Astrophysical data analysis of the weak-field predictions support the claim that modified gravity (MOG) theories provide a self-consistent, scale-invariant, universal description of galaxy rotation curves, without the need of non-baryonic dark matter. Comparison to the predictions of Milgrom's modified dynamics (MOND) provide a best-fit and experimentally determined universal value of the MOND acceleration parameter. The predictions of the modified gravity theories are compared to the predictions of cold non-baryonic dark matter (CDM), including a constant density core-modified fitting formula, which produces excellent fits to galaxy rotation curves including the low surface brightness and dwarf galaxies. Upon analysing the mass profiles of clusters of galaxies inferred from X-ray luminosity measurements, from the smallest nearby clusters to the largest of the clusters of galaxies, it is shown that while MOG provides consistent fits, MOND does not fit the observed shape of cluster mass profiles for any value of the MOND acceleration parameter. Comparison to the predictions of CDM confirm that whereas the Navarro-Frenk-White (NFW) fitting formula does not fit the observed shape of galaxy cluster mass profiles, the core-modified dark matter fitting formula provides excellent best-fits, supporting the hypothesis that baryons are dynamically important in the distribution of dark matter halos.

Astrophysical data analysis of the weak-field predictions support the claim that modified gravity (MOG) theories provide a self-consistent, scale-invariant, universal description of galaxy rotation curves, without the need of non-baryonic dark matter. Comparison to the predictions of Milgrom's modified dynamics (MOND) provide a best-fit and experimentally determined universal value of the MOND acceleration parameter. The predictions of the modified gravity theories are compared to the predictions of cold non-baryonic dark matter (CDM), including a constant density core-modified fitting formula, which produces excellent fits to galaxy rotation curves including the low surface brightness and dwarf galaxies. Upon analysing the mass profiles of clusters of galaxies inferred from X-ray luminosity measurements, from the smallest nearby clusters to the largest of the clusters of galaxies, it is shown that while MOG provides consistent fits, MOND does not fit the observed shape of cluster mass profiles for any value of the MOND acceleration parameter. Comparison to the predictions of CDM confirm that whereas the Navarro-Frenk-White (NFW) fitting formula does not fit the observed shape of galaxy cluster mass profiles, the core-modified dark matter fitting formula provides excellent best-fits, supporting the hypothesis that baryons are dynamically important in the distribution of dark matter halos.

Observations from the 1930s until the present have established the existence of dark matter with an abundance that is much larger than that of luminous matter. Because none of the known particles of nature have the correct properties to be identified as the dark matter, various exotic candidates have been proposed. The neutralino of supersymmetric theories is the most promising example. Such cold dark matter candidates, however, lead to a conflict between the standard simulations of the evolution of cosmic structure and observations. Simulations predict excessive structure formation on small scales, including density cusps at the centers of galaxies, that is not observed. This conflict still persists in early 2007, and it has not yet been convincingly resolved by attempted explanations that invoke astrophysical phenomena, which would destroy or broaden all small scale structure. We have investigated another candidate that is perhaps more exotic: Lorentz-violating dark matter, which was originally motivated by an unconventional fundamental theory, but which in this dissertation is defined as matter which has a nonzero minimum velocity. Furthermore, the present investigation evolved into the broader goal of exploring the properties of Lorentz-violating matter and the astrophysical consequences – a subject which to our knowledge has not been previously studied. Our preliminary investigations indicated that this form of matter might have less tendency to form small-scale structure. These preliminary calculations certainly established that Lorentz-violating matter which always moves at an appreciable fraction of the speed of light will bind less strongly. However, the much more thorough set of studies reported here lead to the conclusion that, although the binding energy is reduced, the small-scale structure problem is not solved by Lorentz-violating dark matter. On the other hand, when we compare the predictions of Lorentz-violating dynamics with those of classical special relativity and general relativity, we find that differences might be observable in the orbital motions of galaxies in a cluster. For example, galaxies – which are composed almost entirely of dark matter – observed to have enlarged orbits about the cluster center of mass may be an indication of Lorentz violation.

