Browsing by Subject "Star formation"

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Application of convolutional neural networks to identify stellar feedback structures
(2021-08) Xu, Duo, M.A.; Offner, Stella; Gutermuth, Robert; Huang, Qixing; Kraus, Adam; Dinerstein, Harriet
Stellar feedback, such as stellar winds and outflows, plays a significant role in both physical and chemical evolution of molecular clouds. This energy and momentum leave identifiable signatures (bubbles and outflows) that affect the dynamics and structure of the cloud. Most feedback feature searches are performed “by-eye”, which are usually time-consuming, subjective and difficult to calibrate. Automatic classifications based on machine learning make it possible to perform systematic, quantifiable and repeatable searches for stellar feedback features. In this dissertation, I adopt a deep learning method CASI (Convolutional Approach to Shell Identification) and extend it to 3D (CASI-3D) to identify signatures of stellar feedback, i.e., bubbles and outflows, in molecular line spectra, such as ¹²CO and ¹³CO. First, I adopt magnetohydrodynamic simulations that study the impact of stellar winds in a turbulent molecular cloud as an input to generate synthetic observations. I apply the 3D radiative transfer code RADMC-3D to model ¹³CO (J=1-0) line emission from the simulated clouds. I train two CASI-3D models to identify stellar wind driven bubbles: ME1 is trained to predict only the position of feedback, while MF is trained to predict the fraction of the mass coming from feedback in each voxel. Both ME1 and MF models are able to identify stellar wind driven bubbles in both synthetic and observational data. I apply our two models on the full 98 deg² FCRAO ¹³CO survey of the Taurus cloud. I find that feedback properties computed based on visual identifications are significantly over-estimated due to line of sight confusion and contamination from background and foreground gas. Then I carry out magnetohydrodynamics simulations that model forming stars that launch protostellar outflows and use these to generate synthetic observations. I apply RADMC-3D to model ¹²CO (J = 1 − 0) line emission from the simulated clouds. I train two CASI-3D models to identify protostellar outflows similar to those of bubbles. The two models successfully identify all 60 previously visually identified outflows in Perseus. Additionally, CASI-3D finds 20 new high-confidence outflows. The mass, momentum, and energy of individual outflows in Perseus predicted by model MF is comparable to the previous estimations. This similarity is due to a cancellation in errors: previous calculations missed outflow material with velocities comparable to the cloud velocity; however, they compensate for this by overestimating the amount of mass at higher velocities that has contamination from nonoutflow gas. I also find that outflows likely driven by older sources have more high-velocity gas compared to those driven by younger sources. Next, I employ CASI-3D to systemically identify protostellar outflows in four nearby molecular clouds, Ophiuchus, Taurus, Perseus and Orion. The total outflow masses are 267 M[subscript ☉], 795 M[subscript ☉], 1305 M[subscript ☉] and 6332 M[subscript ☉] for Ophiuchus, Taurus, Perseus and Orion, respectively. The total 3D outflow energies are 9✕10⁴⁵ ergs, 6✕10⁴⁶ ergs, 1.2✕10⁴⁷ ergs and 6✕10⁴⁷ ergs for Ophiuchus, Taurus, Perseus and Orion, respectively. I compare the energy associated with outflows to the rate of turbulent dissipation and find that feedback is sufficient to maintain turbulent dissipation at the current epoch for all four clouds. All four clouds exhibit a break point in the spatial power spectrum of the outflow prediction map, which might indicate the typical outflow mass/energy injection scale. I also find the outflow mass is linearly proportional to the total number of YSOs for all four clouds. In the future, I will extend CASI-3D to explore more intrinsic physical properties related to feedback, such as 3D momentum and turbulent driving modes by feedback. Additionally, I will exploit CASI-3D to infer fundamental properties of the turbulence in molecular clouds.
Assessing the performance of a machine learning algorithm in identifying bubbles in dust emission
(2019-01-30) Xu, Duo, M.A.; Offner, Stella
Stellar feedback created by radiation and winds from massive stars plays a significant role in both physical and chemical evolution of molecular clouds. This energy and momentum leaves an identifiable signature (“bubbles") that affect the dynamics and structure of the cloud. Most bubble searches are performed “by-eye", which are usually time-consuming, subjective and difficult to calibrate. Automatic classifications based on machine learning make it possible to perform systematic, quantifiable and repeatable searches for bubbles. We employ a previously developed machine learning algorithm, Brut, and quantitatively evaluate its performance in identifying bubbles using synthetic dust observations. We adopt magneto-hydrodynamics simulations, which model stellar winds launching within turbulent molecular clouds, as an input to generate synthetic images. We use a publicly available three-dimensional dust continuum Monte-Carlo radiative transfer code, HYPERION, to generate synthetic images of bubbles in three Spitzer bands (4.5 μm, 8 μm and 24 μm). We designate half of our synthetic bubbles as a training set, which we use to train Brut along with citizen-science data from the Milky Way Project. We then assess Brut's accuracy using the remaining synthetic observations. We find that after retraining Brut's performance increases significantly, and it is able to identify yellow bubbles, which are likely associated with B-type stars. Brut continues to perform well on previously identified high-score bubbles, and over 10% of the Milky Way Project bubbles are reclassified as high-confidence bubbles, which were previously marginal or ambiguous detections in the Milky Way Project data. We also investigate the size of the training set, dust model, evolution stage and background noise on bubble identification.
