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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
271

A Radio Study Of Gas Loss Processes In Nearby Galaxies

Hota, Ananda 06 1900 (has links)
The work in this thesis involves detailed multi-frequency radio continuum (from 325 MHz to 15 GHz) and Hi spectroscopic studies of a few represent tative nearby galaxies which are experiencing gas-loss from their disks due to different physical processes. These processes are starburst-driven superwind, active galactic nucleus (AGN) −driven nuclear outflow, ram pressure stripping and tidal interactions. Gas-loss could affect the evolution of individual galaxies with age as well as their evolution with cosmic epoch. We have made use of both the Giant Metrewave Radio Telescope (GMRT) and the Very Large Array (VLA) for our study. Stars and gases are the two major constituents of a galaxy. The properties of the gaseous medium in them change with galaxy-types, such as the presence of large amount of gas in the late type disk galaxies and little interstellar medium (ISM) in the ellipticals or early−type galaxies. Galaxies in groups and clusters interact with each other and with the low density gaseous medium surrounding them, resulting in a possible evolution of their structures and physical properties. Basic differences in their observed properties depend on their history of interactions with the environment and also on the history of their central activities. Tidal interaction among galaxies may result in the flow of gas into the central region of the galaxy. This can trigger a central starburst and/or feed a central super-massive black hole to trigger an AGN activity. These activities produce either starburst-driven superwind or AGN-driven nuclear outflows (accretion disk wind or bipolar radio jet/bubbles) and the galaxy may lose their metal-enriched central gas concentration to the intra-cluster (ICM) or intergalactic medium (IGM). There are suggestions that when large amount of gas is cleared out from the central region of an ultra-luminous infrared galaxy (ULIRG), the dust enshrouded hidden AGN may unveil itself as a bona fide quasi-stellar object (QSO). Galaxies also lose gas usually from the outer parts when they move through the intra-cluster medium (ICM) or intragroup medium (IGrM) due to ram pressure stripping. When the ram pressure is stronger than the pressure by which the gas is bound to the galaxy, most of the gas may be lost or displaced from the disk thus affecting the star formation and metal formation in the disk of the galaxy. Starburst galaxies and superwinds: The starformation rate or the supernovae rate in some gas rich galaxies are 10−1000 times higher than that of the normal galaxies. This process can consume large amount (1−30×109 M) of gas over a short time scale (107−8yr) in a small region (typically 1 kpc). During such bursts of starformation, the cumulative effect of many supernovae and stellar winds from the massive stars in the central region of a disk galaxy imparts huge amount of mechanical energy to the ISM of the galaxy which then creates a high pressure (4 orders of magnitude higher then the average ISM pressure in the Milky way), high temperature (106−7 K) bubble of gas. This high pressure and high temperature bubble of gas expands and flows outwards in the direction of the steepest pressure gradient. This outflowing hot gas carries cooler gas and dust from the ISM along with it. It carries out heavy metals cooked in the central region of a galaxy to the external environment including the ICM or IGM. The typical outflow rate is 10−100Myr−1 with typical outflow velocity of few 100−1500 km s−1and it persists over few to 10 million yr. The observed sizes of such outflows are 1−20 kpc. We have studied a remarkable starburst-superwind system, NGC1482. This early type galaxy has been discovered to have a bi-conical shaped soft X-ray outflow as well as Hα and [Nii] outflow. The low-frequency radiocontinuum flux density was used to estimate the supernova rate, which could be used to constrain the dynamics of the driving force. The high-frequency high-resolution VLA observations revealed the central starforming region which is at the base of the bi-conical structure and presumably driving this outflow. The Hi observations also have resulted in finding two blobs of Hi on opposite sides of the galaxy defining an axis perpendicular to the outflow axis and rotating about it. We have also detected a multi-component broad (∼ 250 km s−1) Hi-absorption spectrum against the central continuum source. The absorption spectrum is nearly 70 km s−1 asymmetric towards the blue side with a component blue-shifted by 120 km s−1. The observed absorption could be due to the Hi-clouds driven outward by the central superwind outflow. Active galaxies and nuclear outflows: The inflow of gas to the central region of a galaxy triggers the starformation as well as the AGN activity. Co-existence of both these phenomena in some