FERENGI: Redshifting galaxies from SDSS to GEMS, STAGES and COSMOS

We describe the creation of a set of artificially "redshifted" galaxies in the range 0.1<z<1.1 using a set of ~100 SDSS low redshift (v<7000 km/s) images as input. The intention is to generate a training set of realistic images of galaxies of diverse morphologies and a large range of redshifts for the GEMS and COSMOS galaxy evolution projects. This training set allows other studies to investigate and quantify the effects of cosmological redshift on the determination of galaxy morphologies, distortions and other galaxy properties that are potentially sensitive to resolution, surface brightness and bandpass issues. We use galaxy images from the SDSS in the u, g, r, i, z filter bands as input, and computed new galaxy images from these data, resembling the same galaxies as located at redshifts 0.1<z<1.1 and viewed with the Hubble Space Telescope Advanced Camera for Surveys (HST ACS). In this process we take into account angular size change, cosmological surface brightness dimming, and spectral change. The latter is achieved by interpolating a spectral energy distribution that is fit to the input images on a pixel-to-pixel basis. The output images are created for the specific HST ACS point spread function and the filters used for GEMS (F606W and F850LP) and COSMOS (F814W). All images are binned onto the desired pixel grids (0.03" for GEMS and 0.05" for COSMOS) and corrected to an appropriate point spread function. Noise is added corresponding to the data quality of the two projects and the images are added onto empty sky pieces of real data images. We make these datasets available from our website, as well as the code - FERENGI: "Full and Efficient Redshifting of Ensembles of Nearby Galaxy Images" - to produce datasets for other redshifts and/or instruments.Comment: 11 pages, 10 figures, 3 table

Ultrafast pump-probe dynamics in ZnSe-based semiconductor quantum-wells

Pump-probe experiments are used as a controllable way to investigate the properties of photoexcited semiconductors, in particular, the absorption saturation. We present an experiment-theory comparison for ZnSe quantum wells, investigating the energy renormalization and bleaching of the excitonic resonances. Experiments were performed with spin-selective excitation and above-bandgap pumping. The model, based on the semiconductor Bloch equations in the screened Hartree-Fock approximation, takes various scattering processes into account phenomenologically. Comparing numerical results with available experimental data, we explain the experimental results and find that the electron spin-flip occurs on a time scale of 30 ps.Comment: 10 pages, 9 figures. Key words: nonlinear and ultrafast optics, modeling of femtosecond pump-probe experiments, electron spin-flip tim

On-axis spectroscopy of the host galaxies of 20 optically luminous quasars at z~0.3

We present the analysis of a sample of 20 bright low-redshift quasars (M_B<-23 and z < 0.35) observed spectroscopically with the VLT. The FORS1 spectra, obtained in Multi Object Spectroscopy (MOS) mode, allow to observe simultaneously the quasars and several reference stars used to spatially deconvolve the data. Applying the MCS deconvolution method, we are able to separate the individual spectra of the quasar and of the underlying host galaxy. Contrary to some previous claims, we find that luminous quasars are not exclusively hosted by massive ellipticals. Most quasar host galaxies harbour large amounts of gas, irrespective of their morphological type. Moreover, the stellar content of half of the hosts is a young Sc-like population, associated with a rather low metallicity interstellar medium. A significant fraction of the galaxies contain gas ionized at large distances by the quasar radiation. This large distance ionization is always associated with signs of gravitational interactions (as detected from images or disturbed rotation curves). The spectra of the quasars themselves provide evidence that gravitational interactions bring dust and gas in the immediate surrounding of the super massive black hole, allowing to feed it. The quasar activity might thus be triggered (1) in young gas-rich spiral galaxies by local events and (2) in more evolved galaxies by gravitational interactions or collisions. The latter mechanism gives rises to the most powerful quasars. Finally, we derive mass models for the isolated spiral host galaxies and we show that the most reliable estimators of the systemic redshift in the quasar spectrum are the tips of the Ha and Hb lines.Comment: 30 pages, 19 figures, 9 tables, accepted for publication in MNRAS, major revisio

