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

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

Parents’ online self-disclosure and parental social media trusteeship: How parents manage the digital identity of their children

Obwohl Eltern versuchen ihre Privatsphäre zu schützen, offenbaren sie in sozialen Medien oftmals persönliche Informationen ihrer Kinder. Der Beitrag untersucht das «privacy paradox» und die Selbstoffenbarung in sozialen Medien und schlägt das Konzept elterlicher Medientreuhänderschaft als ergänzende theoretische Perspektive vor, um zu verstehen, wie Eltern die digitale Identität ihrer Kinder verwalten. Die theoretischen Überlegungen werden durch Ergebnisse von 46 Tiefeninterviews ergänzt. Sie zeigen, dass sich die Eltern der Verantwortung ihrer Treuhänderschaft kaum bewusst sind und die Folgen ihrer Social Media-Aktivitäten nur mit Einschränkungen abschätzen können. In der Analyse der Interviews lassen sich insgesamt drei unterschiedliche Ansätze elterlicher Medientreuhänderschaft identifizieren: Während (1) einige Eltern versuchen ihre Kinder weitgehend von sozialen Medien abzuschirmen, scheinen (2) andere nur eingeschränkt in der Lage zu sein, angemessen auf Risiken sozialer Medien zu reagieren oder (3) blenden diese weitgehend aus. Schliesslich wird mit Blick auf die elterliche Vermittlung von Medienkompetenz deutlich, dass die befragten Eltern keine konsistente Vorstellung davon haben, wie sie ihren Kindern einen achtsamen und verantwortungsvollen Umgang mit sozialen Medien vermitteln können.Although parents consider online privacy important, they insouciantly include personal information about their children. Reviewing research on the privacy paradox and online self-disclosure, this article suggests the concept of media trusteeship as an additional theoretical perspective to understand how parents shape the digital identity of their children. The results of 46 in-depth interviews indicate that parents are largely unaware of the described role duality and are only partially able to foresee the consequences of their activities. The analysis identifies three distinct types of parental media trusteeship: While some parents shield their offspring from social media, others appear unable to respond adequately to the risks of social media activities or seem to ignore them completely. Finally, it became clear that the parents surveyed had no idea how to teach media literacy and guide their children to a safe and careful use of social media

Mass density slope of elliptical galaxies from strong lensing and resolved stellar kinematics

We discuss constraints on the mass density distribution (parameterized as $\rho\propto r^{-\gamma}$) in early-type galaxies provided by strong lensing and stellar kinematics data. The constraints come from mass measurements at two `pinch' radii. One `pinch' radius $r_1=2.2 R_{Einst}$ is defined such that the Einstein (i.e. aperture) mass can be converted to the spherical mass almost independently of the mass-model. Another `pinch' radius $r_2=R_{opt}$ is chosen so that the dynamical mass, derived from the line-of-sight velocity dispersion, is least sensitive to the anisotropy of stellar orbits. We verified the performance of this approach on a sample of simulated elliptical galaxies and on a sample of 15 SLACS lens galaxies at $0.01 \leq z \leq 0.35$, which have already been analysed in Barnabe et al. (2011) by the self-consistent joint lensing and kinematic code. For massive simulated galaxies the density slope $\gamma$ is recovered with an accuracy of $\sim 13\%$, unless $r_1$ and $r_2$ happen to be close to each other. For SLACS galaxies, we found good overall agreement with the results of Barnabe et al. (2011) with a sample-averaged slope $\gamma=2.1\pm0.05$. While the two-pinch-radii approach has larger statistical uncertainties, it is much simpler and uses only few arithmetic operations with directly observable quantities.Comment: accepted for publication in MNRA

Stellar orbits in cosmological galaxy simulations: the connection to formation history and line-of-sight kinematics

We analyze orbits of stars and dark matter out to three effective radii for 42 galaxies formed in cosmological zoom simulations. Box orbits always dominate at the centers and $z$-tubes become important at larger radii. We connect the orbital structure to the formation histories and specific features (e.g. disk, counter-rotating core, minor axis rotation) in two-dimensional kinematic maps. Globally, fast rotating galaxies with significant recent in situ star formation are dominated by $z$-tubes. Slow rotators with recent mergers have significant box orbit and $x$-tube components. Rotation, quantified by the $\lambda_R$-parameter often originates from streaming motion of stars on $z$-tubes but sometimes from figure rotation. The observed anti-correlation of $h_3$ and $V_0 / \sigma$ in rotating galaxies can be connected to a dissipative formation history leading to high $z$-tube fractions. For galaxies with recent mergers in situ formed stars, accreted stars and dark matter particles populate similar orbits. Dark matter particles have isotropic velocity dispersions. Accreted stars are typically radially biased ($\beta \approx 0.2 - 0.4$). In situ stars become tangentially biased (as low as $\beta \approx -1.0$) if dissipation was relevant during the late assembly of the galaxy. We discuss the relevance of our analysis for integral field surveys and for constraining galaxy formation models.Comment: 21 pages, 19 figure