Minor mergers and their impact on the kinematics of old and young stellar populations in disk galaxies

By means of N-body simulations we investigate the impact of minor mergers on the angular momentum and dynamical properties of the merger remnant. Our simulations cover a range of initial orbital characteristics and gas-to-stellar mass fractions (from 0 to 20%), and include star formation and supernova feedback. We confirm and extend previous results by showing that the specific angular momentum of the stellar component always decreases independently of the orbital parameters or morphology of the satellite, and that the decrease in the rotation velocity of the primary galaxy is accompanied by a change in the anisotropy of the orbits. However, the decrease affects only the old stellar population, and not the new population formed from gas during the merging process. This means that the merging process induces an increasing difference in the rotational support of the old and young stellar components, with the old one lagging with respect to the new. Even if our models are not intended specifically to reproduce the Milky Way and its accretion history, we find that, under certain conditions, the modeled rotational lag found is compatible with that observed in the Milky Way disk, thus indicating that minor mergers can be a viable way to produce it. The lag can increase with the vertical distance from the disk midplane, but only if the satellite is accreted along a direct orbit, and in all cases the main contribution to the lag comes from stars originally in the primary disk rather than from stars in the satellite galaxy. We also discuss the possibility of creating counter-rotating stars in the remnant disk, their fraction as a function of the vertical distance from the galaxy midplane, and the cumulative effect of multiple mergers on their creation.

A spectroscopic and photometric study of stellar populations in a sample of clusters in the Magellanic Clouds

This Ph.D. Thesis has been carried out in the framework of a long-term and large project devoted to describe the main photometric, chemical, evolutionary and integrated properties of a representative sample of Large and Small Magellanic Cloud (LMC and SMC respectively) clusters. The globular clusters system of these two Irregular galaxies provides a rich resource for investigating stellar and chemical evolution and to obtain a detailed view of the star formation history and chemical enrichment of the Clouds. The results discussed here are based on the analysis of high-resolution photometric and spectroscopic datasets obtained by using the last generation of imagers and spectrographs. The principal aims of this project are summarized as follows: • The study of the AGB and RGB sequences in a sample of MC clusters, through the analysis of a wide near-infrared photometric database, including 33 Magellanic globulars obtained in three observing runs with the near-infrared camera SOFI@NTT (ESO, La Silla). • The study of the chemical properties of a sample of MCs clusters, by using optical and near-infrared high-resolution spectra. 3 observing runs have been secured to our group to observe 9 LMC clusters (with ages between 100 Myr and 13 Gyr) with the optical high-resolution spectrograph FLAMES@VLT (ESO, Paranal) and 4 very young (<30 Myr) clusters (3 in the LMC and 1 in the SMC) with the near-infrared high-resolution spectrograph CRIRES@VLT. • The study of the photometric properties of the main evolutive sequences in optical Color- Magnitude Diagrams (CMD) obtained by using HST archive data, with the final aim of dating several clusters via the comparison between the observed CMDs and theoretical isochrones. The determination of the age of a stellar population requires an accurate measure of the Main Sequence (MS) Turn-Off (TO) luminosity and the knowledge of the distance modulus, reddening and overall metallicity. For this purpose, we limited the study of the age just to the clusters already observed with high-resolution spectroscopy, in order to date only clusters with accurate estimates of the overall metallicity.

A panchromatic study of stellar populations in Galactic Globular Clusters

The aim of this Thesis is to investigate (i) how common the bimodal Blue Straggler Stars (BSS) radial distribution is in stellar clusters and (ii) which are the physical processes that can produce this bimodality. We discuss possible relations between the properties of the BSS radial distribution and the dynamical state of the hosting clusters by making use of dynamical models and simulations. When relevant, we also discuss the possible links with some cluster "anomalies" and the effects of a massive object (like Imtermediate Mass Black Hole) in the cluster center. To this purpose we present the observational multiwavelength studies of the BSS populations and their radial distributions in 5 GGCs: M5, M55, M2, NGC 2419 and NGC 6388.

