The Structural Parameters of Bulges, Bars and Discs in the Local Universe

Image decomposition of galaxies is now routinely used to estimate the structural parameters of galactic components. In this work, I address questions on the reliability of this technique. In particular, do bars and AGN need to be taken into account to obtain the structural parameters of bulges and discs? And to what extent can we trust image decomposition when the physical spatial resolution is relatively poor? With this aim, I performed multi-component (bar/bulge/disc/AGN) image decomposition of a sample of very nearby galaxies and their artificially redshifted images, and verified the effects of removing the bar and AGN components from the models. Neglecting bars can result in a overestimation of the bulge-to-total luminosity ratio of a factor of two, even if the resolution is low. Similar effects result when bright AGN are not considered in the models, but only when the resolution is high. I also show that the structural parameters of more distant galaxies can in general be reliably retrieved, at least up to the point where the physical spatial resolution is about 1.5 Kpc, but bulge parameters are prone to errors if its effective radius is small compared to the seeing radius, and might suffer from systematic effects. I briefly discuss the consequences of these results to our knowledge of the stellar mass budget in the local universe, and finish by showing preliminary results from a large SDSS sample on the dichotomy between classical and pseudo-bulges.Comment: 4 pages, 3 figures; contributed talk to appear in "Formation and Evolution of Galaxy Bulges", proceedings of the IAU Symp. 245, held in Oxford, UK, July 2007, M. Bureau, E. Athanassoula, B. Barbuy (eds.

Stellar Populations, Bars and Secular Evolution in Late-Type Galaxies

We have done a robust statistical analysis of UBV color profiles of 257 Sbc barred and unbarred galaxies. We found that there is an excess of barred galaxies among the objects with null or positive (bluish inward) color gradients, which seems to indicate that bars act as a mechanism of homogenization of the stellar population along galaxies. Moreover, the relationship found between total and bulge colors shows that, in the process of homogenization, the stellar population of bulges are getting bluer, whereas the total color of galaxies remains the same. These characteristics are expected in a secular evolutionary scenario, and seem incompatible with both the monolithic and the hierarchical scenarios for spiral galaxy formation.Comment: 2 pages, 1 table, no figures. To appear in ASP Conference Series, "Galaxy Disks and Disk Galaxies", J. G. Funes S. J. and E. M. Corsini, ed

On the Lengths, Colours and Ages of Bars

In an effort to obtain further observational evidences for secular evolution processes in galaxies, as well as observational constraints to current theoretical models of secular evolution, we have used BVRI and Ks images of a sample of 18 barred galaxies to measure the lengths and colours of bars, create colour maps and estimate global colour gradients. In addition, applying a method we developed in a previous article, we could distinguish for 7 galaxies in our sample those whose bars have been recently formed from the ones with already evolved bars. We estimated an average difference in the optical colours between young and evolved bars that may be translated to an age difference of the order of 10 Gyr, meaning that bars may be long standing structures. Moreover, our results show that, on average, evolved bars are longer than young bars. This seems to indicate that, during its evolution, a bar grows longer by capturing stars from the disk, in agreement with recent numerical and analytical results.Comment: To appear in Galaxy Evolution Across the Hubble Time, proceedings of the IAU Symp. 235, F. Combes and J. Palous (eds.); 1 page; the poster can be found at http://www.mpa-garching.mpg.de/~dimitri/iauga.pd

When the signal is in the noise: Exploiting Diffix's Sticky Noise

Anonymized data is highly valuable to both businesses and researchers. A large body of research has however shown the strong limits of the de-identification release-and-forget model, where data is anonymized and shared. This has led to the development of privacy-preserving query-based systems. Based on the idea of "sticky noise", Diffix has been recently proposed as a novel query-based mechanism satisfying alone the EU Article~29 Working Party's definition of anonymization. According to its authors, Diffix adds less noise to answers than solutions based on differential privacy while allowing for an unlimited number of queries. This paper presents a new class of noise-exploitation attacks, exploiting the noise added by the system to infer private information about individuals in the dataset. Our first differential attack uses samples extracted from Diffix in a likelihood ratio test to discriminate between two probability distributions. We show that using this attack against a synthetic best-case dataset allows us to infer private information with 89.4% accuracy using only 5 attributes. Our second cloning attack uses dummy conditions that conditionally strongly affect the output of the query depending on the value of the private attribute. Using this attack on four real-world datasets, we show that we can infer private attributes of at least 93% of the users in the dataset with accuracy between 93.3% and 97.1%, issuing a median of 304 queries per user. We show how to optimize this attack, targeting 55.4% of the users and achieving 91.7% accuracy, using a maximum of only 32 queries per user. Our attacks demonstrate that adding data-dependent noise, as done by Diffix, is not sufficient to prevent inference of private attributes. We furthermore argue that Diffix alone fails to satisfy Art. 29 WP's definition of anonymization. [...

