Variabilité climatique et statistiques. Etude par simulation de la puissance et de la robustesse de quelques tests utilisés pour vérifier l'homogénéité de chroniques

L'analyse statistique de séries chronologiques de données hydrométéorologiques est un des outils d'identification de variations climatiques. Cette analyse consiste le plus souvent à la mise en œuvre et à l'interprétation de tests statistiques d'homogénéité des séries. Les séries hydrologiques (données de pluie ou de débit) se caractérisent fréquemment par des effectifs faibles, et ne répondent que rarement aux conditions requises par l'application des tests statistiques dont certains sont paramétriques.Nous avons cherché à évaluer, en terme de puissance et de robustesse, le comportement de quelques méthodes statistiques largement employées dans les études de variabilité climatique. Ce travail a été mené dans chaque cas étudié au moyen de procédures de simulations type Monte-Carlo de 100 échantillons de 50 valeurs conformes aux caractéristiques souvent rencontrées dans les séries naturelles. La variabilité simulée est celle d'un changement brutal de la moyenne. Les procédures concernées sont le test de corrélation sur le rang, le test de Pettitt, le test de Buishand, la procédure bayésienne de Lee et Heghinian, et la procédure de segmentation des séries hydrométéorologiques de Hubert et Carbonnel. Des séries artificielles soit stationnaires, soit affectées par une rupture de la moyenne, normales, non-normales, autocorrélées, présentant une tendance linéaire ou un changement brutal de la variance ont été générées.Les conclusions de ce travail doivent être nuancées selon la méthode considérée. D'une manière générale la puissance maximale estimée se situe autour de 50% pour des taux de rupture de la moyenne de l'ordre de 75% de la valeur de l'écart-type. Par ailleurs il apparaît que l'autocorrélation et la présence d'une tendance dans les séries sont les deux caractéristiques qui pénalisent le plus les performances des procédures.Statistical analysis of hydrometeorological time series is often used to identify climatic variations. Most often this analysis consists of applying and interpreting statistical tests of time series homogeneity. Hydrological time series (rainfall and runoff data) are often short and do not always comply with the hypotheses of the statistical methods. Through simulation we have investigated the power and the robustness of some tests which are widely used in the studies dealing with climatic variability. In each case studied, one hundred samples of fifty elements have been generated based on the main characteristics of natural rainfall series. A shift in the mean has been used to represent a possible climatic variation. The procedures used are the rank correlation test, Pettitt's test, Buishand's test, Lee and Heghinian's bayesian procedure, and Hubert and Carbonnel's segmentation procedure for hydrometeorological series.Each simulation of one hundred samples is used to assess the performances of the methods considering a specific characteristic of the series: normality or non-normality, autocorrelation, trend, shift in the variance. First of all, stationary series have been simulated to evaluate the type I error of the tests. Then the series have been simulated with a break in the mean with different levels of amplitude, from 25% to 100% of the standard deviation value. The rank correlation test, Pettitt's test, Buishand's test and the segmentation procedure with a significance level of 1% (significance level of Scheffé's test) reject as heterogeneous less than ten series over one hundred homogeneous simulated series. This result is consistent with the type I error of a statistical test. On the other hand, Lee and Heghinian's bayesian method rejects about 40% of the series. This result means that this latter procedure must only be applied under the hypothesis of heterogeneity. The estimated power of the methods exceeds 40% to 50% when the break in the mean is more than 75% of the standard deviation value.Independent series have been simulated from normal, log-normal and Pearson distributions to compare the performances of the methods requiring normality. The results show that normality has no significant impact on the performances of these methods. However, the simulations do show that the condition of independence of the successive elements of the series is essential to keep performances constant. Otherwise a trend in the series makes the tests inefficient, except for the rank correlation test for which the alternative is a trend. No method seems to be robust against both negative and positive autoregressive dependencies. The procedures requiring a constant variance are robust when the series keep a constant mean, but seem more or less slightly influenced by a break both in the mean and in the standard deviation

Survey for Emission-Line Galaxies: Universidad Complutense de Madrid List 3

A new low-dispersion objective-prism search for low-redshift (z<0.045) emission-line galaxies (ELG) has been carried out by the Universidad Complutense de Madrid with the Schmidt Telescope at the Calar-Alto Observatory. This is a continuation of the UCM Survey, which was performed by visual selection of candidates in photographic plates via the presence of the Halpha+[NII]6584 blend in emission. In this new list we have applied an automatic procedure, fully developed by us, for selecting and analyzing the ELG candidates on the digitized images obtained with the MAMA machine. The analyzed region of the sky covers 189 square degrees in nine fields near R.A.=14h & 17h, Dec=25 deg. The final sample contains 113 candidates. Special effort has been made to obtain a large amount of information directly from our uncalibrated plates by using several external calibrations. The parameters obtained for the ELG candidates allow for the study of the statistical properties for the sample.Comment: 13 pages, 18 PostScript figures, 6 JPEG figures, Table 2 corrected. Accepted for publication in Astrophysical Journal Supplements, also available at http://www.ucm.es/info/Astrof/opera/LIST3_ApJS99

