Thesis (PhD) - Swinburne University of Technology, 2006. / A dissertation presented in fulfillment of the requirements for the degree of Doctor of Philosophy, Swinburne University of Technology - 2006. Typescript. Bibliography: p. 83-91.

Thesis (Ph. D. in Physics)--Vanderbilt University, May 2010. / Title from title screen. Includes bibliographical references.

Thesis (Ph. D.)--Ohio State University, 2006. / Title from first page of PDF file. Includes bibliographical references (p. 134-137).

Thesis (Ph. D.)--University of Washington, 2003. / Vita. Includes bibliographical references (p. 161-167).

Today, it is well known that galaxies like the Milky Way consist not only of stars but also of gas and dust. The galactic halo, a sphere of gas that surrounds the stellar disk of a galaxy, is especially interesting. It provides a wealth of information about in and outflowing gaseous material towards and away from galaxies and their hierarchical evolution. For the Milky Way, the so-called high-velocity clouds (HVCs), fast moving neutral gas complexes in the halo that can be traced by absorption-line measurements, are believed to play a crucial role in the overall matter cycle in our Galaxy. Over the last decades, the properties of these halo structures and their connection to the local circumgalactic and intergalactic medium (CGM and IGM, respectively) have been investigated in great detail by many different groups. So far it remains unclear, however, to what extent the results of these studies can be transferred to other galaxies in the local Universe. In this thesis, we study the absorption properties of Galactic HVCs and compare the HVC absorption characteristics with those of intervening QSO absorption-line systems at low redshift. The goal of this project is to improve our understanding of the spatial extent and physical conditions of gaseous galaxy halos in the local Universe. In the first part of the thesis we use HST /STIS ultraviolet spectra of more than 40 extragalactic background sources to statistically analyze the absorption properties of the HVCs in the Galactic halo. We determine fundamental absorption line parameters including covering fractions of different weakly/intermediately/highly ionized metals with a particular focus on SiII and MgII. Due to the similarity in the ionization properties of SiII and MgII, we are able to estimate the contribution of HVC-like halo structures to the cross section of intervening strong MgII absorbers at z = 0. Our study implies that only the most massive HVCs would be regarded as strong MgII absorbers, if the Milky Way halo would be seen as a QSO absorption line system from an exterior vantage point. Combining the observed absorption-cross section of Galactic HVCs with the well-known number density of intervening strong MgII absorbers at z = 0, we conclude that the contribution of infalling gas clouds (i.e., HVC analogs) in the halos of Milky Way-type galaxies to the cross section of strong MgII absorbers is 34%. This result indicates that only about one third of the strong MgII absorption can be associated with HVC analogs around other galaxies, while the majority of the strong MgII systems possibly is related to galaxy outflows and winds. The second part of this thesis focuses on the properties of intervening metal absorbers at low redshift. The analysis of the frequency and physical conditions of intervening metal systems in QSO spectra and their relation to nearby galaxies offers new insights into the typical conditions of gaseous galaxy halos. One major aspect in our study was to regard intervening metal systems as possible HVC analogs. We perform a detailed analysis of absorption line properties and line statistics for 57 metal absorbers along 78 QSO sightlines using newly-obtained ultraviolet spectra obtained with HST /COS. We find clear evidence for bimodal distribution in the HI column density in the absorbers, a trend that we interpret as sign for two different classes of absorption systems (with HVC analogs at the high-column density end). With the help of the strong transitions of SiII λ1260, SiIII λ1206, and CIII λ977 we have set up Cloudy photoionization models to estimate the local ionization conditions, gas densities, and metallicities. We find that the intervening absorption systems studied by us have, on average, similar physical conditions as Galactic HVC absorbers, providing evidence that many of them represent HVC analogs in the vicinity of other galaxies. We