We revisit the proposed extended Schmidt law, which posits that the star formation efficiency in galaxies depends on the stellar mass surface density, by investigating spatially resolved star formation rates (SFRs), gas masses, and stellar masses of star formation regions in a vast range of galactic environments, from the outer disks of dwarf galaxies, to spiral disks and to merging galaxies, as well as individual molecular clouds in M33. We find that these regions are distributed in a tight power law as Sigma(SFR) proportional to (Sigma(0.5)(star)Sigma(gas))(1.09), which is also valid for the integrated measurements of disk and merging galaxies at high-z. Interestingly, we show that star formation regions in the outer disks of dwarf galaxies with Sigma(SFR) down to 10(-5) M(circle dot)yr(-1) kpc(-2), which are outliers of both the Kennicutt-Schmidt and Silk-Elmegreen laws, also follow the extended Schmidt law. Other outliers in the Kennicutt-Schmidt law, such as extremely metal-poor star formation regions, also show significantly reduced deviation from the extended Schmidt law. These results suggest an important role for existing stars in helping to regulate star formation through the effect of their gravity on the midplane pressure in a wide range of galactic environments.

Motivated by the desire to investigate two of the largest outstanding problems in galactic evolution -- stellar feedback and galactic chemical evolution -- we develop the first set of galaxy-scale simulations that simultaneously follow star formation with individual stars and their associated multi-channel stellar feedback and multi-element metal yields. We developed these simulations to probe the way in which stellar feedback, including stellar winds, stellar radiation, and supernovae, couples to the interstellar medium (ISM), regulates star formation, and drives outflows in dwarf galaxies. We follow the evolution of the individual metal yields associated with these stars in order to trace how metals mix within the ISM and are ejected into the circumgalactic and intergalactic media (CGM, IGM) through outflows. This study is directed with the ultimate goal of leveraging the ever increasing quality of stellar abundance measurements within our own Milky Way galaxy and in nearby dwarf galaxies to understand galactic evolution. Our simulations follow the evolution of an idealized, isolated, low mass dwarf galaxy (Mvir ∼ 10^9 M ) for ∼ 500 Myr using the adaptive mesh refinement hydrodynamics code Enzo. We implemented a new star formation routine which deposits stars individually from 1 M to 100 M . Using tabulated stellar properties, we follow the stellar feedback from each star. For massive stars (M∗ > 8 M ) we follow their stellar winds, ionizing radiation (using an adaptive ray-tracing radiative transfer method), the FUV radiation which leads to photoelectric heating of dust grains, Lyman-Werner radiation, which leads to H2 dissociation, and core collapse supernovae. In addition, we follow the asymptotic giant branch (AGB) winds of low-mass stars (M∗ < 8 M ) and Type Ia supernovae. We investigate how this detailed model for stellar feedback drives the evolution of low mass galaxies. We find agreement with previous studies that these low mass dwarf galaxies exhibit bursty, irregular star formation histories with significant feedback-driven winds. Using these simulations, we investigate the role that stellar radiation feedback plays in the evolution of low mass dwarf galaxies. In this regime, we find that the local effects of stellar radiation (within ~ 10 pc of the massive, ionizing source star) act to regulate star formation by rapidly destroying cold, dense gas around newly formed stars. For the first time, we find that the long-range radiation effects far from the birth sites are vital for carving channels of diffuse gas in the ISM which dramatically increase the effect of supernovae. We find this effect is necessary to drive strong winds with significant mass loading factors and has a significant impact on the metal content of the ISM. Focusing on the evolution of individual metals within this galaxy, it remains an outstanding question as to what degree (if any) metal mixing processes in a multi-phase ISM influence observed stellar abundance patterns. To address this issue, we characterize the time evolution of the metal mass fraction distributions of each of the tracked elements in our simulation in each phase of the ISM. For the first time, we demonstrate that there are significant differences in how individual metals are sequestered in each gas phase (from cold, neutral gas up to hot, ionized gas) that depend upon the energetics of the enrichment sources that dominate the production of a given metal species. We find that AGB wind elements have much broader distributions (i.e. are poorly mixed) as compared to elements released in supernovae. In addition, we demonstrate that elements dominated by AGB wind production are retained at a much higher fraction than elements released in core collapse supernovae (by a factor of ~ 5). We expand upon these findings with a more careful study of how varying the energy and spatial location of a given enrichment event changes how its metal yields mix within the ISM. We play particular attention to events that could be associated with different channels of r-process enrichment (for example, neutron star - neutron star mergers vs. hypernovae) as a way to characterize how mixing / ejection differences may manifest themselves in observed abundance patterns in low mass dwarf galaxies. We find that -- on average -- the injection energy of a given enrichment source and the galaxy's global SFR at the time of injection play the strongest roles in regulating the mixing and ejection behavior of metals. Lower energy events are retained at a greater fraction and are more inhomogeneously distributed than metals from more energetic sources. However, the behavior of any single source varies dramatically, particularly for the low energy enrichment events. We further characterize the effect of radial position and local ISM density on the evolution of metals from single enrichment events. Finally, we summarize how this improved physical model of galactic chemical evolution that demonstrates that metal mixing and ejection from galaxies is not uniform across metal species can be used to improve significantly upon current state of the art galactic chemical evolution models. These improvements stand to help improve our understanding of galactic chemical evolution and reconcile outstanding disagreements between current models and observations.

