| 1. |
- Hultman, John
(författare)
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Dissipative formation of galaxies in hierarchical cosmology
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A description and detailed tests of a numerical code for simulations of galaxy formation is presented. The dynamics is followed with fully adaptive and Lagrangian Smooth Particle Hydrodynamics (SPH) combined with a gravitational tree method. The severe numerical problem of sub-resolution clustering is solved by merging of gas particles.The code is used for Hydrodynamical simulations of the formation of galaxies in the mass range 109 - 1013M☉. The effect of an ambient UV radiation field, and the effect of metal enrichment of halo gas due to star formation and stellar evolution, are investigated. The UV radiation field is found to strongly affect galaxies with circular velocities less than ≈ 50 km/s. The effect is, however, not large enough to reconcile hierarchical clustering models with observations.I present numerical simulations of galaxy formation in a CDM scenario, including star formation, using the SPH and tree method on scales ranging from 20 Mpc down to less than 1 kpc. The gas component is treated as a two-phase medium, governed by thermal instability and supernova feedback. For a certain degree of supernova evaporation, several general properties of elliptical galaxies are reproduced, like shapes, r1/4-profiles, half-light radii, slow rotation and anisotropic velocity dispersions. A Faber-Jackson -like relation is observed, being of the form M ~ Σ2.6.No disk shaped objects form. The elliptical objects form through a hierarchical, star-forming "inside-out" collapse. The SFR has a rapidly raising and later exponentially decaying behaviour in individual objects. The stellar contribution to the closure density is around one percent. The co-moving average SFR shows an increasing behaviour with redshift. Low mass objects are strongly suppressed. Generally, the more massive objects are in place at z ~ 0.5 - 2. Although many problems and questions still remain, the overall impression is that surprisingly many properties of ellipticals are reproduced.
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| 2. |
- Östlin, Göran A.
(författare)
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On the origin and evolution of blue compact galaxies
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is focused on the origin and evolution of blue compact galaxies (BCGs). BCGs are low mass galaxies characterised by blue colours, low chemical abundances and high star formation rates. The gas consumtion time scale is considerable shorter than the age of the universe and thus the high star formation rate must be transient. Such a non sustainable star formation rate is commonly referred to as a "starburst". It has been proposed that BCGs are genuinely young galaxies or alternatively that theyoccasionally undergo starbursts, separated by long quiesent periods. It is of importance for the understanding of the evolution of galaxies, and in particular dwarf galaxies, to understand under what conditions a galaxy can become a starbursting BCG.The properties of in particular intrinsically luminous BCGs have been examined photometrically and kinematically. The results show that the studied galaxies have a population of old stars underlying the luminous starbursts. Thus they are not young galaxies. It is shown that infrared photometry is a powerful method to detect underlying old populations in BCGs. Kinematical studies of the velocity field suggests that the galaxies have been perturbed by mergers, and it is proposed that mergers betweengas rich dwarf galaxies is the triggering mechanism for starbursts in luminous BCGs. Low surface brightness galaxies are promising ingredients in such mergers.Observations with the Hubble Space Telescope of the BCG ESO 338- IG04 reveal the presence of a very rich population of globular clusters (GCs) of various ages. Apparently GC formation is still going on in this starbursting galaxy. The age dating of GCs offers an efficient way of studying the starburst history of BCGs and galaxies in general. The relation between BCGs and other types of low mass galaxies is discussed.