Imagen de destino en redes sociales virtuales

Since destination image is an important construct for tourism research, cities and regions try to understand and to develop a positive image in order to guarantee the number of visitors and the tourism revenue. The main objective of this article was to develop a model of evaluation of destination image through the use of pictures shared in virtual social networks. We propose that the image of a destination can be analyzed through the way tourists picture the destination in virtual social networks. Hence, we have developed a measurement to evaluate destination image based on pictures posted on Instagram, and we made a model to analyze destination image based on pictures shared in virtual social network. Our method was composed by an analysis of 1,500 pictures of three important destinations in Brazil (Foz do Iguaçú, Rio de Janeiro, and Salvador). Our results contribute to tourism research by indicating that it is possible to determine the main characteristics of a destination by the pictures in virtual social networks, and by providing a 5-dimension model to do so. This study contributes to tourism research in two venues. First, we propose a method for evaluating destination image based on pictures posted in social media rather than the usual questionnaires. This may help scholars by providing an alternative way of evaluating destination image without recall, social desirability and non-response bias. And secondly, this study also contributes to practitioners and public policy in tourism by showing which characteristics of a destination image are more prominent to the destination image, based on virtual social networks.Como a imagem de destino é um importante construto para a pesquisa em turismo, as cidades e regiões tentam desenvolver uma imagem positiva para garantir o número de visitantes e a receita. O objetivo deste artigo é desenvolver um modelo de avaliação de imagem de destino através do uso de imagens compartilhadas em redes sociais virtuais. Propomos que a imagem de um destino possa ser analisada pela maneira como os turistas imaginam o destino em redes sociais virtuais. Assim, desenvolvemos uma medida para avaliar a imagem de destino com base em fotos publicadas no Instagram. Nosso método foi composto por uma análise de 1500 fotos de três importantes destinos no Brasil (Foz do Iguaçu, Rio de Janeiro e Salvador). Nossos resultados contribuem para a pesquisa em turismo, indicando que é possível determinar as principais características de um destino pelas imagens em redes sociais virtuais e fornecendo um modelo de 5 dimensões para fazê-lo. Este estudo contribui para a pesquisa em turismo em dois locais: propomos um método para avaliar a imagem de destino com base em imagens postadas em mídias sociais, em vez dos questionários usuais. Isso pode ajudar os estudiosos, fornecendo uma maneira alternativa de avaliar a imagem de destino sem recordação, desejo social e viés de não-resposta. Este estudo também contribui para os profissionais e políticas públicas em turismo, mostrando quais características de uma imagem de destino são mais proeminentes para a imagem de destino com base em redes sociais virtuais.Dado que la imagen de destino es una construcción importante para la investigación turística, las ciudades y regiones intentan entender y desarrollar una imagen positiva para garantizar el número de visitantes y los ingresos del turismo. El objetivo principal de este artículo es desarrollar un modelo de evaluación de la imagen de destino mediante el uso de imágenes compartidas en redes sociales virtuales. Proponemos que la imagen de un destino se pueda analizar a través de la forma en que los turistas representan el destino en las redes sociales virtuales. Por lo tanto, desarrollamos una medida para evaluar la imagen de destino basada en imágenes publicadas en Instagram y desarrollamos un modelo para analizar la imagen de destino basada en imágenes compartidas en una red social virtual. Nuestro método fue compuesto por un análisis de 1500 imágenes de tres destinos importantes en Brasil (Foz do Iguaçú, Río de Janeiro y Salvador). Nuestros resultados contribuyen a la investigación turística en tanto indican que es posible determinar las características principales de un destino por las imágenes en las redes sociales virtuales, y proporcionan un modelo de cinco dimensiones para hacerlo. Este estudio contribuye a la investigación turística en dos espacios. Por un lado, proponemos un método para evaluar la imagen de destino basada en imágenes publicadas en las redes sociales en lugar de los cuestionarios habituales; esto puede ayudar a los académicos al proporcionar una forma alternativa de evaluar la imagen de destino sin recordar la conveniencia social y el sesgo de no respuesta. Y por otro lado, este estudio también resulta provechoso para los profesionales y las políticas públicas en turismo al mostrar qué características de una imagen de destino son más prominentes a la imagen de destino basada en redes sociales virtuales.https://revistas.udem.edu.co/index.php/anagramas/article/view/252

Estimating the Ages of Bars: Implications for the Bar-AGN-Star Formation Connection

In an effort to elevate to higher grounds our understanding on the impact of the formation and evolution of bars in the formation and evolution of galaxies, we have developed a diagnostic tool to distinguish between recently formed and evolved bars. Our method was applied in the study of a sample of 14 galaxies and revealed that, apparently, AGN activity tends to appear in galaxies which have young bars rather than evolved bars. This suggests that the time scale for the fueling of AGN by bars is short, and may help to explain, for instance, why there is not a clear correlation between the presence of bars and AGN in galaxies.Comment: 4 pages including 2 figures and 1 table; contributed talk to appear in the proceedings of the IAU Symp. 222, The Interplay among Black Holes, Stars and ISM in Galactic Nuclei, Th. Storchi Bergmann, L.C. Ho, H.R. Schmitt, eds., held in Gramado, March 200

Using Virtual Observatory techniques to search for Adaptive Optics suitable AGN

Until recently, it has been possible only for nearby galaxies to study the scaling relations between central black hole and host galaxy in detail. Because of the small number densities at low redshift, (luminous) AGN are underrepresented in such detailed studies. The advent of adaptive optics (AO) at large telescopes helps overcoming this hurdle, allowing to reach small linear scales over a wide range in redshift. Finding AO-suitable targets, i.e., AGN having a nearby reference star, and carrying out an initial multiwavelength classification is an excellent use case for the Virtual Observatory. We present our Virtual-Observatory approach to select an AO-suitable catalog of X-ray-emitting AGN at redshifts 0.1<z<1.Comment: 4 pages, 5 figures, submitted to "EURO-VO AIDA workshop: Multiwavelength astronomy and Virtual Observatory", ESAC, Spain, 1-3 Dec. 200