cases are well known. The accretion disk of the AGN can produce a hot wind emitting in X-rays in addition to the radio continuum jet perpendicular to the accretion disk. It is known that the AGN jets, in particular Seyfert jets have no correlation with the rotation axis of the host disk-galaxy. In such a scenario a very complex geometry can arise. Radio jets may interact with the starburst-driven winds or winds from the accretion disk or with the clouds of ISM of the galaxy accelerating and ionizing the gas. Sometimes the mass outflow rate may be ten times the mass accretion rate necessary to fuel the AGN, suggesting heavy mass loading of these AGN outflows. Depending on the energy of the outflow processes gas may be ejected out of the gravitational field of the galaxy. We have studied a composite galaxy, NGC6764 with an AGN and a very young starburst with two episodes of starformation, one 3−5 and another 15−50 Myr ago. The high-resolution high-frequency radio-continuum obser- vations reveal a radio core and roughly bi-conical radio emission oriented along the major axis of the galaxy with a feature which could be a jet. The lower-resolution images show bi-polar radio bubbles oriented along the minor-axis of the galaxy. These edge-brightened radio bubbles are asymmetric in size, extent (north-south) and luminosity. There is an east-west asymmetry in the spectral index of the bubbles. In a representative sample of dozen such radio bubbles in nearby galaxies we find that invariably all of these have an AGN. Similar to few other galaxies in the sample we also found that the radio continuum bubbles in NGC6764 are well correlated with the Hα filaments extending along the minor-axis of the galaxy. The CO(J=2-1) and CO(J=1-0) flux density ratio is also higher at the tip of the bubbles. Molecular gas plumes are seen extending along minor axis and have components blue-shifted by 140 km s−1. Our high-resolution Hi observations also show an absorption component at the systemic velocity with a weak component blue shifted by 120 km s−1. We have discussed the possibility of the radio plasma ejected from the AGN being carried outwards along the minor-axis by the superwind created by the young circumnuclear starburst. That bubble of hot gas from the superwind mixed with the relativistic plasma from the AGN is interacting with the cooler Hi and molecular gas of the ISM and driving it outwards. This interaction which is possibly in an early phase of expansion is also giving rise to the outflowing Hα filaments in this interesting composite galaxy. Cluster galaxies and stripping processes: When a galaxy moves through the hot and dense ICM with velocities 1000 km s−1, the ram pressure exerted by the ICM can strip the loosely bound and more tenuous gas of the galaxy. As a result of this the dense molecular gas or the stars in the galaxy remain almost unaffected but the tenuous gas moving out of the galaxy’s gravitational field could reach the ICM. The fate of such stripped gas is not well constrained. Recently very long tails with sizes of 50−125 kpc have been discovered. Some of these are magnetised, some ionised, some neutral and some are million degree hot. These tails may cool and eventually form galaxies or may evaporate and mix with the ICM. They enrich the ICM with metals and magnetic fields. In some cases galaxies are known to have become as high as 90 % deficient in Hi in comparison to the corresponding field galaxy of same type, size and luminosity. In the cluster/group environment tidal interactions with other group/cluster members or the cluster potential well could also take place affecting the observed properties of the galaxy. In addition, tidal interactions could also facilitate the removal of gas by ram pressure due to the ICM or IGrM. NGC4438 which we have studied in detail is an archetypal example of a galaxy which has been severely affected by the cluster environment. This late-type galaxy in the central region of the Virgo cluster is known to have interacted with the northern companion NGC4435. We have unambiguously resolved the radio nucleus from the lobes of radio continuum emission, and have shown it to have an inverted spectrum confirming it to be the nucleus. The lobes are almost perpendicular to the central molecular or stellar disk which is seen nearly edge-on. Projected onto the plane of the sky the lobeaxis is roughly parallel to the direction of the ram pressure wind. The lobes are very asymmetric in its extent, size and luminosity. The lobes are shelllike in structure and are interacting strongly with the asymmetric ISM. In the region of interaction both Hα and soft X-ray emission shells are seen. We explore possible reasons for the asymmetry in the lobes which is unlikely to be only due to the asymmetry in the density of the ISM on opposite sides of the galaxy. On a larger scale we have imaged the diffuse lower-frequency radio-continuum emission 5 kpc away from the central region seen on the western side