Galaxy formation and evolution

Abstract:

We present a comparative study of four physical dust models and two single-temperature modified blackbody models by fitting them to the resolved WISE, Spitzer, and Herschel photometry of M101 (NGC 5457). Using identical data and a grid-based fitting technique, we compare the resulting dust and radiation field properties derived from the models. We find that the dust mass yielded by the different models can vary by up to a factor of 3 (factor of 1.4 between physical models only), although the fits have similar quality. Despite differences in their definition of the carriers of the mid-IR aromatic features, all physical models show the same spatial variations for the abundance of that grain population. Using the well-determined metallicity gradient in M101 and resolved gas maps, we calculate an approximate upper limit on the dust mass as a function of radius. All physical dust models are found to exceed this maximum estimate over some range of galactocentric radii. We show that renormalizing the models to match the same Milky Way high-latitude cirrus spectrum and abundance constraints can reduce the dust mass differences between models and bring the total dust mass below the maximum estimate at all radii.

Abstract:

ABSTRACT In the standard Lambda cold dark matter paradigm, pure dark matter simulations predict dwarf galaxies should inhabit dark matter haloes with a centrally diverging density ‘cusp’. This is in conflict with observations that typically favour a constant density ‘core’. We investigate this ‘cusp-core problem’ in ‘ultra-faint’ dwarf galaxies simulated as part of the ‘Engineering Dwarfs at Galaxy formation’s Edge’ project. We find, similarly to previous work, that gravitational potential fluctuations within the central region of the simulated dwarfs kinematically heat the dark matter particles, lowering the dwarfs’ central dark matter density. However, these fluctuations are not exclusively caused by gas inflow/outflow, but also by impulsive heating from minor mergers. We use the genetic modification approach on one of our dwarf’s initial conditions to show how a delayed assembly history leads to more late minor mergers and, correspondingly, more dark matter heating. This provides a mechanism by which even ultra-faint dwarfs ($M_* \lt 10^5\, \text{M}_{\odot }$), in which star formation was fully quenched at high redshift, can have their central dark matter density lowered over time. In contrast, we find that late major mergers can regenerate a central dark matter cusp, if the merging galaxy had sufficiently little star formation. The combination of these effects leads us to predict significant stochasticity in the central dark matter density slopes of the smallest dwarfs, driven by their unique star formation and mass assembly histories.

Abstract:

How does a public policy reform become seemingly inevitable? How and why does bureaucratic “common sense” override empirical evidence? What are the overlaps between accountability and corruption? These open-ended questions drive my study of a neglected object of analysis: the well-intentioned people and institutions working within Australia’s Indigenous affairs local government policy arena. In July 2008 the Northern Territory’s local government sector underwent the most sweeping reform of its history, when 53 councils (governing predominantly rural and majority-Indigenous communities) were forcibly amalgamated into eight regional shires. According to the official narrative, the sector had reached a point of crisis: many councils were administratively too small to manage their growing responsibilities. Despite trenchant popular opposition, amalgamations were justified as technically effective, financially efficient -- even morally imperative. Based on a decade of ethnographic research in the Northern Territory including extensive interviews and professional involvement over many years, my thesis moves beyond an empirical evaluation of these events to explore how government actors establish, maintain and self-assess a policy reform. Despite bureaucracy’s claims on rationality and evidence, I argue that factors such morality, routines and aesthetics play more important roles in policy formation than acknowledged. Influenced by the Deleuzian concepts of assemblage and the minor event, I analyse this reform through mundane bureaucratic habits, obscured settler colonial social relations, and near-forgotten events: the rendering of Indigenous totemic art into a corporate logo; a sport and recreation funding agreement as a site of bureaucratic violence; the mobilisation of a created statistic. This perspective invites an alternative to orthodox policy evaluation, whereby the policy cycle is understood ecologically, as a complex assemblage of force, violence and effect. In July 2008 the Northern Territory’s local government sector underwent the most sweeping reform of its history, when 53 councils (governing predominantly rural and majority-Indigenous communities) were forcibly amalgamated into eight regional shires. According to the official narrative, the sector had reached a point of crisis: many councils were administratively too small to manage their growing responsibilities. Despite trenchant popular opposition, amalgamations were justified as technically effective, financially efficient -- even morally imperative. Based on a decade of ethnographic research in the Northern Territory including extensive interviews and professional involvement over many years, my thesis moves beyond an empirical evaluation of these events to explore how government actors establish, maintain and self-assess a policy reform. Despite bureaucracy’s claims on rationality and evidence, I argue that factors such morality, routines and aesthetics play more important roles in policy formation than acknowledged. Influenced by the Deleuzian concepts of assemblage and the minor event, I analyse this reform through mundane bureaucratic habits, obscured settler colonial social relations, and near-forgotten events: the rendering of Indigenous totemic art into a corporate logo; a sport and recreation funding agreement as a site of bureaucratic violence; the mobilisation of a created statistic. This perspective invites an alternative to orthodox policy evaluation, whereby the policy cycle is understood ecologically, as a complex assemblage of force, violence and effect

Abstract:

Abstract To determine the importance of merging galaxies to galaxy evolution, it is necessary to design classification tools that can identify the different types and stages of merging galaxies. Previously, using GADGET-3/SUNRISE simulations of merging galaxies and linear discriminant analysis (LDA), we created an accurate merging galaxy classifier based on imaging predictors. Here, we develop a complementary tool, based on stellar kinematic predictors, derived from the same simulation suite. We design mock stellar velocity and velocity dispersion maps to mimic the specifications of the Mapping Nearby Galaxies at Apache Point (MaNGA) integral field spectroscopy (IFS) survey, and utilize an LDA to create a classification, based on a linear combination of 11 kinematic predictors. The classification varies significantly with mass ratio; the major (minor) merger classifications have a mean statistical accuracy of 80% (70%), a precision of 90% (85%), and a recall of 75% (60%). The major mergers are best identified by predictors that trace global kinematic features, while the minor mergers rely on local features that trace a secondary stellar component. While the kinematic classification is less accurate than the imaging classification, the kinematic predictors are better at identifying post-coalescence mergers. A combined imaging + kinematic classification has the potential to reveal more complete merger samples from imaging and IFS surveys such as MaNGA. We note that since the suite of simulations used to train the classifier covers a limited range of galaxy properties (i.e., the galaxies are of intermediate mass, and disk-dominated), the results may not be applicable to all MaNGA galaxies.