The Edge of the Galactic Disc

As part of a stellar population sampling program, a series of photometric probes at various field sizes and depths have been obtained in a low extinction window in the galactic anticentre direction. Such data set strong constraints on the radial structure of the disc. At the forefront of this "drilling" program, very deep CCD frames probe the most external parts of the disc. Over the whole effective magnitude range (18 to 25), all contributions in the statistics which should be expected from old disc stars beyond 6 kpc vanish, although such stars dominate by far at distances less than 5 kpc. This is the signature of a sharp cut-off in the star density: the edge of the galactic disc between 5.5 and 6 kpc. As a consequence, the galactic radius does not exceed 14 kpc (assuming $R/-/(//sun/)$=8.5). Colours of elliptical galaxies measured in the field rule out the risk of being misled by undetected extinction.Comment: 8 pages, Besancon Observatory preprint n.

Extended HI spiral structure and the figure rotation of triaxial dark halos

The HI disk of the blue compact dwarf (BCD) galaxy NGC 2915 extends to 22 optical scalelengths and shows spiral arms reaching far beyond the optical component. None of the previous theories for spiral structure provide likely explanations for these very extended spiral arms. Our numerical simulations first demonstrate that such large spiral arms can form in an extended gas disk embedded in a massive triaxial dark matter halo with slow figure rotation, through the strong gravitational torque of the rotating halo. We then show that the detailed morphological properties of the developed spirals and rings depend strongly on the pattern speed of the figure rotation, the shape of the triaxial halo, and the inclination of the disk with respect to the plane including the triaxial halo's long and middle axes. These results strongly suggest that the dark matter halo of NGC 2915 is triaxial and has figure rotation. Based on these results, we also suggest that dynamical effects of triaxial halos with figure rotation are important in various aspect of galaxy formation and evolution, such as formation of polar ring galaxies, excitation of non-axisymmetric structures in low surface-brightness galaxies, and gas fueling to the central starburst regions of BCDs.Comment: 13 pages 2 figures (fig.2 = jpg format), accepted by ApJ

THE IMPACT OF CALORIC AND NON-CALORIC SWEETENER CONSUMPTION ON THE BEHAVIORAL, HORMONAL AND BRAIN RESPONSESTO FOOD

Sight is a primary channel conveying information about food, in turn influencing appetite control via homeostatic, hedonic and cognitive factors. Brain responses to visual food cues have been increasingly studied in the past decades. However, the influence of specific dietary factors such as caloric (sugar) and non-caloric sweetener (NNS) consumption on subsequent visual responses to food remains poorly understood. Yet, both sweeteners have been associated to long-term weight gain. The thesis at hand aims at a more integrative view to understand the impact of sugar and NNS consumption on visual food perception and intake behavior, by combining measures of behavioral, physiological and brain responses towards food. The first exploratory project (study A) investigated changes in behavioral and brain responses to food viewing via a 3-month replacement of sugar-sweetened beverages with NNS-sweetened beverages. We showed intervention-induced modulations in neural activity in response to high-fat, sweet food viewing that were mostly apparent in dorsal prefrontal and precentral cortices, i.e. brain areas associated with inhibitory control and attention. The decrease in activity within the dorsal prefrontal cortex was inversely correlated with changes in body weight, i.e. participants who failed to lose weight also showed decreased activity to palatable food cues in brain areas that have been related to food intake control. The second project (study B) investigated the acute effects of sucrose- and NNS-beverage consumption, as compared to water, on the subsequent brain responses to food viewing and later spontaneous food intake at an ad libitum buffet. Sucrose consumption elicited a differential pattern of neural activity to food viewing as compared to water, and a subsequent decrease in spontaneous food intake. NNS consumption, on the other hand, did not affect food intake, but modified post-prandial brain responses to food viewing, most pronounced in prefrontal areas and the insula, i.e. brain regions that have been associated with food intake control and nutrient-flavor conditioning. Altogether, the thesis at hand provides insights on the impact of caloric and non-caloric sweetener consumption on the visual perception of tempting food cues. This is of great relevance in our modern environment where visual cues are ubiquitous and guide consumption behavior in daily life. Detailed mechanisms as to how NNS might impact behavior when repeatedly consumed yet need to be investigated in more detail in the future, in particular to disentangle effects driven by NNS-containing foods and beverages as such, as opposed to individuals’ expectations related to the consumption of such non-caloric products. La vision est utilisée comme principal vecteur d’informations lorsqu’un individu est confronté à la nourriture, influençant de ce fait le contrôle de l’appétit par des facteurs homéostatiques, hédoniques et cognitifs. Les réponses cérébrales lors de la perception visuelle de nourriture ont été fortement étudiées dans les dernières décennies. Cependant, l’influence de facteurs alimentaires spécifiques tels que la consommation d’agents sucrants caloriques (les sucres) et non-caloriques (les édulcorants) sur les réponses visuelles ultérieures reste encore peu claire. Les sucres et édulcorants ont pourtant été associés à une prise de poids corporel sur le long terme. Cette thèse a pour but de mieux comprendre l’impact de la consommation de sucres et d’édulcorants sur la perception visuelle de nourriture et sur le comportement alimentaire, en combinant des mesures comportementales, physiologiques et cérébrales. Le premier projet (étude A) a exploré les changements dans les réponses cérébrales et comportementales à la vision de nourriture induits par un remplacement de la consommation de boissons sucrées par leurs équivalents édulcorés. A la suite de trois mois d’intervention, nous avons mis en évidence des modulations de l’activité neuronale lors de la vision d’aliments sucrés et riches en gras dans des aires cérébrales préfrontales dorsales et précentrales, associées au contrôle inhibiteur et à l’attention. Une diminution d’activité dans l’aire préfrontale dorsale était inversement corrélée au changement de poids corporel, c’est-à-dire que les participants qui n’ont pas perdu de poids ont aussi montré les plus grandes baisses d’activités dans cette aire cérébrale liée au contrôle inhibiteur de la prise alimentaire. Le deuxième projet (étude B) a étudié les effets aigus d’une consommation de boissons sucrées ou édulcorées, en comparaison à l’eau, sur les réponses cérébrales subséquentes à la vision de nourriture, ainsi que sur le comportement alimentaire lors d’un buffet ad libitum. La consommation de sucre, en comparaison à l’eau, a modifié l’activité cérébrale à la vue de nourriture. Ceci était associé à une moindre prise alimentaire lors du buffet. En revanche, la consommation d’édulcorants n’a pas affecté le comportement alimentaire, mais a modifié les réponses cérébrales postprandiales en particulier dans les aires préfrontales ainsi que dans l’insula, des régions associées aux habilités de contrôle de la prise alimentaire et au conditionnement goût-nutriment. Ensemble, les études réalisées dans le cadre de cette thèse ont fourni des indications sur l’impact d’une consommation de sucres et d’édulcorants sur la perception visuelle de nourriture appétissante. Ceci est particulièrement important dans notre environnement alimentaire moderne, dans lequel les stimuli visuels de nourriture sont omniprésents et guident notre comportement alimentaire quotidien. Les mécanismes d’action des édulcorants sur notre comportement lorsqu’ils sont consommés de manière répétée restent cependant à étudier de manière plus détaillée, en particulier dans le but de distinguer les effets des édulcorants eux-mêmes des attentes individuelles liées à la consommation de ces produits