of the disk of the galaxy. This extended emission has flatter spectral index at higher frequencies which suggests it to be a mixture of thermal and non-thermal components. In this region Hα, soft X-ray, Hi, molecular gas and relativistic plasma (i.e. all phases of the ISM) have been detected. We have found a linear structure on the western side near the same region with mass of nearly 200 million M We have imaged the Hi−emission from the stellar disk for the first time. The Hi -velocity field shows that the extra-planar gas could be rotating slower then the disk as seen in cases of ram pressure stripping. At lower resolution we detect more Hi from the halo of the galaxy. The iso-velocity contours appear to curve towards the axis of rotation or direction of the ram pressure wind, as you go away from the mid plane. We discuss whether this might be due due to the interaction of NGC4438 with NGC4435. We have discovered a 50 kpc long faint tail of Hi having a mass of 140 million solar mass to the north-west of the NGC4438−NGC4435 system. This Hi−tail partially coincides with an extremely faint (µv> 28) stellar tail, which has been seen in the deep optical imaging of intra-cluster light. Such tails have not been predicted by the simulations of interaction between NGC4435 and NGC4438. Hence it seems to be a remnant of some past event in the evolution of this interesting system. To further study the effects of ram pressure stripping and tidal interaction in galaxies in a group, we have studied the group Ho 124. We found that the radio continuum bridge of tidal interaction between NGC2820 and NGC2814 has a very steep (α=−1.8) spectrum possibly due to the older relativistic plasma left in it. The Hi of NGC2820 has sharp truncation on the southeastern side parallel to the edge on disk, while it has a unipolar huge loop on the north-west. NGC2814 has both an Hi and radio continuum tail different from the connecting bridge with sharp truncation again on the side opposite to the tail. Although there is reasonable radio continuum emission from the disk of NGC2820, there is no detectable emission corresponding to the huge one sided Hi loop. The velocity field of the Hi-loop trails that of the underlying stellar disk. Also in the galaxy NGC2805, a member of the same group, we find the Hi to have accumulated on the northern side while there is a bow-shock shaped starformation arc on the southern side of the disk. All these features namely starformation arc, sharp cut off in the Hi-disk, Hi-loop and Hiand radio continuum tails are signatures of ram pressure stripping. Ram pressure stripping in groups is relatively rare but this could get assistance from tidal interactions which help loosen the gravitational bound of the stellar disk on the tenuous ISM. A more spectacular case of ram pressure stripping is seen in the cluster Abell 1367. We have studied a region of the cluster A1367 where three of its galaxies namely CGCG 09773, CGCG 09779 and CGCG 09787 exhibit amazingly long (50−75 kpc) tails of radio continuum and optical emission lines (Hα) pointing roughly away from the cluster centre. They also show arcs of starformation on the side facing the ram pressure of the cluster medium. In our Histudy we found that all three of them have higher mass of Hi on the down-stream side. Two of the galaxies (CGCG 09773 and CGCG 09779) exhibit sharper gradients in Hiintensity on the side of the tail or on the down-stream side. However the Hi emission in all the three galaxies extends to much smaller distances than the radio-continuum and Hαtails, and are possibly still bound to the gravitational pull of the respective galaxies. These results are in good agreement with the hydrodynamical simulations of ram pressure stripping in cluster medium. In this study we have found a number of interesting results on a few nearby galaxies where different gas-loss processes have modified the morphology and kinematics of the ISM and/or the stellar distribution of the respective parent galaxies. We have found evidence of blue-shifted Hi absorption lines driven outwards by the starburst-driven superwinds and/or AGN-driven nuclear outlows. The synchrotron plasma outflowing from an AGN in a composite galaxy has been suggested to be interacting with the superwind which also drives other components of the ISM outwards. In groups or clusters of galaxies we have discovered an Hiloop, Hitails, regions of compressed Hi, trailing velocity fields, slow-rotating extra-planar gas, displaced ISM and asymmetries in various radio continuum or Hifeatures as evidences of ram pressure stripping mechanism affecting the member galaxies. The results obtained from this study illustrates the manifestations of gas loss proceeses in galaxies existing in different environments, and should provide valuable insights for future investigations with larger statistical samples towards a more complete understanding of gas loss processes in galaxies and their implications on galaxy evolution
272