Chemical evolution in low-mass star forming cores
(2010-08) Chen, Jo-Hsin; Evans, Neal J.; Edwin, Bergin A.; Volker, Bromm; Paul, Harvey M.; Daniel, Jaffe T.; John, Lacy H.
In this thesis, I focus on the physical and chemical evolution at the earliest stages of low-mass star formation. I report results from the Spitzer Space Telescope and molecular line observations of 9 species toward the dark cloud L43, a survey of 10 Class 0 and 6 Class I protostars with 8 molecular lines, and a survey of 9 Very Low Luminosity Objects (VeLLOs) with 11 molecular lines. From the observational results, CO depletion is extensively observed with C¹⁸O(2-1) maps. A general evolutionary trend is also seen toward the Class 0 and I samples: higher deuterium fractionation at higher CO depletion. For the VeLLO candidates and starless cores with N₂D⁺(3-2) detection, we found the deuterium ratio of N₂D⁺/N₂H⁺ is higher comparing with the Class 0 and I samples. We use DCO⁺(3-2) maps to trace the velocity structures. Also, HCO⁺(3-2) blue profiles are seen toward the VeLLO candidate L328, indicating possible infall. To test theoretical models and to interpret the observations, we adopt a modeling sequence with self-consistent calculations of dust radiative transfer, gas energetics, chemistry, and line radiative transfer. In the L43 region described in Chapter 2, a starless core and a Class I protostar are evolving in the same environment. We modeled both sources with the same initial conditions to test the chemical characteristics with and without protostellar heating. The physical model consists of a series of Bonner-Ebert spheres describing the pre-protostellar (PPC) stages following by standard inside-out collapse (Shu 1977). The model best matches the observed lines suggests a longer total timescale at the PPC stage, with faster evolution at the later steps with higher densities. In Chapter 3, we modeled the entire group of Class 0 and I protostars. The trend of decreasing deuterium ratio can be seen after the temperature is high enough for CO to evaporate. After the evaporation, the history of heavy depletion (e.g, from longer PPC timescales or different grain surface properties) no longer affects the line intensities of gas-phase CO. The HCO⁺ blue profiles, which are used as infall indicators, are predicted to be observed when infall is beyond the CO evaporation front. The low luminosity of VeLLOs cannot be explained by standard models with steady accretion, and we tested an evolutionary model incorporating episodic accretion to investigate the thermal history and chemical behaviors. We tested a few chemical parameters to compare with the observations and the results from Chapter 2 and 3. The modeling results from episodic accretion models show that CO and N₂ evaporate from grain mantle surfaces at the accretion bursts and can freeze back onto grain surfaces during the long periods of quiescent phases. Deuterated species, such as N₂D⁺ and H₂D⁺, are most sensitive to the temperature. Possible good tracers for the thermal history include the line intensities of gas-phase N₂H+ relative to CO, as well as CO₂ and CO ice features.
The Class 0 protostar BHR71: Herschel observations and dust continuum models
(2015-12) Yang, Yao-Lun; Evans, Neal J.; Lacy, John H
We performed a comprehensive analysis of the Herschel spectra of BHR71, an embedded Class 0 protostar. We recovered 66 lines in the central spaxel. Counting detections in all spaxels in PACS and SPIRE, more than 700 lines were detected. A CO rotational diagram analysis shows four excitation temperature components, 51 K, 153 K, 409 K, and 1053 K. Low-J CO lines trace the outflow while the high-J CO lines are centered on the infrared source. The low-excitation emission lines of water trace the large-scale outflow, while the high-excitation emission lines trace a small scale distribution around the equatorial plane. We model the structure of the envelope using the dust radiative transfer code, Hyperion, to fit the spectral energy distribution (SED) observed by Spitzer and Herschel. The model incorporates rotational collapse and an outer static envelope as well as an outflow cavity and disk. Our exploration of parameter space shows that the evolution of a collapsing envelope can be constrained by the Herschel SED and that the structure of the outflow cavity plays a critical role at shorter wavelengths. A cavity with a constant-density inner region and a power-law density outer region can reproduce the observations. The best fit model has a mass of 22 solar mass inside a radius of 0.2 pc and a central luminosity of 15.18 solar luminosity. The time since collapse began is 1.2x10^4 year with considerable uncertainty. The central luminosity in the best-fit model is greater than the observed luminosity because radiation is channeled out the outflow cavity. Even with this correction, the current mass accretion rate determined from the luminosity is about a factor of three less than the mass infall rate, suggestive of episodic accretion.