De différents aspects de la variabilité de la pluviométrie en Afrique de l'Ouest et Centrale non sahélienne

La sécheresse observée depuis plus d'une vingtaine d'années dans les pays sahéliens se fait également ressentir plus au sud dans des régions d'Afrique aux climats plus humides. Cette baisse de la pluviométrie et la diminution des apports en eau de surface qu'elle entraîne y sont de nature à pénaliser les différents projets de développements liés à l'eau. Le programme ICCARE mené par l'ORSTOM a pour objet l'identification et les conséquences de cette variabilité climatique dans l'ensemble de la zone non sahélienne d'Afrique de l'Ouest et Centrale, en s'appuyant sur les données de deux cents postes pluviométriques et sur un ensemble de méthodes alliant représentations cartographiques et procédures statistiques de détection de ruptures dans les séries chronologiques, univariées et multivariées. La simple étude des séries chronologiques de hauteurs précipitées annuelles fait apparaître une nette et brutale fluctuation du régime pluviométrique dans toute la région considérée, à la fin des années 1960 et au début des années 1970. D'une manière générale, il apparaît que ce sont les zones à régime pluviométrique extrême qui ont subi les modifications les plus importantes : les plus arrosées (de la Guinée à la Côte d'Ivoire) et les plus arides (la bordure sahélienne au nord de la zone étudiée). Entre les deux, le phénomène est d'intensité plus nuancée. Les différentes procédures statistiques appliquées aux séries de hauteurs annuelles précipitées soulignent l'existence d'une rupture survenue à la fin des années 1960 ou au début des années 1970, et donc en phase avec ce qui a été observé et étudié au Sahel. D'autres variables permettant une caractérisation plus "qualitative" du phénomène ont également été étudiées. Elles apportent un complément d'information quant aux manifestations de cette variabilité pluviométrique et montrent que la variabilité climatique se traduit à différents niveaux (durée des saisons des pluies, quantités précipitées hors saisons des pluies, etc.). L'examen des séries chronologiques depuis l'origine des stations a permis de resituer l'événement observé dans une perspective historique faite d'alternances de périodes sèches et de périodes humides. Le phénomène observé à la fin des années 1960 et au début des années 1970 apparaît, cependant comme le plus significatif du point de vue statistique. Si les causes premières d'apparition du phénomène sont, à l'heure actuelle, encore insuffisamment expliquées, et ce même si certaines activités humaines y ont, sans aucun doute, contribué, cette baisse de la pluviométrie a, bien entendu, des conséquences importantes sur la disponibilité des ressources en eau dans ces régions. Si la carence pure et simple n'est pas à craindre dans ces régions où les quantités précipitées restent importantes dans l'absolu, les effets de cette variabilité climatique peuvent, malgré tout, se révéler désastreux, en ce sens qu'ils modifient les données d'un équilibre déjà souvent mis à mal par ailleurs (pression anthropique et déforestation par exemple).The drought observed for more than twenty years in the sahelian countries has also affected those located more to the South with more humid climates (SUTCLIFFE and KNOTT, 1987; NICHOLSON et al, 1988; MAHE and OLIVRY, 1991; OLIVRY et al, 1993 PATUREL et al, 1995). The decrease in rainfall and consequently that in runoff might penalise development projects linked with water supply. The ICCARE programme led by ORSTOM aimed at identifying and measuring the consequences of this climatic variability in the non-sahelian parts of the West and Central African region as a whole. The study was based on rainfall data from more than two hundred stations, break detection methods in the time series as well as cartographic tools were used. This study allowed to highlight the manifestations of the climatic variability observed for nearly 25 years in West and Central Africa. Whereas it had been thought for a long time that the rainfall deficit was restricted to the sahelian region, this study showed that the forest covered regions were also affected and generally speaking the so-called 'humid Africa'. The decrease in rainfall entails that in runoff and thus a change in water resources availability which is the cornerstone of a fairly great number of development projects. Hydrological regimes variability and possible modifications of rainfall-runoff relationship are to be the next stages of the ICCARE programme, partial results were already published (SERVAT et al, 1997). A simple study of the time series showed straightforward fluctuations of the rainfall patterns (figures 2 and 3), which happened during the late sixties or the early seventies over the whole region. The mapping of the time series analysis depicts a clear general trend towards a shift of the isohyets to the south-south-west from the 1950's to the 1980's. This shift reflects a sharp drop in the annual rainfall over the whole of the non-sahelian west and central Africa. Generally speaking it appears that the zones with extreme rainfall patterns underwent the most important modifications, namely : the wettest ones from Guinea to Ivory Coast and the driest ones, bordering the sahelian region to