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

Σκοπός του διδακτορικού αυτού είναι να αναδείξει τις ομοιότητες και τις διαφορές των ενεργών πυρήνων, μελετώντας το περιβάλλον γαλαξιών τύπου Sy1, Sy2, αλλά και λαμπρών υπέρυθρων γαλαξιών (BIRG, οι οποίοι ως επί το πλείστον είναι τύπου Starburst και Sy2) και συγκρίνοντας το με το περιβάλλον κανονικών μη ενεργών γαλαξιών. Διερευνάται επίσης εις βάθος, η σχέση Starburst και AGN γαλαξιών και περιλαμβάνεται η αναλυτική φασματοσκοπική μελέτη και κατηγοριοποίηση των γειτόνων των Seyfert και BIRG, σε μία προσπάθεια να βρεθεί η αναμενόμενη αμφίδρομη σχέση μεταξύ των αλληλεπιδρώντων γαλαξιών. Εν κατακλείδι, προτείνεται ένα συνολικό εξελικτικό σενάριο, που περιλαμβάνει όλους τους τύπους των ενεργών γαλαξιών που παρατηρούνται στο τοπικό σύμπαν. Το τελευταίο τμήμα της διατριβής προσεγγίζει το πρόβλημα του περιβάλλοντος των ενεργών γαλαξιών από μία διαφορετική πλευρά, αυτή των σμηνών γαλαξιών. Η ανεύρεση των ενεργών πυρήνων σε αυτή την περίπτωση γίνεται με χρήση δεδομένων ακτινών-Χ από το δορυφόρο XMM-Newton. Η ορθή ερμηνεία των αποτελεσμάτων προϋποθέτει την σύγκριση των αποτελεσμάτων με οπτικά δεδομένα, η οποία ακολουθεί σε δεύτερη φάση. / The purpose of the present thesis is to bring out the similarities and the differences of the Active Galactic Nuclei (AGN), by studying the environment of Seyfert and of Bright IRAS galaxies (BIRG, which in their majority are Starburst and Sy2 galaxies) and compare it with the environment of normal (non-active) galaxies. The Starburst/AGN connection is also studied and the spectroscopic analysis and classification of all the neighboring galaxies of Seyferts and BIRGs is included, in an attempt to find the expected bidirectional relation between interacting galaxies. We propose an evolutionary scenario, which includes all types of active galaxies present in the local universe. The last part approaches the problem from a different angle, that of the galaxy clusters. In this case the selection of the AGNs is based on their X-ray emmision, using data from the XMM-Newton satellite. Finally, we compare our findings with optical data from the Sloan Digital Sky Survey.

No description available.

The interstellar matter in the Galaxy is concentrated in a thin layer close to the galactic plane, mostly in spiral arms. Most of the interstellar matter is in a form of atomic and molecular gas and only 1% of its mass is in a form of small (0.01 – 0.1 µm) dust grains which absorb and scatter the light of stars and cause the interstellar extinction. A part of interstellar gas and dust are in a diffuse form and fill the space with density which exponentially decreases with the distance from the galactic plane. However, in the spiral arms gigantic molecular clouds are present which play an outstanding role in the evolution of the Galaxy, since in them star forming processes take place. Molecular and dust clouds, young star clusters and associations and other star forming regions are concentrated in the Milky Way plane. However, due to gravitational and radiation interactions between the molecular clouds and the young massive stars, some fragments of spiral arms deviate from the galactic plane. One of such deviating branches from the Local (Orion) spiral arm is a huge elongated system of molecular and dust clouds located in the Cepheus constellation which is known as the Cepheus Flare. Most of the objects investigated in this dissertation belong to this branch of the Local spiral arm. The objects selected for the investigation are star forming regions in the vicinity of the reflection nebula NGC 7023, a group of dark clouds TGU 619, the young open cluster NGC 7129 and the... [to full text] / Paukščių tako Galaktikoje didžioji dalis tarpžvaigždinės medžiagos yra susitelkusi į ploną sluoksnį Galaktikos plokštumoje, daugiausiai spiralinėse vijose. Didžiąją dalį tarpžvaigždinės medžiagos sudaro atominės ir molekulinės dujos ir tik maždaug 1% masės yra mažos (0.01 – 0.1 µm) dydžio dulkelės, kurios sugeria ir išsklaido šviesą ir sukelia tarpžvaigždinės ekstinkcijos reiškinį. Spiralinėse vijose esantys molekuliniai debesys yra svarbūs Galaktikos evoliucijos procese, nes juose vyksta aktyvūs žvaigždžių formavimosi procesai. Molekuliniai ir dulkių debesys, jaunų žvaigždžių spiečiai ir asociacijos bei kitos žvaigždžių susidarymo sritys dažniausiai yra Galaktikos plokštumoje. Tačiau dėl gravitacinės ir radiacinės sąveikos tarp jaunų didelės masės žvaigždžių ir molekulinių debesų kai kuriose Paukščių Tako srityse žvaigždėdaros rajonai nukrypsta nuo Galaktikos plokštumos. Viename iš tokių rajonų Cefėjo žvaigždyno kryptimi dalis tarpžvaigždinių debesų ir žvaigždėdaros rajonų yra nukrypę nuo Galaktikos plokštumos link šiaurinio dangaus poliaus ir sudaro Paukščių Tako atšaką, literatūroje žinomą kaip Cepheus Flare. Dauguma šioje disertacijoje tyrinėjamų objektų priklauso šiai Cefėjo atšakai. Šio darbo tikslas yra Cefėjo žvaigždėdaros rajonų, žinomų kaip atspindžio ūkas NGC 7023, tamsiųjų debesų kompleksas TGU 619 ir jaunas spiečius NGC 7129 fotometrinis tyrimas, siekiant nustatyti šių objektų nuotolį ir tarpžvaigždinę ekstinkciją, jauno spiečiaus NGC 7129 ir... [toliau žr. visą tekstą]