Exploring AGN and star formation activity of massive galaxies at cosmic noon
(2020-08-17) Florez, Jonathan; Jogee, Shardha; Boylan-Kolchin, Michael
We investigate the relation between AGN and star formation (SF) activity at 0.5 < z < 3 by analyzing 898 galaxies with X-ray luminous AGN (L [subscript x] > 10⁴⁴ erg s⁻¹) and a large comparison sample of ~ 320,000 galaxies without X-ray luminous AGN. Our samples are selected from a large (11.8 deg²) area in Stripe 82 that has multi-wavelength (X-ray to far-IR) data. The enormous comoving volume (~ 0.3 Gpc³) at 0.5 < z < 3 minimizes the effects of cosmic variance and captures a large number of massive galaxies (~ 30,000 galaxies with M [subscript *] > 10¹¹ \ [solar mass]) and X-ray luminous AGN. While many galaxy studies discard AGN hosts, we fit the SED of galaxies with and without X-ray luminous AGN with Code Investigating GALaxy Emission (CIGALE) and include AGN emission templates. We find that without this inclusion, stellar masses and star formation rates (SFRs) in AGN host galaxies can be overestimated, on average, by factors of up to ~ 5 and ~ 10, respectively. The average SFR of galaxies with X-ray luminous AGN is higher by a factor of ~ 3 to 10 compared to galaxies without X-ray luminous AGN at fixed stellar mass and redshift, suggesting that high SFRs and high AGN X-ray luminosities may be fueled by common mechanisms. The vast majority (> 95%) of galaxies with X-ray luminous AGN at z = 0.5 - 3 do not show quenched SF: this suggests that if AGN feedback quenches SF, the associated quenching process takes a significant time to act and the quenched phase sets in after the highly luminous phases of AGN activity.
Exploring the formation and evolutionary pathways of young stars and planetary systems at high precision
(2022-08-12) Krolikowski, Daniel Milker; Kraus, Adam L.; Bowler, Brendan; Covey, Kevin; Hawkins, Keith; Morley, Caroline
The star and planet formation process is one continuous event on scales from a giant molecular cloud to an individual circumstellar disk. There are a multitude of competing theories for the dominant physical mechanisms acting at each stage of this process, ranging from cloud conditions to how planet characteristics change over time. The properties of young stellar associations, the stars within them, and their planetary systems are crucial direct tests of these competing models. However, finding and characterizing isolated young systems is hard. Our best opportunity is to measure the ensemble age of a coeval group of stars, and observe systems within them to study the inherently intertwined star and planet formation and evolution process. In my dissertation, I use precision observations of young stars and planetary systems to explore their formation and evolutionary pathways. I first present a comprehensive census of the Taurus star forming region to reconstruct its complex star forming history. I use Gaia astrometry to reveal its highly substructured nature, identifying subgroups with age spreads indicating a prolonged star forming event, and kinematics indicating a typical turbulent environment and early dynamical evolution. Taurus is likely connected to a long-lasting, larger-scale galactic star forming event that can only be uncovered in the Gaia era. I then discuss exoplanet-related projects using high precision NIR spectroscopy from the Habitable-zone Planet Finder. Young stars are highly active, which introduces significant noise in spectroscopic observations. In a sample of young transiting planet hosts, I characterize the NIR helium spectral feature, which is an important probe of atmospheric mass loss and conditions in the stellar chromosphere. Stellar helium variability decreases with age, reflecting the higher activity levels in youth, but the line strength is constant beyond 100 Myr implying similar line formation conditions across the sample. Stellar variability should not preclude detection of mass loss at young ages. With this same data set, I search for giant planets exterior to the known transiting planets to measure their occurrence rate and constrain the typical dynamical history of a planetary system. I find three candidate signals of long-period companions, although the occurrence rate remains largely unconstrained. My dissertation exemplifies the power that precision observations of young stars has to improve our understanding of the complicated and interrelated processes of star and planet formation and evolution.