Evolving Starburst Model of FIR/sub-mm/mm Line Emission and Its Applications to M82 and Nearby Luminous Infrared Galaxies

Yao, Lihong 08 March 2011 (has links)
This thesis presents a starburst model for far-infrared/sub-millimeter/millimeter (FIR/sub-mm/mm) line emission of molecular and atomic gas in an evolving starburst region, which is treated as an ensemble of non-interacting hot bubbles which drive spherical shells of swept-up gas into a surrounding uniform gas medium. These bubbles and shells are driven by winds and supernovae within massive star clusters formed during an instantaneous starburst. The underlying stellar radiation from the evolving clusters affects the properties and structure of photodissociation regions (PDRs) in the shells, and hence the spectral energy distributions (SEDs) of the molecular and atomic line emission from these swept-up shells and the associated parent giant molecular clouds (GMCs) contains a signature of the stage evolution of the starburst. The physical and chemical properties of the shells and their structure are computed using a a simple well known similarity solution for the shell expansion, a stellar population synthesis code, and a time-dependent PDR chemistry model. The SEDs for several molecular and atomic lines ($^{12}$CO and its isotope $^{13}$CO, HCN, HCO$^+$, C, O, and C$^+$) are computed using a non-local thermodynamic equilibrium (non-LTE) line radiative transfer model. By comparing our models with the available observed data of nearby infrared bright galaxies, especially M 82, we constrain the models and in the case of M 82, provide estimates for the age of the recent starburst activity. We also derive the total H$_2$ gas mass in the measured regions of the central 1 kpc starburst disk of M 82. In addition, we apply the model to represent various stages of starburst evolution in a well known sample of nearby luminous infrared galaxies (LIRGs). In this way, we interpret the relationship between the degree of molecular excitation and ratio of FIR to CO luminosity to possibly reflect different stages of the evolution of star-forming activity within their nuclear regions. We conclude with an assessment of the strengths and weaknesses of this approach to dating starbursts, and suggest future work for improving the model.
273

Evolving Starburst Model of FIR/sub-mm/mm Line Emission and Its Applications to M82 and Nearby Luminous Infrared Galaxies

Yao, Lihong 08 March 2011 (has links)
This thesis presents a starburst model for far-infrared/sub-millimeter/millimeter (FIR/sub-mm/mm) line emission of molecular and atomic gas in an evolving starburst region, which is treated as an ensemble of non-interacting hot bubbles which drive spherical shells of swept-up gas into a surrounding uniform gas medium. These bubbles and shells are driven by winds and supernovae within massive star clusters formed during an instantaneous starburst. The underlying stellar radiation from the evolving clusters affects the properties and structure of photodissociation regions (PDRs) in the shells, and hence the spectral energy distributions (SEDs) of the molecular and atomic line emission from these swept-up shells and the associated parent giant molecular clouds (GMCs) contains a signature of the stage evolution of the starburst. The physical and chemical properties of the shells and their structure are computed using a a simple well known similarity solution for the shell expansion, a stellar population synthesis code, and a time-dependent PDR chemistry model. The SEDs for several molecular and atomic lines ($^{12}$CO and its isotope $^{13}$CO, HCN, HCO$^+$, C, O, and C$^+$) are computed using a non-local thermodynamic equilibrium (non-LTE) line radiative transfer model. By comparing our models with the available observed data of nearby infrared bright galaxies, especially M 82, we constrain the models and in the case of M 82, provide estimates for the age of the recent starburst activity. We also derive the total H$_2$ gas mass in the measured regions of the central 1 kpc starburst disk of M 82. In addition, we apply the model to represent various stages of starburst evolution in a well known sample of nearby luminous infrared galaxies (LIRGs). In this way, we interpret the relationship between the degree of molecular excitation and ratio of FIR to CO luminosity to possibly reflect different stages of the evolution of star-forming activity within their nuclear regions. We conclude with an assessment of the strengths and weaknesses of this approach to dating starbursts, and suggest future work for improving the model.
274