Paukščių Tako Galaktikoje didžioji dalis tarpžvaigždinės medžiagos yra susitelkusi į ploną sluoksnį Galaktikos plokštumoje, daugiausiai spiralinėse vijose. Didžiąją dalį tarpžvaigždinės medžiagos sudaro atominės ir molekulinės dujos ir tik maždaug 1% masės yra mažos (0.01 – 0.1 µm) dydžio dulkelės, kurios sugeria ir išsklaido šviesą ir sukelia tarpžvaigždinės ekstinkcijos reiškinį. Spiralinėse vijose esantys molekuliniai debesys yra svarbūs Galaktikos evoliucijos procese, nes juose vyksta aktyvūs žvaigždžių formavimosi procesai. Molekuliniai ir dulkių debesys, jaunų žvaigždžių spiečiai ir asociacijos bei kitos žvaigždžių susidarymo sritys dažniausiai yra Galaktikos plokštumoje. Tačiau dėl gravitacinės ir radiacinės sąveikos tarp jaunų didelės masės žvaigždžių ir molekulinių debesų kai kuriose Paukščių Tako srityse žvaigždėdaros rajonai nukrypsta nuo Galaktikos plokštumos. Viename iš tokių rajonų Cefėjo žvaigždyno kryptimi dalis tarpžvaigždinių debesų ir žvaigždėdaros rajonų yra nukrypę nuo Galaktikos plokštumos link šiaurinio dangaus poliaus ir sudaro Paukščių Tako atšaką, literatūroje žinomą kaip Cepheus Flare. Dauguma šioje disertacijoje tyrinėjamų objektų priklauso šiai Cefėjo atšakai. Šio darbo tikslas yra Cefėjo žvaigždėdaros rajonų, žinomų kaip atspindžio ūkas NGC 7023, tamsiųjų debesų kompleksas TGU 619 ir jaunas spiečius NGC 7129 fotometrinis tyrimas, siekiant nustatyti šių objektų nuotolį ir tarpžvaigždinę ekstinkciją, jauno spiečiaus NGC 7129 ir... [toliau žr. visą tekstą] / The interstellar matter in the Galaxy is concentrated in a thin layer close to the galactic plane, mostly in spiral arms. Most of the interstellar matter is in a form of atomic and molecular gas and only 1% of its mass is in a form of small (0.01 – 0.1 µm) dust grains which absorb and scatter the light of stars and cause the interstellar extinction. A part of interstellar gas and dust are in a diffuse form and fill the space with density which exponentially decreases with the distance from the galactic plane. However, in the spiral arms gigantic molecular clouds are present which play an outstanding role in the evolution of the Galaxy, since in them star forming processes take place. Molecular and dust clouds, young star clusters and associations and other star forming regions are concentrated in the Milky Way plane. However, due to gravitational and radiation interactions between the molecular clouds and the young massive stars, some fragments of spiral arms deviate from the galactic plane. One of such deviating branches from the Local (Orion) spiral arm is a huge elongated system of molecular and dust clouds located in the Cepheus constellation which is known as the Cepheus Flare. Most of the objects investigated in this dissertation belong to this branch of the Local spiral arm. The objects selected for the investigation are star forming regions in the vicinity of the reflection nebula NGC 7023, a group of dark clouds TGU 619, the young open cluster NGC 7129 and the... [to full text]

The focus of this dissertation is to study the star-forming regions of the interstellar medium (ISM), using two very diverse environments: the Polaris Flare, high-galactic latitude, cirrus cloud complex consisting of several starless molecular cores with no nearby hot stars; and the Orion Nebula, which is the closest massive star forming region. The two environments provide a wide range of physical conditions. It is commonly assumed that the Herschel far-infrared (FIR) fluxes are a good measure of column density, hence, mass of interstellar clouds. We find that the FIR fluxes are insensitive to the column density if AV ≳ 2. The Polaris Flare has been previously observed with the Herschel Space Telescope. We use Cloudy to model the molecular cores in MCLD 123.5+24.9 of the Polaris Flare. The Polaris Flare, 150 pc distant, is well within the Galactic disc. There are no nearby hot stars. Therefore, the cloud is illuminated by an external far-ultraviolet (FUV) flux (6-13 eV) due to the galactic background interstellar radiation field (ISRF). The dust grains absorb the incident FUV flux and re-emit in the FIR continuum emission. We use detailed grain models that suggest that the grains in dense regions are coated with water and ammonia ices, increasing their sizes and opacities. In our models, dust temperatures decline rapidly into the cloud. Therefore, the cloud interiors contribute very little additional FIR flux, leading to an underestimate of inferred column density. Cloudy also predicts mm-wavelength molecular lines for comparison with published observations. Our models suggest that at low temperatures (≲ 20K), molecules freeze-out on grain surfaces, and desorption by cosmic rays becomes important. Our models of inter-core regions in MCLD 123.5+24.9 significantly under predict molecular line strengths unless the gas is clumped into high-density regions. We use Cloudy to construct a detailed model of the Orion H ii region. This study is an improvement over the work of Baldwin et al. 1991 with the new atomic data and stellar atmosphere models, and a wealth of archival observational data obtained over last two decades. We use collisionally excited lines to determine the elemental abundance of the region.

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