Measurements of hydrogen are important in our understanding of the Universe. Following reionization at z ∼ 6, most of the hydrogen outside galaxies is in an ionized state. Within galaxies, hydrogen passes through a neutral phase as it cools and collapses into molecular hydrogen and then to stars. This work centers around how galactic reservoirs of neutral hydrogen (HI) evolve over cosmic time. We know that cosmic star formation peaks at z ∼ 2 and sharply declines to the present day, yet we know very little about the gas reservoirs in individual galaxies that lead to star formation through these redshifts. The Very Large Array’s (VLA) recent upgrade has made it possible to probe a large instantaneous bandwidth with HI imaging surveys beyond the local Universe. The COSMOS HI Large Extragalactic Survey (CHILES) is a 1000-hour program using the Karl G. Jansky VLA that will image HI in a redshift range of 0 < z < 0.45. With our first epoch of data, we study the galaxy properties of a sample of ten nearby galaxies. We find that our data follow known scaling relations. Both theory and observations suggest that large-scale structure impacts galaxy evolution in addition to known trends in local density. We find that galaxy spins tend to be aligned with cosmic web filaments and a hint of the predicted transition mass associated with the spin angle alignment. With our second epoch of data from the CHILES survey, we probe the high-redshift regime. We present two new HI detections at z = 0.257 and z = 0.258, plus a stacked result at z ∼ 0.36. We combine these results with the previously published CHILES samples. This provides, for the first time, a continuous look at directly detected HI in emission over redshift range 0 < z < 0.45. We strengthen our epoch one comic web results, finding a perpendicular galaxy spin alignment with the cosmic web for a high-mass HI detection and a parallel galaxy spin alignment for a gas-rich low-mass HI detection embedded within a cosmic web filament. Having HI content, morphology, and kinematics, along with knowledge of the large-scale environments across substantial cosmic time spanning one-third the age of the Universe, will help shed light on the overall origin and fate of gas in galaxies.

A major goal of the field of Galactic archeology is to understand the formation and evolution of the Milky Way disk. Stars migrate to different Galactic radii throughout their lifetimes, often leaving little dynamical signature of their initial orbits. Therefore, we need to look at the archaeological record preserved in stellar chemical compositions, which is indicative of their birth environment. In this thesis, we use the measurable properties of stars (chemical compositions and ages) to reconstruct the Milky Way disk's past. First, using hydrodynamical simulations, we find that a star's birth radius and age are linked to its chemical abundances. Subsequently, we learn that even with current-day measurement uncertainty and sample sizes, chemical abundances of Milky Way stars provide a route to reconstructing its formation over time. Extending the insights from hydrodynamical simulations to 30,000 stars observed across the Milky Way disk in the APOGEE survey reveals the importance of using the high-dimensional chemical abundance space. Specifically, we determine that we can use groups of chemically similar stars with 19 measured abundances to trace different underlying formation conditions. Using the high-dimensional abundance data for 10,000 stars from two spectroscopic surveys, APOGEE and GALAH, we empirically describe the chemical abundance trends across a vast radial extent of the Milky Way disk. To do this, we employ a novel approach of quantifying radial variations for individual abundances conditioned on supernovae enrichment history. This enables us to assess the information content in each of the 15 abundances examined and capture the fine-grained signatures in the disk's chemical evolution history. This thesis outlines the potential of using stellar chemistry to trace different evolutionary events of the Milky Way disk, particularly in a time where survey data sample size and precision are growing rapidly.

