Η παρούσα διατριβή παρουσιάζει τα αποτελέσματα μιας συστηματικής έρευνας των πληθυσμών Υπολειμμάτων Υπερκαινοφανών (Υ/Υ) σε έξι κοντινούς γαλαξίες (NGC 2403, NGC 3077, NGC 4214, NGC 4395, NGC 4449 και NGC 5204) βασισμένη σε αρχειακά δεδομένα του δορυφόρου ακτίνων-Χ Chandra, και σε βαθειές οπτικές παρατηρήσεις με τα στενά φίλτρα Hα (λ 6563) και [SΙΙ] (λλ 6716, 6731) καθώς και φασματοσκοπικές παρατηρήσεις. Η ταξινόμηση των Υ/Υ επιλεγμένων στις ακτίνες-Χ βασίστηκε στα μαλακά, θερμικά φάσματα (kT < 3 keV) των πηγών στις ακτίνες-Χ ή στα χρώματά τους στις ακτίνες-Χ. Αντίστοιχα, η ταξινόμηση των οπτικών Υ/Υ βασίστηκε στο καθιερωμένο κριτήριο του λόγου των γραμμών εκπομπής [SΙΙ](λλ 6716, 6731)/Hα > 0.4. Εντοπίστηκαν 37 θερμικά Υ/Υ στις ακτίνες-Χ, 30 εκ των οποίων είναι νέες ανακαλύψεις και ~400 (~350 από αυτά είναι νέες ανιχνεύσεις) φωτομετρικά Υ/Υ, για 67 από τα οποία πιστοποιήθηκε φασματοσκοπικά η φύση τους ως Υ/Υ. Πολλοί από τους γαλαξίες στο δείγμα μας μελετώνται για πρώτη φορά στις ακτίνες-Χ (NGC 4214, NGC 4395 και NGC 5204) ή στο οπτικό μέρος του φάσματος (NGC 4395, NGC 3077) με συστηματικό τρόπο, καταλήγοντας στην ανακάλυψη αρκετών νέων Υ/Υ. Σε πολλές περιπτώσεις, η ταξινόμηση των πηγών ως Υ/Υ στις ακτίνες-Χ ή στο οπτικό μέρος του φάσματος επιβεβαιώνεται από ομόλογα Υ/Υ που έχουν ανιχνευθεί σε άλλα μήκη κύματος, δείχνοντας ότι οι μέθοδοι ανίχνευσης που χρησιμοποιούμε είναι αξιόπιστες. Συζητάμε τις ιδιότητες (π.χ. φωτεινότητα, θερμοκρασία, πυκνότητα, ταχύτητα σοκ) των Υ/Υ σε διάφορους τύπους γαλαξιών και ως εκ τούτου διαφορετικά περιβάλλοντα, προκειμένου να δούμε την εξάρτησή τους από το μεοσαστρικό μέσο. Συσχετίζουμε παραμέτρους των ανιχνευμένων οπτικών Υ/Υ (λόγος [SΙΙ]/Hα, φωτεινότητα) με τις παραμέτρους των αντίστοιχων Υ/Υ στις ακτίνες-Χ (θερμοκρασία, φωτεινότητα, πυκνότητα) προκειμένου να κατανοήσουμε την εξέλιξή τους. Μερικά από τα πιο ενδιαφέροντα αποτελέσματα αυτής της έρευνας είναι τα ακόλουθα: α) Βρίσκουμε ότι τα Υ/Υ που είναι ανιχνευμένα στις ακτίνες-Χ και βρίσκονται σε άμορφους γαλαξίες φαίνεται να είναι πιο λαμπρά από εκείνα στους σπειροειδείς γαλαξίες. Αποδίδουμε αυτό το γεγονός στη χαμηλότερη μεταλλικότητα των άμορφων γαλαξιών από αυτή των σπειροειδών (η χαμηλότερη μεταλλικότητα δημιουργεί πρόγονους αστέρες μεγαλύτερης μάζας) ή στις υψηλότερες τοπικές πυκνότητες που παρατηρούνται στο μεσοαστρικό μέσο των άμορφων γαλαξιών, β) Η σύγκριση του αριθμού των παρατηρούμενων λαμπρών Υ/Υ στις ακτίνες-Χ με τον αριθμό αυτών που αναμένονται με βάση τις κατανομές φωτεινότητας των Υ/Υ στις ακτίνες-Χ στα Νέφη του Μαγγελάνου και στον M33, δείχνουν ότι κατανομές φωτεινότητας των Υ/Υ μεταξύ σπειροειδών και άμορφων γαλαξιών είναι διαφορετικές, από αυτές που αφορούν τα Υ/Υ στους άμορφους γαλαξίες και τείνουν να είναι πιο επίπεδες, γ) Βρίσκουμε ότι υπάρχει διαφορά στους λόγους [NΙΙ]/Hα των Υ/Υ μεταξύ διαφορετικών τύπων γαλαξιών, το οποίο κατά πάσα πιθανότητα οφείλεται σε διαφορές στη μεταλλικότητά τους και δ) Υπάρχουν ισχυρές ενδείξεις για μια γραμμική σχέση μεταξύ του αριθμού των λαμπρών Υ/Υ στο οπτικό και στις ακτίνες-Χ και του ρυθμού αστρογένεσης των γαλαξιών του δείγματος. / This thesis presents the results of a comprehensive investigation of the Supernova Remnant (SNR) populations in six nearby galaxies (NGC 2403, NGC 3077, NGC 4214, NGC 4395, NGC 4449 and NGC 5204) based on Chandra archival data and deep optical narrow-band Hα and [SΙΙ] images, as well as spectroscopic observations. The classification of X-ray emitting SNRs was based on their soft thermal spectra (kT < 3 keV) or their X-ray colors and for optically-emitting SNRs on the well-established emission-line flux criterion of [SΙΙ](λλ 6716, 6731)/Hα(λ 6563) > 0.4. We have identified 37 X-ray selected thermal SNRs, 30 of which are new discoveries and ~400 optical SNRs (~350 are new detections), for 67 of which we spectroscopically verified their shock-excited nature. Many of the galaxies in our sample are studied for the first time in the X-ray (NGC 4214, NGC 4395, and NGC 5204) or optical (NGC 4395, NGC 3077) band in a self-consistent way, resulting in the discovery of many new SNRs. In many cases, the X-ray and optical classifications are confirmed based on the identification of SNR counterparts