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).Comment: 4 pages, 3 figure

Exploiting boundary states of imperfect spin chains for high-fidelity state transfer

We study transfer of a quantum state through XX spin chains with static imperfections. We combine the two standard approaches for state transfer based on (i) modulated couplings between neighboring spins throughout the spin chain and (ii) weak coupling of the outermost spins to an unmodulated spin chain. The combined approach allows us to design spin chains with modulated couplings and localized boundary states, permitting high-fidelity state transfer in the presence of random static imperfections of the couplings. The modulated couplings are explicitly obtained from an exact algorithm using the close relation between tridiagonal matrices and orthogonal polynomials [Linear Algebr. Appl. 21, 245 (1978)]. The implemented algorithm and a graphical user interface for constructing spin chains with boundary states (spinGUIn) are provided as Supplemental Material.Comment: 7 pages, 3 figures + spinGUIn description and Matlab files iepsolve.m, spinGUIn.fig, spinGUIn.

A Virtual Sky with Extragalactic HI and CO Lines for the SKA and ALMA

We present a sky simulation of the atomic HI emission line and the first ten CO rotational emission lines of molecular gas in galaxies beyond the Milky Way. The simulated sky field has a comoving diameter of 500/h Mpc, hence the actual field-of-view depends on the (user-defined) maximal redshift zmax; e.g. for zmax=10, the field of view yields ~4x4 sqdeg. For all galaxies, we estimate the line fluxes, line profiles, and angular sizes of the HI and CO emission lines. The galaxy sample is complete for galaxies with cold hydrogen masses above 10^8 Msun. This sky simulation builds on a semi-analytic model of the cosmic evolution of galaxies in a Lambda-cold dark matter (LCDM) cosmology. The evolving CDM-distribution was adopted from the Millennium Simulation, an N-body CDM-simulation in a cubic box with a side length of 500/h Mpc. This side length limits the coherence scale of our sky simulation: it is long enough to allow the extraction of the baryon acoustic oscillations (BAOs) in the galaxy power spectrum, yet the position and amplitude of the first acoustic peak will be imperfectly defined. This sky simulation is a tangible aid to the design and operation of future telescopes, such the SKA, the LMT, and ALMA. The results presented in this paper have been restricted to a graphical representation of the simulated sky and fundamental dN/dz-analyzes for peak flux density limited and total flux limited surveys of HI and CO. A key prediction is that HI will be harder to detect at redshifts z>2 than predicted by a no-evolution model. The future verification or falsification of this prediction will allow us to qualify the semi-analytic models.Comment: 16 pages, 9 figures, 1 tabl

Compactness of Cold Gas in High-Redshift Galaxies

Galaxies in the early Universe were more compact and contained more molecular gas than today. In this paper, we revisit the relation between these empirical findings, and we quantitatively predict the cosmic evolution of the surface densities of atomic (HI) and molecular (H2) hydrogen in regular galaxies. Our method uses a pressure-based model for the H2/HI-ratio of the Interstellar Medium, applied to ~3*10^7 virtual galaxies in the Millennium Simulation. We predict that, on average, the HI-surface density of these galaxies saturates at Sigma_HI<10 Msun/pc^2 at all redshifts (z), while H2-surface densities evolve dramatically as Sigma_H2(1+z)^2.4. This scaling is dominated by a (1+z)^2 surface brightness scaling originating from the (1+z)^-1 size scaling of galaxies at high z. Current measurements of Sigma_H2 at high z, derived from CO-observations, tend to have even higher values, which can be quantitatively explained by a selection bias towards merging systems. However, despite the consistency between our high-z predictions and the sparse empirical data, we emphasize that the empirical data potentially suffer from serious selection biases and that the semi-analytic models remain in many regards uncertain. As a case study, we investigate the cosmic evolution of simulated galaxies, which resemble the Milky Way at z=0. We explicitly predict their HI- and H2-distribution at z=1.5, corresponding to the CO-detected galaxy BzK-21000, and at z=3, corresponding to the primary science goal of the Atacama Large Millimeter/submillimeter Array (ALMA).Comment: 5 pages, 3 figures, 2 table

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.Comment: 17 pages, 11 figures, 3 table

SKA HI end2end simulation

The current status of the HI simulation efforts is presented, in which a self consistent simulation path is described and basic equations to calculate array sensitivities are given. There is a summary of the SKA Design Study (SKADS) sky simulation and a method for implementing it into the array simulator is presented. A short overview of HI sensitivity requirements is discussed and expected results for a simulated HI survey are presented.Comment: 7 pages, 6 figues, need skads2009.cls file to late