Rozprawy doktorskie na temat „Dwarf Satellites”

The dynamical histories of three Galactic satellite systems have been investigated. These include the Fornax and Sculptor dwarf spheroidal galaxies, and the globular cluster Omega Centauri. Wide-field CCD photometry was obtained for these objects in two colours, V and I, covering a total sky area of 30 square degrees. These data were used to construct a colour-magnitude diagram for each system, which then provided a filter to select probable member stars. This technique reduced contamination from foreground stars, allowing a comprehensive examination of the poorly-known outer regions of these three Galactic satellites.¶ An initial analysis of the central region of Fornax was made using the deep photometric data provided by Stetson et al. The analysis revealed a shell-like feature located approximately 1.5 core radii southeast from the centre of the dwarf galaxy, with an integrated luminosity of M_V ~ -4. The colour-magnitude data for this shell indicate it to be dominated by stars with an age ~2 Gyr.¶ The complete analysis of Fornax utilised two colour data covering a 3.2 deg times 3.2 deg area on the sky. The colour-magnitude selection technique revealed a second shell-like feature situated 1.3 deg northwest from the Fornax centre, approximately 30' beyond the nominal tidal radius at this position angle. This feature displays an integrated luminosity of M_V ~ -7. The alignment of this second shell is parallel to the original shell, and both are situated on the minor axis. Additionally, a statistical analysis of the extra-tidal region of Fornax revealed two large, faint surface brightness structures located on the minor axis. These structures, combined with the two shells, present strong evidence for shell structure in Fornax. This is the first such structure observed in a dwarf galaxy, and implies that Fornax has experienced a merger event in the recent past.¶ The photometric survey of Sculptor was complete to the depth of the horizontal branch stars. The red horizontal branch stars were found to be significantly more concentrated than the blue horizontal branch stars. An analysis of the distribution of red giant branch-selected stars revealed no significant extra-tidal structure. To further test this result, spectra at far red wavelengths were obtained for over 700 candidate red giant stars over the 10 sq deg region using the 2dF instrument on the Anglo-Australian Telescope. Radial velocities and metallicities for these stars were measured using Ca ii triplet lines, providing additional constraints to select Sculptor members beyond the nominal tidal radius. The distribution of the 179 probable Sculptor members indicated a lack of extra-tidal stars. These results support, at most, a mild level of interaction between this system and the Galaxy. An upper mass limit for extra-tidal material was measured to be 10% of the Sculptor luminous mass.¶ The analysis of Omega Centauri was similar to that of Sculptor. V and I band photometry was obtained to search for the tidal tails proposed by a previous study. As noted by others, dust correction was found to remove these structures. Spectra covering the region 370-450 nm were obtained for approximately 4000 candidate cluster members, and radial velocities were used to distinguish members from field stars. A total of 24 probable members of Omega Cen were found in the extra-tidal regions. Hence, if Omega Cen does possess tidal tails, they comprise at most 1% of the cluster mass.

This PhD project is aimed at investigating the chemical composition of the stellar populations in the closest satellites of the Milky Way (MW), namely the Large and Small Magellanic Cloud (LMC and SMC, respectively) and the remnant of the Sagittarius (Sgr) dwarf spheroidal galaxy. Their proximity allows us to resolve their individual stars both with spectroscopy and photometry, studying in detail the characteristics of their stellar populations. All these objects are interacting galaxies: LMC and SMC are in an early stage of a minor merger event, and Sgr is being disrupted by the tidal field of the MW. There is a plenty of literature regarding the chemical composition of these systems, however, the extension of these galaxies prevents a complete and homogeneous analysis. Therefore, we homogeneously analysed stellar spectra belonging to MW and its satellites galaxies and we derived their chemical compositions. We highlighted the importance of a homogeneous analysis in the comparison among different galaxies or different samples, to avoid systematics due to different methods or physical assumptions.

Transmission spectra are differential measurements that utilize stellar illumination to probe transiting exoplanet atmospheres. Any spectral difference between the illuminating light source and the disk-integrated stellar spectrum due to starspots and faculae will be imprinted in the observed transmission spectrum. However,. few constraints exist for the extent of photospheric heterogeneities in M dwarfs. Here we model spot and faculae covering fractions consistent with observed photometric variabilities for M dwarfs and the associated 0.3-5.5. mu m stellar contamination spectra. We find that large ranges of spot and faculae covering fractions are consistent with observations and corrections assuming a linear relation between variability amplitude, and covering fractions generally underestimate the stellar contamination. Using realistic estimates for spot and faculae covering fractions, we find that stellar contamination can be more than 10x. larger than the transit depth changes expected for atmospheric features in rocky exoplanets. We also find that stellar spectral contamination can lead to systematic errors in radius and therefore the derived density of small planets. In the case of the TRAPPIST-1 system, we show that TRAPPIST-1 ' s rotational variability is consistent with spot covering fractions f(spot) = 8(7)(+18)% and faculae covering fractions f(fac) = 54(-46)(+16)%. The associated stellar contamination signals alter the transit depths of the TRAPPIST-1 planets at wavelengths of interest for planetary atmospheric species by roughly 1-15x. the strength of planetary features, significantly complicating JWST follow-up observations of this system. Similarly, we find that stellar contamination can lead to underestimates of the bulk densities of the TRAPPIST-1 planets of Delta(rho) = -8(-20)(+7)%, thus leading to overestimates of their volatile contents.