The circumgalactic medium (CGM), loosely defined as the region between a galaxy disk and its virial radius, has long been of interest to astronomers and astrophysicists because it acts as an interface between galaxies and their surroundings. Studying it, therefore, gives us hints of how gas flows between galaxies and the intergalactic medium (IGM), fueling star formation for instance. This thesis addresses some of the current and future observation, analysis and instrumentation challenges that should be tackled for a better understanding of the CGM. Chapter 1 is an overview of science related to the CGM and of instruments that our lab works on: the Circumgalactic Hydrogen-Alpha Spectrograph (CHaS) and the Faint Intergalactic-medium Redshifted Emission Balloon (FIREBall). It sets the ground for a better understanding of the science discussed in subsequent chapters. CHaS is an IFU spectrograph installed on a 2.4 m telescope at the MDM Observatory in Arizona (Melso et al. 2022). It has high sensitivity and high spectral resolution, and it collects individual spectra from points across our targets using a microlens array, allowing us to make detailed spectral maps of observed astronomical objects. FIREBall is a balloon-born UV multi-object spectrograph, allowing us to look at yet another emission line prominent in the CGM. In this thesis, we will focus on what a future FIREBall detector might look like. Chapters 2 and 3 present data collected with CHaS in November 2021 from two very distinct objects: NGC 6946 (the Fireworks galaxy) and M76 (the Little Dumbbell nebula). Both chapters address how we process spectral data from CHaS images and the subsequent making of velocity maps. Using CHaS images, we tackle anomalous gas motion and formations in both targets. We compared the data presented in both chapters to previous literature, showing that CHaS velocity maps were more detailed and complimented previous findings. NGC 6946 is known for being a prolific star forming galaxy and also for having holes in its HI distribution, which have historically been attributed to the expansion and bursting of gas bubbles. In Chapter 2, we find that the motion around these holes is indeed consistent with expanding bubbles and galactic fountains on their edges, with velocities in the -20 km/s to 20 km/s range, consistent with what Boomsma et al. (2008) found, going up to +/-60 km/s, similar to the velocities found by Efremov et al. (2002). We also found that Long et al. (2019)'s supernova remnants candidates catalog had a clear position correlation with the boundaries of different holes in the Boomsma et al. (2008) HI hole catalog, suggesting that these holes might indeed be related to gas bubbles resulting from supernova explosions. The Little Dumbbell nebula, on the other hand, show its own set of anomalies. M76 is a butterfly planetary nebula with a central torus and two polar lobes. We find that these lobes are not completely symmetric. In fact, the wester lobe is more rounded and the eastern one is more stretched and fragmented. From our velocity maps, we propose a couple of explanations for how the ISM might interact with the nebula both in the core star's AGB phase and after the nebula is formed to give M76 its shape. Both explanations vary depending on the assumed direction of motion of the star in it its AGB phase, but both are consistent with models by Villaver, Manchado and García-Segura (2012) and Wareing et al. (2007). Moreover, we compare our data to those of other authors and find similar velocity ranges around an axis going from one lobe to another as spectral maps made by Ramos-Larios et al. (2017) and Bryce et al. (1996). Departing from observational data analysis, Chapter 4 focuses on how we can probe further into the CGM by upgrading existing instruments, turning commonplace condensed matter methods into tools for astrophysics. More specifically, Chapter 4 discusses the possibility of switching FIREBall's current UV sensitive emCCD detectors, which rely on coating to be visible-blind and on cryogenic equipment that is heavy for a balloon flight, for devices made out of hexagonal boron nitride (hBN). hBN's main energy bandgap overlaps with the emission lines that FIREBall is interested in capturing, and it can be combined with graphene (which is isomorphic to hBN) to make high quality, quantum efficient devices. While we weren't able to finish full devices, Chapter 4 discusses their fabrication in detail as well as how our Siesta SISL simulations show that even small device defects might be acceptable for a detector. The chapter ends with considerations about how one might fit individual devices as multi-pixel detectors.