Exploring the interplay between star formation and active galactic nuclei and the role of environment in galaxy evolution
(2021-08-13) Florez, Jonathan; Jogee, Shardha; Bromm, Volker; Finkelstein, Steven L; Evans, Neal J; Boylan-Kolchin, Michael; Conselice, Christopher
One of the central goals of extragalactic astronomy is to understand how galaxies grow their stellar mass and central black holes, the connection between star formation and active galactic nuclei (AGN), and the impact of environment on this growth. In this thesis, I utilize multiwavelength surveys that are both deep and wide, advanced computational codes that model the spectral energy distributions of galaxies with and without AGN, as well as state-of-the-art simulations of galaxy evolution in order to explore how galaxy properties are impacted by their surrounding environment and AGN activity. These studies explore galaxies over a redshift range of 0.015 < z < 0.023 (lookback time of ~ 0.2 to ~ 0.3 Gyr), and over a redshift range of 0.5 < z < 3.0 (lookback time of ~ 5 to ~ 12 Gyr). The large-area surveys used here provide some of the largest and most statistically robust samples to-date of rare massive galaxies (with stellar mass M [subscript *] > 10¹¹ M☉) and extremely luminous AGN (with X-ray luminosity L [subscript X] > 10⁴⁴ erg s⁻¹) out to z ~ 3, thereby limiting the effects of cosmic variance and Poisson statistics. I analyze the observed stellar masses and star formation rates of galaxies as a function of environment and AGN activity, compare the empirical results to theoretical models of galaxy evolution, and discuss the implications of such comparisons. This work will provide significant guidance and constraints to the future development of theoretical models of galaxy growth. In Chapter 2 (Florez et al. 2021, ApJ, 906, 97) I measure the environmental dependence, where environment is defined by the distance to the third nearest neighbor, of multiple galaxy properties inside the Environmental COntext (ECO) catalog. I focus primarily on void galaxies at redshifts z = 0.015 - 0.023, which I define as the 10% of galaxies having the lowest local density. I compare the properties of void and non-void galaxies: baryonic mass, color, fractional stellar mass growth rate (FSMGR), morphology, and gas-to-stellar-mass ratio. The void galaxies typically have lower baryonic masses than galaxies in denser environments, and they display the properties expected of a lower mass population: they have more late-types, are bluer, have higher FSMGR, and are more gas rich. I also control for baryonic mass and investigate the extent to which void galaxies are different at fixed mass. I find that void galaxies are bluer, more gas-rich, and more star forming at fixed mass than non-void galaxies, which is a possible signature of galaxy assembly bias and other environmental processes. Furthermore, I show that these trends persist even at fixed mass and morphology, and I find that voids host a distinct population of early-types that are bluer and more star-forming than the typical red and quenched early-types. In addition to these empirical observational results, I also present theoretical results from mock catalogs with built-in galaxy assembly bias. I show that a simple matching of galaxy properties to (sub)halo properties, such as mass and age, can recover the observed environmental trends in the local galaxy population. In Chapter 3 (Florez et al. 2020, MNRAS, 497, 3273) I investigate the relation between AGN and star formation activity at 0.5 < z < 3 by analyzing 898 galaxies with high X-ray luminosity AGN (L [subscript X] > 10⁴⁴ erg s⁻¹) and a large comparison sample of ~ 320,000 galaxies without such AGN. My samples are selected from a large (11.8 deg²) area in Stripe 82 that has multi-wavelength (X-ray to far-IR) data. The enormous comoving volume (~ 0.3 Gpc³) at 0.5 < z < 3 minimizes the effects of cosmic variance and captures a large number of massive galaxies (~ 30,000 galaxies with M [subscript *] > 10¹¹ M☉) and high X-ray luminosity AGN. While it is typical for studies of galaxy evolution to discard AGN host galaxies, I fit the SED of galaxies with and without high X-ray luminosity AGN with Code Investigating GALaxy Emission (CIGALE) and include AGN emission templates. I find that without this inclusion, stellar masses and star formation rates in AGN host galaxies can be overestimated, on average, by factors of up to ~ 5 and ~ 10, respectively. The average star formation rate of galaxies with X-ray luminous AGN is higher by a factor of ~ 3 to 10 compared to galaxies without X-ray luminous AGN at fixed stellar mass and redshift, suggesting that high star formation rates and high AGN X-ray luminosities may be fueled by common mechanisms. The vast majority (> 95%) of galaxies with X-ray luminous AGN at z = 0.5 - 3 do not show quenched star formation: this suggests that if AGN feedback quenches star formation, the associated quenching process takes a significant time to act and the quenched phase sets in after the highly luminous phases of AGN activity. In numerical simulations and theoretical models of galaxy evolution, AGN and star formation activity are closely linked and AGN feedback is invoked to regulate galaxy growth. However, few empirical tests exist on how well the models and simulations implement the growth and interplay between AGN and star formation. To address this issue, in Chapter 4 (Florez et al. submitted) I compare the hydrodynamical simulations IllustrisTNG and SIMBA, and the semi-analytical model SAG to the empirical results on AGN and star formation at cosmic noon reported in Chapter 3. The main results of my comparisons are: (i) SAG and IllustrisTNG both qualitatively reproduce the empirical result that galaxies with high X-ray luminosity AGN have higher mean star formation rates, at a given stellar mass, than galaxies without such AGN. SAG, however, strongly over-produces the number density of high X-ray luminosity AGN by a factor of 10 to 100, while IllustrisTNG shows a lack of high X-ray luminosity AGN at high stellar mass (M [subscript *] > 10¹¹ M☉) at z ~ 2. (ii) In SIMBA, the mean star formation rate of galaxies with high X-ray luminosity AGN is lower than the star formation rate of galaxies without such AGN. Contrary to the data, many high X-ray luminosity AGN in SIMBA have quenched star formation, suggesting that AGN feedback, or other feedback modes in galaxies with such AGN, might be too efficient in SIMBA. I discuss the implications of these results for our understanding of the evolution of galaxies and the growth of their stellar masses and black holes across cosmic time.
