this study identifies important variables and relationships as well as plausible causal networks that shaped local government and governance in Ghana. Using an inductive and theory-building design, it explains a model of decentralized governance and highlights potential partnership arrangements for the effective engagement of Community-Based Organizations (CBOs) in complementing the efforts of local governments.The results of decentralization, interpreted through questionnaires as well as stories and conversations with local people in two Ghanaian District Assemblies, was a combination of success and failure. In the two case study districts, the assemblies have resulted in a slight increase in development projects and services. However, the poor level of local revenue mobilization has limited the ability of the assemblies to finance significant development projects in their districts. Consequently, this has forced the assemblies to depend on the District Assemblies' Common Fund as well as on external donor funded projects and programs and on local people in self-help projects. The analysis of revenue and expenditure patterns in the two districts showed that per capita development spending was low, while recurrent expenditure and spending on local government infrastructure was high. District assemblies and CBOs often remain unwilling partners, and both are faced with serious capacity constraints which militate against structuring effective partnerships for service delivery. The successful implementation of decentralization depends on the degree to which national political leaders are committed to decentralization, and the ability and willingness of the national bureaucracy to facilitate and support decentralized development. Therefore, the ongoing process of decentralization in Ghana must be seen in the broader context of a deliberate redirection and change in the internal regulatory framework of the state

Major Mergers and the Origin of Elliptical Galaxies

The formation of elliptical galaxies as a result of the merging of spiral galaxies is discussed. We analyse a large set of numerical N-Body merger simulations which show that major mergers can in principle explain the observed isophotal fine structure of ellipticals and its correlation with kinematical properties. Equal-mass mergers lead to boxy, slowly rotating systems, unequal-mass mergers produce fast rotating and disky ellipticals. However, several problems remain. Anisotropic equal mass mergers appear disky under certain projections which is not observed. The intrinsic ellipticities of remnants are often larger than observed. Finally, although unequal-mass mergers produce fast rotating ellipticals, the remnants are in general more anisotropic than expected from observations. Additional processes seem to play an important role which are not included in dissipationless mergers. Resolving these problems might provide interesting new information on the structure and gas content of the progenitors of early-type galaxies.Comment: 13 pages, 5 figures, research review, to appear in "Galaxies and Chaos", eds. G. Contopoulos and N. Voglis (Springer

The Surprising Anisotropy of Fast Rotating, Disky Elliptical Galaxies

The projected kinematical properties of unequal-mass merger remnants of disk galaxies are analysed and shown to agree well with observations of disky, fast rotating elliptical galaxies. This supports the major merger hypothesis of early-type galaxy formation. However, in contrast to previous claims, the merger remnants are very anisotropic with values of the anisotropy parameter that are similar to equal-mass merger remnants that form boxy, slowly rotating ellipticals. Including gas in the simulations does not change this result although the line-of-sight velocity profile and the intrinsic orbital structure are strongly affected by the presenceof gas. The kinematical difference between boxy and disky ellipticals appears not to be the amount of anisotropy but rather rotation and the shape of the velocity dispersion tensor. The apparent isotropy of observed disky ellipticals is shown to result from inclination effects. Even small inclination angles strongly reduce the measured anisotropy of fast rotating systems, seen in projection. A second problem is the limited amount of information that is available when measuring only the central velocity dispersion and a characteristic rotation and ellipticity. Methods are investigated that allow a better determination of the intrinsic anisotropy of fast rotating early-type galaxies with known inclination angles.Comment: 7 pages, 6 figures, submitted to MNRA

Spinning dark matter halos promote bar formation

Stellar bars are the most common non-axisymmetric structures in galaxies and their impact on the evolution of disc galaxies at all cosmological times can be significant. Classical theory predicts that stellar discs are stabilized against bar formation if embedded in massive spheroidal dark matter halos. However, dark matter halos have been shown to facilitate the growth of bars through resonant gravitational interaction. Still, it remains unclear why some galaxies are barred and some are not. In this study, we demonstrate that co-rotating (i.e., in the same sense as the disc rotating) dark matter halos with spin parameters in the range of $0 \le \lambda_{\mathrm{dm}} \le 0.07$ - which are a definite prediction of modern cosmological models - promote the formation of bars and boxy bulges and therefore can play an important role in the formation of pseudobulges in a kinematically hot dark matter dominated disc galaxies. We find continuous trends for models with higher halo spins: bars form more rapidly, the forming slow bars are stronger, and the final bars are longer. After 2 Gyrs of evolution, the amplitude of the bar mode in a model with $\lambda_{\mathrm{dm}} = 0.05$ is a factor of ~6 times higher, A_2/A_0 = 0.23, than in the non-rotating halo model. After 5 Gyrs, the bar is ~ 2.5 times longer. The origin of this trend is that more rapidly spinning (co-rotating) halos provide a larger fraction of trailing dark matter particles that lag behind the disc bar and help growing the bar by taking away its angular momentum by resonant interactions. A counter-rotating halo suppresses the formation of a bar in our models. We discuss potential consequences for forming galaxies at high-redshift and present day low mass galaxies which have converted only a small fraction of their baryons into stars.Comment: 14 pages, 14 figures, 2 tables. Accepted for publication in MNRA

Galaxy mergers at high resolution: From elliptical galaxies to tidal dwarfs and globular clusters

Numerical simulations of galaxy mergers are a powerful tool to study these fundamental events in the hierarchical built-up of galaxies. Recent progress have been made owing to improved modeling, increased resolution and large statistical samples. We present here the highest-resolution models of mergers performed so far. The formation of a variety of substructures ranging from kinematically decoupled cores to globular-like clusters is directly resolved. In a resolution study, we show that the large-scale structure of elliptical-like merger remnants can be affected by the resolution, and a too modest resolution may affect the numerical predictions on the properties of major merger remnants: understanding precisely which kind of event or succession of events has formed the various types of elliptical galaxies remains an open challenge.Comment: To be published in the proceedings of the "Galactic and Stellar Dynamics 2008" conference. 4 page