Observational evidence that star formation proceeds in a clustered manner raises a question on the mass function of star clusters and their evolution. However, we have a limited scope of these processes in the Milky Way galaxy. The M31 galaxy is the nearest stellar system similar to our Galaxy, therefore, it is the most suitable one to provide clues for understanding the star cluster population and the evolution of galactic structures. However, detailed study of stellar populations and star clusters is a challenging task for ground-based observations due to crowded stellar fields. Using Subaru telescope Suprime-Cam wide-field images, a survey of clusters was carried out in the disk region of the M31 galaxy southwest field, which is a close analogue to that of the Solar neighborhood in terms of chemical composition, stellar density, and quiescent star formation. Data analysis methods and programs were developed, tested, and applied for crowded wide-field image reduction and evolutionary parameter determination of semi-resolved star clusters. The main results are: (1) enhanced star cluster formation activity in M31 occurred ~70 Myr ago; (2) approximately 10% of stars born in star clusters remain there at 100 Myr age; (3) the characteristic lifetime of a cluster of ~10^4 solar masses mass is ~300 Myr; (4) the mass function of star clusters in M31 is similar to that in other low star formation activity galaxies and it is best described by the Schechter's function with a... [to full text] / Daugėjant įrodymų, kad žvaigždės formuojasi spiečiais, jų masių funkcijos ir evoliucijos savybes kol kas galima nustatyti empiriniais stebėjimais. Tačiau tirti žvaigždėdaros procesą mūsų Galaktikoje galima ribotai. Andromedos galaktika (M31) yra artimiausia žvaigždžių sistema, panaši į Galaktiką. Todėl ji tinkamiausia žvaigždžių spiečių populiacijai tirti, tačiau ilgą laiką buvo tirta ribotai dėl tankių žvaigždžių laukų stebėjimo sudėtingumo. Naudojant „Subaru“ teleskopo Suprime-Cam plataus lauko nuotraukas, buvo ištirti žvaigždžių spiečiai M31 galaktikos disko srityje, kuri pagal cheminę sudėtį, žvaigždinį tankį ir mažą žvaigždėdaros spartą yra analogiška Saulės aplinkai mūsų Galaktikoje. Buvo sukurti, išbandyti ir pritaikyti duomenų analizės metodai ir programos tankiems plataus lauko vaizdams apdoroti ir pusiau išskiriamų žvaigždžių spiečių evoliuciniams parametrams nustatyti. Pagrindiniai rezultatai: (1) žvaigždžių spiečių formavimosi spartos sustiprėjimas M31 galaktikoje įvyko prieš ~70 mln. m.; (2) maždaug 10% žvaigždžių spiečių gyvena ilgiau nei 100 mln. m.; (3) būdingoji ~10^4 Saulės masių spiečiaus gyvavimo trukmė yra ~300 mln. m.; (4) žvaigždžių spiečių masių pasiskirstymas panašus į kitų mažos žvaigždėdaros spartos galaktikų žvaigždžių spiečių pasiskirstymą ir jį gerai nusako Schechter'io funkcija, kurios charakteringoji masė ~2x10^5 Saulės masių. Tai rodo, kad M31 galaktikoje yra gausi vidutinės masės žvaigždžių spiečių populiacija, kuri mūsų Galaktikoje yra... [toliau žr. visą tekstą]

