The topic of this thesis is the study of the vertical structure of the disk galaxies and their dark matter halos through theoretical modeling and numerical calculations. The basic theoretical model of the galactic disk used involves gravitationally-coupled stars and gas under the force-field of a dark matter halo; the disk is rotationally-supported in the plane and pressure-supported perpendicular to the plane of the galaxy. The first part of the thesis involves evaluating the vertical structure of stars and gas in normal as well as dwarf spiral galaxies. The second part of the thesis deals with probing the dark matter halo density profiles of disk galaxies using both the observed rotation curve and the H i scale height data. Following is the layout of the thesis. Chapter 1 gives a general introduction to the topic of vertical structure of spiral galaxies and their dark matter halos, followed by a broad overview of the theoretical development of the topic and ends with highlighting the motivation and challenges met in this thesis. Chapters 2 & 3 deal with the vertical structure of stars and gas in galaxies, Chapters 4-6 focus on obtaining the dark matter halo density profiles of disk galaxies from the observed rotation curve and the H i scale height data whereas Chapter 7 is devoted to the summary of results and future research plans. Vertical structure of stars and gas in galaxies The vertical thickness of the stars and the gas, namely atomic hydrogen (H i) and molecular hydrogen (H2) in a spiral galaxy, is crucial in regulating the disk dynamics close to the mid-plane, especially in the inner galaxy. However, measuring it observationally is not in general practicable due to the limitations of astronomical observations, and often impossible as in the case of face-on galaxies. Therefore, it is imperative to develop a theoretical model of the galaxy which can predict the thickness of the disk components by using as input parameters the physical quantities, which are more observationally-amenable compared to the disk thickness. The vertical thickness of the disk components is determined by a trade-off between the upward kinetic pressure and the net downward gravitational pull of the galaxy. The fraction of the disk mass due to the stars is an order of magnitude higher than that of the gas in ordinary spiral galaxies, and therefore the gas contribution to the disk gravity is ignored in general. We have developed a multi-component model of gravitationally-coupled stars, HI and H2 subjected to the force-field of an external dark matter halo, and conclusively demonstrated the importance of the inclusion of gas gravity in explaining the steep vertical stellar distribution observed in galaxies. These apart, this model does not implicitly assume a flat rotation curve for the galaxy and therefore is applicable in general to obtain the thickness of stars and gas in dwarfs (with linearly rising rotation curves) as well as in ordinary spirals. In Chapter 2, we investigate the origin of the steep vertical stellar distribution in the Galactic disk. One of the direct fall outs of our above model of the galaxy, which incor¬porates the self-gravity of the gas unlike the earlier theoretical models, lies in explaining the long-standing puzzle of the steep vertical stellar density distribution of the disk galax¬ies near the mid-plane. Over the past two decades, observations revealed that the vertical density distribution of stars in galaxies near the mid-plane is substantially steeper than the sech2 function that is expected for a self-gravitating system of stars under isothermal ap¬proximation. However, the physical origin for this has not been explained so far. We have clearly demonstrated that the inclusion of the self-gravity of the gas in the dynamical model of the Galaxy solves the problem even under the purview of isothermal approximation for the disk components. Being a low dispersion component, the gas resides closer to the mid¬plane compared to the stars, and forms a thin, compact layer near the mid-plane, thereby strongly governing the local disk dynamics. This novel idea, highlighting the significance of gas gravity has produced substantial impact on the field and triggered research activities by other groups in related areas of disk dynamics. The strong effect of the gas gravity on the vertical density profile of the stellar disk indicates that it should also bear its imprint on the Milky way thick disk, as the epoch of its formation 109 years ago is marked by a value of gas fraction, almost an order of magnitude higher than its present day value. Interest-ingly, the findings of the upcoming Gaia mission can be harnessed to verify this theoretical prediction. It may also hold the clue as to the reason behind the absence of thick disk in superthin galaxies. In Chapter 3, we use the same model to theoretically determine the H i vertical scale heights in the dwarf galaxies: DDO 154, Ho II, IC 2574 & NGC 2366 for which most of the necessary input parameters are available from observations. We stress the fact that the observational determination of the gas thickness in these dwarf irregulars