in other wavelengths, giving us confidence that the detection methods we use are robust. We discuss the properties (e.g. luminosity, temperature, density, shock velocity) of the X-ray/optically detected SNRs in different types of galaxies and hence different environments, in order to address their dependence on their interstellar medium. We compare optical ([SΙΙ]/Hα ratio, luminosity) and X-ray parameters (temperature, luminosity, density) of the detected SNRs, in order to understand their evolution and investigate possible selection effects. The most intriguing results of this survey are the following: a) We find that X-ray selected SNRs in irregular galaxies appear to be more luminous than those in spirals. We attribute this either to the lower metallicities and therefore more massive progenitor stars of irregular galaxies or to the higher local densities of the interstellar medium, b) A comparison of the numbers of observed luminous X-ray selected SNRs with those expected from the luminosity functions of X-ray SNRs in the Magellanic Clouds and M33 suggest different luminosity distributions between the SNRs in spiral and irregular galaxies, with the latter tending to have flatter distributions, c) We find that there is a difference in [NΙΙ]/Hα line ratios of the SNR populations between different types of galaxies which is the result of the low metalicity of irregular galaxies, and d) We find evidence for a linear relation between the number of luminous optical or X-ray SNRs and Star Formation Rate in our sample of galaxies.

Fifty years after the discovery of the first pulsating neutron star, the field of pulsar science has grown into a multidisciplinary research field, working to address a wide range of problems in astrophysics - from stellar evolution models to high precision tests of General Relativity to analysing the detailed structure of the Interstellar Medium in the Milky Way. Over 2500 Galactic pulsars have been discovered. The next generation telescopes, such as the Square Kilometre Array, promise to discover the complete observable Milky Way population, of several tens of thousands, over the next decade. These point sources in the sky have extreme properties, with matter densities comparable to that of an atomic nucleus, and surface magnetic fields a trillion times stronger than Earth's magnetic field. Observationally, the most valuable property is their rotational stability - allowing us to anticipate and sum their beamed radio emission, as the pulsar spins around its axis, on millisecond to second timescales. The detected radio wave signals carry with them information of the ionised interstellar medium (IISM) paths they traveled along. The imprints reveal that the pulsar signals we detect travel along multiple paths. While the bulk of the emitted signal propagates along a straight line, we also receive delayed emission scattered through small angles, back into our line of sight. This scattering is caused by fluctuations in the free electron densities of the IISM. The impact of these inhomogeneities is exaggerated at low observing frequencies, where averaged pulsar profiles are observed to be broadened, and showcase exponential scattering tails characterised by a scattering timescale &gcy;. Simple theoretical models predict a power law dependence of &gcy; on frequency, with a spectral index &alpha; = 4. Despite these predictions, my analysis of pulsar data in this thesis, reveal a more complex frequency dependence on &gcy;. I investigate the scattering characteristics of a set of pulsars observed by the Low Frequency Array (LOFAR), at 110~MHz to 190~MHz. These data are ideal datasets for accurate studies of pulsar scattering, providing broad frequency bands at low frequencies. I find anomalously low power law spectral indices, &alpha;, describing the frequency dependence of &gcy;. These indices are likely due to anisotropic scattering mechanisms or small scattering clouds in the IISM. To conduct effective data analysis, I develop scattering fitting techniques by first analysing IISM effects on simulated pulsar data. I investigate the effects of two different types of scattering mechanisms, isotropic and anisotropic scattering, and