Disks around brown dwarfs (BDs) are excellent laboratories to study the first steps of planet formation in cold and low-mass disk conditions. The radial-drift velocities of dust particles in BD disks higher than in disks around more massive stars. Therefore, BD disks are expected to be more depleted in millimeter-sized grains compared to disks around T Tauri or Herbig Ae/Be stars. However, recent millimeter observations of BD disks revealed low millimeter spectral indices, indicating the presence of large grains in these disks and challenging models of dust evolution. We present 3 mm photometric observations carried out with the IRAM/Plateau de Bure Interferometer (PdBI) of three BD disks in the Taurus star-forming region, which have been observed with ALMA at 0.89 mm. The disks were not resolved and only one was detected with enough confidence (similar to 3.5 sigma) with PdBI. Based on these observations, we obtain the values and lower limits of the spectral index and find low values (alpha(mm) less than or similar to 3.0). We compare these observations in the context of particle trapping by an embedded planet, a promising mechanism to explain the observational signatures in more massive and warmer disks. We find, however, that this model cannot reproduce the current millimeter observations for BD disks, and multiple-strong pressure bumps globally distributed in the disk remain as a favorable scenario to explain observations. Alternative possibilities are that the gas masses in the BD disk are very low (similar to 2 x 10(-3) M-Jup) such that the millimeter grains are decoupled and do not drift, or fast growth of fluffy aggregates.

A l’échelle galactique, le paradigme lambda-CDM n’est pas prédictif. Afin d’approfondir nos connaissances dans cette gamme de taille, les satellites du Groupe Local (GL) sont les systèmes galactiques les plus simples et les plus proches pour tester nos différentes hypothèses. Ainsi, nous présentons d’abord une méthode permettant d’obtenir analytiquement l’ellipticité intrinsèque des galaxies naines. Les résultats de cette technique appliquée sur un échantillon de 25 satellites de la galaxie Andromède (M31) laissent présumer que le GL est plus perturbé qu’il n’était envisagé jusqu’alors. Après cette approche individuelle, nous exposons un résultat sur la cinématique globale du système M31. Cette estimation montre pour la première fois que la vitesse transverse de ce système par rapport à la Voie Lactée est élevée. Cela peut mener à de fortes implications sur le GL, notamment quant à la détermination de sa masse et de son évolution passée et future
The Lambda-CDM cosmological model represents nowadays the best understanding of the formation and the evolution of large scale structures in our Universe. Nevertheless, this paradigm is not predictive and successful yet at smaller scales. In this context, satellites in the Local Group (LG), the simpler and closer galactic systems, are one of our best chance to test this model and to improve our comprehension of galaxy formation. Thus, we present here a method to derive analytically the intrinsic (3D) morphology of dwarf galaxies. Results of this technic applied to 25 Andromeda (M31) satellites suggest that the LG is in fact more disturbed than what was previously thought. After this individual approach, we further expose a recent result on the global kinematics of the M31 system. This new estimation suggests for the first time a high transverse velocity for this system with respect to the Milky Way. These values could lead to redefine the entire dynamic of the LG and its surroundings

We present multi-wavelength radio observations obtained with the VLA of the protoplanetary disk surrounding the young brown dwarf 2MASS J04442713+2512164 (2M0444) in the Taurus star-forming region. 2M0444 is the brightest known brown dwarf disk at millimeter wavelengths, making this an ideal target to probe radio emission from a young brown dwarf. Thermal emission from dust in the disk is detected at 6.8 and 9.1 mm, whereas the 1.36 cm measured flux is dominated by ionized gas emission. We combine these data with previous observations at shorter sub-mm and mm wavelengths to test the predictions of dust evolution models in gas-rich disks after adapting their parameters to the case of 2M0444. These models show that the radial drift mechanism affecting solids in a gaseous environment has to be either completely made inefficient, or significantly slowed down by very strong gas pressure bumps in order to explain the presence of mm/cm-sized grains in the outer regions of the 2M0444 disk. We also discuss the possible mechanisms for the origin of the ionized gas emission detected at 1.36 cm. The inferred radio luminosity for this emission is in line with the relation between radio and bolometric luminosity valid for for more massive and luminous young stellar objects, and extrapolated down to the very low luminosity of the 2M0444 brown dwarf.

This study focuses on the spatial distribution of the M31 satellite system. A new Bayesian technique for determining object distances from the Tip of their Red Giant Branch is developed and used to obtain distance probability distributions for M31and 27 of its satellite galaxies. These distances are then used to calculate the satellite positions in three dimensions. Subsequent analysis of the resulting spatial distribution reveals striking inhomogeneity, with roughly half of the satellites confined to a curiously oriented thin disk. The distribution is also markedly asymmetric, with the majority of satellites lying on the Milky Way side of M31.

