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Galactic bars play an important role in the dynamical evolution of their host galaxy, but their own evolution and impact on the local gas reservoir and star formation rate are still open questions. Recent work by the Physics at High Angular resolution in Nearby GalaxieS (PHANGS) collaboration found higher molecular gas surface densities and velocity dispersions in barred galaxies compared to unbarred galaxies. The higher turbulence found in bars is expected to create the observed increases. In this work, I explore bar turbulence in molecular gas using published high resolution measurements of CO(2-1) from the PHANGS-ALMA survey. I compare properties of the molecular gas, such as surface density, velocity dispersion and star formation rate, in the centres of barred and unbarred galaxies. I consider the effect of galaxy environment on these properties from a local perspective (at cloud scales, ~100 pc) for galaxies with and without an AGN. On global scales, I consider these properties in the context of the environment in which a galaxy lives, whether in a cluster or in the field. All three quantities (gas surface density, velocity dispersion, and star formation rate) are found to be enhanced in barred galaxy centres, even without an AGN and regardless of global environment. / Thesis / Master of Science (MSc)

<p> One of the most difficult and interesting aspects of the physics of collapse and outflow formation, as well as the evolution of the protostellar disk, is the role of hydromagnetic forces. However, magnetic fields are only coupled to the charged species present in poorly ionized molecular clouds. Ambipolar diffusion-the process by which magnetic fields "slip" in poorly ionized gas-strongly affects the initial cloud as well as the final observable structure through collisional heating. Also, as the gas becomes opaque to cosmic rays, the ionized structure of the accreting gas may become more complex, leading to a neutral 'dead zone' in a layered accretion disk (vital in determining planet masses in planet formation theories (Matsumura & Pudritz, 2005)). We omit possible effects of ionizing radiation in these early stages of formation. </p> <p> In this thesis, we perform fully 3D simulations (using the FLASH AMR code) and have implemented ambipolar diffusion in the MHD module of the code in addition to a broad treatment of cooling (Banerjee et al., 2006). This has allowed us to track the ionized gas and magnetic fields properly from the beginning of collapse down to the onset of outflows. We find that high accretion rates persist on the order 1 of 10-3 M0 yr-(where the core mass has reached about 0.1 M0 ) due to efficient extraction of angular momentum through magnetic processes. Magnetic braking is reduced by about 3/4 in the initial collapse relative to an ideal collapse of same initial conditions. This, with a reduction in magnetic pressure in the disk, leads to an increased rate of fragmentation. One of the major new results of this work is the discovery that outflows from disks still occur even in the presence of ambipolar diffusion. Surprisingly, they are initiated even earlier than outflows from idealized, completely ionized disks. They are generated by a magnetic tower mechanism at central densities of 1012 cm-3, as effective ram pressure on the wound up toroidal field is reduced, allowing it to push away from the disk earlier. </p> <p> We have also shown that the formation of a dead zone in these early stages is dependent on shielding of cosmic rays, in the absence of which a decoupled zone in the disk midplane forms. This region, where the accreting gas is effectively decoupled from the magnetic field, extends 10 AU in radius and (2-3) AU in height from the midplane. The global magnetic field threading such a complex accretion disk shows a dragged out structure, as coupled surface layers of the disk pull in the field. The disk is puffy due to drift heating and the initial stages of the outflow pushing out into the ambient medium. However, overall magnetic field build-up is still efficient, as values of the magnetic field in the disk are only reduced by half. </p> / Thesis / Master of Science (MSc)