Large Eddy Simulationen von isolierten Scheibengalaxien / Large Eddy Simulations of Isolated Disk Galaxies

Braun, Harald Udo 05 December 2014 (has links)
In dieser Arbeit stelle ich ein neu entwickeltes, dynamisches Modell für das turbulente sternbildende interstellare Medium auf Skalen von einigen zehn Parsecs vor, welches den Namen MIST (Multi-phase Interstellar medium model with Star formation and Turbulence) trägt. Das Verhalten von MIST wurde mittels seiner Ein-Zonen-Gleichgewichtslösungen aber auch im Rahmen von Large Eddy Simulationen untersucht, wobei verschiedenste Beobachtungsresultate gleichzeitig reproduziert werden konnten.
275

A case for an ultra massive black hole in the galaxy cluster MS0735.6+7421

Movassaghi Jorshari, Razzi 22 June 2012 (has links)
In this work, we study the galaxy cluster MS0735.6+7421 that hosts the most energetic observed active galactic nucleus (AGN) outburst so far. Explaining this very energetic AGN outburst is found to be challenging. McNamara et al. 2009 grappled with this problem and proposed two possible solutions: either the black hole (BH) must be an ultra massive one (with mass $> 10^{10} \ \text{M}_\odot$), or the efficiency of the mass to energy conversion ($\epsilon$) should be higher than the generally assumed value of $\epsilon \sim 0.1$. However, the efficiency of the mass to energy conversion depends on the BH's spin {Benson and Babul 2009}; higher $\epsilon$ can be achieved with a higher spinning BH. Here, we explore the second solution in detail, and ask the question: How did the BH spin up to the very high spins in advance of the outburst? We also explore the attendant physical processes, such as star formation, during the spin-up mode and investigate the associated observational implications. Comparing our results with what is generally expected from simulations and observational studies suggests that for all intents and purposes, the existence of an ultra massive BH is the simplest solution. / Graduate
276

Shocks, Superbubbles, and Filaments: Investigations into Large Scale Gas Motions in Giant Molecular Clouds