Comprendre l’évolution des galaxies suppose d’expliquer leur apparence, qui résulte de leur contenu stellaire. Or les étoiles se forment à partir de gaz froid. Les processus de rétroaction baryonique peuvent empêcher la formation d’étoiles en chauffant le gaz, en générant des vents galactiques qui expulsent le gaz ou l’empêchent de parvenir dans la galaxie. Sans ces processus, les modèles peinent à reproduire les observations. Certaines étoiles, lorsqu’elles explosent en supernovae (SNe), sont considérées comme la source principale de rétroaction dans les plus petites galaxies – les galaxies naines. Cependant, selon des travaux récents, leur capacité de rétroaction est insuffisante. Par ailleurs, les galaxies naines font l'objet de divergences entre théorie et observations concernant la nature de la matière noire, qui reste méconnue. Il se peut que ces divergences soient résolues en invoquant des processus de rétroaction. En parallèle, les noyaux actifs – des trous noirs massifs qui croissent –, constituent une autre source de rétroaction. Des observations récentes suggèrent qu’ils pourraient avoir de l’importance dans les galaxies naines. Premièrement, je montre analytiquement que la capacité des noyaux actifs à générer des vents galactiques dépasse souvent celle des SNe. Ensuite, je montre numériquement que le noyau actif d’une galaxie peut impacter l’évolution galaxie voisine. Enfin, je montre par une série de simulations numériques que l’injection de rayonnement cosmique par les SNe réduit la formation d’étoiles et augmente la génération de vents, qui sont alors en bien meilleur accord avec les observations, bien qu'encore en deçà
Understanding galaxies calls for the understanding of their appearance, which results from their stellar content, and stars form out of cold gas. Feedback processes can impede star formation: by heating the gas or preventing it from cooling and forming stars; by triggering galactic outflows that eject the gas; and by preventing the inflow of gas. In the lowest mass galaxies – dwarf galaxies –powerful and luminous stellar explosions, supernovae (SNe), are thought to be the predominant feedback process. However, recent work suggests that SN feedback is too weak. Moreover, several puzzles surrounding dwarf galaxies challenge our understanding of dark matter – of which the exact nature that is still unknown –, and these discrepancies might be solved by feedback processes. Besides, Active Galactic Nuclei (AGN) – i.e. growing massive black holes –, are another source of feedback in galaxies. The effect of AGN in dwarf galaxies is often thought to be minimal, because their growth is suppressed. But there is growing evidence that their presence in dwarf galaxies might be stronger than once thought. First, I show, using analytical methods, that the capacity of AGN to trigger outflows in dwarf galaxies is stronger than that of SNe in most of the parameter space. I then show numerically, that the AGN of a galaxy can influence the evolution of a neighboring galaxy. Finally, I assess the impact of the injection of cosmic rays by SNe: I find that it reduces the rate of star formation and increases the efficiency of SNe at driving winds, bringing simulated wind properties of dwarf galaxies much closer to observations, but the effect is not sufficient to fully match the observations

The growing number of observations of brown dwarfs (BDs) has provided evidence for strong atmospheric circulation on these objects. Directly imaged planets share similar observations and can be viewed as low-gravity versions of BDs. Vigorous condensate cycles of chemical species in their atmospheres are inferred by observations and theoretical studies, and latent heating associated with condensation is expected to be important in shaping atmospheric circulation and influencing cloud patchiness. We present a qualitative description of the mechanisms by which condensational latent heating influences circulation, and then illustrate them using an idealized general circulation model that includes a condensation cycle of silicates with latent heating and molecular weight effect due to the rainout of the condensate. Simulations with conditions appropriate for typical T dwarfs exhibit the development of localized storms and east-west jets. The storms are spatially inhomogeneous, evolving on a timescale of hours to days and extending vertically from the condensation level to the tropopause. The fractional area of the BD covered by active storms is small. Based on a simple analytic model, we quantitatively explain the area fraction of moist plumes and show its dependence on the radiative timescale and convective available potential energy (CAPE). We predict that if latent heating dominates cloud formation processes, the fractional coverage area of clouds decreases as the spectral type goes through the L/T transition from high to lower effective temperature. This is a natural consequence of the variation of the radiative timescale and CAPE with the spectral type.

We present simultaneous Hubble Space Telescope (HST) WFC3+Spitzer IRAC variability monitoring for the highly variable young (similar to 20 Myr) planetary-mass object PSO J318.5-22. Our simultaneous HST + Spitzer observations covered approximately two rotation periods with Spitzer and most of a rotation period with the HST. We derive a period of 8.6. +/-. 0.1 hr from the Spitzer light curve. Combining this period with the measuredvsinifor this object, we find an inclination of 56 degrees.2. +/-. 8 degrees.1. We measure peak-to-trough variability amplitudes of 3.4%. +/-. 0.1% for Spitzer Channel 2 and 4.4%-5.8% (typical 68% confidence errors of similar to 0.3%) in the near-IR bands (1.07-1.67 mu m) covered by the WFC3 G141 prism-the mid-IR variability amplitude for PSO J318.5-22 is one of the highest variability amplitudes measured in the mid-IR for any brown dwarf or planetary-mass object. Additionally, we detect phase offsets ranging from 200 degrees to 210 degrees (typical error of similar to 4 degrees) between synthesized near-IR light curves and the Spitzer mid-IR light curve, likely indicating depth-dependent longitudinal atmospheric structure in this atmosphere. The detection of similar variability amplitudes in wide spectral bands relative to absorption features suggests that the driver of the variability may be inhomogeneous clouds (perhaps a patchy haze layer over thick clouds), as opposed to hot spots or compositional inhomogeneities at the top-of-atmosphere level.

