In this thesis, I explore the rate at which molecular gas is converted to stars through detailed studies of a sample of molecular clouds in the Milky Way, IFU spatially resolved observations of gas-rich nearby interacting galaxies, as well as the environmental dependence of star formation and galaxy morphology in a galaxy supercluster. This thesis is composed of three individual projects that investigate nearby star formation within the local 500 pc of our Sun, to neighboring extreme star forming environments of interacting starburst galaxies, and finally studying how star formation varies with galaxy morphology in a galaxy supercluster a z~0.165. I discuss the relation between the star formation rate (SFR) and molecular gas surface densities (e.g., Schmidt-Kennicutt relation) in Galactic star forming regions and find there is a discrepancy between my study and extragalactic relations. The discrepancy is attributed to extragalactic measurements that are averaged over large >kpc scales and include star forming molecular gas (above some threshold) and molecular gas the is not dense enough to form stars. I find a steep increase in the Galactic SFR-gas surface density relation indicative of a threshold for efficient star formation that is best fit to a broken power law with a linear slope above 129 Msun pc⁻². I introduce the VIRUS-P Investigation of the eXtreme ENviroments of Starbursts (VIXENS) project which is a survey of interacting is a large integral field unit survey of nearby infrared bright (L_IR>3x10¹⁰ Lsun) interacting/starburst galaxies. The main goal of VIXENS is to investigate the relation between star formation and gas content on spatially resolved scales of ~0.1-1 kpc in the extreme star forming environments of interacting/starburst galaxies. The VIXENS sample is composed of systems in a range interaction stages with morphological signatures from early phase (close pairs) to late stage mergers (single system with multiple nuclei), SFRs, and gas surface densities. I highlight the first results from the VIXENS survey in the late interaction phase galaxy merger Arp 299. I find 1.3 kpc regions in Arp 299 to lie along the SFR-gas surface density relation found for mergers at high redshift, but this relation is highly dependent on the CO to molecular hydrogen (H₂) conversion factor. I find evidence for a Galactic CO-to-H₂ conversion factor using metallicity and dust temperature measurements, which would place 1.3 kpc regions in the Arp 299 merger in between the high redshift and Kennicutt-Schmidt relations. Comparing the SFR to dense gas surface densities as traced by HCN and HCO⁺, I find an agreement between the spatially resolved measurements and that found on global scales in spirals and (ultra)luminous infrared galaxies. Finally, I present an investigation of the influence of environment on frequency, distribution, color, and star formation properties of galaxy mergers and non-interacting galaxies in the Abell 901/902 supercluster at z~0.165. I find galaxy mergers be preferentially blue in color and have an enhanced SFR by a factor of ~2 compared to non-interacting galaxies. This result may be due to a decrease in galaxy velocity dispersion in the cluster outskirt, favoring galaxy-galaxy interactions, or to interacting galaxies that are part of groups or field galaxies being accreted along cosmological filaments by the clusters. I compare to N-body simulations of groups and field galaxies accreting onto the clusters and find the fraction of mergers are similar to that predicated at group overdensities. I find the SFR of galaxies in the supercluster to be depressed compared to field galaxies in both the core and cluster outskirts, suggesting that an environmental process such as ram pressure stripping is effective throughout the cluster. The results of a modest SFR enhancement and a low merger fraction culminate in my finding that mergers contribute only a small fraction (between 10% and 15%) of the total SFR density of the Abell 901/902 clusters. / text

The Bolocam Galactic Plane Survey (BGPS) has surveyed the northern Galactic plane at 1.1 mm and detected 8,358 sources. The BGPS catalog is large enough to characterize the properties of massive star formation in a statistically significant way. In this dissertation, I have conducted a survey of NH₃ lines toward 771 BGPS sources located throughout the Galactic plane. The NH₃ and 1.1 mm continuum observations together have allowed for complete characterization of the physical properties of these sources. I detected the NH₃(1,1) line toward 408 BGPS sources in the inner Galaxy, allowing for determination of their kinematic distances. At distances less than roughly 1 kpc, the BGPS detects predominately cores which will form a single star or small multiple system, while at distances between 1 and 7 kpc the BGPS detects predominately clumps which will form entire stellar clusters. At distances greater than 7 kpc, the BGPS detects the large scale clouds which contain clumps and cores. I have correlated the BGPS catalog with mid-IR catalogs of massive young stellar objects (MYSOs), and found that 49% of the BGPS sources contain signs of active star formation. The masses, densities, H₂ and NH₃ column densities, gas kinetic temperatures, and NH₃ velocity dispersions are higher in BGPS sources with associated mid-IR sources. I have also studied the physical properties of the BGPS sources as a function of Galactocentric radius, R[subscript Gal]. I find that the mean radius and mass decrease with increasing R[subscript Gal] but peak within the 5 kpc molecular ring where the gas kinetic temperature reaches a minimum. The fraction of BGPS sources with associated mid-IR sources decreases by 10% within the molecular ring. I postulate that these trends can be explained by an ambient gas density which decreases with R[subscript Gal], but peaks within the molecular ring. Similarly, the NH₃ column density and abundance decrease by almost an order of magnitude from the inner to outer Galaxy. / text

Thesis (Ph. D.)--Rijksuniversiteit te Leiden, 1988. / Includes bibliographical references.