Pon, Andrew Richard 25 April 2013 (has links)
Giant molecular clouds (GMCs), out of which stars form, are complex, dynamic systems, which both influence and are shaped by the process of star formation. In this dissertation, I examine three different facets of the dynamical motions within GMCs. Collapse modes in different dimensional objects. Molecular clouds contain lower dimensional substructures, such as filaments and sheets. The collapse properties of finite filaments and sheets differ from those of spherical objects as well as infinite sheets and filaments. I examine the importance of local collapse modes of small central perturbations, relative to global collapse modes, in different dimensional objects to elucidate whether strong perturbations are required for molecular clouds to fragment to form stars. I also calculate the dependence of the global collapse timescale upon the aspect ratio of sheets and filaments. I find that lower dimensional objects are more readily fragmented, and that for a constant density, lower dimensional objects and clouds with larger aspect ratios collapse more slowly. An edge-driven collapse mode also exists in sheets and filaments and is most important in elongated filaments. The failure to consider the geometry of a gas cloud is shown to lead to an overestimation of the star formation rate by up to an order of magnitude. Molecular tracers of turbulent energy dissipation. Molecular clouds contain supersonic turbulence that simulations predict will decay rapidly via shocks. I use shock models to predict which species emit the majority of the turbulent energy dissipated in shocks and find that carbon monoxide, CO, is primarily responsible for radiating away this energy. By combining these shock models with estimates for the turbulent energy dissipation rate of molecular clouds, I predict the expected shock spectra of CO from molecular clouds. I compare the results of these shock models to predictions for the emission from the unshocked gas in GMCs and show that mid-to-high rotational transitions of CO (e.g., J = 8 to 7), should be dominated by shocked gas emission and should trace the turbulent energy being dissipated in molecular clouds. Orion-Eridanus superbubble. The nearby Orion star forming region has created a large bubble of hot plasma in the local interstellar medium referred to as the Orion-Eridanus superbubble. This bubble is unusual in that it is highly elongated, is believed to be oriented roughly parallel to the galactic plane, and contains bright filamentary features on the Eridanus side. I fit models for a wind driven bubble in an exponential atmosphere to the Orion-Eridanus superbubble and show that the elongation of the bubble cannot be explained by such a model in which the scale height of the galactic disk is the typical value of 150 pc. Either a much smaller scale height must be adopted or some additional physics must be added to the model. I also show that the Eridanus filaments cannot be equilibrium objects ionized by the Orion star forming region. / Graduate / 0606 / andyrpon@gmail.com
277

The nearby young [special character] Chamaeleontis cluster as a laboratory for star formation and evolution

Lyo, A-Ran, Physical, Environmental & Mathematical Sciences, Australian Defence Force Academy, UNSW January 2004 (has links)
[Special characters cannot be displayed. Please see the pdf version of the Abstract for an accurate reproduction.] We studied the circumstellar discs, the initial mass function (IMF), mass distribution, binarity and the fundamental properties of the [special character] 9 Myr-old pre-main sequence (PMS) [special character] Chamaeleontis cluster. Using JHKL colour-colour and colour-excess diagrams, we found the circumstellar disc fraction to be [special character] 0.60 among the late-type members. Four stars with [special character] (K - L) > 0.4 were identified as experiencing ongoing accretion which was later confirmed by high-resolution spectroscopic study. Quantitative analysis of the H[special character] profiles found accretion in these four stars at rates comparable to that of two members of the similarly-aged TW Hydrae Association (TWA); rates 1 - 3 orders of magnitude lower than in younger classical T Tauri stars. Together these results suggest that, while the mass accretion rate decreases with age, PMS stars can retain their inner discs for [special character] 10 Myr. An optical photometric survey spanning 1.3 ?? 1.3 pc added two low-mass stars to the cluster inventory. Together with other recent surveys the population is likely to be significantly complete for primaries with masses M > 0.15M[special character]. The cluster now consists of 18 primaries and 9 confirmed and candidate secondaries, with [special character] 2-4 times higher multiplicity than seen in field dwarfs. The cluster IMF is consistent with that of rich young clusters and field stars. By extending the IMF to lower masses, we predict 20-29 low-mass stars and brown dwarfs may remain undiscovered. From study of the cluster???s spatial and mass distribution, we find the [special character] Cha cluster has significant mass segregation, with > 50 per cent of the stellar mass residing within the central 0.17 pc. Lastly we classified members of the cluster with low-resolution spectra, providing information about the fundamental properties of the PMS stars by comparison to standard dwarfs. Broadband VRI colours and pseudocontinuum indices derived for the cluster stars are indistinguishable from dwarfs at visual and red wavelengths. This suggests the temperature sequence for the PMS [special character] Cha cluster is similar to that of the dwarf sequence. Narrow-band spectral indices for the [special character] Cha cluster possibly indicate higher metallicity and strongly indicate lower surface gravity than the dwarf indices.
278

Ανίχνευση και μελέτη εξωγαλαξιακών υπολειμμάτων υπερκαινοφανών σε πολλαπλά μήκη κύματος / Detection and study of extragalactic multi-wavelength supernova remnants