I have used blue near-infrared colours to select a group of UKIDSS T dwarfs with spectral types later than T4. From amongst these I identify two kinematic halo T-dwarf candi- dates. Blue near-infrared colours have been attributed to collisionally-induced hydrogen absorption, which is enhanced by either high surface gravity or low metallicity. Proper motions are measured and distances estimated, allowing the determination of tangential velocities. U and V components are estimated for our objects by assuming Vrad = 0. From this, ULAS J0926+0835 is found to have U = 62 kms−1 and V = −140 kms−1 and ULAS J1319+1209 is found to have U = 192 kms−1 and V = −92 kms−1. These values are consistent with potential halo membership. However, surprisingly, these are not the bluest objects in the selection. The bluest is ULAS J1233+1219, with J −K = −1.16±0.07, and surprisingly this object is found to have thin disc-like U and V . Our sample also contains Hip 73786B, which I find to be a companion to the star Hip 73786. Hip 73786 is a metal- poor star, with [Fe/H]= −0.3 ± 0.1 and is located at a distance of 19±0.7 pc. U, V,W space velocity components are calculated for Hip 73786A and B, finding that U = −48±7 kms−1, V = −75 ± 4 kms−1 and W = −44 ± 8 kms−1. From the properties of the pri- mary, Hip 73786B is found to be at least 1.6Gyr old. As a metal poor object, Hip 73786B represents an important addition to the sample of known T dwarf benchmarks. Using mid-infrared data from WISE, I also identify T dwarfs with abnormally-red H − W2 and consider possible causes for their extreme colours. In particular I exam- ine three prominent examples of this phenomenon, ULAS J1416+1348B, 2MASS J0939- 2448 and BD+01o 2920B. A plot of spectral type against MW2-magnitude suggests that ULAS J1416+1348B is potentially an unresolved binary, similar to 2MASS J0939-2448. However, the plot also indicates that BD+01o 2920B is not an unresolved binary. I also present new FIRE spectroscopy for ULAS J1416+1348B and 2MASS J0939-2448. These data show that ULAS J1416+1348B has a similar shape to the Y -band spectrum to that of BD+01o 2920B, thus suggesting that the two objects have a similar metallicity, whereas 2MASS J0939-2448 appears to be a more metal-rich object. Using a new parallactic dis- tance, I derive a luminosity of (6.9±0.7)×1020W for ULAS J1416+1348B. I also find a radial velocity of −39 ± 1 kms−1 for this object. The agreement between this and that of the L dwarf SDSS J1416+1348A confirms that these two objects are physically-associated. I also present a set of simulated unresolved binaries; the colours of these systems do not appear to redden significantly with the addition of cooler companions. From this, I suggest that the colours of ULAS J1416+1348B and BD+01o 2920B cannot be solely attributed to any possible unresolved companions; for these two objects, composition and/or surface gravity must be playing a substantial role. Consideration of model predictions provides extra evidence for this argument, showing as it does that high log g and low metallicity can redden H − W2 colours by as much as »0.5mag as compared to a high-metallicity and low log g object of the same effective temperature. I also present kinematics and photometry for several more new candidate low-metallicity T dwarfs. Spectra are also presented, where available. In addition I provide new follow-up JHK spectroscopy for ULAS J0926+0835, ULAS J1233+1219 and ULAS J1319+1209. These new spectra allow full JHK-based spectral typing for these objects.

We present multi-wavelength detections of nine candidate gravitationally lensed dusty starforming galaxies (DSFGs) selected at 218 GHz (1.4 mm) from the Atacama Cosmology Telescope (ACT) equatorial survey. Among the brightest ACT sources, these represent the subset of the total ACT sample lying in Herschel SPIRE fields, and all nine of the 218 GHz detections were found to have bright Herschel counterparts. By fitting their spectral energy distributions (SEDs) with a modified blackbody model with power-law temperature distribution, we find the sample has a median redshift of z = 4.1(-1.0)(+1.1) (68 per cent confidence interval), as expected for 218 GHz selection, and an apparent total infrared luminosity of log10(mu LIR/L-circle dot) = 13.86(-0.30)(+0.33), which suggests that they are either strongly lensed sources or unresolved collections of unlensed DSFGs. The effective apparent diameter of the sample is root mu d = 4.2(-1.0)(+1.7) kpc, further evidence of strong lensing or multiplicity, since the typical diameter of DSFGs is 1.0-2.5 kpc. We emphasize that the effective apparent diameter derives from SED modelling without the assumption of optically thin dust (as opposed to image morphology). We find that the sources have substantial optical depth (tau = 4.2(-1.9)(+3.7)) to dust around the peak in the modified blackbody spectrum (lambda(obs) <= 500 mu m), a result that is robust to model choice.

