Simulations of Dust in Interacting Galaxies I: Dust Attenuation

A new Monte-Carlo radiative-transfer code, Sunrise, is used in conjunction with hydrodynamic simulations of major galaxy mergers to calculate the effects of dust in such systems. The simulations are in good agreement with observations of dust absorption in starburst galaxies, and the dust has a profound effect on their appearance. The dust attenuation increases with luminosity such that at peak luminosities ~90% of the bolometric luminosity is absorbed by dust. In general, the detailed appearance of the merging event depends on the stage of the merger and the geometry of the encounter. The fraction of bolometric energy absorbed by the dust, however, is a robust quantity that can be predicted from the intrinsic properties bolometric luminosity, baryonic mass, star-formation rate, and metallicity of the system. This paper presents fitting formulae, valid over a wide range of masses and metallicities, from which the absorbed fraction of luminosity (and consequently also the infrared dust luminosity) can be predicted. The attenuation of the luminosity at specific wavelengths can also be predicted, albeit with a larger scatter due to the variation with viewing angle. These formulae for dust attenuation appear to be valid for both isolated and interacting galaxies, are consistent with earlier studies, and would be suitable for inclusion in theoretical models, e.g. semi-analytic models of galaxy formation.Comment: 12 pages, 10 figures, submitted to Ap

Modeling high-redshift galaxies: what can we learn from high and ultra-high resolution hydrodynamical simulations?

We present results from a high resolution cosmological galaxy formation simulation called Mare Nostrum and a ultra-high resimulation of the first 500 million years of a single, Milky Way (MW) sized galaxy. Using the cosmological run, we measure UV luminosity functions and assess their sensitivity to both cosmological parameters and dust extinction. We find remarkably good agreement with the existing data over the redshift range 4 < z < 7 provided we adopt the favoured cosmology (WMAP 5 year parameters) and a self-consistent treatment of the dust. Cranking up the resolution, we then study in detail a z = 9 protogalaxy sitting at the intersection of cold gas filaments. This high-z MW progenitor grows a dense, rapidly spinning, thin disk which undergoes gravitational fragmention. Star formation in the resulting gas clumps rapidly turns them into globular clusters. A far reaching galactic wind develops, co-powered by the protogalaxy and its cohort of smaller companions populating the filaments. Despite such an impressive blow out, the smooth filamentary material is hardly affected at these redshifts

Modeling high-redshift galaxies: what can we learn from high and ultra-high resolution hydrodynamical simulations?

We present results from a high resolution cosmological galaxy formation simulation called Mare Nostrum and a ultra-high resimulation of the first 500 million years of a single, Milky Way (MW) sized galaxy. Using the cosmological run, we measure UV luminosity functions and assess their sensitivity to both cosmological parameters and dust extinction. We find remarkably good agreement with the existing data over the redshift range 4 < z < 7 provided we adopt the favoured cosmology (WMAP 5 year parameters) and a self-consistent treatment of the dust. Cranking up the resolution, we then study in detail a z = 9 protogalaxy sitting at the intersection of cold gas filaments. This high-z MW progenitor grows a dense, rapidly spinning, thin disk which undergoes gravitational fragmention. Star formation in the resulting gas clumps rapidly turns them into globular clusters. A far reaching galactic wind develops, co-powered by the protogalaxy and its cohort of smaller companions populating the filaments. Despite such an impressive blow out, the smooth filamentary material is hardly affected at these redshift

Galactic star formation in parsec-scale resolution simulations

The interstellar medium (ISM) in galaxies is multiphase and cloudy, with stars forming in the very dense, cold gas found in Giant Molecular Clouds (GMCs). Simulating the evolution of an entire galaxy, however, is a computational problem which covers many orders of magnitude, so many simulations cannot reach densities high enough or temperatures low enough to resolve this multiphase nature. Therefore, the formation of GMCs is not captured and the resulting gas distribution is smooth, contrary to observations. We investigate how star formation (SF) proceeds in simulated galaxies when we obtain parsec-scale resolution and more successfully capture the multiphase ISM. Both major mergers and the accretion of cold gas via filaments are dominant contributors to a galaxy's total stellar budget and we examine SF at high resolution in both of these context

Prediction of dynamic strains on a monopile offshore wind turbine using virtual sensors

The monitoring of the condition of the offshore wind turbine during its operational states offers the possibility of performing accurate assessments of the remaining life-time as well as supporting maintenance decisions during its entire life. The efficacy of structural monitoring in the case of the offshore wind turbine, though, is undermined by the practical limitations connected to the measurement system in terms of cost, weight and feasibility of sensor mounting (e.g. at muddline level 30m below the water level). This limitation is overcome by reconstructing the full-field response of the structure based on the limited number of measured accelerations and a calibrated Finite Element Model of the system. A modal decomposition and expansion approach is used for reconstructing the responses at all degrees of freedom of the finite element model. The paper will demonstrate the possibility to predict dynamic strains from acceleration measurements based on the aforementioned methodology. These virtual dynamic strains will then be evaluated and validated based on actual strain measurements obtained from a monitoring campaign on an offshore Vestas V90 3 MW wind turbine on a monopile foundation