The stellar halo of the Milky Way is populated by mostly old and metal-poor stars. As dynamical timescales are of order ~Gyrs at these large distances, accreted stellar substructures, such as dwarf galaxies or globular clusters, survive here as coherent entities longer than anywhere else in the Galaxy. These substructures represent our “fossil record” which can be used to reconstruct the Galaxy’s complex past. In this work, we seek to identify the structures found in the far reaches of the stellar halo as a step towards a correct interpretation of this fossil record. The advent of all-sky surveys in the Gaia era has ignited a prosperous period for this field of Galactic archaeology, but exploring the distant Milky Way (>10 kpc) with Gaia is difficult. Parallax measurements are much less accurate beyond the Solar neighborhood, though Gaia’s proper motions remain useful out to large radii. To push Gaia into the distant Galaxy, we combined these astrometric data with u-band photometry from the Canada-France Imaging Survey (CFIS). We exploited CFIS’ excellent photometric quality and depth (which extends 3 magnitudes deeper than that of the Sloan Digital Sky Survey) to use blue horizontal branch stars (BHBs) as a tracer population with well-measured distances. We first examined the distribution of BHBs using the OPTICS (Ordering Points To Identify the Clustering Structure) clustering algorithm to visualize the hierarchical nature of outer halo substructure. We then identified several well-known satellites, including a group of stars in the vicinity of a distant globular cluster (NGC 5466). Analysis of their kinematics suggested a few of these BHBs outside the cluster’s tidal radius were co-moving with NGC 5466, implying they may be tidal debris from this system. Interestingly, a stream had previously been detected extending from this globular cluster. However, its properties had not been studied in the decade since its discovery, and previous dynamical models were unable to reproduce many of the reported features. As one of the (allegedly) longest globular cluster streams on the sky - and given its distance and utility to constrain the Milky Way’s mass at large Galactic radius - we sought to explore this structure further. We subsequently used red giant branch stars (RGBs) identified in CFIS to try to better quantify the characteristics of the putative stream. We were able to filter these data and obtain a sample of stars that are fully consistent with stream membership and which span approximately 31 degrees of sky. Combined with the BHBs, we used these populations to trace the path of the stream, its distance and distance gradient across the stream’s longitude, and additionally estimated a lower limit to the stream’s luminosity. Our measurements suggest that the stream is at least 11% of the luminosity of the cluster. We then compared our observational data to dynamical models, which showed generally good agreement with the observed stream. This success reflects the updated properties of data measured in this work, and the inclusion of new data (especially proper motions). Our model suggests that the pericenter and apocenter of NGC 5466's orbit are 6.4 and 43 kpc, respectively, resulting in a very eccentric orbit (ε = 0.74). We also find evidence that the cluster experienced a recent interaction (within the past ~100 Myrs) with the Galactic disk, suggesting that the primary source of mass loss in this system may be caused by disk-shocking. The NGC 5466 stellar stream also exhibits an interesting heliocentric gradient in the leading arm, which our simplistic spherical halo model does not fully reproduce. Dynamical experiments with various halo shapes fit to this stream will prove interesting for future work. For local cosmology in particular, long, thin, dynamically cold stellar streams are ideal systems for constraining properties of the Milky Way’s dark matter halo, and streams at large radius are especially useful for measuring the Galaxy's mass interior to the stream. In this respect, we anticipate that NGC 5466 will be exceptionally useful as a probe of the shape, mass, and dark substructure of the Milky Way's distant dark matter halo. / Graduate

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Despite valiant effort by astronomers, the mass of the Milky Way (MW) Galaxy is poorly constrained, with estimates varying by a factor of two. A range of techniques have been developed and different types of data have been used to estimate the MW’s mass. One of the most promising and popular techniques is to use the velocity and position information of satellite objects orbiting the Galaxy to infer the gravitational potential, and thus the total mass. Using these satellites, or Galactic tracers, presents a number of challenges: 1) much of the tracer velocity data are incomplete (i.e. only line-of-sight velocities have been measured), 2) our position in the Galaxy complicates how we quantify measurement uncertainties of mass estimates, and 3) the amount of available tracer data at large distances, where the dark matter halo dominates, is small. The latter challenge will improve with current and upcoming observational programs such as Gaia and the Large Synoptic Survey Telescope (LSST), but to properly prepare for these data sets we must overcome the former two. In this thesis work, we have created a hierarchical Bayesian framework to estimate the Galactic mass profile. The method includes incomplete and complete data simultaneously, and incorporates measurement uncertainties through a measurement model. The physical model relies on a distribution function for the tracers that allows the tracer and dark matter to have different spatial density profiles. When the hierarchical Bayesian model is confronted with the kinematic data from satellites, a posterior distribution is acquired and used to infer the mass and mass profile of the Galaxy. This thesis walks through the incremental steps that led to the development of the hierarchical Bayesian method, and presents MW mass estimates when the method is applied to the MW’s globular cluster population. Our best estimate of the MW’s virial mass is 0.87 (0.67, 1.09) x 10^(12) solar masses. We also present preliminary results from a blind test on hydrodynamical, cosmological computer-simulated MW-type galaxies from the McMaster Unbiased Galaxy Simulations. These results suggest our method may be able to reliably recover the virial mass of the Galaxy. / Thesis / Doctor of Philosophy (PhD)

