No description available.

Interstellar molecular clouds play an essential role in the Universe. Such clouds are invoked for the production and destruction of stars, galaxies and gas and also for energy transport in galaxies. The Galaxy, or the Milky Way, is a large spiral galaxy, with a central bar structure, that harbours a few hundred billion stars and large amounts of gas and dust. At the centre of the Galaxy, a 4 million solar mass supermassive black hole resides, surrounded by a dense core of millions of stars, as well as molecular and dust clouds. The Galactic centre (GC) is hidden by gas and dust, such that only astronomical observations of radio-, infrared-, X-rays and gamma-rays are available for a gathering of information at the centre. In this work, I have studied neutral molecular clouds in absorption at the innermost 50 light years from the centre with the Karl Jansky Very Large Array Observatory in New Mexico in the USA, and with data from observations with the Swedish-ESO Submillimetre Telescope in Chile, and also from the orbital observatory Odin. I have detected a new stream-like feature of gas that seems to link a previously known ring of gas clouds (the CND) and the GC. Moreover, the hypothesis of feeding the CND from an outside cloud is supported by this work. Contemporary discussions in the literature that the central bar structure would act as a pump of material inwards from the spiral arms towards the GC via molecular clouds are also suggested by the data. A number of maser sources have been observed and some of those are shown to reside at shock fronts or anticipated regions of collisions between molecular clouds or at star forming regions. Unusually high water abundance was detected at the south-west part of the CND, indicative of shocks and strong turbulence. Moreover, I have produced high-resolution spectral line maps of hydroxyl (OH) absorption intensity in the four main transition lines of OH at 1612, 1720, 1665 and 1667 MHz, as well as apparent opacity and position-velocity maps of the GC region.

Thesis (Ph. D.)--Northwestern University, 2007. / Includes vita. Includes references.

No description available.

No description available.

Thesis (Ph. D.)--Northwestern University, 2007. / Includes vita. Includes references.

Recent times have seen an increase in observational campaigns aimed at contributing to the overall sample of active galactic nuclei (AGNs) with central super-massive black hole (SMBH) mass estimates using reverberation mapping (RM) technique. RM makes use of the time variability of the broad emission lines from the nearby broad-line region (BLR) to measure their time response or delay (τ) relative to the incident ionizing continuum from the accretion disc. This delay is a consequence of light-travel time effects and directly enables the size of the BLR to be determined as RBLR = cτ (c is speed of light). Assuming virialized motions of the gas clouds in the BLR, and combining the velocity width (∆V ) of a Doppler broadened emission line with RBLR leads to the estimation of the black hole mass M. RM can also be used to measure the accretion-disc size in which the X-ray variability source is assumed to drive UV/optical variations in the accretion disc leading to wavelength-dependent delays that scale as τ ∝ λ 4/3 . Here, both aspects of RM are explored using photometric and spectroscopic observations of 3C 120, PG 2304+043 and SDSS J132447.65+032432.6, with observations emanating from the Southern African Large Telescope (SALT) and Las Cumbres Observatory (LCO) robotic telescopes. The broad emission-line and inter-band continuum delays in these AGNs are measured using the Interpolated Cross-Correlation Function (ICCF) and Continuum REverberation AGN Markov Chain Monte Carlo (CREAM).

During the close approach of two galaxies in a merger, tidally induced gas inflows can trigger simultaneous black hole accretion which are observed as dual active galactic nuclei (dAGNs). Merger simulations predict that the resulting increased nuclear gas reservoirs will obscure the X-ray emissions from the AGNs. We investigate whether dAGNs in mergers are observed to be more obscured than their isolated counterparts by combining the results of previous radio and optical spectroscopy studies with new Chandra X-ray observations for a sample of 4 dAGN systems in the Stripe 82 field. For the 6 detected components, we find the rest-frame X-ray luminosities range between 39.8 < log LX /erg s-1 < 42.0. The sources have redshifts between 0.04 < z < 0.22 and projected separations between 4.3 and 9.2 kpc, as well as multi-wavelength properties most closely resembling low-luminosity AGNs. However, we determine that the X-ray emissions for 2 of the sources likely has strong contributions from hot interstellar medium, and star-formation and X-ray binaries may contribute to the X-ray luminosities of several sources. We do not find evidence of enhanced obscuration through our analysis of X-ray hardness ratios, optical [O III] emission line luminosities, and mid-infrared luminosities. Therefore, we suggest that the unobscured low-level accretion observed for the AGNs in this sample is driven through stochastic processes rather than the massive gas inflows predicted for a merger-driven scenario.

<p> Through HI observations, galactic gas discs can be observed to extend past the edge of the star forming disk. Observations of HI in these extended galactic disks consistently show significant velocity dispersion, which suggests a non-thermal origin. This suggests that turbulence in the gas is contributing significantly to the observed velocity dispersion. To address this, a new parallel adaptive mesh three dimensional shearing-box implementation with adaptions for evening numerical diffusion effects, self-gravity in the shearing boundary conditions and appropriate vertical boundary conditions has been built, based on the FLASH code. This code is used to perform local simulations, incorporating differential rotation, self-gravity, stratification, hydrodynamics and cooling. These simulations explore possible mechanisms for driving turbulent motions through thermal and self-gravitational instabilities coupling to differential rotation. In isothermal simulations a marginally stable disk is found to be stable against forming a gravitoturbulent quasi-steady state. In simulations including cooling, where the disk conditions do not trigger the formation of a two-phase medium, it is found that perturbations to the flow damp without leading to a sustained mechanism for driving turbulence. In cases where a two-phase medium develops, gravitational angular momentum transporting stresses are much greater, creating a possible mechanism for transferring energy from galactic rotation to turbulence, though a gravitoturbulent quasi-steady state is not found. The differing angular momentum transport properties of the single phase and two phase regimes of the disk suggests a significant dynamical division can be drawn between the two, which may occur far outside the star formation cutoff in a galactic disk. </p> / Thesis / Master of Science (MSc)

published_or_final_version / Physics / Master / Master of Philosophy