the North of the studied area. Elsewhere changes are less drastic (SERVAT et al, 1996). The different statistical procedures applied to the series of annual rainfall showed breaks taking place during the late 1960's or the early 1970's (figure 4), which was in keeping with what had been observed in the sahelian region. Rainfall deficits were in the order of 20% and they could reach values higher than 25% (table 1), in particular along the Atlantic Coast or in the North, which upholds the fact that 'humid Africa' was also severely affected by the rainfall variability. Other variables which allow a more qualitative characterisation of the phenomenon were also studied (table 2) (figures 5 and 6) ( PATUREL et al, 1997, SERVAT et al, 1997). They brought complementary information about the ways the rainfall variability expresses itself. The pattern of the rainy season was slightly different from what it had been before the 1970's, its length was generally shorter either because it started later than before or because it ended earlier. Likewise, the rainfall distribution was modified, which resulted in a more 'homogeneous' pattern for the zone with only one rainy season and in a sensible change in the ratio of the rainfall heights of the two rainy seasons. Some of the regions of the so-called 'wooded' savannah saw a modification of their climate with a shift from a 'guinean' climate toward a 'sudanese' one. In west Africa some regions saw also a decrease of the amount of precipitation occurring outside the rainy season, which led to a strengthening of the dry season and contributed, if need be, to the perception of the phenomenon by local populations. The decrease in the number of rainy days, where it was possible to study it, was in line with the rainfall deficit. A complementary statistical approach was carried out, it consists of a spatio-temporal study using a multidimensional exploratory analysis (KHODJA et al, 1998). This led to using a multivariate test for detecting a shift in the mean value. This approach confirmed the results obtained with univariate analyses whether it is for the time location of the break (late 1960's, early 1970's) or whether for the heterogeneous character of the phenomenon both from a spatial or temporal point of view. A major characteristic of this persisting rainfall deficit seems to be the existence of two axes of heterogeneity along the north-south and east-west directions (table 3) (figure 7). The survey of the rainfall time series from the origin allowed to place the drought in a historical perspective. So, it appears that, since the beginning of the century, the region underwent a succession of dry and wet periods, although it is difficult to speak of cycles. The phenomenon observed during the late 60's and the early 70's appears, however, as the most significant from a statistical point of view. Besides, the still lasting period of deficit has displayed a length and an intensity quite remarkable, in particular in the north and west sectors of the studied zone where the phenomenon presents an even more exceptional character (figures 8,9,10 and 11). Even though what brought about this diminution in rainfall remains, until now, unexplained, certain human activities undoubtedly contributed to the aggravation of the phenomenon. Although deforestation cannot be held entirely responsible for the drought, the fact remains that overlogging helped to increase the rainfall deficit in numerous regions along the Atlantic coast and the Guinean gulf. Of course, this rainfall deficit has important consequences on the availability of water resources in those regions. Agriculture, the filling of dams and therefore the hydroelectric production, to mention only a few domains, are strongly penalised by this decrease in resources. If a real shortage is not to be feared in those regions where the quantities of precipitation remain high in absolute terms, the effects of that variability can still prove to be disastrous, as they modify the elements of a balance that is already threatened by other factors (anthropic pressure and deforestation, for example). The ICCARE programme, which goes on with the study of the modifications of the river hydrological regimes, will give answers as to the effect of the rainfall deficit upon water resources availability

Shape of the Galactic Orbits in Clusters

A kinematical analysis applied to a sample of galaxy clusters indicates that the differences between the velocity distribution of elliptical and spiral galaxies are associated with the shape of their orbit families. The orbital anisotropies present on each morphological population could be measured with the use of a parameter which is the ratio of the radial and tangential velocity dispersions, and can be recovered through the observed line-of-sight velocity distribution. When a Gaussian velocity distribution is assumed, having different dispersions along the radial and tangential directions, we conclude that the orbits of elliptical galaxies in clusters are close to radial, while spirals have more circular shaped or isotropic orbits. Lenticulars galaxies shares an intermediate orbital parameter, between spirals and ellipticals.Comment: 23 pages including 6 EPS-figures, and 4 tables. Accepted for publication by ApJ, April 199