is not viable. Nevertheless, it is important to estimate it theoretically as it plays a crucial role in calculating the star-formation activities and other related phenomena. However, two vital aspects have to be taken care of while modeling these dwarf galaxies. Firstly, the mass fraction in gas in these galaxies is comparable to that of the stars, and hence the gas gravity cannot be ignored on any account unlike in the case of large spirals. Secondly, dwarf galaxies have a rising rotation curve over most of the disk unlike the flat rotation curves of ordinary spirals. Both these factors have been considered in developing our model of the dwarf galaxies. We find that three out of the four galaxies studied show a flaring of their H i disks with increasing radius, by a factor of a few within several disk scale lengths. The fourth galaxy (Ho II) has a thick H1 disk throughout. A comparison of the size distribution of H1 holes in the four sample galaxies reveals that of the 20 type 3 holes, all have radii that are in agreement with them being still fully contained within the gas layer. Probing the dark matter halo profiles of disk galaxies The next part of the thesis involves the dynamical study of the shapes and density profiles of galactic dark matter halos using observational constraints on our theoretical model of a spiral galaxy. The density distribution of the dark matter halo is generally modeled using the observed rotation curve of the spiral galaxies. The rotational velocity at any radius is determined by the radial component of the net gravitational force of the galaxy, which, however, is weakly dependent on the shape of the dark matter halo. Therefore, one cannot trace the dark matter halo shape by the observed rotation curve alone. The vertical thickness of the stars and gas, on the other hand, is strongly dependent on the flattening of the dark matter halo, and therefore the observed gas thickness can be used as a diagnostic to probe the halo shape. In this thesis, we have used the double constraints of the rotation curve and the H i thickness data to obtain the best-fit values of the core density, core radius and the vertical-to-planar axis ratio (or flattening) of the dark matter halos of our largest nearby galaxy Andromeda (or M31), a low-surface brightness (LSB) superthin galaxy UGC 7321 and to study the dark matter halo shape of our Galaxy. In Chapter 4, we study the dark matter halo of M31 or Andromeda, the largest nearby galaxy to the Milky Way. We find that M31 has a highly flattened isothermal dark matter halo with the vertical-to-horizontal axis ratio equal to 0.4, which interestingly lies at the most oblate end of the halo shapes found in cosmological simulations. This indicates that either M31 is a unusual galaxy, or the simulations need to include additional physics, such as the effect of the baryons, that can affect the shape of the halo. This is quite a remarkable result as it challenges the popular practice of assuming a spherical dark matter halo in the dynamical modeling of the galaxy In Chapter 5, we have applied this technique to the superthin galaxy UGC 7321. Su¬perthins are somewhat the “extreme” objects in the local Universe because of their high gas fraction and absence of a thick disk component. It is interesting to analyze their so-called extreme characteristics in the light of the physical mechanisms which determined them to understand better the properties of ordinary spirals. We find that UGC 7321 has a spher¬ical isothermal halo, with a core radius almost equal to the disk scale length. This reveals that the dark matter dominates the dynamics of this galaxy at all radii, including the inner parts of the galaxy. This is unlike the case for the large spiral galaxies, where the core radius is typically about 3-4 disk scale lengths. Interestingly, the best-fit halo core density and the core radius are consistent, with deviations of a few percent, with the dark matter fundamental plane correlations, which depict the systematic properties of the dark matter halo in late-type and dwarf spheroidal galaxies. This apart, a high value of the gas velocity dispersion is required to get a better fit to the H i scale height data, although the superthin nature of the stellar disk implies a dynamically cold dynamic galactic disk. However, it explains the low star-formation rates in these galaxies since the Toomre Q criterion (Q < 1) for instability is less likely to be satisfied, and hence the disk is liable to be more stable to star formation. In Chapter 6, we investigate the shape of the dark matter halo in the outer Galaxy. We find that the halo is prolate, with the vertical-to-planar axis ratio monotonically increasing to 2.0 at 24 kpc, or 8 radial disk scale lengths. The resulting prolate-shaped halo can explain several long-standing puzzles in galactic dynamics, for example, it permits long-lived warps thus explaining their ubiquitous nature. It also imposes novel constraints on the galaxy formation models. Finally, in Chapter 7, the thesis is concluded with a summary of the main results and a brief discussion of the scope for future work.