therefore determine typical halo sizes for SiII, SiIII, and CIII for L = 0.01L∗ and L = 0.05L∗ galaxies. Based on the covering fractions of the different ions in the Galactic halo, we find that, for example, the typical halo size for SiIII is ∼ 160 kpc for L = 0.05L∗ galaxies. We test the plausibility of this result by searching for known galaxies close to the QSO sightlines and at similar redshifts as the absorbers. We find that more than 34% of the measured SiIII absorbers have galaxies associated with them, with the majority of the absorbers indeed being at impact parameters ρ ≤160 kpc. / Galaxien bestehen nicht nur aus Planeten und Sternen, sondern sind u.a. auch von einer Hülle aus Gas und Staub, dem Halo, umgeben. Dieser Halo spielt für die Entwicklung der Galaxie eine zentrale Rolle, da er mit der galaktischen Scheibe wechselwirken kann. Für das Verständnis des galaktischen Materiekreislaufs ist es daher entscheidend, die Prozesse und Vorgänge sowie das Zusammenspiel der verschiedenen Gasphasen in diesem Übergangsbereich zum intergalaktischen Medium charakterisieren und verstehen zu können. In der vorliegenden Arbeit werden lokale Phänomene, die sogenannten Hochgeschwindigkeitswolken (HVCs), im Halo der Milchstraße mit Hilfe des Hubble-Weltraumteleskops analysiert und ausgewertet. Im Gegensatz zu dem normalen Halo Gas bewegen sich diese HVCs mit ungewöhnlich hohen Geschwindigkeiten durch die ̈ äußeren Bereiche der Milchstraße. Sie passen daher nicht in das Galaktische Ge- schwindigkeitsmodell und stellen eine eigene, wichtige Klasse von Objekten dar, welche mit der Galaxie wechselwirken und diese beeinflussen. Für die Analyse dieser HVCs werden mehr als 40 Spektren von extragalaktischen Hintergrundquellen statistisch untersucht, um u.a. den Bedeckungsanteil von verschiedenen niedrig-/mittel- und hochionisierten Metallen zu ermitteln. Wegen der Ähnlichkeit der Ionisationsparameter von einfach ionisiertem Silizium, SiII, und einfach ionisiertem Magnesium, MgII, ist es möglich, den Beitrag von HVCs zum Wirkungsquerschnitt von starken MgII Absorbern im lokalen Universum zu bestimmen. Es stellt sich heraus, dass, würde man von außen auf die Milchstraße schauen, Galaktische HVCs etwa 52 % zum totalen Wirkungsquerschnitt von starken MgII Absorptionssystemen in der Milchstraße beitragen. Weiterhin ergibt sich, dass nur etwa ein Drittel der starken MgII Absorptionssysteme in der Umgebung von Milchstraßen-ähnlichen Galaxien als HVC Gegenstücke identifziert werden kann. Betrachtet man die große Anzahl an bekannten MgII Absorptionssystemen folgt daraus, dass das HVC-Phänomen nicht alleine auf unsere Galaxie beschränkt ist, sondern im Gegenteil, weit verbreitet zu sein scheint. Weiterhin werden die Eigenschaften von Metallsystemen bei niedriger Rotverschiebung in Quasarspektren analysiert. Die Suche nach extragalaktischen Metallsystemen in einer Vielzahl von Spektren und deren statistische Auswertung bezogen auf ihre Ursprungsgalaxien ermöglicht es, neue Erkenntnisse über die typische Struktur von Halos Milchstraßen-ähnlicher Galaxien zu erlangen. Eine der Hauptfragestellungen ist die Identifizierung von entfernten Metallsystemen als HVC-Analoga. Dazu wurden weitere Quasarspektren des Hubble-Teleskops ausgewertet und mit den Ergebnissen über Galaktische HVCs verglichen. Es zeigt sich hierbei, dass z.B. in der Säulendichteverteilung von neutralem Wasserstoff eine deutliche zweikomponentige Struktur zu erkennen ist. Diese könnte das Resultat von zwei verschiedenen Absorber Populationen sein, wobei eine HVC-ähnliche Eigenschaften aufweist. Diese Absorptionssysteme besitzen im Mittel sehr ähnliche Eigenschaften wie Galaktische Absorber, z.B. in Bezug auf die Eigenschaften des Gases oder dessen Zusammensetzung. Das impliziert u.a., dass sich auch dazugehörige Galaxien innerhalb eines bestimmten Abstandes um diese Absorber befinden sollten. Diese Vermutung wird anhand der Daten exemplarisch für zweichfach ionisiertes Silizium, SiII, untersucht. Es stellt sich heraus, dass sich in mehr als 34 % der Fälle zugehörige Galaxien bei SiIII Absorbern befinden, wobei die Mehrheit sogar innerhalb des von uns ermittelten mittleren Detektionsradius von 160 kpc zu finden ist. Allgemein können wir viele Hinweise darauf finden, dass das HVC-Phänomen nicht nur auf die Milchstraße beschränkt, sondern weit verbreitet ist. Zusätzlich scheinen Metallsysteme in Quasarspektren gute Indikatoren für HVC-Analoga in der Umgebung von anderen entfernten Galaxien zu sein.