M dwarfs constitute around 70% of all stars in the local Galaxy. Their multitude together with their long main-sequence lifetimes make them important for studies of global properties of the Galaxy such as the initial mass function or the structure and kinematics of stellar populations. In addition, the exoplanet community is showing an increasing interest for those small, cold stars. However, very few M dwarfs are well characterized, and in the case of exoplanetary systems the stellar parameters have a direct influence on the derived planet properties. Stellar parameters of M dwarfs are difficult to determine because of their low surface temperatures that result in an optical spectrum dominated by molecular lines. Most previous works have therefore relied on empirical calibrations. High-resolution spectrographs operating in the infrared, a wavelength region less affected by molecular lines, have recently opened up a new window for the investigation of M dwarfs. In the two first papers of this thesis we have shown that we can determine the metallicity, and in some cases the effective temperature, using synthetic spectral fitting with improved accuracy. This method is time consuming and therefore not practical or even feasible for studies of large samples of M dwarfs. When comparing our results from the high-resolution studies with available photometric calibrations we find systematic differences. In the third paper we therefore used our sample to determine a new photometric metallicity calibration. Compared to previous calibrations our new photometric calibration shows improved statistical characteristics, and our calibration gives similar results as spectroscopic calibrations. In a comparison with theoretical calculations we find a good agreement of the shapes and slopes of iso-metallicity lines with our empirical relation. Applying the photometric calibration to a sample of M dwarfs with confirmed exoplanets we find a possible giant planet-metallicity correlation for M dwarfs.

The Great Barrier Reef hosts the only known reliable aggregation of dwarf minke whale (Balaenoptera acutorostrata subspecies) in Australian waters. While this short seasonal aggregation is quite predictable, the distribution and movements of the whales during the rest of their annual cycle are poorly understood. In particular, feeding and resting areas on their southward migration which are likely to be important have not been described. Using satellite telemetry data, I modelled the habitat use of seven whales during their southward migration through waters surrounding Tasmania. The whales were tagged with LIMPET satellite tags in the GBR in July 2013 (2 individuals) and 2014 (5 individuals). The study area around Tasmania was divided into 10km² cells and the time spent by each individual in each cell was calculated and averaged based on the number of animals using the cell. Two areas of high residency time were highlighted: south-western Bass Strait and Storm Bay (SE Tasmania). Remotely sensed ocean data were extracted for each cell and averaged temporally during the entire period of residency. Using Generalised Additive Models I explored the influence of key environmental characteristics. Nine predictors (bathymetry, distance from coast, distance from shore, gradient of sea surface temperature, sea surface height (absolute and variance), gradient of current speed, wind speed and chlorophyll-a concentration) were retained in the final model which explained 68% of the total variance. Regions of higher time-spent values were characterised by shallow waters, proximity to the coast (but not to the shelf break), high winds and sea surface height but low gradient of sea surface temperature. Given that the two high residency areas corresponded with regions where other marine predators also forage in Bass Strait and Storm Bay, I suggest the whales were probably feeding, rather than resting in these areas.

The Hubble Space Telescope Wide Field Camera 3 (WFC3) near-IR channel is extensively used in time-resolved observations, especially for transiting exoplanet spectroscopy as well as. brown dwarf and directly imaged exoplanet rotational phase mapping. The ramp effect is the dominant source of systematics in the WFC3 for time-resolved observations, which limits its photometric precision. Current mitigation strategies are based on empirical fits and require additional orbits to help the telescope reach a thermal equilibrium. We show that the ramp-effect profiles can be explained and corrected with high fidelity using charge trapping theories. We also present a model for this process that can be used to predict and to correct charge trap systematics. Our model is based on a very small number of parameters that are intrinsic to the detector. We find that these parameters are very stable between the different data sets, and we provide best-fit values. Our model is tested with more than 120 orbits (similar to 40 visits) of WFC3 observations. and is proved to be able to provide near photon noise limited corrections for observations made with both staring and scanning modes of transiting exoplanets as well as for starting-mode observations of brown dwarfs. After our model correction, the light curve of the first orbit in each visit has the same photometric precision as subsequent orbits, so data from the first orbit no longer need. to. be discarded. Near-IR arrays with the same physical characteristics (e.g., JWST/NIRCam) may also benefit from the extension of this model if similar systematic profiles are observed.

We present the survey strategy and early results of the "Satellites Around Galactic Analogs" (SAGA) Survey. The SAGA. Survey's goal is to measure the distribution of satellite galaxies around 100 systems analogous to the Milky Way down to the luminosity of the Leo I dwarf galaxy (M-r < -12.3). We define a Milky Way analog based on K-band luminosity and local environment. Here, we present satellite luminosity functions for eight Milky-Way-analog galaxies between 20 and 40. Mpc. These systems have nearly complete spectroscopic coverage of candidate satellites within the projected host virial radius down to r(o) < 20.75 using low-redshift gri color criteria. We have discovered a total of 25 new satellite galaxies: 14. new satellite galaxies meet our formal criteria around our complete host systems, plus 11 additional satellites in either incompletely surveyed hosts or below our formal magnitude limit. Combined with 13 previously known satellites, there are a total of 27 satellites around 8 complete Milky-Way-analog hosts. We find a wide distribution in the number of satellites per host, from 1 to 9, in the luminosity range for which there are 5 Milky Way satellites. Standard abundance matching extrapolated from higher luminosities predicts less scatter between hosts and a steeper luminosity function slope than observed. We find that the majority of satellites (26 of 27) are star-forming. These early results indicate that the Milky Way has a different satellite population than typical in our sample, potentially changing the physical interpretation of measurements based only on the Milky Way's satellite galaxies.