Where a galaxy is located has a strong effect on its properties. The dense cluster environment is home to a large population of red, quiescent elliptical galaxies, whereas blue, star-forming, spiral galaxies are common in lower-density environments. This difference is intricately linked to the ability of the galaxy to form new stars and therefore ultimately to the fuel for star formation, the atomic and molecular gas. In this thesis, I use two large JCMT surveys to explore the effects of environment on the atomic gas, molecular gas, and star formation properties of a large sample of nearby gas-rich galaxies. From the NGLS and follow-up studies, I select a sub-sample of 98 HI-flux selected spiral galaxies. I measure their total molecular gas mass using the CO J=3-2 line and combine this data with measurements of their total atomic gas mass using the 21-cm line and star formation rate using attenuation-corrected H-alpha luminosity. I find an enhancement in the mean H2 mass and a higher H2-to-HI ratio for the Virgo Cluster sample. Virgo Cluster galaxies also have longer molecular gas depletion times (H2/SFR), which suggests that they are forming stars at a lower rate relative to their molecular gas reservoirs than non-Virgo galaxies. Next, I collect VLA 21 cm line maps from the VIVA survey and follow-up VLA studies of selected galaxies in the NGLS. I measure the surface density maps of the atomic gas, molecular gas, and star formation rate in order to determine radial trends. I find that the H2 distribution is enhanced near the centre for Virgo Cluster galaxies, along with a steeper total gas (HI + H2) radial profile. I suggest that this is due to the effects of moderate ram pressure stripping, which would strip away low-density gas in the outskirts while enhancing high-density gas near the centre. There are no trends with radius for the molecular gas depletion times, but the longer depletion times for the Virgo Cluster sample is still present. Finally, I use 850 micron continuum observations for 105 star-forming galaxies and CO J=2-1 line observations for 35 galaxies in the initial data release (DR1) of the JINGLE survey. I match the JINGLE galaxies to a SDSS group catalogue and measure environmental parameters such as the host halo mass, environment density, and location in phase space. I find that the molecular gas masses estimated from the 850 μm and CO J=2-1 line observations are well-correlated. The H2-to-HI ratio and the molecular gas depletion times do not appear to vary with stellar mass. I did not find any significant variation with environment in the DR1 sample, but I will apply this framework to the full JINGLE sample once the complete dataset is available. / Thesis / Doctor of Philosophy (PhD)

Within this thesis, we present an extended multiwavelength analysis of the rich massive Galactic star-forming complex G305. We have focused our attention on studying the both the embedded massive star-forming population within G305, while also identifying the intermediate-, to lowmass content of the region also. Though massive stars play an important role in the shaping and evolution of their host galaxies, the physics of their formation still remains unclear. We have therefore set out to studying the nature of star formation within this complex, and also identify the impact that such a population has on the evolution of G305. We firstly present a Herschel far-infrared study towards G305, utilising PACS 70, 160 μm and SPIRE 250, 350, and 500 μm observations from the Hi-GAL survey of the Galactic plane. The focus of this study is to identify the embedded massive star-forming population within G305, by combining far-infrared data with radio continuum, H2O maser, methanolmaser,MIPS, and Red MSX Source survey data available from previous studies. From this sample we identify some 16 candidate associations are identified as embedded massive star-forming regions, and derive a two-selection colour criterion from this sample of log(F70/F500)! 1 and log(F160/F350)! 1.6 to identify an additional 31 embedded massive star candidates with no associated starformation tracers. Using this result, we are able to derive a star formation rate (SFR) of 0.01 - 0.02 M! yr−1. Comparing this resolved star formation rate, to extragalactic star formation rate tracers (based on the Kennicutt-Schmidt relation), we find the star formation activity is underestimated by a factor of !2 in comparison to the SFR derived from the YSO population. By next combining data available from 2MASS and VVV, Spitzer GLIMPSE and MIPSGAL, MSX, and Herschel Hi-GAL, we are able to identify the low-, to intermediate-mass YSOs present within the complex. Employing a series of stringent colour selection criteria and fitting reddened stellar atmosphere models, we are able remove a significant amount of contaminating sources from our sample, leaving us with a highly reliable sample of some 599 candidate YSOs. From this sample, we derive a present-day SFR of 0.005±0.001M! yr−1, and find the YSOmass function (YMF) of G305 to be significantly steeper than the standard Salpeter-Kroupa IMF. We find evidence of mass segregation towards G305, with a significant variation of the YMF both with the active star-forming region, and the outer region. The spatial distribution, and age gradient, of our 601 candidate YSOs also seem to rule out the scenario of propagating star formation within G305, with a more likely scenario of punctuated star formation over the lifetime of the complex.