Polarons in semiconductor quantum-dots and their role in the quantum kinetics of carrier relaxation

While time-dependent perturbation theory shows inefficient carrier-phonon scattering in semiconductor quantum dots, we demonstrate that a quantum kinetic description of carrier-phonon interaction predicts fast carrier capture and relaxation. The considered processes do not fulfill energy conservation in terms of free-carrier energies because polar coupling of localized quantum-dot states strongly modifies this picture.Comment: 6 pages, 6 figures, accepted for publication in Phys.Rev.

Optical properties of self-organized wurtzite InN/GaN quantum dots: A combined atomistic tight-binding and full configuration interaction calculation

In this work we investigate the electronic and optical properties of self-assembled InN/GaN quantum dots. The one-particle states of the low-dimensional heterostructures are provided by a tight-binding model that fully includes the wurtzite crystal structure on an atomistic level. Optical dipole and Coulomb matrix elements are calculated from these one-particle wave functions and serve as an input for full configuration interaction calculations. We present multi-exciton emission spectra and discuss in detail how Coulomb correlations and oscillator strengths are changed by the piezoelectric fields present in the structure. Vanishing exciton and biexciton ground state emission for small lens-shaped dots is predicted.Comment: 3 pages, 2 figure

Effective environments: Preparation of stationary states with inverse temperature ranging from positive to negative values

In this paper, we discuss how effective environments incorporating periodic measurements can be used to prepare a two-level system (TLS) in almost arbitrary thermal states: Concretely, we study a TLS coupled to a spin environment, the magnetization of which is measured periodically. In ensemble average these measurements cause a relaxation of the TLS into a thermal (diagonal) state. By adjusting the time between the measurements and the detuning of the environmental spins, the creation of very low temperatures as well as inversion becomes possible. Our analytical results derived for large environments are numerically shown to be valid even for quite small environments, down to only a few spins.Comment: 20 pages, 3 figures, accepted for publication in Phys. Rev.

Spatial decomposition of on-nucleus spectra of quasar host galaxies

In order to study the host galaxies of type 1 (broad-line) quasars, we present a semi-analytic modelling method to decompose the on-nucleus spectra of quasars into nuclear and host galaxy channels. The method uses the spatial information contained in long-slit or slitlet spectra. A routine determines the best-fitting combination of the spatial distribution of the point like nucleus and extended host galaxy. This is fully complementary to a numerical spatial deconvolution technique that we applied to the same data in a previous analysis, which allows a cross-calibration of the two methods. Inputs are a simultaneously observed point spread function, and external constraints on galaxy morphology from imaging. We demonstrate the capabilities of the method to two samples of a total of 18 quasars observed with EFOSC at the ESO 3.6-m telescope and FORS1 at the ESO VLT. ∼50 per cent of the host galaxies with successful decomposition show distortions in their rotation curves or peculiar gas velocities above normal maximum velocities for discs. This is consistent with the fraction from optical imaging. All host galaxies have quite young stellar populations, typically 1-2 Gyr. For the disc dominated hosts these are consistent with their inactive counterparts, the luminosity-weighted stellar ages are much younger for the bulge dominated hosts, compared to inactive early-type galaxies. While this presents further evidence for a connection of galaxy interaction and active galactic nucleus (AGN) activity for half of the sample, this is not clear for the other half. These are often undistorted disc dominated host galaxies, and interaction on a smaller level might be detected in deeper high-resolution images or deeper spectroscopic data. The velocity information does not show obvious signs for large scale outflows triggered by AGN feedback - the data are consistent with velocity fields created by galaxy interactio

My Little Jitney Girl

Illustration of a little boy chasing a little girl across a hill. A photograph of a little girl on a five wheel bike is inset in the lower right.https://scholarsjunction.msstate.edu/cht-sheet-music/2568/thumbnail.jp