consider each of their particular frequency-dependent impacts on pulsar data. The work on simulated data provides a robust fitting technique for extracting scattering parameters and a framework for the interpretation of the LOFAR data used in this study. The fitting technique simultaneously models scattering effects and standard frequency-dependent pulse profile evolution. I present results for 13 pulsars with simple pulse shapes, and find that &gcy;, associated with scattering by a single thin screen, has a power law dependence on frequency with &alpha; ranging from 1.50 to 4.0. My results show that extremely anisotropic scattering can cause low &alpha; measurements. The anomalous scattering properties can also be caused by the presence of small scattering clumps in the IISM, as opposed to the conventionally modelled large scattering screens. Evidence for both anisotropic scattering and small scattering clouds with high electron densities come from other areas of research. Indications of the anisotropic nature of the local IISM mostly come from high resolution pulsar scintillation analyses, while evidence for high density scattering clouds is often based on extreme scattering events measured through quasar observations. My results suggest that these anomalous scattering properties are more prevalent than formerly thought, prompting us to reconsider the physical conditions of the IISM, where traditionally high electron densities are reserved for H_II regions and anisotropy is not modelled. High quality, low frequency pulsar data, where anomalous propagation effects become measurable, are a valuable addition in assisting us to distinguish between the different physical mechanisms that can be at play. The more complex these IISM characteristics reveal themselves to be, the harder it will be to disentangle intrinsic profile emission from IISM propagation imprints. Successfully separating these effects, however, promises to improve our understanding of the intrinsic pulsar radio emission - a process that is still poorly understood.

L'ère des observations interférométriques mène à la nécessité d'une description plus précise de la structure physique et de la dynamique des régions de formation d'étoiles, des coeurs pré-stellaires et des disques proto-planétaires. L'émission moléculaire et du continuum de la poussière peuvent être décrites par de multiples composantes physiques. Pour comparer avec les observations, un modèle de transfert radiatif précis et complexe de ces régions est nécessaire. J'ai développé au cours de cette thèse une application autonome appelée GASS (Generator of Astrophysical Sources Structures, Quénard et al., soumis) à cette fin. Grâce à son interface, GASS permet de créer, de manipuler et de mélanger différents composants physiques tels que des sources sphériques, des disques et des outflows. Dans cette thèse, j'ai utilisé GASS pour travailler sur différents cas astrophysiques et, entre autres, j'ai étudié en détail l'eau et l'émission de l'eau deutérée dans le coeur pré-stellaire L1544 (Quénard et al., 2016) ainsi que l'émission des ions dans la proto-étoile de faible masse IRAS16293-2422 (Quénard et al., soumis). / The era of interferometric observations leads to the need of a more and more precise description of physical structure and dynamics of star-forming regions, from pre-stellar cores to proto-planetary disks. The molecular and dust continuum emission can be described with multiple physical components. To compare with the observations, a precise and complex radiative transfer modelling of these regions is required. I have developed during this thesis a standalone application called GASS (Generator of Astrophysical Sources Structures, Quénard et al., submitted) for this purpose. Thanks to its interface, GASS allows to create, manipulate, and mix several different physical components such as spherical sources, disks, and outflows. In this thesis, I used GASS to work on different astrophysical cases and, among them, I studied in details the water and deuterated water emission in the pre-stellar core L1544 (Quénard et al., 2016) and the emission of ions in the low-mass proto-star IRAS16293-2422 (Quénard et al., submitted).