Observations of galaxy environments have revealed numerous correlations associated with their intrinsic properties. It is therefore clear that if we are to understand the processes by which galaxies form and evolve, we have to consider the role of their immediate environment and how these trends change across cosmic time. In this thesis, I investigate the relationship between the environmental densities of galaxies and their associated properties by developing and implementing a novel approach to measuring galaxy environments on individual galaxy scales with Voronoi tessellations. Using optical spectroscopy and photometry from GAMA and SDSS, with 250μm far-infrared observations from the Herschel-ATLAS SDP and Phase-One fields, the environmental and star formation properties of far-infrared detected and non–far-infrared detected galaxies are compared out to z ∼ 0.5. Applying statistical analyses to colour, magnitude and redshift-matched samples, I show there to be significant differences between the normalised density distributions of the optical and far-infrared selected samples, at the 3.5σ level for the SDP increasing to > 5σ when combined with the Phase-One data. This is such that infrared emission (a tracer of star formation activity) favours underdense regions, in agreement with previous studies that have proposed such a correlation. I then apply my method to synthetic light cones generated from semianalytic models (SAMs), finding that over the whole redshift distribution the same correlations between star-formation rate and environmental density are found. However, as the SAMs restrict the role of ram-pressure stripping, the fact that we find the same qualitative results may preclude ram-pressure as a key mechanism in truncating star formation. I also find significant correlations between isothermal dust temperature and environment, such that the coldest sources reside in the densest regions at the 3.9σ level, indicating that the observed far-infrared emission in these densest regions is the product of ISM heating by the older stellar populations. I then extend my analysis to a deeper sample of galaxies out to z ∼ 2.2, combining near-infrared and optical photometry from the VIDEO and CFHTLS-D1 observations, cross-matched in colour, magnitude and redshift against 1.4 GHz VLA radio observations. Across the entire radio sample, galaxies with radio detected emission are found to reside in more overdense environments at a 4.0σ significance level. I then divide my radio sample to investigate environmental dependence on both radio detected star-forming galaxies and radio detected AGN individually, based upon a luminosity selection defined as L = 1023 W Hz−1. The same trends with environment are shown by my Radio-AGN sample (L > 1023 W Hz−1) which favour overdense regions at the 4.5σ level, suggestive of the interaction processes (i.e. major mergers) that are believed to trigger accretion, in agreement with earlier work that has suggested such a relationship. At lower radio luminosities, my Radio-SF sample (L < 1023 W Hz−1) also display a significant trend towards overdense regions in comparison to my nonradio detected sample, at the less significant level of 2.7σ. This is suggestive of the low overall bolometric luminosity of radio emission in star forming galaxies, leading to only the brightest radio emitting star forming galaxies being observed and a bias towards overdense regions. This is in addition to the fact that the luminosity selection used to separate AGN from star forming galaxies is not a perfect selection and open to AGN contamination in the low-luminosity sample. I conclude that the next generation of deep radio surveys, which are expected to reach many orders of magnitude deeper than current observations, will remove radio-loud AGN contamination and allow for the detection of low-luminosity star forming galaxies via radio emission out to high redshifts. This work has allowed for the environments of galaxies to be probed on smallerscales and across both wider and deeper samples than previous studies. With significant environmental correlations being returned, this indicates that the established processes responsible for such trends must have influence on the most local of scales.

This thesis presents a detailed study of the impact of radio-AGN on star formation and the interstellar medium (ISM) of galaxies across cosmic time. To do this, this thesis uses far-IR/sub-mm data from the Herschel Space Observatory. I create a well-selected sample of 1599 radio sources using the NRAO VLA Sky Survey (NVSS) and Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) data in combination with the UKIRT Infrared Deep Sky Survey - Large Area Survey (UKIDSS - LAS) data. I find that the far-IR luminosities and dust temperatures of radio galaxies are lower in comparison to those of non-radio-detected galaxies. This luminosity deficit grows with increasing stellar mass. I argue that the reasons for these differences is probably due to indirect radio-AGN feedback, i.e. radio jets mechanically heat the halo-environment, preventing external sources of cold gas from entering the host and forming stars. The far-IR luminosity and dust temperature is found to decrease as a function of radio source size. I find the most likely explanation for this is jet-induced star formation while the jets are confined to the ISM. Finally, a method for identifying reliable high-z, star-bursting radio sources in the H-ATLAS is described with which statistically significant studies of radio-jet induced star formation may be undertaken.