Hubble Space Telescope Photometry of the Dwarf Spheroidal Galaxy ESO 410-G005

We present HST WFPC2 imaging of the nearby low-surface-brightness dwarf spheroidal galaxy ESO 410-G005, which has been resolved into stars for the first time. The resulting color-magnitude diagram for about 2500 stars shows a red giant branch branch with a tip at I=(22.4+-0.15), which yields a distance of D=(1.9+-0.2) Mpc. ESO 410-G005 is found to be metal-poor with a mean metallicity of (-1.8+-0.4) dex estimated from its red giant branch. Upper asymptotic giant branch stars appear to be present near the center of the galaxy, indicative of a substantial, centrally concentrated intermediate-age population, unless these objects are artifacts of crowding. Previous studies did not detect ESO 410-G005 in H alpha or in HI. ESO 410-G005 is a probable member of the Sculptor group. Its linear separation from the nearest spiral, NGC 55, is 230 kpc on the sky. The deprojected separation ranges from 340 to 615 kpc depending on the assumed distance of NGC 55. ESO 410 G005 appears to be a relatively isolated dSph within the Sculptor group. Its absolute magnitude, Mv = (-12.1+-0.2) mag, its central surface brightness, mu_V = (22.7+-0.1) mag/arcsec^2, and its mean metallicity, [Fe/H] = (-1.8+-0.4) dex, follow the trend observed for dwarf galaxies in the Local Group. (abridged)Comment: Accepted for publication in the Astrophysical Journal, Vol. 542 (Oct 20). 23 pages in AASTEX style, 9 figures, partially in gif format to save spac

The AMIGA sample of isolated galaxies XI. A First Look at Isolated Galaxy Colors

The basic properties of galaxies can be affected by both nature (internal processes) or nurture (interactions and effects of environment). Deconvolving the two effects is an important current effort in astrophysics. Observed properties of a sample of isolated galaxies should be largely the result of internal (natural) evolution. It follows that nurture-induced galaxy evolution can only be understood through comparative study of galaxies in different environments. We take a first look at SDSS (g-r) colors of galaxies in the AMIGA sample involving many of the most isolated galaxies in the local Universe. This leads us to simultaneously consider the pitfalls of using automated SDSS colors. We focus on median values for the principal morphological subtypes found in the AMIGA sample (E/S0 and Sb-Sc) and compare them with equivalent measures obtained for galaxies in denser environments. We find a weak tendency for AMIGA spiral galaxies to be redder than objects in close pairs. We find no clear difference when we compare with galaxies in other (e.g. group) environments. However, the (g-r) color of isolated galaxies shows a Gaussian distribution as might be expected assuming nurture-free evolution. We find a smaller median absolute deviation in colors for isolated galaxies compared to both wide and close pairs. The majority of the deviation on median colors for spiral subtypes is caused by a color-luminosity correlation. Surprisingly isolated and non-isolated early-type galaxies show similar (g-r). We see little evidence for a green valley in our sample with most spirals redder than (g-r)=0.7 having spurious colors. The redder colors of AMIGA spirals and lower color dispersions for AMIGA subtypes -compared with close pairs- is likely due to a more passive star formation in very isolated galaxies.Comment: Accepted for publication in A&A. 9 pages, 7 Figures, and 2 tables, one only available onlin

Properties of Quasar-Galaxy Associations and Gravitational Mesolensing by Halo Objects

A new catalog of 8382 close quasar-galaxy pairs is presented. The catalog was composed using published catalogs of quasars and active galactic nuclei containing 11358 objects, as well as the LEDA catalog of galaxies, which contains on the order of 100 thousand objects. The search for pairs was carried out in such a way that the linear distance between the galaxy and projected quasar does not exceed 150kpc. Based on these new data, the dependence of the number of pairs on a=z_G/z_Q is analysed, where z_G and z_Q are the redshifts of the galaxy and quasar, respectively, revealing an excess of pairs with a<0.1 and a>0.9. This means that the galaxies in pairs are preferably located close to either the observer or the quasar and avoid intermediate distances along the line of sight to the quasar. Computer simulations demonstrate that it is not possible to explain this number of pairs with the observed distribution in a as the result of chance positional coincidences with a uniform spatial distribution of galaxies. Data on globular clusters show that the excess of pairs with a0.9 is consistent with the hypothesis that we are observing distant compact objects that are strongly gravitationally lensed by transparent lenses with a King mass distribution located in the halos of nearby galaxies. The Hubble diagram for galaxies and quasars is presented. Observational tests of the mesolensing hypothesis are formulated.Comment: 11 pages, 7 figure