Λεωνιδάκη, Ιωάννα 28 February 2013 (has links)
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.
279

TOYS : time-domain observations of young stars

Bozhinova, Inna January 2017 (has links)
Stars form inside clouds of molecular gas and dust. In the early stages of stellar evolution the remainders of the initial cloud form a circumstellar disk. For the next few million years the disk will slowly dissipate via accretion, outflows, photoevaporation and planet growth while the star makes its way onto the Main Sequence. This stage of a star's life is referred to as the T Tauri phase and is characterised by high-level spectrophotometric variability. This thesis aims to study and map out the environments of T Tauri stars down to the very low mass regime by the means of time-domain monitoring. Different physical processes in the system manifest themselves as variability on different time- scales as well as produce characteristic spectroscopic and photometric features at various wave- lengths. In order to study young stellar objects in depth, the observing campaigns presented in this work were designed to cover a large range of time-scales - minutes, hours, days and months. Combining all the data, this thesis establishes a baseline of over a decade for some objects. The observations also cover a wide range of wavelengths from the optical to the mid-infrared part of the spectrum. The star RW Aur experienced two long-lasting dimming events in 2010 and 2014. This thesis presents a large collection of spectral and photometric measurements carried out just before and during the 2014 event. Spectral accretion signatures indicate no change in the accretion activity of the system. Photometry indicates that parallel to the dimming in the optical the star becomes brighter in the mid-infrared. The observations in this work combined with literature data suggest that the origin of the 2014 event is most likely obscuration of the star by hot dust from the disk being lifted into the disk wind. Very low mass stars (< 0.4 M⊙) are the most common type of star in the Galaxy. In order to understand the early stages of stellar evolution we must study young very low mass stars. This work investigates the photometric and spectroscopic variability of seven brown dwarfs in star forming regions near σ Ori and ε Ori. All targets exhibit optical photometric variability between from 0.1 to over 1.0 magnitude that persists on a time-scale of at least one decade. Despite the photometric variability no change in the spectral type is measured. In the cases where the stars are accreting, modelling of the spectral changes suggest the accretion flow is more homogeneous and less funnelled compared to Sun-like T Tauri stars. The non-accreting variables are more plausibly explained by obscuration by circumstellar material, possibly a ring made out of multiple clouds of dust grains and pebbles with varying optical depths. The star-disk systems studied in this thesis have some broader implications for star and planet formation theory. The case-study of RW Aur has unambiguously demonstrated that the planet- forming environment is very dynamic and can change dramatically on short time-scales, which in turn would have implications for the diversity of planetary systems found in the Galaxy. The Orion stars have shown that the current theory for the T Tauri stage of stellar evolution is valid down to the very low mass regime. The seven dwarfs are a good example for the evolutionary path of circumstellar disks, showing the transition from gas-high, flared accretion disks (σ Ori) to dust-dominated, depleted, structured debris disks (ε Ori).
280

Formation of stars and stellar clusters in galactic environment

Smilgys, Romas January 2018 (has links)
Star and stellar cluster formation in spiral galaxies is one of the biggest questions of astrophysics. In this thesis, I study how star formation, and the formation of stellar clusters, proceeds using SPH simulations. These simulations model a region of 400 pc and 107 solar masses. Star formation is modelled through the use of sink particles which represent small groups of stars. Star formation occurs in high density regions, created by galactic spiral arm passage. The spiral shock compresses the gas and generates high density regions. Once these regions attain sufficiently high density, self-gravity becomes dominant and drives collapse and star formation. The regions fragment hierarchically, forming local small groups of stars. These fall together to form clusters, which grow through subsequent mergers and large scale gas infall. As the individual star formation occurs over large distances before forming a stellar cluster, this process can result in significant age spreads of 1-2 Myrs. One protocluster is found to fail to merge due to the large scale tidal forces from the nearby regions, and instead expands forming a dispersed population of young stars such as an OB association.

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