This dissertation is a study of the physical conditions in star-forming regions, and combines observational data and theoretical calculations. We studied the physical conditions of Orions Veil, which is an absorbing screen that lies along the line of sight to the Orion H II region. We computed photoionization models of the Veil. We combined calculations with UV, radio, and optical spectra that resolve the Veil into two velocity components. We derive many physical parameters for each component seen in 21 cm absorption. We find the magnetic field energy dominates turbulent and thermal energies in one component while the other component is close to equipartition between turbulent and magnetic energies. We observe H2 absorption for highly excited levels. We find that the low ratio of H2/H0 in the Veil is due to the high UV flux incident upon the Veil. We detect blueshifted S+2 and P+2 ions which must arise from ionized gas between the neutral portions of the Veil and the Trapezium and shields the Veil from ionizing radiation. We determine the ionized and neutral layers of the Veil will collide in less than 85,000 years. The second part of this dissertation involved self-consistently calculating the thermal and chemical structure of an H II region and photodissociation region (PDR) that are in pressure equilibrium. This differs from previous work, which used separate calculations for each gas phase. Our calculations span a wide range of initial conditions. We describe improvements made to the spectral synthesis code Cloudy which made these calculations possible. These include the addition of a molecular network with ~1000 reactions involving 68 molecules and improved treatment of the grain physics. Archival data are used to derive important physical characteristics of observed H II regions and PDRs. These include stellar temperatures, electron densities, ionization parameters, UV flux, and PDR density. The contribution of the H II region to PDR emission line diagnostics is also calculated. Finally, these calculations are used to derive emission line ratios than can tell us the equation of state in star-forming regions.

The ignition of the first sources of light marked the end of the cosmic dark ages, an era when the Universe transitioned from the relatively simple conditions following the Big Bang to the complex tapestry of dark matter, baryons, and pervasive cosmic radiation fields we see today. To better understand this uncharted cosmic epoch, we primarily utilize hydrodynamical, N-body simulations to model the assembly of the first galaxies at redshifts greater than ten and the stars that form within them. These simulations begin from cosmological initial conditions, employ a robust, non-equilibrium chemo-thermodynamic model, and take advantage of adaptive-grid-refinement to probe the multi-scale, complex process of star formation from ab initio principles. We explore the consequences that metal enrichment has on the process of star formation, confirming the presence of a critical metallicity for low-mass star formation. To assess the observational prospects of these primeval stellar populations with next-generation telescopes, like the James Webb Space Telescope, we constrain the star formation efficiency of both metal-enriched and metal-free star formation in a typical first galaxy. We also resolve the formation of individual metal-enriched stars in simulations that ultimately began from cosmological scales, allowing meaningful comparisons between our simulations and the recently discovered ultra-faint dwarf satellite galaxies, the suspected analogs of the first galaxies in the local Universe. / text

Massive stars play a key role in the evolution of the Galaxy; hence they are important objects of study in astrophysics. Although they are rare compared to low mass stars, they are the principal source of heavy elements and UV radiation, affecting the process of formation of stars and planets, and the physical, chemical, and morphological structure of galaxies. Star clusters form in dense "clumps" (~few parsecs in size) within giant molecular clouds, while individual stars form in cores (subparsec scale). An important step in the observational study of massive star formation is the identification and characterization of clumps. More detailed studies can then show how these clumps fragment into cores. Studies of clumps in our Galaxy will provide fundamental guidelines for the analysis of other galaxies, where individual clumps and cores cannot be resolved, and provide a catalog of interesting sources for observations of the Milky Way with a new generation of instruments, such as the Atacama Large Millimeter/Submillimeter Array. Large-scale blind surveys of the Galactic plane at millimeter and submillimeter wavelengths have recently been completed, allowing us to identify star forming clumps and improve our understanding of the early stages of massive stars. One of these studies, the Bolocam Galactic Plane Survey (BGPS), mapped the continuum emission at 1.1 mm over a large region of the northern Galactic plane at a resolution of 33'', identifying 8559 compact sources throughout the Galaxy. In this dissertation, I present observations of a sample of sources from the BGPS catalog, obtained with the Submillimeter High Angular Resolution Camera II (SHARC-II). I present in this work 107 continuum emission maps at 350 microns at high angular resolution (8.5'') toward clump-like sources and construct a catalog of BGPS substructures. I estimate clump properties such as temperatures and multiplicity of substructures, and compare my results with 350 microns continuum maps from the Hi-GAL survey. I also present a detailed analysis, using molecular line and dust continuum observations, of the region G331.5-0.1, one of the most luminous regions of massive star formation in the Milky Way, located at the tangent region of the Norma spiral arm. Molecular line and millimeter continuum emission maps reveal the presence of six compact and luminous molecular clumps, with physical properties consistent with values found toward other massive star forming sources. This work includes the discovery of one of the most energetic and luminous molecular outflows known in the Galaxy, G331.512-0.103. For this high-speed outflow, I present ALMA observations that reveal a very compact, extremely young bipolar outflow and a more symmetric outflowing shocked shell surrounding a very small region of ionized gas. The source is one of the youngest examples of massive molecular outflows associated with the formation of a high-mass star. / text