Pattern speed is an important parameter of the density wave theory for spiral galaxies. In this thesis we have determined the pattern speed for warped galaxies (flat galaxies being a special case of this generalization) using the observable data of the surface brightness and line-of-sight velocity distribution of the galaxy. We have also extracted the transverse velocity field for the warped galaxy using the same data. Here we have simulated the data and applied our method to it and we found that our method works well in warped galaxy. We assume a parameterized model of the warp and by the method of minimizing χ2 error we can determine the parameters of the model also and thus we can construct the warp equation. We have also discussed the implications and the limitations of this method.

A triaxialidade dos halos das galáxias espirais foi estudada através das funções de distribuição de razões axiais aparentes dos seus discos. Utilizou-se uma amostra de galáxias limitada por magnitude (g<16). Foram selecionadas ~1600 galáxias espirais do SDSS (Sloan Digital Sky Survey), que foram classificadas morfologicamente por inspeção visual. Para evitar contaminações espúrias, removeram-se da amostra galáxias em interação, com companheiras próximas ou aproximadamente alinhadas com estrelas da Galáxia. As razões axiais das galáxias foram medidas individualmente e com critérios uniformes, ajustando-se isofotas elípticas a cada imagem no filtro r. Um algoritmo de Monte Carlo foi empregado para inferir a distribuição de razões axiais intrínsecas. O modelo de disco oblato é incapaz de reproduzir a observada falta de galáxias aparentemente circulares. A fração reduzida de galáxias aparentemente circulares, que se observa sistematicamente nas espirais de todos os sub-tipos morfológicos, é uma forte indicação de que os discos destas galáxias sejam intrinsecamente elípticos. O modelo triaxial mostrou-se mais estatisticamente significativo, além de ser capaz de prever a fração correta de galáxias quase circulares. Concluímos, portanto, que o modelo de disco não-circular descreve melhor as observações e que a razão dos eixos no plano do disco deve ser de p = 0,849 +/- 0,063. Empregando uma abordagem analítica simplificada, nós propusemos uma descrição da formação de halos escuros triaxiais no contexto de colisões de protogaláxias. Aproximando as protogaláxias por elipsóides homogêneos de Jacobi, notamos que a triaxialidade do objeto resultante de uma fusão depende da velocidade de colisão e calculamos a época em que estes encontros devem ter ocorrido, para gerar elipsóides com a triaxialidade em questão. Estimamos que neste cenário os discos das galáxias espirais tenham sido construídos a partir de aproximadamente z = 0,69 +/- 0,32. / The triaxiality of the halos of spiral galaxies was studied through the distribution function of the apparent axial ratios of their disks. We used a magnitude-limited sample of galaxies (g<16). We selected ~1600 spiral galaxies from the SDSS (Sloan Digital Sky Survey), which were morphologically classified by visual inspection. In order to avoid spurious contaminations, interacting galaxies were removed from the sample, as well as those with nearby companions or with foreground stars. The axial ratios of the galaxies were measured individually and with uniform criteria, by fitting elliptical isophotes to each r-filter image. A Monte Carlo algorithm was used to infer the distribution of intrinsic axial ratios. The oblate disk model is unable to account for the observed lack of apparently circular galaxies. The small fraction of apparently circular galaxies, which is systematically observed in spirals of all morphological subtypes, is as strong indication that the disks are indeed intrinsically elliptical. The triaxial model is more statistically significant and also, it is able to predict the correct fraction of nearly circular galaxies. We find, therefore, that the non-circular disk model provides a better description of the observations and that the axes on the plane of the disk should be in the ratio p = 0.849 +/- 0.063. By adopting a simplified analytical approach, we propose a description of the formation of triaxial dark halos in the context of collisions of protogalaxies. Assuming that the protogalaxies might be represented by homogeneous Jacobi ellipsoids, we noted that the triaxiality of the resulting object depends on the collision velocity. We also find the epoch at which such encounters would yield the required triaxiality. We estimate that in this scenario the disks of spiral galaxies would have been assembled at z = 0.69 +/- 0.32.