Multiple coherent supernova explosions (SNe) in an OB association can produce a strong shock that moves through the interstellar medium (ISM). These shocks fronts carve out hot and tenuous regions in the ISM known as superbubbles. The density contour plot at three different times (0.5 Myr (left panel), 4 Myr (middle panel), 9.5 Myr (right panel)) showing different stages of superbubble evolution for n0 = 0.5 cm−3, z0 = 300 pc, and for NOB = 104. This density contour plot is produced using ZEUS-MP 2D hydrodynamic simulation with a resolution of 512 × 512 with a logarithmic grid extending from 2 pc to 2.5 kpc. For a detailed description of this figure, see Roy et. al., 2015. The evolution of a superbubble is marked by different phases, as it moves through the ISM. Consider an OB association at the center of a disk galaxy. Initially the distance of the shock front is much smaller than the disk scale height. The superbubble shell sweeps up the ISM material, and once the amount of swept up material becomes comparable to the ejected material during SNe, the superbubble enters a self-similar phase (analogous to the Sedov-Taylor phase of individual SNe). As the superbubble shell sweeps up material, its velocity decreases, and thus the corresponding post-shock temperature drops. At a temperature of ∼ 2 × 105 K (where the cooling function peaks), the superbubble shell becomes radiative and starts losing energy via radiative cooling. This radiative phase is shown in the left panel of Figure 1. The superbubble shell starts fragmenting into clumps and channels due to Rayleigh-Taylor instabilities (RTI) (which is seeded by the thermal instability; for details see Roy et. al., 2013) when the superbubble shell crosses a few times the scale height. This is represented in the middle panel of the same figure. At a much later epoch, RTI has a strong effect on the shell fragmentation and the top of the bubble is completely blown off (the right panel). In the first chapter of the thesis (reported in Sharma et. al., 2014), we show using ZEUS-MP hydrodynamic simulations that an isolated supernova loses almost all its mechanical energy within a Myr whereas superbubbles can retain up to ∼ 40% of the input energy over the lifetime of the starcluster (∼ few tens of Myr), consistent with the analytic estimate of the second chapter. We also compare different recipes (constant luminosity driven model (LD model), kinetic energy driven model (KE model) to implement SNe feedback in numerical simulations. We determine the constraints on the injection radius (within which the SNe input energy is injected) so that the supernova explosion energy realistically couples to the interstellar medium (ISM). We show that all models produce similar results if the SNe energy is injected within a very small volume ( typically 1–2 pc for typical disk parameters). The second chapter concentrates on the conditions for galactic disks to produce superbubbles which can give rise to galactic winds after breaking out of the disk. The Kompaneets formalism provides an analytic expression for the adiabatic evolution of a superbubble. In our calculation, we include radiative cooling, and implement the supernova explosion energy in terms of constant luminosity through out the life-time of the OB stars in an exponentially stratified medium (Roy et. al., 2013). We use hydrodynamic simulations (ZEUS-MP) to determine the evolution of the superbubble shell. The main result of our calculation is a clear demarcation between the energy scales of sources causing two different astrophysical phenomenon: (i) An energy injection rate of ∼ 10−4 erg cm−2 s−1 (corresponding Mach number ∼ 2–3, produced by large OB associations) is relevant for disk galaxies with synchrotron emitting gas in the extra-planar regions. (ii) A larger energy injection scale ∼ 10−3 erg cm−2 s−1, or equivalently a surface density of star formation rate ∼ 0.1 M⊙ yr−1 kpc−2 corresponding to superbubbles with high Mach number (∼ 5–10) produces galactic-scale superwinds (requires superstar clusters to evolve coherently in space and time). The stronger energy injection case also satisfies the requirements to create and maintain a multiphase halo (matches with observations). Roy et. al., 2013 also points out that Rayleigh-Taylor instability (RTI) plays an important role in the fragmentation of superbubble shell when the shell reaches a distance approximately 2–3 times the scale-height; and before the initiation of RTI, thermal instability helps to corrugate the shell and seed the RTI. Another important finding of this chapter is the analytic estimation of the energetics of superbubble shell. The shell retains almost ∼ 30% of the thermal energy after the radiative losses at the end of the lifetime of OB associations. The third chapter considers the escape of hydrogen ionizing (Lyc) photons arising from the central OB-association that depends on the superbubble shell dynamics. The escape fraction of Lyc photons is expected to decrease at an initial stage (when the superbubble is buried in the disk) as the dense shell absorbs most of the ionizing photons, whereas the subsequently formed channels (created by RTI and thermal instabilities) in the shell creates optically thin pathways at a later time (∼ 2–3 dynamical times) which help the ionizing photons to escape. We determine an escape fraction (fesc) of Lyc photons of ∼ 10 ± 5% from typical disk galaxies (within 0 ≤ z (redshift) ≤ 2) with a weak variation with disk masses (reported in Roy et. al., 2015). This is consistent with observations of local galaxies as well as constraints from the epoch of reionization. Our work connects the fesc with the fundamental disk parameters (mid-plane density (n0), scale-height (z0)) via a relation that fescαn20z03 (with a ≈ 2.2) is a constant. In the fourth chapter, we have considered a simple model of molecule formation in the superbubble shells produced in starburst nuclei. We determine the threshold conditions on the disk parameters (gas density and scale height) for the formation of molecules in superbubble shells breaking out of disk galaxies. This threshold condition implies a gas surface density of ≥ 2000 M⊙ pc−2, which translates to a SFR of ≥ 5 M⊙ yr−1 within the nuclear region of radius ∼ 100 pc, consistent with the observed SFR of galaxies hosting molecular outflows. Consideration of molecule formation in these expanding superbubble shells predicts molecular outflows with velocities ∼ 30–40 km s−1 at distances ∼ 100–200 pc with a molecular mass ∼ 106–107 M⊙, which tally with the recent ALMA observations of NGC 253. We also consider different combinations of disk parameters and predict velocities of molecule bearing shells in the range of ∼ 30–100 km s−1 with length scales of ≥ 100 pc, in rough agreement with the observations of molecules in NGC 3628 and M82 (Roy et. al., 2016, submitted to MNRAS).

