Harmonic analysis of the Ha velocity field of NGC 4254

The ionized gas kinematics of the Virgo Cluster galaxy NGC 4254 (Messier 99) is analyzed by an harmonic decomposition of the velocity field into Fourier coefficients. The aims of this study are to measure the kinematical asymmetries of Virgo cluster galaxies and to connect them to the environment. The analysis reveals significant $m=1,2,4$ terms which origins are discussed.Comment: 3 pages, 2 figures, to appear in "Science Perspectives for 3D Spectroscopy", ESO Astrophysics Symposia, M. Kissler-Patig, M.M. Roth & J.R. Walsh ed

Structure, mass and distance of the Virgo cluster from a Tolman-Bondi model

We have applied a relativistic Tolman-Bondi model of the Virgo cluster to a sample of 183 galaxies with measured distances within a radius of 8 degrees from M87. We find that the sample is significantly contaminated by background galaxies which lead to too large a cluster mean distance if not excluded. The Tolman-Bondi model predictions, together with the HI deficiency of spiral galaxies, allows one to identify these background galaxies. One such galaxy is clearly identified among the 6 calibrating galaxies with Cepheid distances. As the Tolman-Bondi model predicts the expected distance ratio to the Virgo distance, this galaxy can still be used to estimate the Virgo distance, and the average value over the 6 galaxies is 15.4 +- 0.5 Mpc. Well-known background groups of galaxies are clearly recovered, together with filaments of galaxies which link these groups to the main cluster, and are falling into it. No foreground galaxy is clearly detected in our sample. Applying the B-band Tully-Fisher method to a sample of 51 true members of the Virgo cluster according to our classification gives a cluster distance of 18.0 +- 1.2 Mpc, larger than the mean Cepheid distance. Finally, the same model is used to estimate the Virgo cluster mass, which is M = 1.2 10^{15} Msun within 8 degrees from the cluster center (2.2 Mpc radius), and amounts to 1.7 virial mass.Comment: 12 pages, 7 figures. Astronomy and Astrophysics, in press (accepted May 31, 2001

The K luminosity-metallicity relation for dwarf galaxies and the tidal dwarf galaxies in the tails of HCG 31

We determine a K-band luminosity-metallicity (K-Z) relation for dwarf irregular galaxies, over a large range of magnitudes, -20.5 < M_K < -13.5, using a combination of K photometry from either the 2-micron all sky survey (2MASS) or the recent study of Vadivescu er al. (2005), and metallicities derived mainly with the T_e method, from several different studies. We then use this newly-derived relation, together with published K_s photometry and our new spectra of objects in the field of HCG 31 to discuss the nature of the possible tidal dwarf galaxies of this group. We catalogue a new member of HCG 31, namely "R", situated ~40 kpc north of the group center, composed by a ring of H alpha knots which coincides with a peak in HI. This object is a deviant point in the K-Z relation (it has too high metallicity for its luminosity) and its projected distance to the parent galaxy and large gas reservoir makes it one of the most promising tidal dwarf galaxy candidates of HCG 31, together with object F. The subsystems A1, E, F, H and R all have metallicities similar to that of the galaxies A+C and B, result that is expected in a scenario where those were formed from material expelled from the central galaxies of HCG 31. While objects A1, E and H will most probably fall back onto their progenitors, F and R may survive as tidal dwarf galaxies. We find that two galaxies of HCG 31, G and Q, have A+em spectral signatures, and are probably evolving toward a post-starburst phase.Comment: 32 pages, 4 figures - Submitted to AJ - A version of this paper with full resolution figures can be found at http://www.astro.iag.usp.br/~eduardo/HCG31-KZrelation.pd

Astronomy Education in France

International audienceAfter asking why there are less students in science and how the situation could be improved, a description of Astronomy Education in France at different levels will be given

Formation des galaxies lenticulaires

Les galaxies lenticulaires, systèmes plats sans bras spiraux possèdent à la fois des propriétés des galaxies elliptiques et des galaxies spirales.Leur mode de formation fait actuellement l’objet de nombreux débats. Elles pourraient soit résulter des conditions initiales particulières subies lors de l’effondrement de leur disque soit être d’anciennes spiralés ayant perdu leur gaz sous l’action de leur environnement.Une théorie qui concilie ces 2 points de vue est à l’heure actuelle celle qui répond le mieux aux contraintes observationnelles