Exploring the limits of star formation from the extreme environment of galaxy interactions to the Milky Way
(2012-12) Heiderman, Amanda Lea; Evans, Neal J.; Gebhardt, Karl; Milosavljevic, Milos; Hill, Gary; Calzetti, Daniela; Papovich, Casey
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.
Innovative technologies for and observational studies of star and planet formation
(2015-05) Gully-Santiago, Michael Anthony; Jaffe, D. T.; Lacy, John H; Evans, Neal J; Kraus, Adam L.; Weinberger, Alycia J
I summarize the optical design, fabrication, and performance of silicon diffractive optics for astronomical spectrographs. The first set of optical devices includes diffraction-limited, high-throughput silicon grisms for JWST-NIRCam. These grisms served as pathfinders to Silicon immersion gratings, which offer size and cost savings for high-resolution near-infrared spectrographs. I demonstrate the production and optical evaluation of the immersion grating that enabled IGRINS at the McDonald Observatory. This grating provides spectral resolution R=40,000 over the H and K near-infrared band atmospheric windows 1.5-2.5 micron. Electron-beam lithography offers much higher precision over contact mask photolithography for the production of Si immersion gratings. Electron-beam patterned prototypes are stepping-stones to monolithic Si gratings for iSHELL and GMTNIRS. The monolithic design of Si immersion gratings presents a limitation for scaling up the grating size, since existing fabrication equipment cannot handle monolithic silicon pucks. The size limitation can be overcome by direct-bonding Si substrates to optical prisms. I demonstrate a technique to measure interfacial gaps as small as 14 nm between the bonding interfaces, which produce 0.2% transmission loss. These technologies will enable the direct measurement of the atmospheric properties of extrasolar planets in the next decade. IGRINS is now measuring fundamental properties of young solar-mass stars; low luminosity young brown dwarfs are below the sensitivity limit of existing high spectral resolution near-IR spectrographs. My approach to the discovery and characterization of young brown dwarfs therefore employs low-resolution R~2000 near-IR spectroscopy. I confirm and characterize 17 candidate young stars and brown dwarfs reported by Allers and collaborators. All 17 sources have circumstellar disks. Using deep optical, near-infrared, and mid-infrared photometry, I search an off-core region towards the nearby ~1 Myr Ophiuchus star forming cluster for candidate young stars and brown dwarfs. Multi-object I-band spectroscopy of 419 candidates reveals 12 new members. Ten of these have no evidence for mid-IR excess emission from 3.6 to 8.0 micron. The disk fraction for spectral types M4 and later towards this region of Ophiuchus is 5/15. Two of the disk sources have edge-on disks, pointing to a high edge-on disk fraction. I discuss possible sources of contamination in the survey.
An investigation of the physical parameters of young stellar objects
(2011-12) Deen, Casey Patrick; Jaffe, D. T.; Lacy, John; Sneden, Chris; Scalo, John; Johns-Krull, Christopher; Evans, Neal J.
Studies of the temporal evolution of young stars and their associated properties rely upon the ability of astronomers to determine ages and masses of objects in different evolutionary states. The best method for determining the age and mass of a young stellar object is to place the object on the Hertzsprung-Russell (HR) diagram and to compare to theoretical evolutionary tracks. Accurate ages allow the investigation of the temporal evolution of properties associated with stellar youth (accretion rates, X-ray activity, circumstellar excess, etc...). One property intimately linked with stellar youth is the presence (or absence) of an optically thick primordial circumstellar disk. Objects in "young" star forming regions are more likely to show evidence for a disk than objects in "older" clusters. Within a single cluster, the picture is not as clear. There exist objects in very young clusters (~1 Myr) which show no evidence for circumstellar disks, and there exist objects in very old clusters (~10 Myr), which show evidence for robust disks, suggesting a variable other than stellar age is driving the evolution of the disks. To investigate whether these outliers are due to age spreads, initial conditions, or simply appear anomalous due to erroneous age determinations, we must determine better placements in the HR diagram by carefully transforming observable quantities (spectral type and apparent magnitude) into the quantities necessary for comparison evolutionary models (effective temperature and luminosity). In the Ophiuchus star forming region, I investigate whether or not objects with disks are younger than disk-less objects. I find no difference in the ages of the two populations, but the systematic and random uncertainties are large enough to mask all but the largest age differences. In the hope of better determining the physical parameters of young stellar objects, I embark on a spectral synthesis campaign to produce comparison synthetic spectra which account for the effects of magnetic fields. This requires the modification of the MOOG spectral synthesis program to handle the full Stokes vector treatment for polarized radiation through a magnetized medium. I create a grid of synthetic spectra covering ranges in effective temperature, surface gravity, and average magnetic field strength relevant for studies of young stellar objects, and develop a Chi-squared minimization routine to determine the best fit synthetic spectrum for a given observed spectrum at an arbitrary resolving power. This grid of synthetic spectra will be an invaluable complement to future near infrared, large band-pass, high-resolving power spectrographs (i.e. IGRINS). In addition to these observational and theoretical attempts to reduce systematic errors, I also helped to develop a suite of silicon and KRS-5 grisms for use in the FORCAST instrument, a mid infrared camera on the SOFIA telescope. These grisms will afford the imaging instrument a mid infrared spectroscopic capability at wavelengths normally inaccessible from the ground. I also report on my work to help write FG Widget, the quick-look reduction software package developed to support grism observations.