Multiple stellar populations in globular clusters with photometry and low resolution spectroscopy

Our view of Globular Clusters has deeply changed in the last decade. Modern spectroscopic and photometric data have conclusively established that globulars are neither coeval nor monometallic, reopening the issue of the formation of such systems. Their formation is now schematized as a two-step process, during which the polluted matter from the more massive stars of a first generation gives birth, in the cluster innermost regions, to a second generation of stars with the characteristic signature of fully CNO-processed matter. To date, star-to-star variations in abundances of the light elements (C, N, O, Na) have been observed in stars of all evolutionary phases in all properly studied Galactic globular clusters. Multiple or broad evolutionary sequences have also been observed in nearly all the clusters that have been observed with good signal-to-noise in the appropriate photometric bands. The body of evidence suggests that spreads in light-element abundances can be fairly well traced by photometric indices including near ultraviolet passbands, as CNO abundance variations affect mainly wavelengths shorter than ~400 nm owing to the rise of some NH and CN molecular absorption bands. Here, we exploit this property of near ultraviolet photometry to trace internal chemical variations and combined it with low resolution spectroscopy aimed to derive carbon and nitrogen abundances in order to maximize the information on the multiple populations. This approach has been proven to be very effective in (i) detecting multiple population, (ii) characterizing their global properties (i.e., relative fraction of stars, location in the color-magnitude diagram, spatial distribution, and trends with cluster parameters) and (iii) precisely tagging their chemical properties (i.e., extension of the C-N anticorrelation, bimodalities in the N content).

Resolved stellar populations in milky way and andromeda's new satellites

This thesis concerns the study of the variable stars and resolved stellar populations in four recently discovered dSphs, namely, Hercules and Ursa Major I (UMa I), which are UFD satellites of the MW; Andromeda XIX (And XIX) and Andromeda XXI (And XXI), which are satellites of M31. The main aim is to obtain detailed informations on the properties (age, metallicity, distance, and Oosterhoff type) of the stellar populations in these galaxies, to compare them with those of other satellites around the MW and M31, both ''classical'' dSphs and UFDs. The observables used to achieve these goals are the pulsating variables, especially the RR Lyrae stars, and the color magnitude diagram (CMD) of the resolved stellar populations. In particular, for UMa I, we combined B, V time-series observations from four different ground-based telescopes (Cassini, TLS, TT1 and Subaru) and for Hercules, we used archival data acquired with the Advanced Camera for Surveys (ACS) on board the HST. We used, instead B and V times-series photometry obtained with the Large Binocular Telescope (LBT) for And XIX and And XXI .

Stellar populations in bulges of spiral galaxies

We present a few results of a work in progress tackling the radial spectroscopic properties of bulges of spiral galaxies.

An analysis of the composite stellar population in M32

We obtained long-slit spectra of high signal-to-noise ratio of the galaxy M32 with the Gemini Multi-Object Spectrograph at the Gemini-North telescope. We analysed the integrated spectra by means of full spectral fitting in order to extract the mixture of stellar populations that best represents its composite nature. Three different galactic radii were analysed, from the nuclear region out to 2 arcmin from the centre. This allows us to compare, for the first time, the results of integrated light spectroscopy with those of resolved colour-magnitude diagrams from the literature. As a main result we propose that an ancient and an intermediate-age population co-exist in M32, and that the balance between these two populations change between the nucleus and outside one effective radius (1r(eff)) in the sense that the contribution from the intermediate population is larger at the nuclear region. We retrieve a smaller signal of a young population at all radii whose origin is unclear and may be a contamination from horizontal branch stars, such as the ones identified by Brown et al. in the nuclear region. We compare our metallicity distribution function for a region 1 to 2 arcmin from the centre to the one obtained with photometric data by Grillmair et al. Both distributions are broad, but our spectroscopically derived distribution has a significant component with [Z/Z(circle dot)] <= -1, which is not found by Grillmair et al.