Binary Galaxies in the Local Supercluster and Its Neighborhood

We report a catalog of 509 pairs identified among 10403 nearby galaxies with line-of-sight velocities V_LG < 3500 km/s.We selected binary systems in accordance with two criteria (bounding and temporal), which require the physical pair of galaxies to have negative total energy and its components to be located inside the zero-velocity surface. We assume that individual galaxy masses are proportional to their total K-band luminosities, M = L_K x 6M/L. The catalog gives the magnitudes and morphological types of galaxies and also the projected (orbital) masses and pair isolation indices. The component line-of-sight velocity differences and projected distances of the binary systems considered have power-law distributions with the median values of 35 km/s and 123 kpc, respectively. The median mass-to-K-band luminosity ratio is equal to 11 M/L, and its uncertainty is mostly due to the errors of measured velocities. Our sample of binary systems has a typical density contrast of d ro/ro_c ~ 500 and a median crossing time of about 3.5 Gyr. We point out the substantial fraction of binary systems consisting of late-type dwarf galaxies, where the luminosities of both components are lower than that of the Small Magellanic Cloud. The median projected distance for 41 such pairs is only 30 kpc, and the median difference of their line-of-sight velocities is equal to 14 km/s which is smaller than the typical error for radial-velocity (30 km/s). This specific population of gas-rich dwarf binary galaxies such as I Zw 18 may be at the stage immediately before merging of its components. Such objects, which are usually lost in flux-limited (and not distance-limited) samples deserve a thorough study in the HI radio line with high spatial and velocity resolution.Comment: published in Astrophysical Bulletin, 2008, Vol. 63, No. 4, pp. 299-34

Five supernova survey galaxies in the southern hemisphere. I. Optical and near-infrared database

The determination of the supernova (SN) rate is based not only on the number of detected events, but also on the properties of the parent galaxy population. This is the first paper of a series aimed at obtaining new, refined, SN rates from a set of five SN surveys, by making use of a joint analysis of near-infrared (NIR) data. We describe the properties of the 3838 galaxies that were monitored for SNe events, including newly determined morphologies and their DENIS and POSS-II/UKST I, 2MASS and DENIS J and Ks and 2MASS H magnitudes. We have compared 2MASS, DENIS and POSS-II/UKST IJK magnitudes in order to find possible systematic photometric shifts in the measurements. The DENIS and POSS-II/UKST I band magnitudes show large discrepancies (mean absolute difference of 0.4 mag), mostly due to different spectral responses of the two instruments, with an important contribution (0.33 mag rms) from the large uncertainties in the photometric calibration of the POSS-II and UKST photographic plates. In the other wavebands, the limiting near infrared magnitude, morphology and inclination of the galaxies are the most influential factors which affect the determination of photometry of the galaxies. Nevertheless, no significant systematic differences have been found between of any pair of NIR magnitude measurements, except for a few percent of galaxies showing large discrepancies. This allows us to combine DENIS and 2MASS data for the J and Ks filters.Comment: 17 pages, 3 figures, 5 tables, published in Astrophysics, Vol. 52, No. 1, 2009 (English translation of Astrofizika

The Nearby Optical Galaxy Sample: The Local Galaxy Luminosity Function

In this paper we derive the galaxy luminosity function from the Nearby Optical Galaxy (NOG) sample, which is a nearly complete, magnitude-limited (B<14 mag), all-sky sample of nearby optical galaxies (~6400 galaxies with cz<5500 km/s). For this local sample, we use galaxy distance estimates based on different peculiar velocity models. Therefore, the derivation of the luminosity function is carried out using the locations of field and grouped galaxies in real distance space. The local field galaxy luminosity function in the B system is well described by a Schechter function. The exact values of the Schechter parameters slightly depend on the adopted peculiar velocity field models. The shape of the luminosity function of spiral galaxies does not differ significantly from that of E-S0 galaxies. On the other hand, the late-type spirals and irregulars have a very steeply rising luminosity function towards the faint end, whereas the ellipticals appreciably decrease in number towards low luminosities. The presence of galaxy systems in the NOG sample does not affect significantly the field galaxy luminosity function, since environmental effects on the total luminosity function appear to be marginal.Comment: 35 pages including 7 figures and 4 tables. Accepted for publication in Ap