In this thesis we test and expand our current knowledge of Star Formation Laws (SF laws) in the extreme environment of dwarf irregular galaxies. We focus on the SF characteristics of our 18 galaxies sample, extending current investigations of the Schmidt-Kennicutt law to the low luminosity, low metallicity regime. The Hi data used in this project have been observed, calibrated and imaged according to the LITTLE THINGS Survey prescription to which I brought my own contribution as a member of the team. Apart from high resolution, VLA data in B, C and D array configurations, this project makes use of an extensive set of multi- wavelength data (H , FUV, 24 m, 3.6 m, V-band and K-band). Molecular gas in dwarfs is very difficult to observe, mainly because due to the low metallicity environment, we lose our only molecular tracer, the CO which becomes under luminous. Therefore the gas distribution is represented by Hi gas only. We create our Star Formation Rate (SFR) maps mainly based on FUV maps because our analysis shows that FUV is the SF tracer that allows us the most extensive sampling of the SFR surface density (SFRD) and Hi surface density relation. The main results of our study are: Whereas in spiral galaxies Bigiel et al. (2008) have found a one to one relation between star formation rate and molecular gas and no relation between the SFR and the neutral gas, in a small sample of dwarfs as well as in the outskirts of spiral galaxies Bigiel et al. (2010b) has found that SFRD does correlate with Hi surface density. We confirm the existence of the SFRD vs. Hi surface density relation in dwarf irregular galaxies and a linear fitting through all our data (all 18 galaxies combined) yields a power law relation ΣSFR ∝ Σ1.87±0.3/HI . We find that the interiors of Hi shells, at 400 pc scales, become resolved and show up in SFRD versus Hi surface density plots although within the shell interior we have SFRD values but no Hi surface density related to them. Thus, the points originating from those regions contribute significantly to the increase of the scatter in the plot. We show that by excluding those points the correlation between SFRD and Hi surface density improves between 10% and 20%. Eight of the 18 galaxies in our sample have Hi maxima higher than the 10M pc-2 value found by Bigiel et al. (2008) for spiral galaxies. Krumholz et al. (2011) predicted that the 10M pc-2 threshold is metallicity dependent in galaxies with sub-solar metallicity, however the theoretically predicted values for our galaxies only match the observed Hi maxima in one case (DDO168). We find that metallicity cannot be the only factor setting the Hi to H2 transition. In fact, we find evidence that the higher the interstellar radiation field (ISRF), the higher the Hi maximum is, hence we suggest that the ISRF should also be taken into consideration in predicting the Hi to H2 transition threshold. We find that even tighter than the SFRD vs. Hi surface density relation is the SFRD vs. V-band surface density relation. Unlike the SFRD vs. Hi surface density relation the SFRD vs. V-band surface density relation follows a power law and can be written as follows: ΣSFR ∝ (10^μv)^-0.43±0.03. The SFRD vs. V-band surface density relation suggests that the existing stars also play a role in the formation of the next generation of stars. Within our sample of dwarf galaxies the average pressure per resolution element and the SFRD are in a 1:1 linear relation: ΣSFR ∝ P_h^1.02±0.05. A similar relation has been found by Blitz & Rosolowsky (2006) for the low-pressure regimes of spiral galaxies. In conclusion we find that in the extreme environments of dwarf galaxies the metal deficiency and the lack of the classic SF stimulators (spiral arms, shear motions) do not impede the star forming process. In these galaxies, dust-shielding becomes predominantly self-shielding and there is plenty of Hi available to achieve this additional task. Existing stars assume the role of pressure enhancers, which in turn will stimulate SF without the need of spiral arms or shear motion.

Le milieu interstellaire ainsi que certaines atmosphères de corps planétaires, en particulier Titan, un des plus grands satellites du système solaire, présentent une grande diversité d'espèces chimiques. Cette chimie complexe est très différente de celle que nous connaissons sur Terre. Pour comprendre les phénomènes globaux qui s'y déroulent, une connaissance des réactions chimiques, de leur vitesse et de leurs produits est requise. A l'aide de la technique CRESU, nous sommes capables de reproduire certaines conditions des milieux les plus froids de l'espace et d'étudier ces réactions. / The interstellar medium and some atmospheres of planetary bodies, in particular Titan, one of the largest satellites of Saturn, present a large variety of chemical species. This complex chemistry is very different from the one we know on Earth. To understand the global phenomenon that happen in these environments, we need to understand the chemical reactions, their reaction rate and their products. With the CRESU technique, we are able to reproduce partially the coldest environments of space to study these reactions.