We directly detect dust emission in an optically detected, multiply imaged galaxy lensed by the Frontier Fields cluster MACSJ0717.5+3745. We detect two images of the same galaxy at 1.1 mm with the AzTEC camera on the Large Millimeter Telescope leaving no ambiguity in the counterpart identification. This galaxy, MACS0717_Az9, is at z > 4 and the strong lensing model (mu = 7.5) allows us to calculate an intrinsic IR luminosity of 9.7 x 10(10) L-circle dot and an obscured star formation rate of 14.6 +/- 4.5 M-circle dot yr(-1). The unobscured star formation rate from the UV is only 4.1 +/- 0.3 M-circle dot yr(-1), which means the total star formation rate (18.7 +/- 4.5 M-circle dot yr(-1)) is dominated (75%-80%) by the obscured component. With an intrinsic stellar mass of only 6.9 x 10(9) M circle dot, MACS0717_Az9 is one of only a handful of z. >. 4 galaxies at these lower masses that is detected in dust emission. This galaxy lies close to the estimated star formation sequence at this epoch. However, it does not lie on the dust obscuration relation (IRX-beta) for local starburst galaxies and is instead consistent with the Small Magellanic Cloud attenuation law. This remarkable lower mass galaxy, showing signs of both low metallicity and high dust content, may challenge our picture of dust production in the early universe.

Galaxies, and clusters of galaxies, can act as gravitational lenses and magnify the light of objects behind them. The effect enables observations of very distant supernovae, that otherwise would be too faint to be detected by existing telescopes, and allows studies of the frequency and properties of these rare phenomena when the universe was young. Under the right circumstances, multiple images of the lensed supernovae can be observed, and due to the variable nature of the objects, the difference between the arrival times of the images can be measured. Since the images have taken different paths through space before reaching us, the time-differences are sensitive to the expansion rate of the universe. One class of supernovae, Type Ia, are of particular interest to detect. Their well known brightness can be used to determine the magnification, which can be used to understand the lensing systems. In this thesis, galaxy clusters are used as gravitational telescopes to search for lensed supernovae at high redshift. Ground-based, near-infrared and optical search campaigns are described of the massive clusters Abell 1689 and 370, which are among the most powerful gravitational telescopes known. The search resulted in the discovery of five photometrically classified, core-collapse supernovae at redshifts of 0.671&lt;z&lt;1.703 with significant magnification from the cluster. Owing to the power of the lensing cluster, the volumetric core-collapse supernova rates for 0.4 ≤ z &lt; 2.9 were calculated, and found to be in good agreement with previous estimates and predictions from cosmic star formation history. During the survey, two Type Ia supernovae in A1689 cluster members were also discovered, which allowed the Type Ia explosion rate in galaxy clusters to be estimated. Furthermore, the expectations of finding lensed supernovae at high redshift in simulated search campaigns that can be conducted with upcoming ground- and space-based telescopes, are discussed. Magnification from a galaxy lens also allows for detailed studies of the supernova properties at high redshift that otherwise would not be possible. Spectroscopic observations of lensed high-redshift supernovae Type Ia are of special interest since they can be used to test for evolution of the standard candle nature of these objects. If systematic redshift-dependent properties are found, their utility for future surveys could be challenged. In the thesis it is shown that the strongly lensed and very distant supernova Type Ia PS1-10afx at z=1.4, does not deviate from the well-studied nearby and intermediate populations of normal supernovae Type Ia. In a different study, the discovery of the first resolved multiply-imaged gravitationally lensed supernova Type Ia is also reported. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.</p>

We present a detailed, multi-wavelength study of star formation (SF) and active galactic nucleus (AGN) activity in 11 near-infrared (IR) selected, spectroscopically confirmed massive (greater than or similar to 10(14)M(circle dot)) galaxy clusters at 1 < z < 1.75. Using new deep Herschel/PACS imaging, we characterize the optical to far-IR spectral energy distributions (SEDs) for IR-luminous cluster galaxies, finding that they can, on average, be well described by field galaxy templates. Identification and decomposition of AGNs through SED fittings allows us to include the contribution to cluster SF from AGN host galaxies. We quantify the star-forming fraction, dust-obscured SF rates (SFRs) and specific SFRs for cluster galaxies as a function of cluster-centric radius and redshift. In good agreement with previous studies, we find that SF in cluster galaxies at z greater than or similar to 1.4 is largely consistent with field galaxies at similar epochs, indicating an era before significant quenching in the cluster cores (r < 0.5 Mpc). This is followed by a transition to lower SF activity as environmental quenching dominates by z similar to 1. Enhanced SFRs are found in lower mass (10.1< logM(kappa)/M-circle dot < 10.8) cluster galaxies. We find significant variation in SF from cluster to cluster within our uniformly selected sample, indicating that caution should be taken when evaluating individual clusters. We examine AGNs in clusters from z = 0.5-2, finding an excess AGN fraction at z greater than or similar to 1, suggesting environmental triggering of AGNs during this epoch. We argue that our results-a transition from field-like to quenched SF, enhanced SF in lower mass galaxies in the cluster cores, and excess AGNs-are consistent with a co-evolution between SF and AGNs in clusters and an increased merger rate in massive halos at high redshift.