Structure morphologique et populations stellaires d’un échantillon de galaxies spirales

Afin de caractériser la structure morphologique et les populations stellaires d’un échantillon de treize galaxies spirales, j’ai analysé des images WISE et GALEX, et j’ai construit des diagrammes magnitude-magnitude et couleur-magnitude pixel à pixel. Les diagrammes présentent des groupes de pixels qui correspondent spatialement aux composantes structurales des galaxies. Les diagrammes ainsi que les profils radiaux de brillance de surface indiquent que les variations de la densité surfacique de masse de la vieille population stellaire jouent un rôle important dans la différenciation des structures. On estime l’âge des jeunes complexes stellaires et l’extinction dans ces galaxies en les comparant à des modèles de populations stellaires simples nées de sursauts de formation stellaire instantanée. L’étude de ces propriétés est possible grâce à la combinaison des données ultraviolettes et infrarouge et à la grande sensibilité de la couleur ultraviolette à la variation de l’âge. On observe un gradient d’extinction dont la pente est liée à la présence d’une barre ou d’une activité nucléaire : en effet, l’extinction décroît avec la distance galactocentrique et la pente est plus petite pour les galaxies ayant une barre ou une activité nucléaire. On observe également un gradient d’âge où les régions externes sont moins évoluées que celles du centre sauf pour les galaxies de type tardif.

Swing amplification of nonaxisymmetric perturbations in stars and gas in a sheared galactic disk

We present a study of the growth of local, nonaxisymmetric perturbations in gravitationally coupled stars and gas in a differentially rotating galactic disk. The stars and gas are treated as two isothermal fluids of different velocity dispersions, with the stellar velocity dispersion being greater than that for the gas. We examine the physical effects of inclusion of a low-velocity dispersion component (gas) on the growth of non-axisymmetric perturbations in both stars and gas, as done for the axisymmetric case by Jog & Solomon. The amplified perturbations in stars and gas constitute trailing, material, spiral features which may be identified with the local spiral features seen in all spiral galaxies. The formulation of the two-fluid equations closely follows the one-fluid treatment by Goldreich & Lynden-Bell. The local, linearized perturbation equations in the sheared frame are solved to obtain the results for a temporary growth via swing amplification. The problem is formulated in terms of five dimensionless parameters-namely, the Q-factors for stars and gas, respectively; the gas mass fraction; the shearing rate in the galactic disk; and the length scale of perturbation. By using the observed values of these parameters, we obtain the amplifications and the pitch angles for features in stars and gas for dynamically distinct cases, as applicable for different regions of spiral galaxies. A real galaxy consisting of stars and gas may display growth of nonaxisymmetric perturbations even when it is stable against axisymmetric perturbations and/or when either fluid by itself is stable against non-axisymmetric perturbations. Due to its lower velocity dispersion, the gas exhibits a higher amplification than do the stars, and the amplified gas features are slightly more tightly wound than the stellar features. When the gas contribution is high, the stellar amplification and the range of pitch angles over which it can occur are both increased, due to the gravitational coupling between the two fluids. Thus, the two-fluid scheme can explain the origin of the broad spiral arms in the underlying old stellar populations of galaxies, as observed by Schweizer and Elmegreen & Elmegreen. The arms are predicted to be broader in gas-rich galaxies, as is indeed seen for example in M33. In the linear regime studied here, the arm contrast is shown to increase with radius in the inner Galaxy, in agreement with observations of external galaxies by Schweizer. These results follow directly due to the inclusion of gas in the problem.

Relevé spectroscopique et étude des propriétés physiques des étoiles naines blanches à moins de 40 parsecs du Soleil