No description available.

The story of a typical spiral galaxy like the Milky Way is a tale of the transformation of metal-poor hydrogen gas to heavier elements through nuclear burning in stars. This gas is thought to arrive in early times during the assembly phase of a galaxy and at late times through a combination of hot and cold “flows” representing external evolutionary processes that continue to the present. Through a somewhat still unclear mechanism, the atomic hydrogen is converted to molecules that collect into clouds, cool, condense, and form stars. At the end of these stars’ lives, much of their constituent gas is returned to the galaxy to participate in subsequent generations of star formation. In earlier times in the history of the universe, frequent and large galaxy mergers brought additional gas to further fuel this process. However, major merger activity began an ongoing decline several Gyr ago and star formation is now diminishing; the universe is in transitioning to an era in which the structural evolution of disk galaxies is dominated by slow, internal (“secular”) processes. In this evolutionary regime, stars and the gas from which they are formed participate in resonant gravitational interactions within disks to build ephemeral structures such as bars, rings, and small scale-height central bulges. This regime is expected to last far into the future in a galaxy like the Milky Way, punctuated by the periodic accretion of dwarf satellite galaxies but lacking in the “major” mergers that kinematically scramble disks into ellipticals. This thesis examines details of the story of gas from infall to structure-building in three major parts. The High- and Intermediate-Velocity Clouds (HVCs/IVCs) are clouds of H i gas at velocities incompatible with simple models of differential Galactic rotation. Proposed ideas explaining their observed properties and origins include (1) the infall of low-metallicity material from the Halo, possibly as cold flows along filaments of a putative “Cosmic Web”; (2) gas removed from dwarf satellite galaxies orbiting the Milky Way via some combination of ram pressure stripping and tidal disruption; and (3) the supply and return feeds of a “Galactic Fountain” cycling gas between the Disk and Halo. Numerical values of their observed properties depend strongly on the Clouds’ distances. In Chapter 2, we summarize results of an ongoing effort to obtain meaningful distances to a selection of HVCs and IVCs using the absorption-line bracketing method. We find the Clouds are not at cosmological distances, and with the exception of the Magellanic Stream, they are generally situated within a few kiloparsecs of the Disk. The strongest discriminator of the above origin scenarios are the heavy element abundances of the Clouds, but to date few reliable Cloud metal- licities have been published. We used archival UV spectroscopy, supplemented by new observations with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope and H I 21 cm emission spectroscopy from a variety of sources to compute elemental abundances relative to hydrogen for 39 HVC/IVC components along 15 lines of sight. Many of these are previously unpublished. We find support for all three origin scenarios enumerated above while more than doubling the number of robust measurements of HVCs/IVCs in existence. The results of this work are detailed in Chapter 3. In Chapter 4 we present the results of a spectroscopic study of the high-mass protostellar object NGC 7538 IRS 9 made with the Texas Echelon Cross Echelle Spectrograph (TEXES), a sensitive, high spectral resolution, mid-infrared grating spectrometer and compare our observations to published data on the nearby object NGC 7538 IRS 1. Forty-six individual lines in vibrational modes of the molecules C₂H₂, CH₄, HCN, NH₃ and CO were detected, including two isotopologues (¹³CO, ¹²C¹⁸O) and one combination mode ([nu]₄+[nu]₅ C₂H₂). Fitting synthetic spectra to the data yielded the Doppler shift, excitation temperature, Doppler b parameter, column density and covering factor for each molecule observed; we also computed column density upper limits for lines and species not detected, such as HNCO and OCS. We find differences among spectra of the two objects likely attributable to their differing radiation and thermal environments. Temperatures and column densities for the two objects are generally consistent, while the larger line widths toward IRS 9 result in less saturated lines than those toward IRS 1. Finally, we compute an upper limit on the size of the continuum-emitting region (~2000 AU) and use this constraint and our spectroscopy results to construct a schematic model of IRS 9. In Chapters 5 and 6, we describe studies of the bright, nearby, edge-on spiral galaxies NGC 4565 and NGC 5746, both previously classified as type Sb spirals with measured bulge-to-total luminosity ratios B/T ≃ 0.4. These ratios indicate merger-built, “classical” bulges but in reality represent the photometric signatures of bars seen end-on. We performed 1-D photometric decompositions of archival Hubble Space Telescope, Spitzer Space Telescope, and Sloan Digital Sky Survey images spanning a range of wavelengths from the optical to near-infrared that penetrate the thick midplane dust in each galaxy. In both, we find high surface brightness, central stellar components that are clearly distinct from the boxy bar and from the disk; we interpret these structures as small scale height “pseudobulges” built from disk material via internal, resonant gravitational interactions among disk material − not classical bulges. The brightness profiles of the innermost component of each galaxy is well fitted by a Sersic function with major/minor axis Sersic indices of n = 1.55±0.07 and 1.33±0.12 for NGC 4565 and n = 0.99±0.08 and 1.17 ± 0.24 for NGC 5746. The true “bulge-to-total” ratios of these galaxies are considerably smaller than once believed: 0.061+0.009 and 0.136 ± 0.019, −0.008, respectively. Therefore, more galaxies than we thought contain little or no evidence of a merger-built classical bulge. We argue further that a classical bulge cannot hide behind the dust lane of either galaxy and that other structures built exclusively through secular evolution processes such as inner rings, both revealed through the infrared imagery, argue strongly against any merger violence in the recent past history of these objects. From a formation point of view, NGC 4565 and NGC 5746 are giant, pure-disk galaxies, and we do not understand how such galaxies form in a ΛCDM universe. This presents a challenge to our picture of galaxy formation by hierarchical clustering because it is difficult to grow galaxies as large as these without making big, classical bulges. We summarize the work presented in this thesis in Chapter 7 and conclude with speculations about the future direction of research in this field. / text