The centre of the Milky Way, commonly referred to as the Galactic Centre, is roughly that region within 500 pc of the central black hole, Sagittarius A*. Within the innermost parsec around the supermassive black hole Sagittarius A* are more than a hundred massive young stars whose orbits align to form one or possibly two discs. At about 100 pc is a ring containing more than ten million solar masses of molecular gas which could be the origin of some of the most massive star clusters in the Galaxy. I have performed a number of numerical simulations to help us understand how it is that these structures may have been formed. I firstly describe and test an improvement to the smoothed particle hydrodynamics code I used. This improves conservation of energy and momentum in certain situations such as in strong shocks from supernovae, which were to be included in a later chapter. The discs of massive stars around Sagittarius A* are believed to have been born there within fragmenting gaseous discs. This is problematic, as the formation of two stellar discs would require two gaseous counterparts. A method is described of forming multiple discs around a black hole from a single cloud's infall and subsequent tidal destruction. This is due to its prolate shape providing a naturally large distribution in the direction of the angular momentum vectors within the cloud. The resulting discs may then go on to form stars. Energetically, it would appear that a sequence of supernovae could potentially cause a giant molecular cloud to fall inwards towards the central black hole from an originally large orbit around the Galactic Centre. I simulate the impact on a giant molecular cloud of supernovae originating from a massive stellar cluster located a parsec away. Ultimately, the supernovae are found to have little effect. Finally, I simulate the formation of the dense ring of clouds observed in the Central Molec- ular Zone at a distance of about 100 pc from Sgr A*. Infalling gas is shown to be subject to such extreme tidal forces that a single cloud of gas is extended to form a long stream. The ribbon grows to the point that it self-intersects and forms a ring-like structure. Its complexity depends on the orbit of the original cloud. The position-velocity data is compared with observations, and similarities are noted.

We study the properties of the foreground galaxy of the Ruby, the brightest gravitationally lensed high-redshift galaxy on the sub-millimeter sky as probed by the Planck satellite, and part of our sample of Planck's dusty GEMS. The Ruby consists of an Einstein ring of 1.4" diameter at z = 3.005 observed with ALMA at 0.1" resolution, centered on a faint, red, massive lensing galaxy seen with HST/WFC3, which itself has an exceptionally high redshift, z = 1.525 +/- 0.001, as confirmed with VLT/X-shooter spectroscopy. Here we focus on the properties of the lens and the lensing model obtained with LENSTOOL. The rest-frame optical morphology of this system is strongly dominated by the lens, while the Ruby itself is highly obscured, and contributes less than 10% to the photometry out to the K band. The foreground galaxy has a lensing mass of (3.70 +/- 0.35) x 10(11) M-Theta Magnification factors are between 7 and 38 for individual clumps forming two image families along the Einstein ring. We present a decomposition of the foreground and background sources in the WFC3 images, and stellar population synthesis modeling with a range of star-formation histories for Chabrier and Salpeter initial mass functions (IMFs). Only the stellar mass range obtained with the latter agrees well with the lensing mass. This is consistent with the bottom-heavy IMFs of massive high-redshift galaxies expected from detailed studies of the stellar masses and mass profiles of their low-redshift descendants, and from models of turbulent gas fragmentation. This may be the first direct constraint on the IMF in a lens at z = 1.5, which is not a cluster central galaxy.