Les étoiles naines blanches représentent la fin de l’évolution de 97% des étoiles de notre galaxie, dont notre Soleil. L’étude des propriétés globales de ces étoiles (distribution en température, distribution de masse, fonction de luminosité, etc.) requiert l’élaboration d’ensembles statistiquement complets et bien définis. Bien que plusieurs relevés d’étoiles naines blanches existent dans la littérature, la plupart de ceux-ci souffrent de biais statistiques importants pour ce genre d’analyse. L’échantillon le plus représentatif de la population d’étoiles naines blanches demeure à ce jour celui défini dans un volume complet, restreint à l’environnement immédiat du Soleil, soit à une distance de 20 pc (∼ 65 années-lumière) de celui-ci. Malheureusement, comme les naines blanches sont des étoiles intrinsèquement peu lumineuses, cet échantillon ne contient que ∼ 130 objets, compromettant ainsi toute étude statistique significative. Le but de notre étude est de recenser la population d’étoiles naines blanches dans le voisinage solaire a une distance de 40 pc, soit un volume huit fois plus grand. Nous avons ainsi entrepris de répertorier toutes les étoiles naines blanches à moins de 40 pc du Soleil à partir de SUPERBLINK, un vaste catalogue contenant le mouvement propre et les données photométriques de plus de 2 millions d’étoiles. Notre approche est basée sur la méthode des mouvements propres réduits qui permet d’isoler les étoiles naines blanches des autres populations stellaires. Les distances de toutes les candidates naines blanches sont estimées à l’aide de relations couleur-magnitude théoriques afin d’identifier les objets se situant à moins de 40 pc du Soleil, dans l’hémisphère nord. La confirmation spectroscopique du statut de naine blanche de nos ∼ 1100 candidates a ensuite requis 15 missions d’observations astronomiques sur trois grands télescopes à Kitt Peak en Arizona, ainsi qu’une soixantaine d’heures allouées sur les télescopes de 8 m des observatoires Gemini Nord et Sud. Nous avons ainsi découvert 322 nouvelles étoiles naines blanches de plusieurs types spectraux différents, dont 173 sont à moins de 40 pc, soit une augmentation de 40% du nombre de naines blanches connues à l’intérieur de ce volume. Parmi ces nouvelles naines blanches, 4 se trouvent probablement à moins de 20 pc du Soleil. De plus, nous démontrons que notre technique est très efficace pour identifier les étoiles naines blanches dans la région peuplée du plan de la Galaxie. Nous présentons ensuite une analyse spectroscopique et photométrique détaillée de notre échantillon à l’aide de modèles d’atmosphère afin de déterminer les propriétés physiques de ces étoiles, notamment la température, la gravité de surface et la composition chimique. Notre analyse statistique de ces propriétés, basée sur un échantillon presque trois fois plus grand que celui à 20 pc, révèle que nous avons identifié avec succès les étoiles les plus massives, et donc les moins lumineuses, de cette population qui sont souvent absentes de la plupart des relevés publiés. Nous avons également identifié plusieurs naines blanches très froides, et donc potentiellement très vieilles, qui nous permettent de mieux définir le côté froid de la fonction de luminosité, et éventuellement l’âge du disque de la Galaxie. Finalement, nous avons aussi découvert plusieurs objets d’intérêt astrophysique, dont deux nouvelles étoiles naines blanches variables de type ZZ Ceti, plusieurs naines blanches magnétiques, ainsi que de nombreux systèmes binaires non résolus.

Intragroup diffuse light in compact groups of galaxies - II. HCG 15, 35 and 51

This continuing study of intragroup light in compact groups of galaxies aims to establish new constraints to models of formation and evolution of galaxy groups, specially of compact groups, which are a key part in the evolution of larger structures, such as clusters. In this paper we present three additional groups (HCG 15, 35 and 51) using deep wide-field B- and R-band images observed with the LAICA camera at the 3.5-m telescope at the Calar Alto observatory (CAHA). This instrument provides us with very stable flat-fielding, a mandatory condition for reliably measuring intragroup diffuse light. The images were analysed with the OV_WAV package, a wavelet technique that allows us to uncover the intragroup component in an unprecedented way. We have detected that 19, 15 and 26 per cent of the total light of HCG 15, 35 and 51, respectively, are in the diffuse component, with colours that are compatible with old stellar populations and with mean surface brightness that can be its low as 28.4 B mag arcsec(-2). Dynamical masses, crossing times and mass-to-light ratios were recalculated using the new group parameters. Also tidal features were analysed using the wavelet technique.

GALAXY EVOLUTION THROUGH THE COSMIC TIME

Elliptical galaxies are the best systems to study the early star formation activity in the universe. This work aims to understand the formation and evolution of these objects through the study of the integrated properties of their stellar populations. Here an evolutionary model is developed and their predicted spectrophotometric properties are presented.