En Astrophysique, la compréhension du processus de formation d'étoiles reste l'une des principales questions. Elle est directement reliée à l'évolution du gaz interstellaire dans les galaxies, et en particulier aux processus de refroidissement et de condensation pour lesquels la turbulence et l'instabilité thermique jouent un rôle dominant. Ce travail se concentre sur l'évolution du gaz atomique et diffus qui fournit les conditions initiales à la formation des nuages moléculaires et se base sur une comparaison étroite entre observations à 21 cm et simulations numériques hydrodynamiques. Pour comprendre les rôles de l'instabilité thermique et de la turbulence dans la transition du gaz chaud (WNM, T ~ 8000 K, n = 0.5 cm-³) vers le gaz froid (CNM, T ~ 80 K, n = 50 cm-³), j'ai produit 90 simulations à basse résolution qui ont permis d'étudier l'influence de la densité initiale du WNM et de la compressibilité du forçage de la turbulence sur l'efficacité de la production de CNM. Un résultat important permet de conclure que le gaz chaud, dans les conditions de turbulence caractéristiques de ce qui est observé, ne transite pas vers le gaz froid quelque soit l'amplitude de la turbulence. Ces simulations à basse résolution ont aussi permis de déterminer quelles conditions initiales permettent de reproduire les propriétés déduites des observations telles que le nombre de Mach, la quantité de CNM en masse ou la dispersion de vitesse turbulente. Un processus de compression, que l'on peut reproduire soit en augmentant la densité initiale du WNM (n ≥ 1.5 cm-³) soit en appliquant un champ de forçage compressif, est nécessaire. Ces conditions initiales ont ensuite été utilisées pour produire deux simulations à haute résolution (1024³) pour lesquelles j'ai montré que les propriétés de la turbulence et de l'instabilité du milieu atomique neutre sont bien reproduites. Les histogrammes de température portent en effet la trace d'un milieu biphasique et les distributions de pression sont semblables aux observations. D'autre part, les spectres de puissance de la densité sont caractéristiques d'un milieu fortement contrasté alors que ceux de la vitesse restent caractéristiques d'une turbulence subsonique. Finalement, les structures froides de ces deux simulations reproduisent les relations masse-échelle et dispersion de vitesse-échelle observées dans les nuages moléculaires, suggérant que la structure des nuages moléculaires pourrait être héritée de celle des nuages de HI à partir desquels ils se sont formés. Le dernier aspect de mon travail est relié à la difficulté rencontrée lors de l'interprétation des données qui n'est possible qu'à partir de grandeurs projetées en deux dimensions. J'ai donc comparé en détails les deux simulations à haute résolution à des observations de cirrus en créant des observations artificielles à 21 cm. Les spectres d'émission et les cartes de densité de colonne ainsi produits sont semblables aux observations. De plus, les simulations donnant accès à l'information en trois dimensions, j'ai étudié les effets de l'auto-absorption dans la création de cartes de densité de colonne à partir de spectres de température de brillance. J'ai conclu de cette étude que l'auto-absorption ne peut être négligée mais qu'elle ne concerne que les lignes de visée les plus brillantes et les plus denses et que la correction habituellement appliquée sur les observations est efficace. Finalement, j'ai appliqué une méthode de décomposition en gaussiennes sur les spectres synthétiques. Cette méthode a pour objectif d'étudier les propriétés de chacune des deux phases thermiques du HI. Les résultats montrent qu'elle est prometteuse pour l'analyse des données de spectro-imagerie à 21 cm, bien que nécessitant des améliorations. Elle permet en effet de bien séparer les phases chaude et froide du milieu atomique et d'en déduire la distribution massique de chacune d'elles. / One of the main current questions in Astrophysics is the understanding of the star formation process, directly related to the processes involved in the cooling and the condensation of the gas yielding to intricate filamentary structures of molecular clouds. Thermal instability and turbulence are playing dominant roles in this complex dynamics. The work presented here is focused on the evolution of the atomic and diffuse interstellar medium that provides the initial conditions to the formation of molecular clouds and is based on the comparison of hydrodynamical numerical simulations and observations. To understand the roles of thermal instability and turbulence in the WNM (warm neutral medium, T ~ 8000 K, n = 0.5 cm-³) to CNM (cold neutral medium, T ~ 80 K, n = 50 cm-³) transition, I produced 90 hydrodynamical numerical simulations of thermally bistable HI and used them to study the impact of the WNM initial density and the compressibility of the turbulent stirring on the efficiency of the CNM production. The main result here is that the warm gas in the observed turbulent conditions do not transit naturally to cold gas whatever the amplitude of turbulent motions. These small resolution simulations also allowed me to determine which initial conditions lead to the reproduction of the observed properties, as the Mach number, the amount of CNM or the amplitude of the turbulent motions. A compression is needed to trigger this transition either by increasing the initial density (n ≥ 1.5 cm-³) or by stirring with a compressive field. These initial conditions have been used to produce two high resolution simulations (1024³). I showed that these two simulations reproduce well the properties of the turbulence and the thermal instability. The temperature histograms present the evidences of a bistable gas and the pressure distributions are in agreement with the observations. On the other hand, the power spectra of the density are characteristic of a high contrasted medium while the power spectra of the velocity remain characteristic of subsonic turbulence. Finally the cold structures of these two simulations reproduce well the mass-size and velocity dispersion-size relations observed in molecular clouds. This suggests that the molecular cloud structure could be inherited from the clouds of atomic gas from which they are born. One of the main limitations in the analysis of observations comes from the fact that it can only be done on integrated quantities in two dimensions. In the last part of my work I compared the two high resolution simulations to observations by creating synthetic 21 cm observations. The emission spectra and column density maps produced in that way are similar to the ones observed. Besides, with the three dimensional informations, I was able to study the effect of the self-absorption in the creation of the column density maps from the brightness temperature spectra. I concluded from this study that the self-absorption cannot be neglected but that it only concerns the brightest and densest lines of sight and that the correction usually applied on observations is efficient. Finally I applied a method of gaussian decomposition on the synthetic spectra. This method has been build to study the properties of each thermal phase in the HI. The results show that it is a highly promising method for the analysis of 21 cm spectro-imaging data even if some improvements are needed. Indeed, it allows a good separation of the cold and warm phases of the atomic medium and a reasonable deduction of the massive distribution of each one.