Kernel-phase interferometry for detection of close in companions : binary demographics of brown dwarfs from birth to maturity
(2023-08) Factor, Samuel Meir; Kraus, Adam L.; Bowler, Brendan P.; Offner, Stella S. R.; Lacy, John H.; Ireland, Michael J.; Evans II, Neal J.
Filling out the dearth of detections between direct imaging and radial velocity surveys will test theories of planet formation and (sub)stellar binarity across the full range of semi-major axes, connecting formation of close to wide separation gas giants and sub-stellar companions. Direct detection of close-in companions is notoriously difficult as classical techniques fail near the diffraction limit. In this dissertation, I present a new pipeline which searches for faint companions using kernel-phase interferometry (KPI), a technique utilizing interferometric analysis of the full unobscured telescope aperture. I demonstrate the pipeline, and the power of KPI, by performing two surveys and accompanying demographic analyses. First, I search for companions in the entire HST/NICMOS F110W and F170M image archive of nearby brown dwarfs (BDs), demonstrating significant sensitivity to companions at half the diffraction limit. I discover no new companions but recover and refine astrometry of 19 previous imaging companions and confirm two previous kernel-phase detections. I also present contrast curves to enable a population study and to demonstrate the strength of this technique at separations inaccessible to classical imaging techniques. Second, after estimating physical properties of a subset of this sample, I use a Bayesian framework to compare these detections and detection limits to a model companion population. When correcting for Malmquist bias, I find a smaller population of companions with tighter separations than seen in previous studies. This is due to our use of KPI, which enables us to resolve the peak of the semimajor axis distribution with significant sensitivity to low-mass companions. I confirm the previously-seen trends of decreasing binary fraction with decreasing mass and a strong preference for tight and equal-mass systems in the field-age sub-stellar regime; only ∼ 1% of systems are wider than 20 au or have a mass ratio q < 0.6. Third, I present the results of a second KPI based survey of sub-stellar objects in Taurus and Upper Scorpius using archival HST/ACS. I present 6 new candidate detections at extremely tight separations, in addition to 4 previously known companions. This is the first application of KPI to visible-wavelength observations. I find an observed companion frequency of F [subscript obs] = 0.24 [superscript +0.10][subscript −0.08] which is slightly higher than previous studies, consistent with our new detections. Combining these two surveys and demographic analyses, I find a significant excess of young wide (ρ > 10 au) companions compared to the field. I also find that the field population of wide companions is consistent with my population of systems formed in low density regions if it were to be diluted by single systems formed in high density regions. This is consistent with dynamical evolution preferentially dissolving systems born in high-density star-forming regions over those in low-density regions. I attribute the characteristics of the BD binary population to turbulent fragmentation setting the initial conditions followed by a brief period of dynamical evolution, removing the widest and lowest-mass companions, before the birth cluster dissolves.
Mapping nearby young stellar structures and their star formation histories
(2021-12-02) Kerr, Ronan Murdoch Paterson; Kraus, Adam L.
Young stellar associations hold a star formation record that can persist for millions of years, revealing the progression of star formation long after the dispersal of the natal cloud. To identify nearby young stellar populations that trace this progression, we have designed a comprehensive framework for the identification of young stars, and use it to identify ~3x10⁴ candidate young stars within a distance of 333 pc using Gaia DR2. Applying the HDBSCAN clustering algorithm to this sample, we identify 27 top-level groups, nearly half of which have little to no presence in previous literature. Ten of these groups have visible substructure, including notable young associations such as Orion, Perseus, Taurus, and Sco-Cen. We provide a complete subclustering analysis on all groups with substructure, using age estimates to reveal each region's star formation history. The patterns we reveal include an apparent star formation origin for Sco-Cen along a semicircular arc, as well as clear evidence for sequential star formation moving away from that arc with a propagation speed of ~4 km s⁻¹ (~4 pc Myr⁻¹). We also identify earlier bursts of star formation in Perseus and Taurus that predate current, kinematically identical active star-forming events, suggesting that the mechanisms that collect gas can spark multiple generations of star formation, punctuated by gas dispersal and cloud regrowth. The large spatial scales and long temporal scales on which we observe star formation offer a bridge between the processes within individual molecular clouds and the broad forces guiding star formation at galactic scales.