We present a chemodynamical analysis of the Leo V dwarf galaxy, based on the Keck II DEIMOS spectra of eight member stars. We find a systemic velocity for the system of nu(r) = 170.9(+2.1) (-1.9) km s(-1) and barely resolve a velocity dispersion for the system, with sigma nu(r) = 2.3(+3.2) (-1.6) km s(-1), consistent with previous studies of Leo V. The poorly resolved dispersion means we are unable to adequately constrain the dark-matter content of Leo V. We find an average metallicity for the dwarf of [ Fe/ H] =-2.48 +/- 0.21 and measure a significant spread in the iron abundance of its member stars, with -3.1 <= [ Fe/ H] <=-1.9 dex, which clearly identifies Leo V as a dwarf galaxy that has been able to self-enrich its stellar population through extended star formation. Owing to the tentative photometric evidence for the tidal substructure around Leo V, we also investigate whether there is any evidence for tidal stripping or shocking of the system within its dynamics. We measure a significant velocity gradient across the system, of dv d chi = -4.1(+2.8) (-2.6) km s(-1) arcmin(-1) ( or d nu/d chi=-71.9(vertical bar 50.8) (-45.6) km s(-1) kpc(-1)), which points almost directly towards the Galactic Centre. We argue that Leo V is likely a dwarf on the brink of dissolution, having just barely survived a past encounter with the centre of the Milky Way.

We present an outline of basic assumptions and governing structural equations describing atmospheres of sub-stellar mass objects, in particular the extrasolar giant planets and brown dwarfs. Although most of the presentation of the physical and numerical background is generic, details of the implementation pertain mostly to the code COOLTLUSTY. We also present a review of numerical approaches and computer codes devised to solve the structural equations, and make a critical evaluation of their efficiency and accuracy.

The dynamical histories of three Galactic satellite systems have been investigated. These include the Fornax and Sculptor dwarf spheroidal galaxies, and the globular cluster Omega Centauri. Wide-field CCD photometry was obtained for these objects in two colours, V and I, covering a total sky area of 30 square degrees. These data were used to construct a colour-magnitude diagram for each system, which then provided a filter to select probable member stars. This technique reduced contamination from foreground stars, allowing a comprehensive examination of the poorly-known outer regions of these three Galactic satellites. ...

The current standard cosmological model is a remarkable achievement in providing an excellent description of the formation of largest-scale structures in the observable universe. However, there have been persistent tensions between observations and the theoretical model on galactic and sub-galactic scales. One of the most pressing issues was the “plane of satellite” problem: the distribution of observed Milky Way satellite galaxies appears to form a thin polar plane, which is largely inconsistent with the prediction of the standard cosmological model, i.e. an isotropic distribution of dark matter sub-halos. This problem was often seen as a consequence of observational biases because many of previously known satellite galaxies were discovered by the Sloan Digital Sky Survey whose survey footprint mainly focused on the northern Galactic pole region. Subsequent surveys to map the southern sky were thus required for an unbiased observational assessment of the standard cosmological model. This thesis presents the discovery of five new low-luminosity Milky Way companions i.e. dwarf spheroidal galaxies and star clusters from the Sloan Digital Sky Survey, Dark Energy Survey, and the Stromlo Milky Way Satellite survey project: PegasusIII, Horologium II, and Kim 1–3. Pegasus III is an ultra-faint outer halo dwarf satellite in the constellation of Pegasus. The large mass-to-light ratio inferred from photometric and spectroscopic analysis indicates a substantial amount of dark matter component in Pegasus III. Horologium II is one of the ultra-faint dwarf satellites in the vicinity of the Magellanic Clouds discovered in the Dark Energy Survey Year 1 data. Its proximity to a satellite dwarf Horologium I implies their possible companionship. Kim 1 is an extremely low-mass star cluster in the constellation of Pegasus with an unusually high ellipticity and low central surface brightness, which implies that it is being tidally disrupted. Kim 2 is a low-mass outer halo star cluster in the constellation of Indus. Although this stellar system falls in between the realms of star clusters and dwarf galaxies on the size-luminosity plane, evidence of pronounced mass segregation in the object argues in favour of star cluster-like nature. Kim 3 is another extremely low-luminosity star cluster in the constellation of Centaurus. With the lack of well-defined centre and evidence of mass segregation, Kim 3 is also classified as a star cluster in the final stage of tidal disruption. Both Pegasus III and Horologium II, as well as the majority of new dwarf satellites discovered by other recent studies, turn out tightly aligned with the original plane of satellites defined by previously known objects. This result confirms the presence of the plane of satellites throughout the whole sky, formerly speculated based on the distribution of clas- sical satellite galaxies decades ago. These findings largely conflict with the prediction of the current standard cosmological model and thus request its reexamination or an alternative model to account for the observation. The discovery of Kim 1–3 implies the possible existence of a large population of such extremely low-mass star clusters. These objects are possibly stripped down versions of formerly more massive star clusters or born extremely low-mass clusters that have survived to the present time. Dedicated observational and theoretical follow-up studies are required to determine their origin, which will shed a light on the formation and evolution history of star clusters in general.

