Solar eclipse | EPFL Graph Search

A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby obscuring the view of the Sun from a small part of the Earth, totally or partially. Such an alignment occurs approximately every six months, during the eclipse season in its new moon phase, when the Moon's orbital plane is closest to the plane of the Earth's orbit. In a total eclipse, the disk of the Sun is fully obscured by the Moon. In partial and annular eclipses, only part of the Sun is obscured. Unlike a lunar eclipse, which may be viewed from anywhere on the night side of Earth, a solar eclipse can only be viewed from a relatively small area of the world. As such, although total solar eclipses occur somewhere on Earth every 18 months on average, they recur at any given place only once every 360 to 410 years. If the Moon were in a perfectly circular orbit and in the same orbital plane as Earth, there would be total solar eclipses once a month, at every new moon. Instead, because the Moon's orbit is t

Astrophysical applications of gravitationally lensed quasars

Alexander Eigenbrod

Gravitational lensing describes how light is deflected as it passes in the vicinity of a mass distribution. The amplitude of the deflection is proportional to the mass of the deflector, called "gravitational lens", and is generally weak, even for large masses. The faintness of this phenomenon explains why gravitational lensing remained essentially unobserved until the late 1970s (only gravitational lensing by the Sun has been observed during the solar eclipse of 1919). Before that time, gravitational lensing was considered merely as a theoretical curiosity. However, the situation dramatically changed with the discovery of the first extragalactic gravitational lens in 1979. Since then, together with the technological progress of astronomical instruments, gravitational lensing has turned from a curiosity into a powerful tool to address important astrophysical and cosmological questions. The present thesis focuses on applications related to gravitationally lensed quasars. Quasars are active galactic nuclei, where matter is heated up as it spirals down onto the central supermassive black hole. When a galaxy is located on the line of sight to a distant quasar, it acts as a gravitational lens and produces multiple images of this background source. The light of the quasar follows different paths for each of its images. Thus, variations of the intrinsic quasar luminosity are observed at different times in each image. The time delays between the images can be used to determine the Hubble constant H0, because they are inversely proportional to H0. This constant describes the current expansion rate of the Universe, and is one of the fundamental parameters of cosmological models. Many efforts have been spent over the years to determine H0, but its value is still poorly constrained. Gravitational lensing has the potential to noticeably decrease the uncertainty of H0. In practice, this requires regular and long-term monitoring of lensed quasars. We have run a series of numerical simulations to both optimize the available telescope time, and measure the time delays with an accuracy of a few percent. The results of these simulations are presented in the form of compact plots to be used to optimize the observational strategy of present and future monitoring programs. Once the time delays are measured, one can infer estimates of H0, provided several other observational constraints are available. A key element to accurately convert time delays into H0 is the redshift of the lensing galaxy. These redshift measurements are difficult because lensing galaxies are generally hidden in the glare of the much brighter quasar images. As a consequence, lens redshifts are often poorly constrained or even completely unknown. We have acquired spectroscopic data of sixteen lensing galaxies with the Very Large Telescope located in Chile. In combination with a powerful deconvolution algorithm, we determine the redshift of these sixteen lensing galaxies, which represents about 25% of all currently known quasar lensing galaxies. These results are useful for both H0 determinations and statistical studies of gravitational lenses, which can be used to provide new constraints on cosmological parameters. While the first part of this thesis focuses on the acquisition of observational constraints for the lens models, the main part consists in using the phenomenon of microlensing to determine the energy profile (or spatial structure) of quasar accretion disks. Microlensing is produced by the stars located in the lensing galaxy. These stars act as secondary lenses, and are called microlenses. Since the stars are moving in the galaxy, they induce flux and color variations in the images of the lensed quasar. These effects can be used as a natural telescope to probe the still mysterious inner structures of quasars with a spatial resolution about ten thousand times better than the capacities of current astronomical instruments, including the Very Large Telescope Interferometer. We present a three-year long spectrophotometric monitoring of the lensed quasar QSO 2237+0305, also known as the Einstein Cross, conducted at the Very Large Telescope. This monitoring reveals significant microlensing-induced variations in the spectra of the quasar images. In a subsequent analysis, we find that the source responsible for the optical and ultraviolet continuum has an energy profile well reproduced by a power-law R α λζ with ζ = 1.2 ± 0.3, where R is the size of the source emitting at wavelength λ. This agrees with the predictions of the standard thin accretion disk model and is, so far, the most accurate determination of a quasar energy profile. As a complement to our microlensing study, we have obtained high spectral and spatial resolution observations of the lensing galaxy of QSO 2237+0305. Our spectroscopic data are acquired with the SINFONI, FLAMES, and FORS2 spectrographs of the Very Large Telescope. We describe the reduction of these data, and provide the currently best and most complete determination of the kinematics of a gravitational lens. The comparison of our data with previously published dynamical models suggests that those may have overestimated the mass of the galaxy bulge. Thus, new and more sophisticated models are required. These models, combined with gravitational lensing, will provide two independent constraints on the mass distribution. This will allow to better determine the quantity and distribution of dark matter in this lensing galaxy, and especially in its extended halo.