Galactic Archaeology aims to understand the formation history of the Milky Way (MW). Observations from large spectroscopic and photometric surveys in the recent over the last decade have revolutionized this field. Many substructures and stellar populations have been discovered thanks to full sky surveys, such as Gaia, suggesting the MW is out of equilibrium. However, with numerous missions providing high-quality spectra and photometric time series for billions of stars, it has become increasingly difficult to interpret multidimensional data. One way to address the challenge of large data ensembles is to convey multidimensional information in a more compact way. This can be done by constructing a set of key summary statistics. In my thesis, I use photometric and abundance data to obtain the ages and birth radii of stars in the MW. These two physical quantities of stars, along with stellar abundances and kinematic measurements provide a ``Galactic timetable'' that marks the locations and times of occurrence of different events including mergers and enhancements in the star formation rate. To infer stellar ages, I use gyrochronology, one of the only viable methods to age-date main-sequence (especially for low-mass K/M dwarfs) stars. This technique uses stellar rotation periods and temperature measurements as age indicators. Due to the complexity of magnetic fields in stars, no purely theoretical gyrochronology model currently exists. As a result, gyrochronology relies strongly on empirical calibrations to known stellar ages using other methods. However, none of the age-dating methods for single field stars are suitable for low-mass main sequence stars, as they are faint and their physical properties evolve slowly. To get ages for these stars, I apply the simple assumption that the velocity dispersion of stars increases over time and adopt an age--velocity--dispersion relation (AVR) to estimate average stellar ages, which we called gyro-kinematic ages, for groupings of stars with similar period, temperature, absolute G magnitude, and Rossby number values. Since calculating gyro-kinematic ages requires a large number of stars with period and kinematic measurements, I measured rotation periods for K and M dwarfs using the Zwicky Transient Facility (ZTF). With conservative vetting criteria, I created the largest rotation period catalog (~ 40,000) for low-mass dwarf stars. By combining open cluster ages from literature and gyro-kinematic ages inferred from stars with 6-D kinematic from Gaia DR3 and rotation periods from Kepler and ZTF, I calibrated a fully empirical gyrochronology relation using Gaussian Processes. This approach is suitable for age-dating dwarf stars between 0.67 - 14 Gyr. Using this newly calibrated relation, I provide the community with the largest and most precise stellar ages for ~ 100,000 low-mass dwarf stars. This is the first time that the approach of gyrochronology has been used to date stars older than 4 Gyr. This sample can be used to study exoplanet evolution and the kinematic sub-structure in the solar neighborhood. Stars move away from their birthplaces over time via a process known as radial migration, which blurs chemo-kinematic relations used for reconstructing the MW formation history. One of the ultimate goals of Galactic Archaeology, therefore, is to understand stars’ birth locations. In my thesis, I first tested the reliability and limitation of the only method \cite{Minchev2018} that is able to infer star by star birth radius. I do so by testing the underlying assumption --- the metallicity gradient is linear at all times --- using the cosmology zoomed-in simulation NIHAO-UHD. This analysis concluded that for the MW, we can infer birth radii with an uncertainty of ~0.5 kpc if the metallicity gradient evolution is known and after the rotationally supported stellar disk has started to form. I then developed a method to recover the time evolution of the stellar birth metallicity gradient, d[Fe/H](R, t)/dR, through its inverse relation to the metallicity range as a function of age today. This allows me to place any star with age and metallicity measurements back to its birthplace, Rb. Applying this method to a high-precision large data set of MW disk subgiant stars, I find a steepening of the birth metallicity gradient from 11 to 8 Gyr ago, which coincides with the time of the last major merger, Gaia-Sausage-Enceladus (GSE). By dissecting the disk into mono-Rb populations, clumps in the low-[alpha/Fe] sequence appear, which are not seen in the total sample and coincide in time with known star-formation bursts.

This thesis is an investigation into some properties of brown dwarfs using medium-resolution spectroscopy. In the first part of the thesis, I address the issue of parameter degeneracy in brown dwarfs. In the course of my analysis, I derive a gravity-sensitive spectral index which can be used, statistically at least, to differentiate populations of young objects from field dwarfs. The index is also capable of finding the difference between a population of ~1 Myr objects and a population of ~10 Myr objects and may be used to separate low-mass members from foreground and background objects in young clusters and associations. The second part of my thesis is an investigation into the major opacity sources in the atmospheres of late T dwarfs. I look particularly at CH4 and NH3 absorption features in the near-infrared spectra of these objects. In my analysis, I identify new absorption features produced by these molecules. I also correct features which had previously been wrongly identified. This has been made possible by the use of high quality data, together with a new CH4 synthetic line list, which is more complete at these temperatures than any previously available list.