Is Galactic Structure Compatible with Microlensing Data?

We generalize to elliptical models the argument of Kuijken (1997), which connects the microlensing optical depth towards the Galactic bulge to the Galactic rotation curve. When applied to the latest value from the MACHO collaboration for the optical depth for microlensing of bulge sources, the argument implies that the Galactic bar cannot plausibly reconcile the measured values of the optical depth, the rotation curve and the local mass density. Either there is a problem with the interpretation of the microlensing data, or our line of sight to the Galactic centre is highly atypical in that it passes through a massive structure that wraps only a small distance around the Galactic centre.Comment: Submitted to ApJ Letters. 8 pages LaTeX, 3 figures. Corrected error in description of microlensing observation

The gravitational force and potential of the finite Mestel disk

Mestel determined the surface mass distribution of the finite disk for which the circular velocity is constant in the disk and found the gravitational field for points in the $z=0$ plane. Here we find the exact closed form solutions for the potential and the gravitational field of this disk in cylindrical coordinates over all the space. The Finite Mestel Disk (FMD) is characterized by a cuspy mass distribution in the inner disk region and by an exponential distribution in the outer region of the disk. The FMD is quite different from the better known exponential disk or the untruncated Mestel disk which, being infinite in extent, are not realistic models of real spiral galaxies. In particular, the FMD requires significantly less mass to explain a measured velocity curve

Extracting galactic structure parameters from multivariated density estimation

Multivariate statistical analysis, including includes cluster analysis (unsupervised classification), discriminant analysis (supervised classification) and principle component analysis (dimensionlity reduction method), and nonparameter density estimation have been successfully used to search for meaningful associations in the 5-dimensional space of observables between observed points and the sets of simulated points generated from a synthetic approach of galaxy modelling. These methodologies can be applied as the new tools to obtain information about hidden structure otherwise unrecognizable, and place important constraints on the space distribution of various stellar populations in the Milky Way. In this paper, we concentrate on illustrating how to use nonparameter density estimation to substitute for the true densities in both of the simulating sample and real sample in the five-dimensional space. In order to fit model predicted densities to reality, we derive a set of equations which include n lines (where n is the total number of observed points) and m (where m: the numbers of predefined groups) unknown parameters. A least-square estimation will allow us to determine the density law of different groups and components in the Galaxy. The output from our software, which can be used in many research fields, will also give out the systematic error between the model and the observation by a Bayes rule