The first chapter begins with the observations of 1,882 sources from the Bolocam Galactic Plane Survey (BGPS) at 1.1 mm in HCO⁺ J = 3 − 2 and N₂H⁺ J = 3 − 2. We determine kinematic distances for 529 sources and derive the size, mass, and average density for this subset of clumps. The median size of BGPS clumps is 0.75 pc with a median mass of 330 M⊙ (assuming T(Dust) = 20 K). The median HCO⁺ linewidth is 2.9 km s⁻¹ indicating the clumps are not thermally supported and provide no evidence for a size-linewidth relationship. This collection of objects is a less-biased sample of star-forming regions in the Milky Way that likely span a wide range of evolutionary states. We study in detail the G111 Infrared Dark Cloud northwest of NGC 7538 with the K-band Focal Plane Array. We map NH₃ (1,1) and (2,2), H₂O maser, and CCS emission simultaneously with the GBT. We find the NH₃ gas traces the 1.1 mm BGPS structure very well with gas kinetic temperatures consistently close to 15 K. Typical column densities are 2.5 × 10¹⁴ cm⁻² with a median abundance of NH₃ to H₂ of 5.94 × 10⁻⁸. The median linewidth of the NH₃ emission is 0.64 km s⁻¹ indicating the filament is not thermally supported. The NH₃ is subthermally populated along the entire filament. Individual NH3 peaks have a median size of 0.61 pc, mass of 188M⊙, and density of 3.4×10³ cm⁻³. An activity analysis shows the most active star forming regions are found at the junctions of the subfilaments that make up the larger G111 IRDC. The last chapter describes our systematic examination of individual student responses to the Light and Spectroscopy Concept Inventory national dataset. We use classical test theory to form a framework of results that is used to evaluate item difficulties, item discriminations, and the overall reliability of the LSCI. We perform an analysis of individual student’s normalized gains, providing further insight into the prior results from this data set. This investigation allows us to better understand the efficacy of using the LSCI to measure student achievement.

Star formation in the Milky Way is confined to star-forming regions (OB association, HII regions, and open clusters) in the Galactic plane. It is usually assumed that these regions are found preferably along spiral arms, as is observed in other spiral galaxies. However, young early-type stars are often found at high Galactic latitudes, far away from their birthplaces in the Galactic disc. These stars are called runaway stars, and it is believed that they were ejected from their birth- places early in their lifetimes by one of two mechanisms: ejection from a binary system following the destruction of the massive companion in a supernova type II event (the binary ejection mechanism), or ejection from a dense cluster following a close gravitational encounter between two close binaries (the dynamical ejection mechanism). The aims of our study were: to improve the current understanding of the nature of high Galactic latitude runaway stars, in particular by investigating whether the theoretical ejection mechanisms could explain the more extreme cases; to show the feasibility of using high Galactic latitude stars as tracers of the spiral arms. The main technique used in this investigation was the tracing of stellar orbits back in time, given their present positions and velocities in 3D space. This technique allowed the determination of the ejection velocities, flight times and birthplaces of a sample of runaway stars. In order to obtain reasonable velocity estimates several recent catalogues of proper motion data were used. We found that the evolutionary ages of the vast majority of runaway stars is consistent with the disc ejection scenario. However, we identified three outliers which would need flight times much larger then their estimated ages in order to reach their present positions in the sky. Moreover, the ejection velocity distribution appears to be bimodal, showing evidence for two populations of runaway stars: a “low” velocity population (89 per cent of the sample), with a maximum ejection velocity of about 300 kms−1, and a “high” velocity population, with ejection velo- cities of 400 – 500 kms−1. We argue that the observed bimodality and maximum ejection velocity of 500 kms−1 can be interpreted as a natural consequence of a variation of the binary ejection mechanism. A possible connection between the “high” velocity population and the so-called hypervelocity stars is also explored, resulting in the conclusion that some stars previously identified as hypervelocity may be in fact runaway stars. The feasibility of using stars as tracers of the spiral arms was tested on a local sample, in order to obtain better quality data and larger numbers. We found that the spiral arms pattern speeds estimated from this sample (24.9±5.2 kms−1 kpc−1) and from a selected sample of runaways (22.8 ± 7.8 kms−1 kpc−1) are consistent within the errors and also consistent with other published estimates. We concluded that our estimates combined with the ones obtained in other studies suggest a value in the range 20 − 25 kms−1 kpc−1 for the pattern speed. Moreover, we concluded that an adequate representation of the spiral arms is obtained given the former pattern speed estimate, even when applied to the sample of runaway stars.