>Magister Scientiae - MSc / This research aimed to evaluate the effects of changing length-scales on the star formation laws and star formation efficiencies for our selected sample of galaxies. We have combined high-resolution Hi data from The Hi Nearby Galaxy Survey, CO data from HERA CO–Line Extragalactic Survey and the Nobeyama CO Atlas of Nearby Spiral Galaxies and 12 𝜇m data from the Wide-field Infrared Survey Explorer of a sample of five nearby galaxies to study the relationship between star formation rate surface density, ΣSFR, and gas surface density, Σgas, at various length-scales. In order to probe the star formation law of each galaxy, all image sets were placed on common astrometric grids and evaluated on a pixel-by-pixel basis over a range of sub-kpc length-scales. We investigated whether the star formation law changes with length-scales and found that as resolution becomes coarser, the Kennicutt Schmidt power-law index decreases for the correlation between ΣSFR and ΣH2 . Our results show that the index values are close to unity but are not consistent with it.

New deep ugr imaging was obtained on the Wyoming Infrared Observatory 2.3 meter telescope for NGC4102 and UGC07608, two galaxies in the Extended Disk Galaxy Exploration Science survey. These data are coupled with deep GALEX ultraviolet and Spitzer, WISE and Herschel infrared imaging to study the radial variations in the spectral energy distributions. Results from the CIGALE SED modeling software are presented, including trends in the galaxy star formation histories.

>Magister Scientiae - MSc / Radio galaxies are some of the most unusual and powerful objects in the Universe and are therefore vital for so many reasons. Studies of Radio-Loud Active Galactic Nuclei (AGN) gives us the potential to enhance our understanding of the key processes leading to the ejection of material, connection to the central engine and how the jets are launched. Besides, the energetic input of these sources is thought to have an impact in the star formation history and properties of the ISM and ICM; thus by studying them, we can improve our understanding of their formation, evolution and their environment.

A spectroscopic analysis of 10 intergalactic star-forming objects (ISFOs) and a photometric analysis of 67 ISFOs in a sample of 14 interacting systems is presented. The majority of the ISFOs have relative polycyclic aromatic hydrocarbon (PAH) band strengths similar to those of nearby spiral and starburst galaxies. In contrast to what is observed in blue compact dwarfs (BCDs) and local giant H II regions in the Milky Way (NGC 3603) and the Magellanic Clouds (30 Doradus and N 66), the relative PAH band strengths in ISFOs correspond to models with a significant PAH ion fraction (<50%) and bright emission from large PAHs (∼100 carbon atoms). The [Ne III]/[Ne II] and [S IV]/[S III] line flux ratios indicate moderate levels of excitation with an interstellar radiation field that is harder than the majority of the Spitzer Infrared Nearby Galaxies Survey and starburst galaxies, but softer than BCDs and local giant H II regions. The ISFO neon line flux ratios are consistent with a burst of star formation ∼6 million years ago. Most of the ISFOs have ∼106 M ⊙ of warm H2 with a likely origin in photo-dissociation regions (PDRs). Infrared Array Camera photometry shows the ISFOs to be bright at 8 μm, with one-third having [4.5]-[8.0] > 3.7, i.e., enhanced non-stellar emission, most likely due to PAHs, relative to normal spirals, dwarf irregulars, and BCD galaxies. The relative strength of the 8 μm emission compared to that at 3.6 μm or 24 μm separates ISFOs from dwarf galaxies in Spitzer two-color diagrams. The infrared power in two-thirds of the ISFOs is dominated by emission from grains in a diffuse interstellar medium. One in six ISFOs have significant emission from PDRs, contributing ∼30%-60% of the total power. ISFOs are young knots of intense star formation.