The thesis, in general aims at studying the early history of the Milky way through the chemical abundances of metal poor stars in the Halo and in the satellite galaxies of the Milky way. Halo of a galaxy is one of the early structures that remain pristine to probe the first low mass stars that formed during the first billion years after the Big Bang. According to the hierarchical galaxy formation models, Milky Way Halo could have formed by accretion of smaller satellite galaxies. Several hundreds of such small dwarf galaxies that did not merge to the Halo will be observable now. Low and high resolution, optical and NIR spectroscopic data are used to study the chemical abundances of metal poor stars both from the Galaxy and the satellite galaxies, to understand the common origin of them in the context of Galaxy formation. The details of each project undertaken during the period of the PhD is briefly described below. _ Chapter-1 includes, introduction of the following topics: metal poor stars, different classes metal poor stars based their origin and composition. Current understanding of the origin of satellite galaxies. Milky way formation and its connection with the satellite galaxies. _ Chapter-2 includes observations, data analysis and methodology. We also discuss the stellar atmospheric models, radiative transfer codes, atomic and molecular data used in the analysis. Spectroscopic data analysis technique for low- and high-resolution spectroscopy is presented. _ Chapter 3 provides the details and results of slit less spectroscopic survey of dwarf satellites of Milky way using Himalayan Chandra Telescope and the follow up studies. Large photometric and spectroscopic surveys like SDSS have been successful in identifying faint galaxies around Milky way. Most of the automated photometric searches, use over densities in the HESS diagrams, using a metal poor globular cluster colour-magnitude diagrams as a representative population for the stars in these faint systems. They also pre-select the sample using various colour cuts to avoid contamination from the foreground stars. Most of these methods identify very few RGB and AGB populations. RGB and AGB members are brighter and can be studied for its detailed chemical abundances, to derive clues of the early stellar population. We initiated a slit less spectroscopic survey of 6 satellite galaxies using 2m Himalayan Chandra Telescope, India. We detected many new samples in these systems which fall in either RGB or HB group. This blind search and photometric preselection have been compared. Two of such bright metal poor stars identified are being followed up using Subaru. A proposal has been submitted for the same. The faint stars detected cannot be followed up using current facilities but are ideal candidates for upcoming large telescopes like TMT. _ Chapter 4 describes the abundance analysis of two CEMP stars from Carina dwarf spheroidal galaxy. Carbon-enhanced metal-poor (CEMP) stars bear important imprints of the early chemical enrichment of any stellar system. Plenty of CEMP stars are found in the Milky way halo and their fraction increases with decreasing metallicity. But a very few CEMP stars are observed in the faint dwarf spheroidal (dSph) satellites. So we performed abundance analysis of two metal-poor carbon rich stars in the Carina dSph galaxy using high-resolution spectroscopic data obtained with the ESO/UVES instrument. One of the stars (ALW-1) is showing enhancement in both heavy and light s-process elements thus classified as CEMP-s star. It is showing radial velocity variation indicating the presence of a binary AGB companion. The other star, ALW-8, show no enhancement in neutron capture elements thus classified as a CEMP-no star which is the first CEMP-no star detected in the Carina dwarf spheroidal galaxy. A moderate enhancement in yttrium is observed in this star indicating a weak r-process activity. Majority of elements detected in ALW-8 show similar abundance pattern of Carina ‘s field star population as well as with CEMP stars in other dSph galaxies. The overall abundance pattern of ALW-8 confirms that the formation site for CEMP-no stars has been affected by both faint supernovae and/or fast rotating massive stars and by standard core collapse supernovae. It could also infer that the mechanisms responsible for the heavy element production in CEMP-no stars are universal and act independent of the environment such as in the Galactic halo or in dSphs. The results have been published in Susmitha et al 2016, A&A accepted (arXiv:1706.06599). _ Chapter-5 describes the detailed abundance analysis of an Extremely Metal Poor stars (EMP) from the Milky way halo. EMP stars are one of the subsets of metal poor stars which preserve the chemical composition of first stars in their atmospheres. In order to understand the nature of first stars and the various nucleosynthetic mechanisms existed in the early universe, we initiated high resolution spectroscopic survey of extremely metal poor stars preselected from Multi-object Apache Point Observatory Radial Velocity Exoplanet Large-area Survey (MARVELS) spectroscopic presurvey. One of the stars (SDSS J134338.67+484426.6) with an apparent magnitude V = 12.14, is the lowest metallicity star found in the pre-survey, and is one of the only _20 known EMP stars that are this bright or brighter. Our high-resolution spectroscopic analysis of this star shows that this star is a subgiant with [Fe/H] = -3.42, having ‘normal’ carbon and no enhancement of neutron-capture abundances. Strontium is underabundant, [Sr/Fe] = -0.47, but the derived lower limit on [Sr/Ba] indicates that Sr is likely enhanced relative to Ba. This star belongs to the sparsely populated class of _-poor EMP stars that exhibit low ratios of [Mg/Fe], [Si/Fe], and [Ca/Fe] compared to typical halo stars at similar metallicity. An ISM with contributions from Pop III intermediate mass stars along with a later stage Pop II contributions with a low SFR can explain the abundance pattern seen in this star. The abundance analysis and the results of this star has been published in Susmitha & Sivarani et al 2016, MNRAS,458,2648. _ Chapter 6 discusses the HESP- TIRSPEC synergy of CEMP stars in the Milky way halo. The large fraction of CEMP stars at lower metallicity indicates the physical mechanisms that causes production of carbon at earlier evolutionary stages of the Milky way. High fraction might also indicate that the IMF in the earlier times included large number of intermediate to high mass stars than the present day. Abundance of carbon, nitrogen and oxygen play a crucial role in understanding the nature of the progenitors and origin of carbon in these stars. We studied four cool CEMP stars from the Milky way halo, in optical high resolution and NIR medium resolution to derive the abundances of C, N and O using, C2, CN, CH. [OI], and CO features. We compared the oxygen abundance derived from optical [OI] lines and CO band in the NIR and found reasonable match in the derived abundance. The CNO abundances together with s-process element abundance and radial velocity monitoring in these four samples revealed that, they are having an AGB companion. The properties of the companion and possible origin of the C, N and O has been discussed in the chapter. _ Chapter 7 includes the conclusion and future plans.