EPFL2008

Characterization of Annular Two-Phase Down Flow Using Non Intrusive Optical Techniques for Flow Patterns, Entrainment, Film Thickness and Velocity Profile

Marco Milan

In this study, adiabatic vertical cocurrent downward air-water flow in an 8.8 mm internal diameter channel was investigated using non intrusive optical techniques. Flow pattern tests were performed to assess the influence of the inlet device, of the test section length and of the flow history on the flow pattern. These tests indicated that in each flow pattern region there are important differences in the actual flow structures and confirmed that a "smart" flow pattern map should then not only contain the flow pattern transition lines but give detailed characteristics of the actual flows expected (as previously suggested by Thome et al. (2012)). As a first such step in that direction here, an entrainment mechanism map superimposed on a two-phase flow pattern map has been proposed for the first time in this study, which should help to better predict the entrainment rate at a given air and water flow rate by knowing which type of entrainment will be predominant in a particular flow pattern. Furthermore, a new entrainment mechanism and a new two-phase flow structure, namely entrainment from the nose of an elongated bubble and membrane flow were observed in adiabatic vertical downward air-water flow and described. Some of the most important annular flow characteristic parameters, namely the liquid en- trainment rate, the liquid film velocity profile and the average film thickness were measured in this research. For the first time, the Planar Laser Induced Fluorescence (PLIF) technique was successfully adopted to measure the entrainment rate at low air and water superficial velocities and the results were compared with the Cioncolini and Thome (2012) entrainment prediction method. The liquid film velocity profiles obtained using the Particle Image Velocimetry (PIV) technique were non-dimesionalized and compared with the tubular universal velocity profile: good agreement was found only in the near wall region while towards the air-water interface, where the influence of the interfacial waves on the flow structure becomes more important, the universal velocity profile increasingly over predicts the measured velocity profiles. Furthermore, the PLIF technique provided time-resolved longitudinal images of the liquid film and allowed quantitative information to be obtained on the liquid film thickness and on the wave dynamics. Finally, high amplitude interfacial waves, which form at the inlet of the test section, were found to dictate the level of the entrainment, affect the liquid film velocity profile and the residual film thickness and to be responsible for the transition from annular to slug plus bubbly flow. The obtained results provide a strong basis for the future development of more sophisticated heat transfer prediction models for such geometries and flow regimes.