Les Antennes sont une des fusions de galaxies les plus connues dans l’Univers proche. Sa proximité nous permet d’observer et d’étudier ses gaz à l’échelle de la formation des amas stellaires. C’est une source idéale pour comprendre comment la dynamique dans les fusions de galaxies déclenche la formation d’étoiles. La plupart des étoiles dans les Antennes sont formées dans des amas stellaires compacts et massifs, surnommés super-star clusters (SSC). Les SSC les plus massifs (>106 M⊙) et les plus jeunes (<6 Myr) sont situés dans la région de collision entre les deux galaxies et sont associés aux complexes moléculaires massifs (~108 M⊙) et super-géants (des centaines de pc) (super-giant molecular clouds, SGMCs). La formation de SSC doit impliquer une intéraction complexe entre la dynamique des gaz et une turbulence entraînée par la fusion des galaxies, et la dissipation de l’énergie cinétique des gaz. Dans les SGMC, une hiérarchie de structures doit être produite, incluant des concentrations denses et compactes de gaz moléculaires qui sont suffisamment massifs pour former un SSC, des nuages pre-cluster clouds (PCC). La formation des étoiles se produira si l’énergie mécanique des PCC est émise dans le lointain, permettant à l’auto-gravité de gagner localement les pressions thermique et turbulente du gaz. Des diagnostics spécifiques de dissipation turbulente sont donc des éléments essentiels pour tester la validité de ce scénario.J’étudie la région d’intéraction des Antennes. J’utilise des observations avec le spectro- imageur SINFONI sur le VLT (raies rovibrationnelles de H2) et ALMA (raie CO(3–2) et l’émission du continuum de la poussière). Les données ont des résolutions angulaires pour résoudre les échelles de la formation des SSC et des résolutions spectrales pour résoudre les mouvements à l’intérieur du SGMC. La combinaison des raies CO et H2 est essentielle dans mon travail. J’utilise le CO comme traceur de la distribution et de la cinématique du gaz moléculaire, et H2 comme traceur du taux de dissipation d’énergie mécanique de gaz.Ma thèse se concentre sur des sources traçant des différentes étapes de la formation d’étoiles : le rassemblement des gaz pour former des SGMCs, la formation des PCC dans les SGMCs et la destruction des nuages moléculaires par les SSC. Je montre que la turbulence joue un rôle essentiel à chaque étape. J’ai trouvé que l’énergie cinétique de rencontre des deux galaxies n’est pas thermalisée dans les chocs aux échelles où elle est injectée. Elle entraîne une turbulence dans l’ISM moléculaire à un niveau beaucoup plus élevé que celui observé dans la Voie Lactée. Sauf dans les SSC encore intégrés dans les nuages moléculaires, la raie de H2 est produite par des chocs et trace la dissipation de l’énergie cinétique turbulente du gaz. J’associe l’émission de H2 à la perte d’énergie cinétique nécessaire pour former des nuages gravitationnellement liés. Cette interprétation est étayée par la découverte d’une source lumineuse et compacte en H2, qui n’est associée à aucun SSC connu, située là où les données montrent le plus grand gradient de vitesse. À notre connaissance, c’est la première fois qu’une source extragalactique avec ces caractéristiques est identifiée. Nous observons la formation d’un nuage suffisamment massif pour former un SSC. Les données montrent également la destruction d’un nuage moléculaire par un SSC récemment formé. Sa matière est faiblement liée. Sa gravité serait soutenue par la turbulence, ce qui rend plus facile pour les mécanismes de rétroaction de perturber le nuage parent.Enfin, je présente deux projets. Je propose d’établir d’autres traceurs de dissipation d’énergie observables avec ALMA, proposition du Cycle 1 acceptée en première priorité. Je propose également d’étendre mon travail pour étudier la formation des étoiles entraînées par la turbulence dans différentes sources extragalactiques en combinant les observations dans le proche infrarouge et submillimétrique. / The Antennae (22 Mpc) is one of the most well-known mergers in the nearby Universe. Its distance allow us to observe and study the gas at the scales of stellar cluster formation. It is an ideal source to understand how the galaxy dynamics in mergers trigger the formation of stars. Most of the stars in the Antennae are formed in compact and massive stellar clusters, dubbed super-star clusters (SSCs). The most massive (>106 M⊙) and youngest (<6 Myr) SSCs are located in the overlap region, where the two galaxies collide, and are associated with massive (several 108 M⊙) and super-giant (few hundred of pc) molecular complexes (SGMCs). The formation of SSCs must involve