On Measuring the Infrared Luminosity of Distant Galaxies with the Space Infrared Telescope Facility

The Space Infrared Telescope Facility (SIRTF) will revolutionize the study of dust-obscured star formation in distant galaxies. Although deep images from the Multiband Imaging Photometer for SIRTF (MIPS) will provide coverage at 24, 70, and 160 micron, the bulk of MIPS-detected objects may only have accurate photometry in the shorter wavelength bands due to the confusion noise. Therefore, we have explored the potential for constraining the total infrared (IR) fluxes of distant galaxies with solely the 24 micron flux density, and for the combination of 24 micron and 70 micron data. We also discuss the inherent systematic uncertainties in making these transitions. Under the assumption that distant star-forming galaxies have IR spectral energy distributions (SEDs) that are represented somewhere in the local Universe, the 24 micron data (plus optical and X-ray data to allow redshift estimation and AGN rejection) constrains the total IR luminosity to within a factor of 2.5 for galaxies with 0.4 < z < 1.6. Incorporating the 70 micron data substantially improves this constraint by a factor < 6. Lastly, we argue that if the shape of the IR SED is known (or well constrained; e.g., because of high IR luminosity, or low ultraviolet/IR flux ratio), then the IR luminosity can be estimated with more certainty.Comment: 4 pages, 3 figures (2 in color). Accepted for Publication in the Astrophysical Journal Letters, 2002 Nov

Continuous dynamic monitoring of an offshore wind turbine on a monopile foundation

In order to minimize O&M costs and to extend the lifetime of offshore wind turbines it will be of high interest to continuously monitor the vibration levels and the evolution of the frequencies and damping ratios of the first modes of the foundation structure. This will allow us to identify and avoid resonant behavior and will also be able to give indications about the current state of the offshore wind turbine. State-of-the-art operational modal analysis techniques can provide accurate estimates of natural frequencies, damping ratios and mode shapes. To allow a proper continuous monitoring during operations a fast and reliable solution, which is applicable on industrial scale, needs to be developed. The methods need also to be automated and their reliability improved, so that no human-interaction is required and the system can track changes in the dynamic behavior of the offshore wind turbine. This paper will present and discuss the approach that will be used for the long-term monitoring campaign on an offshore wind turbine in the Belgian North Sea

Color Gradients and Surface Brightness Profiles of Galaxies in the Hubble Deep Field-North

We fit elliptical isophotes to the Hubble Deep Field-North WFPC-2 and NICMOS data to study the rest-frame UV_{218}-U_{300} color profiles and rest-frame B surface brightness profiles of 33 intermediate redshift galaxies (0.5 <= z <= 1.2) with I_{814} < 25 and 50 high redshift galaxies (2.0 <= z <= 3.5) with H_{160}< 27. From the weighted least-squares fit to the color profiles we find that, at intermediate redshifts, the galaxies possess negative color gradients indicating a reddening towards the center of the profile similar to local samples whereas, at high redshifts, the galaxies possess positive color gradients. This indicates that star formation is more centrally concentrated in the distant galaxy sample which differs from the prevalent mode of extended disk star formation that we observe in the local universe. Additionally, we find that it is critical to correct for PSF effects when evaluating the surface brightness profiles since at small scale lengths and faint magnitudes, an r^{1/4} profile can be smoothed out substantially to become consistent with an exponential profile. After correcting for PSF effects, we find that at higher look-back time, the fraction of galaxies possessing exponential profiles have slightly decreased while the fraction of galaxies possessing r^{1/4} profiles have slightly increased. Our results also suggest a statistically insignificant increase in the fraction of peculiar/irregular type galaxies. We compare our results with recent semi-analytical models which treat galaxy formation and evolution following the cold dark matter hierarchical framework.Comment: 31 pages, 10 JPEG figures. To be published in AJ Vol. 124, October 200

Innovative system identification methods for monitoring applications

Monitoring the modal parameters of civil and mechanical system received plenty of interest the last decades. Several approaches have been proposed and successfully applied in civil engineering for structural health monitoring of bridges (mainly based on the monitoring of the resonant frequencies and mode shapes). In applications such as the monitoring of offshore wind turbines and flight flutter testing the monitoring of the damping ratios are essential. For offshore wind turbine monitoring the presence of time-varying harmonic components, close to the modes of interest, can complicate the identification process. The difficulty related to flight flutter testing is that, in general, only short data records are available. The aim of this contribution is to introduce system identification methods and monitoring strategies that result in more reliable decisions and that can cope with complex monitoring applications. Basic concepts of system identification will be recapitulated with attention for monitoring aspects. The proposed monitoring methodology is based on the recently introduced Transmissibility-based Operational Modal Analysis (TOMA) approach