Environmental Effects on Galaxies in Clusters and Groups

International audienceGalaxies in clusters and groups are perturbed by the effects of the environment due to ram pressure stripping of their HI gas by an intra-cluster X-ray gas or to gravitational interactions between galaxies or with the cluster potential well. We will review the most recent results concerning some galaxies in the Virgo Cluster and in the Stephan's Quintet and show how extragalactic HII regions can be formed from debris stripped by ram pressure or tidal interactions

Environmental Effects on Galaxies in Clusters and Groups

International audienceGalaxies in clusters and groups are perturbed by the effects of the environment due to ram pressure stripping of their HI gas by an intra-cluster X-ray gas or to gravitational interactions between galaxies or with the cluster potential well. We will review the most recent results concerning some galaxies in the Virgo Cluster and in the Stephan's Quintet and show how extragalactic HII regions can be formed from debris stripped by ram pressure or tidal interactions

The effects of ram pressure stripping on cluster galaxies

International audienceWe investigate the effects of ram pressure stripping of spiral galaxies using a numerical model that is directly confronted with interferometric observations. The current status of our research is presented

Recherche de galaxies de faible brillance en infra-rouge proche

Depuis ces 20 dernières années, les galaxies de faible brillance de surface (LSB) sont un sujet d'étude en constante évolution. Alors que selon les travaux de Freeman (1970) l'existence de galaxies de faible brillance de surface était improbable, les sondages de ces dix dernières années ont montré que des galaxies de brillance de surface toujours plus faibles existent. Ces galaxies sont même majoritaires dans la population totale de galaxies. Malgrès leur importance, elles demeurent encore mal connues et mal étudiées. Nous avons entrepris une recherche de galaxies LSB dans l'infrarouge proche en sélectionnant de manière homogène 4000 de ces objets dans le but d'étudier leurs propriétés. Les premières analyses des données multilongueur d'onde (infrarouges, HI et photométriques de surface de sous échantillons) ont montré que: (1) Les galaxies sélectionnées ont des brillances de surface centrale en bande Ks comparables aux brillances de surface centrale des galaxies LSB sélectionnées en optique; (2) Les galaxies LSB infrarouges de l'échantillon sont majoritairement bleues dont une partie de couleurs bleues extrêmes. Nous n'avons pas trouvé un grand nombre de galaxies LSB rouges; (3) Les rapports masse de gaz H I/luminosité de nos objets sont similaires à ceux des galaxies HSB et plus faibles que ceux habituellement trouvés pour les galaxies LSB. Nous avons trouvé une corrélation entre le rapport M_HI/L et la magnitude absolue en Ks ; (4) Nous avons identifié une cinquantaine de galaxies riches en gaz HI (M_HI > 10^10 M[Soleil]); (5) Nous avons utilisé nos données ainsi que des données déjà existantes sur d'autres échantillons de galaxies LSB sélectionnées en optique pour une comparaison avec les résultats d'un modèle d'évolution spectro-photométrique et chimique de disques LSB. Les résultats de ce modèle sont en accord avec la majorité des propriétés des divers échantillons.At present, the Low Surface Brightness galaxies (LSB) constitute the least well known fraction of galaxies. Nevertheless, they represent the most important population of galaxies. We selected an homogenous sample of about 4000 LSB galaxies using the all-sky near-infrared (J, H and Ks-band) 2MASS survey brightness galaxies in order to study their properties. The first results of the multiwavelength observations (infrared, optical and HI data) are: (1) The central surface brightness of the selected galaxies (in Ks band) are similar to the central surface brightness of the optical selected LSB galaxy; (2) These infrared LSBs are blue in majority with some extremely blue galaxies. Not many red LSBs were found; (3) The mass HI to light ratio of the infrared LSB are similar to the ratio of High Surface Brightness galaxies. A correlation appears between the ratio M_HI/L and the absolute magnitude in Ks band; (4) 50 infrared LSBs observed in HI present a mass of gaz HI greater than 10^10 M[Sun] ; (5) A comparison between a large range of LSB galaxies (optical and infrared selected) and a simple model of formation of LSB disk has been done and the majority of the properties of LSBs are well reproduced.ORSAY-PARIS 11-BU Sciences (914712101) / SudocMEUDON-Observatoire (920482302) / SudocSudocFranceF