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| 3. |
- Hidalgo-Gámez, Ana Maria
(författare)
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A study of possible chemical inhomogeneities of dwarf irregular galaxies and the influence on the Z-L relationship
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A study on the chemical abundances of dwarf irregular galaxies (dI's) is presented in this work. This type of galaxies have typically low metallicity. First of all, a definition of dI is proposed, in order to obtain a homogeneous group of targets.Due to the small size of the dI galaxies and the absence of strong dynamical processes a homogeneous chemical composition is normally assumed. The main argument is that the gas in the interstellar medium of the galaxies are probably not too much affected by internal dynamics and could therefore be well mixed. This hypothesis has been investigated. Optical spectra for various star-forming regions (HII regions) in five dI's have been obtained and analysed. For all the HII regions, the chemical abundances of the whole region have been determined. Spectra of four of these HII regions are of very high quality which allow an additional study of possible variations of the chemical abundances within the regions. It could be concluded that the existence of variations in the chemical abundances, especially at large scales, may depend on thephysical properties of the galaxy and its environment.Another important aspect to study is the behaviour of the dI galaxies in the metallicity-luminosity plane. A deep study of previous relationships for this kind of objects were performed as well as the possible sources of dispersion from this relationship. With the results on the chemical abundances of eight galaxies and the latest distance determinations a new, weak, relationship between the metallicity and the luminosity was derived. The influence of the environment and other physical properties of the galaxies have also been studied.
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| 4. |
- Festin, Leif
(författare)
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The faintest stars : A study of white, red and brown dwarfs
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The local space density of the faintest stars, including M dwarfs, white dwarfs and brown dwarfs has been studied by two different methods.By using dark nebulae as opaque outer screens, the luminosity functions of M dwarfs and white dwarfs were studied as volume-limited and dynamically unbiassed foreground samples. The surveyed volume corresponds to 464 pc3 in the solar neighbourhood and contains 21 M dwarfs and 7 white dwarfs. The derived M-dwarf luminosity function is consistent with previous findings, showing no substantial upturnbeyond Mv = 16. The white dwarf space density is consistent with a 20% fraction of the dynamical matter in the solar neighbourhood residing in white dwarfs. This is in line with the most recent independent results obtained with different methods, making the white dwarfs the best explanation to the Milky Way dark matter at present.The brown dwarfs were studied in the Pleiades open cluster. The distance and age of the Pleiades make the rapidly fading brown dwarfs still rather bright and easy to detect. 850 arcmin2 were covered in a deep RIJK survey. Nine new possible cluster members were discovered, four of which are below the brown dwarf limit. The faintest of these has an estimated mass of 0.040 M. and is thereby the lowest mass brown dwarf identified in the Pleiades cluster at present. The derived Pleiades substellar luminosity function is consistent with a mass function index between 0 and 1, making the brown dwarfs unlikely to contribute more than a few percent to the cluster mass, which is also consistent with recent dynamical results.
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| 5. |
- Lagerros, Johan S. V.
(författare)
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Thermal physics of asteroids
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The thermal infrared and microwave emission from asteroids have many applications. Fundamental physical properties can be investigated, which have implications for the geophysical evolution of the asteroids, and the early history of the solar system.A new thermophysical model of asteroids is presented. A number of physical processes previously neglected in the Standard Thermal Model are now considered. The new model predicts the thermal emission of asteroids, from mid-infrared to microwave wavelengths. The irregular shapes of asteroids are modelled in detail, and the spin state of the asteroids is taken into account, whereby it is possible to calculate model thermal lightcurves. The heat conduction into the surface material in general lowers the mid-infrared flux from main-belt asteroids. The model is able to explain the observed "beaming" of the emission into the solar direction, under the assumption that it is caused by the small-scale surface roughness. Subsurface scattering processes are considered when calculating the directional- and wavelength-dependent emissivity.The model is applied to a large database of observations in the wavelength range of 7-2 000 µm. The purpose is to derive the thermophysical properties of ten asteroids, selected as calibration targets for the instruments on board the Infrared Space Observatory. The results indicate very rough and porous surfaces, with low levels of heat conduction. The emissivity variessignificantly with wavelength.
