arXiv subjects

D. Obreschkow

Publications and source records attributed to D. Obreschkow.

43 records · Page 3Linked to original sources

arXiv2009

Mapping the SKA Simulated Skies with the S3-Tools

The S3-Tools are a set of Python-based routines and interfaces whose purpose is to provide user-friendly access to the SKA Simulated Skies (S3) set of simulations, an effort led by the University of Oxford in the framework of the European Union's SKADS program (http://www.skads-eu.org). The databases built from the S3 simulations are hosted by the Oxford e-Research Center (OeRC), and can be accessed through a web portal at http://s-cubed.physics.ox.ac.uk. This paper focuses on the practical steps involved to make radio images from the S3-SEX and S3-SAX simulations using the S3-Map tool and should be taken as a broad overview. For a more complete description, the interested reader should look up the user's guide. The output images can then be used as input to instrument simulators, e.g. to assess technical designs and observational strategies for the SKA and SKA pathfinders.

astro-ph.IM↗

arXiv2009

The Cosmic Decline in the H2/HI-Ratio in Galaxies

We use a pressure-based model for splitting cold hydrogen into its atomic (HI) and molecular (H2)components to tackle the co-evolution of HI, H2, and star formation rates (SFR) in ~3e7 simulated galaxies in the Millennium simulation. The main prediction is that galaxies contained similar amounts of HI at redshift z=1-5 than today, but substantially more H2, in quantitative agreement with the strong molecular line emission already detected in a few high redshift galaxies and approximately consistent with inferences from studies of the damped Lyman-alpha absorbers seen in the spectra of quasars. The cosmic H2/HI-ratio is predicted to evolve monotonically as Omega(H2)/Omega(HI) (1+z)^1.6. This decline of the H2/HI-ratio as a function of cosmic time is driven by the growth of galactic disks and the progressive reduction of the mean cold gas pressure. Finally, a comparison between the evolutions of HI, H2, and SFRs reveals two distinct cosmic epochs of star formation: an early epoch (z>3), driven by the evolution of Omega(HI+H2), and a late epoch (z<3), driven by the evolution of Omega(H2)/Omega(HI).

astro-ph.GA↗

arXiv2009

Simulation of the Cosmic Evolution of Atomic and Molecular Hydrogen in Galaxies

We present a simulation of the cosmic evolution of the atomic and molecular phases of the cold hydrogen gas in about 3e7 galaxies, obtained by post-processing the virtual galaxy catalog produced by (De Lucia et al. 2007) on the Millennium Simulation of cosmic structure (Springel et al. 2005). Our method uses a set of physical prescriptions to assign neutral atomic hydrogen (HI) and molecular hydrogen (H2) to galaxies, based on their total cold gas masses and a few additional galaxy properties. These prescriptions are specially designed for large cosmological simulations, where, given current computational limitations, individual galaxies can only be represented by simplistic model-objects with a few global properties. Our recipes allow us to (i) split total cold gas masses between HI, H2, and Helium, (ii) assign realistic sizes to both the HI- and H2-disks, and (iii) evaluate the corresponding velocity profiles and shapes of the characteristic radio emission lines. The results presented in this paper include the local HI- and H2-mass functions, the CO-luminosity function, the cold gas mass--diameter relation, and the Tully-Fisher relation (TFR), which all match recent observational data from the local Universe. We also present high-redshift predictions of cold gas diameters and the TFR, both of which appear to evolve markedly with redshift.

astro-ph.CO↗

arXiv2009

Understanding the H2/HI Ratio in Galaxies

We revisit the mass ratio Rmol between molecular hydrogen (H2) and atomic hydrogen (HI) in different galaxies from a phenomenological and theoretical viewpoint. First, the local H2-mass function (MF) is estimated from the local CO-luminosity function (LF) of the FCRAO Extragalactic CO-Survey, adopting a variable CO-to-H2 conversion fitted to nearby observations. This implies an average H2-density Omega_H2=(6.9+-2.7) 10^5/h and Omega_H2/Omega_HI=0.26+-0.11 in the local Universe. Second, we investigate the correlations between Rmol and global galaxy properties in a sample of 245 local galaxies. Based on these correlations we introduce four phenomenological models for Rmol, which we apply to estimate H2-masses for each HI-galaxy in the HIPASS catalogue. The resulting H2-MFs (one for each model for Rmol) are compared to the reference H2-MF derived from the CO-LF, thus allowing us to determine the Bayesian evidence of each model and to identify a clear best model, in which, for spiral galaxies, Rmol negatively correlates with both galaxy Hubble type and total gas mass. Third, we derive a theoretical model for Rmol for regular galaxies based on an expression for their axially symmetric pressure profile dictating the degree of molecularization. This model is quantitatively similar to the best phenomenological one at redshift z=0, and hence represents a consistent generalization while providing a physical explanation for the dependence of Rmol on global galaxy properties. Applying the best phenomenological model for Rmol to the HIPASS sample, we derive the first integral cold gas-MF (HI+H2+helium) of the local Universe.