EPFL2012

Dwarf Galaxies and Galaxy Halos

Martin Tafelmeyer

The various projects presented in this thesis contribute to the expansion of our knowledge and understanding of the fundamental processes involved in the formation of a galaxy and its stellar populations. For this purpose, two different techniques are applied: A photometric study of the properties of the stellar halo surrounding the edge-on galaxy NGC 3957, and spectroscopic studies of the chemical compositions of stars in the nearby dwarf spheroidal galaxies Sextans, Fornax, and Sculptor. Due to their extreme weakness, studies of halos outside the Local Group are very challenging, and available data are still very scarce with limited quality. The results of previous studies are quite puzzling. Lequeux et al. (1996, 1998); Zibetti et al. (2004); Zibetti & Ferguson (2004), for example, report the discovery of external stellar halos with extremely red integrated colors, which were neither in agreement with the halo stars observed in the Milky Way or M31, nor with any other realistic stellar population. To investigate this problem, we obtained deep images of the edge-on galaxy NGC 3957, to extract minor axis surface brightness profiles, allowing the study of the halo properties in regions which are not contaminated by the much brighter disk or bulge component. While the central regions of our profiles are very well described by a de Vaucouleurs law, we find a clear excess of light at larger radii, indicating the presence of a stellar halo. Our color profiles rise to very red colors at large radii, however, a careful error analysis shows that systematic errors are mainly responsible for this rise. In the radial range where the halo component is dominant and the systematics are still small, our results are compatible with the expected integrated halo colors of the Milky Way, M31 and the slightly more distant galaxy NGC 891. Even though this is no proof, it suggests that the stellar population of the halo of NGC 3957 is a rather typical one. It is shown that the limiting factor is definitely a sufficiently accurate determination of the background level. We propose a few ways how this could be improved. Nevertheless, the sky subtraction remains the largest challenge for future studies of this type, especially if the goal is to trace the stellar halo to larger radii, comparable to the halo of the Milky Way or M31. The targets for the spectroscopic analysis of stars in dwarf galaxies were selected from an extended low resolution survey of the Ca II triplet (CaT) region, containing ∼2000 stars in Sextans, Sculptor, and Fornax. This is the largest survey of this kind done so far targeting stars outside the Milky Way. One of the most striking results of this survey was the absence of stars with metallicity estimations from the strength of the CaT lines which were below -2.8 (Helmi, 2006). This is particularly interesting, since the Milky Way halo contains a wealth of stars clearly below this limit (Cayrel et al., 2004). Such a difference would have consequences for our understanding of galaxy formation, implying that at early epochs dwarf galaxies must have been fundamentally different from the Milky Way halo. However, in some of the recently discovered, so called "ultra faint dwarfs" (UFDs) several stars with very low metallicities ([Fe/H] ∼ -4) have been found (Norris et al., 2009; Kirby et al., 2008; Frebel et al., 2010, 2009). It is therefore essential to test the reliability of the CaT metallicity estimate and confirm or disprove the existence of extremely metal poor stars in classical dwarf spheroidal galaxies (dSphs). For this purpose, the stars with the lowest CaT metallicities were followed up with high resolution spectroscopy in order to accurately determine metallicities and abundances of several other elements. Our analysis shows unambiguously, that these stars are more metal poor than suggested by the CaT estimations, setting the metallicity floor of dwarf galaxies on a level comparable with the Milky Way halo. The chemical abundance patterns of these stars and their comparison with various other dwarf galaxies and the Milky Way provide valuable insights in the earliest chemical enrichment processes. For carbon, we find signs of a large abundance scatter in Sextans, indicating inhomogeneities in the early ISM of this galaxy. Most of the other elements, in contrast, have very small abundance spreads, implying that there must be a specific process leading to a dispersion in carbon abundances. We also find evidence, that the abundance spread is larger for low mass galaxies, indicating that mixing processes in such systems are less efficient. Below [Fe/H] = -3, the abundances of the α- and iron peak elements are very similar in all galaxies, suggesting formation mechanisms independent on the properties of the host galaxy. Other elements show a more ambiguous picture. The neutron-capture elements strontium and barium are similar in all galaxies at the lowest metallicities ([Fe/H] ≲ -3.5). At higher metallicities, however, the more massive galaxies like Sculptor or the Milky Way increase their abundances to solar values (though with some scatter), while the less massive ones like Ursa Major II or Draco stay very low both in [Ba/Fe] and [Sr/Fe]. This is another indication for different chemical evolution, depending on the size of the host galaxy. It shows the importance of studying stars in various galaxies, spanning a large range of masses. Additionally, abundance patterns for a large sample of stars in the Sextans dSph were derived. Previous spectroscopic studies include only a very small number of objects owing to the large observation times required to obtain the spectra, star by star, with single slit spectrographs. This thesis utilizes the multi-object spectrograph GIRAFFE to obtain spectra of a large number of stars simultaneously, allowing us to study chemical abundances for the biggest sample of Sextans stars observed so far. Our targets, which are complemented with literature data, span a metallicity range from -1 to -3, covering different stages of the the chemical evolution history of Sextans. We find that the decrease in [α/Fe] starts at very low metallicities ([Fe/H] ≲ -2), confirming the low star formation rate of Sextans (e.g. Revaz et al., 2009). Comparing our results with other galaxies, some elements (e.g. those of the Fe-peak) have virtually the same abundance patterns everywhere. Others show signs of differential evolution in the metallicity range -1 to -2. One example is Mg, which decreases to lower values in Sextans than in the more massive Sculptor dSph or the Milky Way. Another one is the abundance ratio of 2nd to 1st s-process elements ([Ba/Y]) which is higher in low mass systems such as Sextans and draco than in Sculptor or the Milky Way.

EPFL2010