We provide an in-depth analysis of the He II and H I absorption in the intergalactic medium (IGM) at redshifts z = 2.3-2.9 toward HE 2347-4342, using spectra from the Far Ultraviolet Spectroscopic Explorer and the Ultraviolet-Visual Echelle Spectrograph on the Very Large Telescope. Following up on our earlier study, we focus here on two major topics: (1) small-scale variability (Δz ≈ 10-3) in the ratio η = N(He II)/N(H I) and (2) an observed correlation of high-η absorbers (soft radiation fields) with voids in the (H I) Lyα distribution. These effects may reflect fluctuations in the ionizing sources on scales of 1 Mpc, together with radiative transfer through a filamentary IGM whose opacity variations control the penetration of 1-5 ryd radiation over 30-40 Mpc distances. Given the photon statistics and backgrounds, we can measure optical depths over the ranges 0.1 < τHe II < 2.3 and 0.02 < τH I < 3.9 and reliably determine values of η ≈ 4τHe II/τH I over the range 0.1-460. Values η = 20-200 are consistent with models of photoionization by quasars with observed spectral indices α s = 0-3. Values η > 200 may require additional contributions from starburst galaxies, heavily filtered quasar radiation, or density variations. Regions with η < 30 may indicate the presence of local hard sources. We find that η is higher in "void" regions, where H I is weak or undetected and ∼80% of the path length has η > 100. These voids may be ionized by local soft sources (dwarf starbursts) or by QSO radiation softened by escape from the active galactic nucleus cores or transfer through the "cosmic web." The apparent differences in ionizing spectra may help to explain the 1.45 Gyr lag between the reionization epochs of H I (zH I ∼ 6.2 ± 0.2) and He II (zHe II ∼ 2.8 ± 0.2).

The presented study gives a comprehensive overview of the theory and the evidence for a systematically varying stellar initial mass function (IMF). Then we focus on the impact of this paradigm change, that is, from the universal invariant IMF to a variable IMF, on galaxy chemical evolution (GCE) studies. For this aim, we developed the first GCE code, GalIMF, that is able to incorporate the empirically calibrated environment-dependent IMF variation theory, the integrated galactic initial mass function (IGIMF) theory. In this theory, the galaxy-wide IMF is calculated by summing all the IMFs in all embedded star clusters which formed throughout the galaxy in 10 Myr time epochs. The GalIMF code recalculates the galaxy-wide IMF at each time step because the integrated galaxy- wide IMF depends on the galactic star formation rate and metallicity. The resulting galaxy-wide IMF and metal abundance evolve with time. Using this code, we examine the chemical evolution of early-type galaxies (ETGs) from dwarf to the most massive. We find that the introduction of the non-canonical IMF affects the best estimation of the galaxy properties such as their mass, star formation history, and star formation efficiency. Moreover, we are able to provide an independent estimation on the stellar formation timescale of galaxies, the...

Philosophiae Doctor - PhD / We present a new sample of distant powerful radio galaxies, in order to study their host-galaxy properties and provide targets for future observations of Hi absorption with new radio telescopes. We cross-match the Sydney University Molonglo Sky Survey radio catalogue at 843 MHz with the VISTA Hemisphere Survey near-infrared catalogue using the Likelihood Ratio technique, producing contour plots as a way to inspect by eye a subset of bright sources to validate the automated technique. We then use the Dark Energy Survey optical and near-infrared wavelength data to obtain photometric redshifts of the radio sources. We find a total of 249 radio sources with photometric redshifts over a 148 square degree region. By fitting the optical and near-infrared photometry with spectral synthesis models, we determine the stellar masses and star-formation rates of the radio sources. We find typical stellar masses of 1011−1012 M for the powerful high-redshift radio galaxies. We also find a population of low-mass blue galaxies. We then report results from the first search for associated Hi 21 cm line absorption with the new MeerKAT radio telescope (shared-risk early science programme). We used a 16-antenna sub-array of MeerKAT to carry out a survey for Hi absorption in the host galaxies of nine powerful (L1.4 GHz > 1026 W Hz−1 ) radio galaxies at cosmological distances (z = 0.29 to 0.54). We found no evidence of absorption with 5σ optical depth detection limits. We only obtain a tentative absorption towards a radio source 3C 262 at z = 0.44 with significant ongoing star formation at a rate of 10.5 M yr−1 . The source consists of two radio lobes separated by 28.5 kpc with no evidence of a compact core. If the absorption arises from neutral gas from an extended disc, the line is redshifted by 79(21) km s−1 with respect to the nucleus and has an average Hi column density across the source of NHI ∼ 7 × 1019−20 cm−2 , which is consistent with the rate of star formation. But after further tests, we find that the Hi detection towards 3C 262 is likely to be an artefact. We conclude that the new correlator with 32 k channel resolution will be needed before searching for its associated absorption in MIGHTEE data.