The Milky Way and its satellite population provides the nearest laboratory for the study of galaxy formation and evolution. As a result, it is the subject of detailed analysis, in order to better understand both the formation history of our own galaxy, as well as galaxy formation in the cosmological context. The dwarf galaxy population is of particular interest due to its perceived high dark matter content inferred from velocity dispersion measurements, making the dwarf galaxies candidates for the primordial building blocks of galaxy formation under the ΛCDM cosmological paradigm. As close as the Milky Way satellite population is, much of it remains a mystery. This is largely due to the difficulty in its observation; not only are the satellites resolved into their individual stars, blending in with the foreground of the Milky Way, they are extremely faint in many cases and require time intensive telescope allocation to obtain science grade data. This has resulted in small-scale, piecemeal observations and varying data sets, making the development of accurate theoretical models difficult. The advent of the digital survey camera era has changed this somewhat, and provides a new opportunity to delve deep into the Milky Way satellite population. This theses presents homogeneous observations, in the form of deep, wide-field photome- try, of three of the Milky Ways satellite dwarf galaxies: Hercules, Sextans and BootesI. The wide-field nature of these observations enables a thorough search for stellar substruc- ture associated with these dwarfs, in an attempt to better understand their level of tidal interaction with the Milky Way and how this influences our understanding of their role in galaxy formation. Each of the three dwarfs is found to possess extended stellar substructure, to varying degrees. The brightest, Sextans, demonstrates the least extreme substructure, and poten- tially has the most circular orbital path. The two fainter dwarfs are more representative of the ultra-faint regime and display more elongated structure, with most extreme of the two, Hercules, most likely to have the minimal peri-galactic distance. Interestingly, Hercules also has the most recent infall time, giving it less time to complete multiple orbits than Sextans or BootesI. This pattern suggests that while the size and infall of each galaxy is important, it is the orbital eccentricity and peri-galactic distance which play a larger role in the level of tidal influence. It is also important to note that in this sample, the two dwarfs closest to the ultra-faint regime are also the two showing the most extreme structure. If this finding is representative of the rest of the ultra-faint population, it may indicate a need to review mass-to-light ratios which are based on the assumption that the dwarfs are in dynamic equilibrium.

Current models of cosmology and galaxy formation are possibly at odds with observations of small-scale galaxies. Such is the case for the dwarf spheroidal (dSph) galaxies of the Milky Way (MW), where tension exists in explaining their observed abundance, mass, and internal structure. Here we present an analysis of the substructure surrounding MW-sized haloes in a Lambda Cold Dark Matter (LCDM) simulation suite. Combined with a semi-analytic model of galaxy formation and evolution, we identify substructures that are expected to host dSph galaxies similar to the satellites of the MW. We subsequently use these simulations to investigate the orbital properties of dSph satellite galaxies to make contact with those orbiting the MW. After accretion into the main halo, the higher mass ``luminous'' substructure remains on highly radial orbits while the orbits of lower mass substructure, which are not expected to host stars, tend to scatter off of the luminous substructure, and thus circularize over time. The orbital ellipticity distribution of the luminous substructure shows little dependence on the mass or formation history of the main halo, making this distribution a robust prediction of LCDM. Through comparison with the ellipticity distribution computed from the positions and velocities of the nine MW dSph galaxies that currently have proper motion estimates as a function of the assumed MW mass, we present a novel means of estimating the virial mass of the Milky Way. The best match is obtained assuming a mass of 1.1 x 10^12 M_sun with 95 per cent confidence limits of (0.6 - 3.1) x 10^12 M_sun. The uncertainty in this estimate is dominated by the large uncertainties in the proper motions and small number of MW satellites used, and will improve significantly with better proper motion measurements from Gaia. We also measure the shape of the gravitational potential of subhaloes likely to host dSphs, down to radii comparable to the half-light radii of MW dSphs. Field haloes are triaxial in general, while satellite haloes become more spherical over time due to tidal interactions with the host. Thus through the determination of the shape of a MW dSph's gravitational potential via line of sight velocity measurements, one could in principle deduce the impact of past tidal interactions with the MW, and thus determine its dynamical history. Additionally, luminous subhaloes experience a radial alignment of their major axes with the direction to the host halo over time, caused by tidal torquing with the host's gravitational potential during close pericentric passages. This effect is seen at all radii, even down to the half-light radii of the satellites. Radial alignment must be taken into account when calibrating weak-lensing surveys which often assume isotropic orientations of satellite galaxies surrounding host galaxies and clusters.
Graduate
0606