Probing the Large Magellanic Cloud age gap at intermediate cluster masses

The Large Magellanic Cloud (LMC) has a rich star cluster system spanning a wide range of ages and masses. One striking feature of the LMC cluster system is the existence of an age gap between 3 and 10 Gyr. But this feature is not clearly seen among field stars. Three LMC fields containing relatively poor and sparse clusters whose integrated colours are consistent with those of intermediate-age simple stellar populations have been imaged in BVI with the Optical Imager (SOI) at the Southern Telescope for Astrophysical Research (SOAR). A total of six clusters, five of them with estimated initial masses M < 104 M(circle dot), were studied in these fields. Photometry was performed and colour-magnitude diagrams (CMDs) were built using standard point spread function fitting methods. The faintest stars measured reach V similar to 23. The CMD was cleaned from field contamination by making use of the three-dimensional colour and magnitude space available in order to select stars in excess relative to the field. A statistical CMD comparison method was developed for this purpose. The subtraction method has proven to be successful, yielding cleaned CMDs consistent with a simple stellar population. The intermediate-age candidates were found to be the oldest in our sample, with ages between 1 and 2 Gyr. The remaining clusters found in the SOAR/SOI have ages ranging from 100 to 200 Myr. Our analysis has conclusively shown that none of the relatively low-mass clusters studied by us belongs to the LMC age gap.

Alternative diagnostic diagrams and the `forgotten` population of weak line galaxies in the SDSS

A numerous population of weak line galaxies (WLGs) is often left out of statistical studies on emission-line galaxies (ELGs) due to the absence of an adequate classification scheme, since classical diagnostic diagrams, such as [O iii]/H beta versus [N ii]/H alpha (the BPT diagram), require the measurement of at least four emission lines. This paper aims to remedy this situation by transposing the usual divisory lines between star-forming (SF) galaxies and active galactic nuclei (AGN) hosts and between Seyferts and LINERs to diagrams that are more economical in terms of line quality requirements. By doing this, we rescue from the classification limbo a substantial number of sources and modify the global census of ELGs. More specifically, (1) we use the Sloan Digital Sky Survey Data Release 7 to constitute a suitable sample of 280 000 ELGs, one-third of which are WLGs. (2) Galaxies with strong emission lines are classified using the widely applied criteria of Kewley et al., Kauffmann et al. and Stasinska et al. to distinguish SF galaxies and AGN hosts and Kewley et al. to distinguish Seyferts from LINERs. (3) We transpose these classification schemes to alternative diagrams keeping [N ii]/H alpha as a horizontal axis, but replacing H beta by a stronger line (H alpha or [O ii]), or substituting the ionization-level sensitive [O iii]/H beta ratio with the equivalent width of H alpha (W(H alpha)). Optimized equations for the transposed divisory lines are provided. (4) We show that nothing significant is lost in the translation, but that the new diagrams allow one to classify up to 50 per cent more ELGs. (5) Introducing WLGs in the census of galaxies in the local Universe increases the proportion of metal-rich SF galaxies and especially LINERs. In the course of this analysis, we were led to make the following points. (i) The Kewley et al. BPT line for galaxy classification is generally ill-used. (ii) Replacing [O iii]/H beta by W(H alpha) in the classification introduces a change in the philosophy of the distinction between LINERs and Seyferts, but not in its results. Because the W(H alpha) versus [N ii]/H alpha diagram can be applied to the largest sample of ELGs without loss of discriminating power between Seyferts and LINERs, we recommend its use in further studies. (iii) The dichotomy between Seyferts and LINERs is washed out by WLGs in the BPT plane, but it subsists in other diagnostic diagrams. This suggests that the right wing in the BPT diagram is indeed populated by at least two classes, tentatively identified with bona fide AGN and `retired` galaxies that have stopped forming stars and are ionized by their old stellar populations.

Multi-Conjugate Adaptive Optics VLT imaging of the distant old open cluster FSR 1415

We employ the recently installed near-infrared Multi-Conjugate Adaptive Optics demonstrator (MAD) to determine the basic properties of a newly identified, old and distant, Galactic open cluster (FSR 1415). The MAD facility remarkably approaches the diffraction limit, reaching a resolution of 0.07 arcsec (in K), that is also uniform in a field of similar to 1.8 arcmin in diameter. The MAD facility provides photometry that is 50 per cent complete at K similar to 19. This corresponds to about 2.5 mag below the cluster main-sequence turn-off. This high-quality data set allows us to derive an accurate heliocentric distance of 8.6 kpc, a metallicity close to solar and an age of similar to 2.5 Gyr. On the other hand, the deepness of the data allows us to reconstruct (completeness-corrected) mass functions (MFs) indicating a relatively massive cluster, with a flat core MF. The Very Large Telescope/MAD capabilities will therefore provide fundamental data for identifying/analysing other faint and distant open clusters in the Galaxy III and IV quadrants.