Alas, the dark matter structures were not that trivial

The radial density profile of dark matter structures has been observed to have an almost universal behaviour in numerical simulations, however, the physical reason for this behaviour remains unclear. It has previously been shown that if the pseudo phase-space density, rho/sigma_d^epsilon, is a beautifully simple power-law in radius, with the "golden values" epsilon=3 and d=r (i.e., the phase-space density is only dependent on the radial component of the velocity dispersion), then one can analytically derive the radial variation of the mass profile, dispersion profile etc. That would imply, if correct, that we just have to explain why rho/sigma^3_r ~r^{-alpha}, and then we would understand everything about equilibrated DM structures. Here we use a set of simulated galaxies and clusters of galaxies to demonstrate that there are no such golden values, but that each structure instead has its own set of values. Considering the same structure at different redshifts shows no evolution of the phase-space parameters towards fixed points. There is also no clear connection between the halo virialized mass and these parameters. This implies that we still do not understand the origin of the profiles of dark matter structures.Comment: 4 pages, 3 figures, accepted for publication in ApJ

On the deployment of Mobile Trusted Modules

In its recently published TCG Mobile Reference Architecture, the TCG Mobile Phone Work Group specifies a new concept to enable trust into future mobile devices. For this purpose, the TCG devises a trusted mobile platform as a set of trusted engines on behalf of different stakeholders supported by a physical trust-anchor. In this paper, we present our perception on this emerging specification. We propose an approach for the practical design and implementation of this concept and how to deploy it to a trustworthy operating platform. In particular we propose a method for the take-ownership of a device by the user and the migration (i.e., portability) of user credentials between devices.Comment: To appear in: Proceedings of the Wireless Communications and Networking Conference, IEEE WCNC 2008, Las Vegas, USA, 31 March - 2 April 200

Frontiers of Galaxy Evolution : Time-Domain Observations and 3D Spectroscopy

Understanding the formation and evolution of galaxies through cosmic time has been a central focus of astrophysics in the last decades: how did the interplay between dark matter structure formation, star formation, galaxy merging, and active galactic nuclei (AGN) give rise to the observed galaxy properties at different redshifts? This thesis presents innovative observational approaches to two aspects of this problem: finding and studying AGN through their variability, and making a first systematic census of galaxy mergers at z > 1 through three-dimensional spectroscopy. First we present a new and simple technique for selecting extensive, complete, and pure quasar samples via their intrinsic variability, parameterizing the single-band light-curve structure function through a power-law to identify quasars among other variable and non-variable sources. Using extensive multi-epoch observations from SDSS Stripe 82 containing ~60 epochs taken over 8 years, we demonstrate the power of this approach. The presented algorithm identifies quasars with a completeness and purity above 90% at all redshifts. Even for Pan-STARRS 1 mock data of only 6 epochs over 3 years, variability is still an encouragingly efficient quasar classifier. Data on intrinsic quasar variability enable a wide range of astrophysical science. We quantify the color variability, confirming and greatly fleshing out previous claims, that quasars become bluer as they brighten. We find a strong redshift dependence of this blueing, which we can attribute to emission lines contributing to the SDSS bands at di fferent redshifts. We find that the color variations of single quasars are much more pronounced than the ranges in color seen in time-averaged ensembles of quasars. This indicates, that the observed color variations cannot be explained by changes in the mean steady state AGN accretion rate, but must arise from accretion disk ‘hotspots’ or similar phenomena. In the second, distinct part of the thesis, we present the first large sample of morphologically selected galaxy mergers with three-dimensional spectroscopy at z~1.5. With individual masses and star formation rates derived from multi-band photometry, we created emission line maps from the slitless grism spectroscopy of 3D-HST as proxy for star formation maps, providing a comprehensive empirical picture of where star formation takes place in galaxy mergers at the epoch, where the cosmic star formation and merger rates peaked. We find that a broad range of star formation morphologies occur at all redshifts, irrespective of star formation rate and total stellar mass, in these mergers. An initial illustrative comparison to a set of cosmological simulations shows, that simulated mergers with similar mass and gas content show star formation in both merger components far more often than for the observed 3D-HST mergers. This suggests that mergers at z~1.5 most commonly happen between galaxies of distinctly di erent gas fractions

The Luminosity Function at z~8 from 97 Y-band dropouts: Inferences About Reionization

[Abbreviated] We present the largest search to date for $z\sim8$ Lyman break galaxies (LBGs) based on 350 arcmin$^2$ of HST observations in the V-, Y-, J- and H-bands from the Brightest of Reionizing Galaxies (BoRG) survey. The BoRG dataset includes $\sim$50 arcmin$^2$ of new data and deeper observations of two previous BoRG pointings, from which we present 9 new $z\sim8$ LBG candidates, bringing the total number of BoRG LBGs to 38 with $25.5\leqslant m_{J} \leqslant 27.6$ (AB system). We introduce a new Bayesian formalism for estimating the galaxy luminosity function (LF), which does not require binning (and thus smearing) of the data and includes a likelihood based on the formally correct binomial distribution as opposed to the often used approximate Poisson distribution. We demonstrate the utility of the new method on a sample of $97$ LBGs that combines the bright BoRG galaxies with the fainter sources published in Bouwens et al. (2012) from the HUDF and ERS programs. We show that the $z\sim8$ LF is well described by a Schechter function with a characteristic magnitude $M^\star = -20.15^{+0.29}_{-0.38}$, a faint-end slope of $\alpha = -1.87^{+0.26}_{-0.26}$, and a number density of $\log_{10} \phi^\star [\textrm{Mpc}^{-3}] = -3.24^{+0.25}_{-0.24}$. Integrated down to $M=-17.7$ this LF yields a luminosity density, $\log_{10} \epsilon [\textrm{erg}/\textrm{s/Hz/Mpc}^{3}] = 25.52^{+0.05}_{-0.05}$. Our LF analysis is consistent with previously published determinations within 1$\sigma$. We discuss the implication of our study for the physics of reionization. By assuming theoretically motivated priors on the clumping factor and the photon escape fraction we show that the UV LF from galaxy samples down to $M=-17.7$ can ionize only 10-50% of the neutral hydrogen at $z\sim8$. Full reionization would require extending the LF down to $M=-15$.Comment: Accepted for publication in ApJ, 22 pages, 15 figure