This thesis presents both a study of the cluster-scale environments in which massive stars form, investigating in particular how the ionized gas in these regions relates to the molecular star-forming material, as well as detailed studies of two luminous forming stars, AFGL 2591 and IRAS 20126+4104, to determine whether they are forming similarly to their low-mass counterparts. The results of this work include the identification of 35 HII regions (20 newly discovered) via a radio continuum survey of ionized gas towards 31 molecular cluster-forming clumps. The observed ionized gas was found to be preferentially associated with the clumps, which were shown to have a range of evolutionary stages. The massive star formation efficiency was determined for the clumps with associated ionized gas, and a relationship was found between the mass of the clumps and the mass of their embedded massive stars. By modelling the SEDs and images of AFGL 2591 and IRAS 20126+4104, it was found that the geometry of their circumstellar material was generally consistent with an envelope plus disk, similar to that expected for low-mass protostars. However, within the central ~1800 AU, the mid-IR images of IRAS 20126+4104 were better described by only a flattened envelope, suggesting that the radiation from IRAS 20126+4104 may be affecting the regions closest to the star. Observations of the ionized and molecular gas towards AFGL 2591 were carried out, and a photoionization code was developed to interpret these observations. The results showed that the observed 3.6 cm emission is likely to be produced by both a shock-ionized jet and a hypercompact HII region that does not appear to have disrupted the jet or the large-scale circumstellar environment. In addition, the C¹⁸O(1-0) emission observed towards AFGL2591 traces the densest parts of the outflow, with the blue-shifted emission exhibiting many of the properties of the outflows from low-mass protostars.

Stars are born from dense cores of gas within molecular clouds. The exact nature of the connection between these gas cores and the stars they form is an important issue in the field of star formation. In this thesis I use numerical simulations of molecular clouds to trace the evolution of cores into stars. The CLUMPFIND method, commonly used to identify gas structures is tested. I find that the core boundaries it yields are unreliable, but in spite of this, the same profile is universally found for the mass function. To facilitate a more robust definition of a core, a modified clumpfind algorithm which uses gravitational potential instead of density is introduced. This allows the earliest fragmentation in a simulated molecular cloud to be identified. The first bound cores have a mass function that closely resembles the stellar IMF, but there is a poor correspondence between individual core masses and the stellar masses formed from them. From this, it is postulated that environmental factors play a significant part in a core’s evolution. This is particularly true for massive stars, as massive cores are prone to further fragmentation. In these simulations, massive stars are formed simultaneously with stellar clusters, and thus the evolution of one can affect the other. In particular, the global collapse of the forming cluster aids accretion by the precursors of the massive stars. By tracing the evolution of the massive stars, I find that most of the material accreted by them comes from diffuse gas, rather than from a well-defined stellar core.

In this thesis, we examine the role of accretion disks in the formation of stellar systems, focusing on young massive disks which regulate the flow of material from the parent molecular core down to the star. We study the evolution of disks with high infall rates that develop strong gravitational instabilities. We begin in chapter 1 with a review of the observations and theory which underpin models for the earliest phases of star formation and provide a brief review of basic accretion disk physics, and the numerical methods which we employ. In chapter 2 we outline the current models of binary and multiple star formation, and review their successes and shortcomings from a theoretical and observational perspective. In chapter 3 we begin with a relatively simple analytic model for disks around young, very massive stars, showing that instability in these disks may be responsible for the higher multiplicity fraction of massive stars, and perhaps the upper mass to which they grow. We extend these models in chapter 4 to explore the properties of disks and the formation of binary companions across a broad range of stellar masses. In particular, we model the role of global and local mechanisms for angular momentum transport in regulating the relative masses of disks and stars. We follow the evolution of these disks throughout the main accretion phase of the system, and predict the trajectory of disks through parameter space. We follow up on the predictions made in our analytic models with a series of high resolution, global numerical experiments in chapter 5. Here we propose and test a new parameterization for describing rapidly accreting, gravitationally unstable disks. We find that disk properties and system multiplicity can be mapped out well in this parameter space. Finally, in chapter 6, we address whether our studies of unstable disks are relevant to recently detected massive planets on wide orbits around their central stars.