We report the large effort that is producing comprehensive high-level young star cluster (YSC) catalogs for a significant fraction of galaxies observed with the Legacy ExtraGalactic UV Survey (LEGUS) Hubble treasury program. We present the methodology developed to extract cluster positions, verify their genuine nature, produce multiband photometry (from NUV to NIR), and derive their physical properties via spectral energy distribution fitting analyses. We use the nearby spiral galaxy NGC 628 as a test case for demonstrating the impact that LEGUS will have on our understanding of the formation and evolution of YSCs and compact stellar associations within their host galaxy. Our analysis of the cluster luminosity function from the UV to the NIR finds a steepening at the bright end and at all wavelengths suggesting a dearth of luminous clusters. The cluster mass function of NGC 628 is consistent with a power-law distribution of slopes similar to-2 and a truncation of a few times 10(5) M-circle dot. After their formation, YSCs and compact associations follow different evolutionary paths. YSCs survive for a longer time frame, confirming their being potentially bound systems. Associations disappear on timescales comparable to hierarchically organized star-forming regions, suggesting that they are expanding systems. We find massindependent cluster disruption in the inner region of NGC 628, while in the outer part of the galaxy there is little or no disruption. We observe faster disruption rates for low mass (<= 10(4) M-circle dot) clusters, suggesting that a massdependent component is necessary to fully describe the YSC disruption process in NGC 628.

Comparamos duas nuvens moleculares, Rosette (RMC) e Monoceros R2 (Mon R2), localizadas no Complexo de Monoceros com o objetivo de estudar suas condições físicas relacionadas às primeiras fases da formação estelar. Tratam-se de regiões interessantes por apresentarem características que podem ser confrontadas com a hipótese de formação estelar provocada pela passagem de nuvens de altas velocidades atravessando o plano Galáctico (HVCs). Avaliamos as propriedades dessas nuvens por meio de mapas de vários traçadores da formação estelar com base em diferentes bandas espectrais visando estudar a estrutura de densidade das nuvens, bem como os objetos estelares jovens, em particular as fontes masers de H2O que apresentam características típicas de protoestrelas massivas. Nossa análise permitiu verificar algumas semelhanças entre RMC e Mon R2, mas também nos revelou diferenças interessantes. De uma forma geral há concordância entre AV, CO e emissão de poeira em 100 microns; RMC possui muitos clumps, entretanto poucos aglomerados e nebulosidades exceto uma única região HII principal (NGC2244) enquanto Mon R2 apresenta poucos clumps, vários aglomerados jovens e pequenas nebulosidades; em RMC há mais estrelas massivas, distribuídas uniformemente; Mon R2 tem poucas estrelas B, distribuídas em estruturas filamentárias com maiores índices de AV, do que em RMC; as fontes emissão maser apresentam cores IRAS compatíveis com candidatas a protoestrelas massivas, mas não parecem estar associadas a fontes de raios-X, sugerindo que masers estão relacionados à fase protoestelar, ao passo que fontes-X representam fase Pré-Sequência Principal. Concluímos que a distribuição de objetos e a estrutura das nuvens estão de acordo com as simulações dos modelos de HVCs. Porém, nossos resultados também são compatíveis com modelos alternativos, que simulam a dinâmica da Galáxia, para explicar o cenário de formação estelar no Complexo de Monoceros. / We compare two molecular clouds of the Monoceros Complex in order to study their physical conditions related to the early stages of star formation. The selected clouds, Rosette (RMC) and Monoceros R2 (Mon R2), are interesting regions due to their characteristics that may be confronted with the hypothesis of star formation triggered by high velocity clouds (HVCs) crossing through the Galactic plane. We evaluate the properties of these clouds using maps obtained on the basis of dierent spectral bands to trace the density of the clouds and the young stellar objects, in particular H2O masers that show typical features of massive protostars. This analysis allowed us to verify some similarities between RMC and Mon R2, but also revealed interesting dierences. In a general way there is an agreement between Av, CO and dust emission at 100 microns; RMC has many clumps, a few clusters and a single main nebulosity that is an HII region around NGC2244, while Mon R2 has a few clumps, several young clusters and small nebulosities. In RMC there is a large number of massive stars, uniformly distributed, while Mon R2 has a few B stars, distributed in lamentary structures with levels of Av higher than in RMC; maser sources have IRAS colors compatible with massive protostars candidates, but do not seem to be associated with X-ray sources, suggesting that masers are more related to the protostellar phase, while X-ray sources are related to pre main sequence phase. We conclude that the distribution of objects and the structure of the clouds are in accordance with the simulations of HVC models. However, our results are also compatible with alternative models of the Galaxy dynamics that explain the scenario of star formation in the Monoceros Complex.