Here I study the “plasma interactions with icy bodies in the solar system”, that is, my quest to understand the fundamental processes that govern such interactions. By using numerical modelling combined with in situ observations, one can infer the internal structure of icy bodies and their plasma environments. After a broad overview of the laws governing space plasmas a more detailed part follows. This contains the method on how to model the interaction between space plasmas and icy bodies. Numerical modelling of space plasmas is applied to the icy bodies Callisto (a satellite of Jupiter), the dwarf planet Ceres (located in the asteroid main belt) and the comet 67P/Churyumov-Gerasimenko. The time-varying magnetic field of Jupiter induces currents inside the electrically conducting moon Callisto. These create magnetic field perturbations thought to be related to conducting subsurface oceans. The flow of plasma in the vicinity of Callisto is greatly affected by these magnetic field perturbations. By using a hybrid plasma solver, the interaction has been modelled when including magnetic induction and agrees well with magnetometer data from flybys (C3 and C9) made by the Galileo spacecraft. The magnetic field configuration allows an inflow of ions onto Callisto’s surface in the central wake. Plasma that hits the surface knocks away matter (sputtering) and creates Callisto’s tenuous atmosphere. A long term study of solar wind protons as seen by the Rosetta spacecraft was conducted as the comet 67P/Churyumov-Gerasimenko approached the Sun. Here, extreme ultraviolet radiation from the Sun ionizes the neutral water of the comet’s coma. Newly produced water ions get picked up by the solar wind flow, and forces the solar wind protons to deflect due to conservation of momentum. This effect of mass-loading increases steadily as the comet draws closer to the Sun. The solar wind is deflected, but does not lose much energy. Hybrid modelling of the solar wind interaction with the coma agrees with the observations; the force acting to deflect the bulk of the solar wind plasma is greater than the force acting to slow it down. Ceres can have high outgassing of water vapour, according to observations by the Herschel Space Observatory in 2012 and 2013. There, two regions were identified as sources of water vapour. As Ceres rotates, so will the source regions. The plasma interaction close to Ceres depends greatly on the source location of water vapour, whereas far from Ceres it does not. On a global scale, Ceres has a comet-like interaction with the solar wind, where the solar wind is perturbed far downstream of Ceres.
Här studerar jag “plasmaväxelverkan med isiga kroppar i solsystemet”, det vill säga, min strävan är att förstå de grundläggande processerna som styr sådana interaktioner. Genom att använda numerisk modellering i kombination med observationer på plats vid himlakropparna kan man förstå sig på deras interna strukturer och rymdmiljöer. Efter en bred översikt över de fysiska lagar som styr ett rymdplasma följer en mer detaljerad del. Denna innehåller metoder för hur man kan modellera växelverkan mellan rymdplasma och isiga kroppar. Numerisk modellering av rymdplasma appliceras på de isiga himlakropparna Callisto (en måne kring Jupiter), dvärgplaneten Ceres (lokaliserad i asteroidbältet mellan Mars och Jupiter) och kometen 67P/Churyumov-Gerasimenko. Det tidsvarierande magnetiska fältet kring Jupiter inducerar strömmar inuti den elektriskt ledande månen Callisto. Dessa strömmar skapar magnetfältsstörningar som tros vara relaterade till ett elektriskt ledande hav under Callistos yta. Plasmaflödet i närheten av Callisto påverkas i hög grad av dessa magnetfältsstörningar. Genom att använda en hybrid-plasma-lösare har växelverkan modellerats, där effekten av magnetisk induktion har inkluderats. Resultaten stämmer väl överens med magnetfältsdata från förbiflygningarna av Callisto (C3 och C9) som gjordes av den obemannade rymdfarkosten Galileo i dess bana kring Jupiter. Den magnetiska konfigurationen som uppstår möjliggör ett inflöde av laddade joner på Callistos baksida. Plasma som träffar ytan slår bort materia och skapar Callistos tunna atmosfär. En långtidsstudie av solvindsprotoner sett från rymdfarkosten Rosetta utfördes då kometen 67P/Churyumov-Gerasimenko närmade sig solen. Ultraviolett strålning från solen joniserar det neutrala vattnet i kometens koma (kometens atmosfär). Nyligt joniserade vattenmolekyler plockas upp av solvindsflödet och tvingar solvindsprotonernas banor att böjas av, så att rörelsemängden bevaras. Denna effekt ökar stadigt då kometen närmar sig solen. Solvinden böjs av kraftigt, men förlorar inte mycket energi. Hybridmodellering av solvindens växelverkan bekräftar att kraften som verkar på solvinden till störst del får den att böjas av, medan kraften som verkar till att sänka dess fart är mycket lägre. Ceres har enligt observationer av rymdteleskopet Herschel under 2012 och 2013 haft högt utflöde av vattenånga från dess yta. Där har två regioner identifierats som källor för vattenångan. Eftersom Ceres roterar kommer källornas regioner göra det också. Plasmaväxelverkan i närheten av Ceres beror i hög grad på vattenångskällans placeringen, medan det inte gör det långt ifrån Ceres. På global nivå har Ceres en kometliknande växelverkan med solvinden, där störningar i solvinden propagerar långt nedströms från Ceres.