Mass accretion in the embedded phase of low-mass star formation
(2010-08) Dunham, Michael Mark; Evans, Neal J.; Bromm, Volker; Harvey, Paul M.; Jaffe, Daniel T.; Lacy, John H.; Myers, Philip; Dullemond, Cornelis
A long-standing problem in low-mass star formation is the "luminosity problem," whereby protostars are underluminous compared to the accretion luminosity expected both from theoretical collapse calculations and arguments based on the minimum accretion rate necessary to form a star within the embedded phase duration. In this dissertation, I present new research on protostars and the protostellar accretion process that addresses the luminosity problem in the following ways: I report new infrared detections of a very low luminosity protostar in Taurus and use all existing data ranging from the infrared through millimeter wavelengths to constrain radiative transfer models and determine physical properties of the source. I argue that the derived source luminosity is lower than that expected based on the properties of a previously detected molecular outflow driven by this source and suggest that this discrepancy can be resolved by variable rather than constant mass accretion. I report the discovery of a new protostar that is also driving a molecular outflow. Following a similar modeling procedure as above, I show that this source has an even lower luminosity that is once again inconsistent with that expected based on the properties of its outflow, again suggesting variable mass accretion. I present the results of a complete search for all protostars with luminosities less than or equal to that of our Sun in a new infrared survey of nearby star-forming regions. I identify 50 protostars with such luminosities. Only a small fraction (15-25%) of dense cores thought to be starless (not yet collapsing to form stars) in fact harbor low luminosity protostars. The distribution of luminosities of these 50 protostars is inconsistent with a constant protostellar mass accretion rate. I present a set of evolutionary models that start with existing models following the inside-out collapse of singular isothermal spheres and add isotropic scattering off dust grains, a circumstellar disk, two-dimensional envelope structure, mass-loss and the opening of outflow cavities, and a simple treatment of episodic mass accretion. I conclude that episodic mass accretion is both necessary and sufficient to resolve the luminosity problem.
Monte Carlo radiation hydrodynamics in the super-Eddington regime
(2018-06-27) Tsang, Tsz Ho; Milosavljević, Miloš; Bromm, Volker; Jogee, Shardha; Evans, Neal J.; Davis, Shane
In this dissertation, we present three projects addressing the dynamical importance of radiation in turbulent media with super-Eddington flux. Examples of such media are massive star-forming environments and supermassive star atmospheres. While there are many theoretical models claiming the pivotal role of radiation in driving strong outflows and setting the star formation efficiency in the course of massive star formation, often they are based on ideal geometries and closure relations of the moment equations for radiation. To directly tackle the challenge of numerically modeling radiation-matter interactions in hydrodynamical simulations, we have adopted and tested a hybrid Monte Carlo radiation transport scheme. In the first project, with a standardized two-dimensional radiation-driven winds setup, we show that low-order methods tend to artificially reinforce the development of the low-density channels and underestimate the strength of radiation pressure. The accuracy of any numerical radiation transport scheme in producing truthful dynamics therefore depends on the validity of its underlying assumptions. In the second project, we carry out radiation hydrodynamical simulations of the formation of super star clusters in supersonically turbulent molecular clouds. The gas distribution is strongly inhomogeneous and that reduces the strength of radiation pressure in halting gas accretion compared to previous predictions. In the last project, unlike the inflow-outflow scenario of the first two, we aim to simulate the radiation hydrodynamics in quasi-hydrostatic media with extreme sensitivity of the opacity to density and temperature. We present the implementation and robustness test of the hybrid Monte Carlo estimators in preparation for direct simulations of the convective, radiation-dominated dynamics prevalent in the atmospheres of supermassive stars.
