Galactic Building Blocks: Searching for Dwarf Galaxies Near and Far

The prevailing model of structure formation that describes how matter is distributed throughout the Universe is known as the Lambda Cold Dark Matter paradigm. A key component of this paradigm is dark matter, which has so far gone undetected in laboratory experiments but is inferred from a wide variety of astrophysical observations. Although the Cold Dark Matter paradigm is extremely successful on large scales, there are significant differences between what computer simulations predict and what we observe on galaxy scales. The purpose of the work presented in this Dissertation is to address some of the issues surrounding the current structure formation paradigm and further develop some tools for investigating small scale structure. An issue that has caused recent controversy is known as the Planes of Dwarf Galaxies problem which describes the curious alignment of the Milky Way\u27s dwarf galaxies into a thin planar structure. We have investigated this structure through time by integrating their orbits using the latest proper motion data as well as compared the distribution with current cosmological simulations and found no significant difference between the Milky Way distribution and simulations. Through analysis of observations of the disturbances in the extended neutral hydrogen disks of spiral galaxies, one can characterize dark matter substructure and the dark matter halo of a host galaxy. This process is called Tidal Analysis. Using a simple test particle code to model satellite interactions with a gas disk, we have developed a scaling relation to relate the observed density response of the disk to the mass and pericenter of a satellite. With this relation, observers can now immediately infer the recent interaction history of a spiral galaxy from neutral hydrogen studies. Changing gears to observational studies of small scale structure, we report observations of Cepheid variables in a putative dwarf galaxy located along the line of sight of the galactic plane that was first predicted through the use of Tidal Analysis. Observations are still ongoing; however, preliminary results indicate that the Cepheids are part of structure that is moving independently of the Milky Way. Finally, in an effort to use Tidal Analysis on other galaxies to constrain substructure, we have begun a 21-cm radio observing campaign of a set of spiral galaxies at redshift z=0.1 to obtain their total mass in neutral hydrogen. This unique set of galaxies also act as strong gravitational lenses, thus allowing us to use both Tidal Analysis and gravitational lensing together for the first time. We report a secure detection and mass measurement for one of our sources and six upper mass limits

The ALMA Interferometric Pipeline Heuristics

We describe the calibration and imaging heuristics developed and deployed in the ALMA interferometric data processing pipeline, as of ALMA Cycle 9. The pipeline software framework is written in Python, with each data reduction stage layered on top of tasks and toolkit functions provided by the Common Astronomy Software Applications package. This framework supports a variety of tasks for observatory operations, including science data quality assurance, observing mode commissioning, and user reprocessing. It supports ALMA and VLA interferometric data along with ALMA and NRO45m single dish data, via different stages and heuristics. In addition to producing calibration tables, calibrated measurement sets, and cleaned images, the pipeline creates a WebLog which serves as the primary interface for verifying the data quality assurance by the observatory and for examining the contents of the data by the user. Following the adoption of the pipeline by ALMA Operations in 2014, the heuristics have been refined through annual development cycles, culminating in a new pipeline release aligned with the start of each ALMA Cycle of observations. Initial development focused on basic calibration and flagging heuristics (Cycles 2-3), followed by imaging heuristics (Cycles 4-5), refinement of the flagging and imaging heuristics with parallel processing (Cycles 6-7), addition of the moment difference analysis to improve continuum channel identification (2020 release), addition of a spectral renormalization stage (Cycle 8), and improvement in low SNR calibration heuristics (Cycle 9). In the two most recent Cycles, 97% of ALMA datasets were calibrated and imaged with the pipeline, ensuring long-term automated reproducibility. We conclude with a brief description of plans for future additions, including self-calibration, multi-configuration imaging, and calibration and imaging of full polarization data.Comment: accepted for publication by Publications of the Astronomical Society of the Pacific, 65 pages, 20 figures, 10 tables, 2 appendice

Discovery of a Group of Receding, Variable Halo Stars toward Norma

We present results from spectroscopic observations of a trio of Cepheid candidates identified from K s -band light curves toward Norma. The spectra show that these stars are moving with a large and similar radial velocity—the heliocentric velocities are 171 ± 32 km s−1, 164 ± 37 km s−1, and 173 ± 20 km s−1. The average radial velocity is ~169 km s−1, which is large and distinct from typical stars in the Galaxy's stellar disk. Given the radial velocities and associated 1σ error, we find that the combined probability that these three stars are foreground Milky Way disk stars is ~7 × 10−4%, and the probability that these are large-amplitude spotted stars in a binary is ~10−5%. These objects at l ~ 333° and b ~ −1° are therefore associated with the stellar halo. The identification of these sources as Type I Cepheids is not certain, and thus the distances of these sources are not yet well established. Assuming the 3.6 μm period–luminosity relation of Type I Cepheids gives a distance of ~78 kpc for these sources