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| 6. |
- Wu, Jian-Guo
(författare)
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Space Weather Physics: Dynamic Neural Network Studies of Solar Wind-Magnetosphere Coupling
- 1997
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis presents studies of solar wind-magnetosphere coupling using dynamic neural networks in combination with statistically correlative analysis. The primary contribution of the thesis is dynamic neural network models that can be implemented for near real-time predictions of geomagnetic storms from the solar wind alone. With acceptable accuracy, the prediction time has been extended up to 5 hours. This is of great socioeconomic significance in space weather forecasting. The secondary contribution of the thesis is the modeling of the magnetospheric dynamics, which optimizes combinations of solar wind parameters and coupling functions. The third contribution of the thesis includes the development of a C-cod of Elman recurrent network models, the development of an algorithm for pruning Elman networks and the algorithms for post network error analyses. The fourth contribution of the thesis is the exploitation of the hybrid of a time-delay network and a recurrent network by examining the role of a time-delay line in recurrent networks. This thesis consists of five chapters. Chapter 1 provides an introduction to solar-terrestrial physics. Chapter 2 is a general description of neural networks. Chapter 3 describes solar wind-magnetosphere coupling and related studies. Chapter 4 is devoted to studies of a time-dependent system, such as the solar wind-driven magnetosphere, using neural networks. Chapter 5 is a summary of the papers included in this thesis. Paper I pioneered exploitation of recurrent neural networks in prediction of geomagnetic activity and presents very accurate one hour ahead prediction of magnetic storms using only solar wind data. Paper II is a study of solar wind-magnetosphere coupling using a partially recurrent neural network to find the optimal coupling functions. The optimal coupling functions found are used to predict magnetic storms up to 5 hours with acceptable accuracy. This study was the first to present real-time one hour ahead prediction of magnetic storms using the satellite WIND real-time data. Paper III made predictions of magnetic storms up to 8 hours. It presents the appropriate combinations of solar wind parameters for predicting magnetic storms, which reveals the relative importance of solar wind parameters. It is found that a magnetic storm is formed in the magnetosphere on a timescale of about 1 hour. In this study, we exploit a time-delay recurrent network which is a hybrid of a time-delay network and an Elman recurrent network, and prove it very helpful in improving predictions. Paper IV studies solar wind-magnetosphere interaction in detail, which finds the best coupling functions for accurate prediction of geomagnetic activity based on neural network modeling, in comparison with the results from cross-correlation analyses. The algorithms for computation of confidence limits on the prediction accuracy are developed in this study.
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| 7. |
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| 8. |
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| 9. |
- Bååth, L.B. 1948-, et al.
(författare)
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0735+178 : The cosmic conspiracy
- 1991
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Ingår i: Astronomy and Astrophysics. - Les Ulis : EDP Sciences. - 0004-6361 .- 1432-0746. ; 250:1, s. 50-56
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Tidskriftsartikel (refereegranskat)abstract
- This paper discusses a radio outburst in the BL Lac-object 0735+178 which started in early 1988, ended during 1990 and was succeeded by a new outburst which is still in progress. Our image obtained with a global VLBI network shows the source to mainly consist of a core-jet like structure ending in diffuse emission region of low surface brightness located to the east of the core. We identify two new components as having been produced during the late 1980's. They are moving out with apparent superluminal motion in a viewing angle of greater-than-or-similar-to 10-degrees at a distance of less-than-or-similar-to 2 mas from the core. The core includes another new component, but our resolution is just sufficient to resolve out this component from the core at our epoch of observation. We suggest that the weak emission region is caused by an older component reaching the point at which the Doppler boosting is at its maximum and that the long time scale flux variation are due to successive events when evolved components reach that point.
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| 10. |
- Mottola, S, et al.
(författare)
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The slow rotation of 253 Mathilde
- 1995
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Ingår i: PLANETARY AND SPACE SCIENCE. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0032-0633. ; 43:12, s. 1609-1613
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Tidskriftsartikel (refereegranskat)abstract
- CCD photometry of the NEAR mission fly-by target 253 Mathilde is presented. Measurements taken during 52 nights of observations, from February to June 1995, allow a rotation period of 17.406 +/- 0.010 days and a lightcurve amplitude of 0.45 +/- 0.02 mag t
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