a complex interplay of merger-driven gas dynamics, turbulence fed by the galaxy interaction, and dissipation of the kinetic energy of the gas. Within SGMCs, a hierarchy of structures must be produced, including dense and compact concentrations of molecular gas massive enough to form SSCs, pre-cluster clouds (PCCs). For star formation to occur, the mechanical energy of PCCs must be radiated away to allow their self-gravity to locally win over their turbulent gas pressure. Specific tracers of turbulent dissipation are therefore key inputs to test the validity of this theoretical scenario. In my thesis, I studied the Antennae overlap region. My work is based on observations with the SINFONI spectro-imager at the VLT, which includes H2 rovibrational and Brγ line emission, and with ALMA, which includes the CO(3-2) line and dust continuum emission. Both data-sets have the needed sub-arcsecond angular resolution to resolve the scales of SSC formation. The spectral resolutions are enough to resolve motions within SGMCs. Combining CO and H2 line emission is key in my PhD work. I use CO as a tracer of the distribution and kinematics of the molecular gas, and H2 as a tracer of the rate at which the gas mechanical energy is dissipated.My thesis focuses on diverse sources in the Antennae overlap region which trace different stages of star formation: the gathering of mass necessary to form SGMCs, the formation of PCCs within SGMCs and the disruption of a parent cloud by a newly formed SSC. I show that at each stage turbulence plays a key role. I found that the kinetic energy of the galaxies is not thermalized in large scale shocks, it drives the turbulence in the molecular ISM at a much higher level than what is observed in the Milky Way. Near-IR spectral diagnostics show that, outside of SSCs embedded in their parent clouds, the H2 line emission is powered by shocks and traces the dissipation of the gas turbulent kinetic energy. I relate the H2 emission to the loss of kinetic energy required to form gravitationally bound clouds. This interpretation is supported by the discovery of a compact, bright H2 source not associated with any known SSC. It has the largest H2/CO emission ratio and is located where the data show the largest velocity gradient in the interaction region. To our knowledge, this is the first time that an extragalactic source with such characteristics is identified. We would be witnessing the formation of a cloud massive enough to form a SSC. The data also allow us to study the disruption of a parent molecular cloud by an embedded SSC. Its matter is loosely bound and its gravity would be supported by turbulence, which makes it easier for feedback to disrupt the parent cloud. I end my manuscript presenting two projects. I propose to establish additional energy dissipation tracers observable with ALMA, which gives us the high spatial and spectral resolution needed to isolate scales at which clusters form. This is a Cycle 1 proposal accepted in first priority. I also plan to expand my work to other nearby extragalactic sources by investigating the turbulence-driven formation of stars in different extragalactic sources by combining near-IR and submillimeter observations.

This thesis is concerned about ion-molecular reactions at low temperatures, which are important the fully understand the chemical evolution in interstellar medium. For realization of experimental part of thesis has been used the apparatus of 22-pole radiofrequency ion trap, which allows study the rate constant of reactions at temperatures 10 - 100 K. Thesis contains measuring results of reaction NH+ + H → N+ + H2, which follows the previous study of reaction N+ + H2 → NH+ + H on the same apparatus, description of the apparatus and the general introduction.

In the present work, the results of the experimental study of reactions of ions with molecular hydrogen in the temperature range 15 - 300 K using a 22-pole ion trap apparatus are presented. The reaction of OD- with para-enriched hydrogen was studied using a combination of the 22-pole ion trap apparatus with a para-hydrogen generator. Also reactions of O- with H2, D2, and HD were studied. These reactions have a channel of water production and a channel of hydrogen or deuterium transfer. Another field of study was a sequence of reactions of oxygen hydride cations with H2 and D2 which leads to the production of H3O+ or its isotopic variant, specifically reactions OH+ with H2, H2O+ with H2, D2O+ with H2, and D2O+ with D2. This reaction chain can be followed by the electron recombination of H3O+ or its isotopologue, which has a channel of water production.