astro-ph.GA↗

arXiv2008

A semi-empirical simulation of the extragalactic radio continuum sky for next generation radio telescopes

We have developed a semi-empirical simulation of the extragalactic radio continuum sky suitable for aiding the design of next generation radio interferometers such as the Square Kilometre Array (SKA). The emphasis is on modelling the large-scale cosmological distribution of radio sources rather than the internal details of individual galaxies. Here we provide a description of the simulation to accompany the online release of a catalogue of 320 million simulated radio sources. The simulation covers 20x20 deg^2 - a plausible upper limit to the instantaneous field of view attainable with future (e.g. SKA) aperture array technologies - out to redshift z=20, and down to flux density limits of 10 nJy at 151, 610 MHz, 1.4, 4.86 and 18 GHz. Five distinct source types are included: radio-quiet AGN, radio-loud AGN of the FRI and FRII structural classes, and star-forming galaxies, the latter split into populations of quiescent and starbursting galaxies. In our semi-empirical approach, the simulated sources are drawn from observed (or extrapolated) luminosity functions and grafted onto an underlying dark matter density field with biases which reflect their measured large-scale clustering. A numerical Press-Schechter-style filtering of the density field is used to identify and populate clusters of galaxies. Radio source structures are built from point source and elliptical sub-components, and for FRI and FRII sources an orientation-based unification and beaming model is used to partition flux between the core and extended lobes and hotspots. The simulation output can be post-processed to achieve more complete agreement with observational data in the years ahead, with the aim of using these 'idealised skies' in telescope simulators to optimise the design of the SKA itself (abridged).

astro-ph↗

arXiv2007

Non-orthogonal Theory of Polarons and Application to Pyramidal Quantum Dots

We present a general theory for semiconductor polarons in the framework of the Froehlich interaction between electrons and phonons. The latter is investigated using non-commuting phonon creation/annihilation operators associated with a natural set of non-orthogonal modes. This setting proves effective for mathematical simplification and physical interpretation and reveals a nested coupling structure of the Froehlich interaction. The theory is non-perturbative and well adapted for strong electron-phonon coupling, such as found in quantum dot (QD) structures. For those particular structures we introduce a minimal model that allows the computation and qualitative prediction of the spectrum and geometry of polarons. The model uses a generic non-orthogonal polaron basis, baptized the "natural basis". Accidental and symmetry-related electronic degeneracies are studied in detail and are shown to generate unentangled zero-shift polarons, which we consistently eliminate. As a practical example, these developments are applied to realistic pyramidal GaAs QDs. The energy spectrum and the 3D-geometry of polarons are computed and analyzed, and prove that realistic pyramidal QDs clearly fall in the regime of strong coupling. Further investigation reveals an unexpected substructure of "weakly coupled strong coupling regimes", a concept originating from overlap considerations. Using Bennett's entanglement measure, we finally propose a heuristic quantification of the coupling strength in QDs.

cond-mat.other↗

arXiv2006

Cavitation Bubble Dynamics inside Liquid Drops in Microgravity

We studied spark-generated cavitation bubbles inside water drops produced in microgravity. High-speed visualizations disclosed unique effects of the spherical and nearly isolated liquid volume. In particular, (1) toroidally collapsing bubbles generate two liquid jets escaping from the drop, and the "splash jet" discloses a remarkable broadening. (2) Shockwaves induce a strong form of secondary cavitation due to the particular shockwave confinement. This feature offers a novel way to estimate integral shockwave energies in isolated volumes. (3) Bubble lifetimes in drops are shorter than in extended volumes in remarkable agreement with herein derived corrective terms for the Rayleigh-Plesset equation.

physics.flu-dyn↗