New insights into primordial star formation
(2011-08) Stacy, Athena Ranice; Bromm, Volker; Milosavljevic, Milos; Wheeler, John C.; Evans, Neal J.; Dinerstein, Harriet; Loeb, Abraham
The formation of the first stars, also known as Population III (Pop III), marked a pivotal point in the universe's evolution from relative smoothness and homogeneity to its current highly structured state. In this dissertation we study key aspects of Pop III star formation. We utilize three-dimensional cosmological simulations to follow the evolution of gas and DM from z ~100 until the first minihalo forms. Once the gas infalls toward the center of the minihalo and condenses, we implement the 'sink particle' method to represent regions that will form a star, and we follow the evolution of the metal-free, star-forming gas for many free-fall times. A disk forms around the initial Pop III star and fragments to form secondary stars with a range of masses (1 - 50 [solar mass]). This is markedly different from the previous paradigm of one single, massive star forming per minihalo. Using a ray-tracing technique, we also examine the effect of radiative feedback on protostellar growth and disk fragmentation. This feedback will not prevent the formation of secondary stars within the disk, but will reduce the final mass reached by the largest Pop III star. Measuring the angular momentum of the gas that falls onto the sink regions, we also find that the more massive Pop III stars accrete sufficient angular momentum to rotate at nearly break-up speeds, and can potentially end their lives as collapsar gamma-ray bursts or hypernovae. We furthermore numerically examine the recently discovered relative streaming motions between dark matter and baryons, originating from the era of recombination. Relative streaming will slightly delay the redshift at which Pop III stars first form, but will otherwise have little impact on Pop III star formation and the history of reionization. We finally evaluate the possible effect of a cosmic ray (CR) background generated by the supernova deaths of massive Pop III stars. A sufficiently large CR background could indirectly enhance the H₂ cooling within the affected minihalos. The resulting lower temperatures would lead to a reduced characteristic stellar mass (~ 10 [solar mass]), providing another possible pathway to form low-mass Pop III stars.
Physical characteristics of dusty star forming galaxies in the past 10 billion years
(2021-12-10) Drew, Patrick Michael; Casey, Caitlin M.; Boylan-Kolchin, Michael; Finkelstein, Steven; Hawkins, Keith; Whitaker, Katherine
Dusty star forming galaxies (DSFGs) are a class of galaxy that are highly obscured and are undergoing a period of intense star formation. While a clear picture of these systems at low redshift has emerged, the physical drivers of their star formation and the properties of their interstellar media at high redshift are uncertain. In this thesis I address fundamental questions about these systems such as, what drives their star formation rates, what is the distribution of matter within these galaxies, and how do key properties of their interstellar media, such as their dust temperatures, evolve with redshift? In Chapter 2 (Drew et al. 2020, ApJ, 892, 104) I investigate whether the star formation rates of three dusty star forming galaxies at z ~ 1.5 have merger-driven star formation or if they are driven by secular processes. Together I analyze their Hα rotation curves, rest-frame UV and optical morphologies, and their locations in Gini--M₂₀ and SFR--M [subscript star symbol] space to look for evidence of ordered rotation or ongoing galaxy mergers or interactions. I find that two galaxies are undergoing mergers or interactions and one is an isolated disk. The location of each galaxy in SFR--M [subscript star symbol] space runs contrary to expectation at low redshift. The isolated disk galaxy lies above the star forming main sequence at z ~ 1.5, one of the merging galaxies lies within the scatter of the main sequence, and the other merging galaxy lies below the main sequence. Our galaxies seems to indicate that the specific star formation rates of high stellar mass galaxies at z ~ 1.5 is not a useful indicator of their recent or ongoing merger histories. In Chapter 3 (Drew et al. 2018, ApJ, 869, 58) I report on the serendipitous discovery of a flat outer rotation curve in the lone isolated disk galaxy detailed in Chapter 2. Flat outer rotation curves are a hallmark of dark matter in galaxies. They are ubiquitous at low redshift but recent studies have reported that galaxies at z > 1 have declining rotation curves caused by low dark matter fractions and high velocity dispersion support in the outer galaxy. This galaxy at z = 1.6 is a disk galaxy seen edge on with flat rotational velocities between ~6--14 kpc (1.2--2.8 disk scale lengths) from the center of the galaxy. The full rotation curve is well fit by an arctangent function and the velocity dispersion curve is well fit by a Gaussian distribution. Both of these signatures are typical of low redshift galaxies. Together with the rotation curve, ALMA observations of dust continuum emission, and H-band imaging we find that the dark matter fraction at the H-band half light radius is 0.44±0.08, which is similar to that of the Milky Way and contrasts with the work of Genzel et al. (2017) who report dark matter fractions between 0 and 0.2 at the half light radii of the star forming galaxies in their sample. This is one of the first examples in the literature of a flat rotation curve at z > 1. In Chapter 4 (Drew and Casey 2021, submitted) I explore whether there is any observational evidence of redshift evolution in the dust temperatures of luminous infrared galaxies between 0 < z < 2. Drawing on three samples containing ~4700 galaxies from the literature that have observations of dust continuum emission from IRAS, Herschel, and SCUBA-2, I find that the peak wavelength of dust emission (an observational proxy for the luminosity-weighted dust temperature) does not evolve with redshift at fixed IR luminosity. This is in contrast to previous works in the literature that find either that dust temperatures are higher at high redshift than in the local universe, or conversely, that dust temperatures are cooler at high redshift than in the local universe. These contradictory claims arise from the use of a number of different methods for fitting dust SEDs as well as the use of data that does not fully constrain the dust SEDs. We model the correlation between the log of the peak wavelengths and log of the IR luminosities of galaxies as a Gaussian distributed about a line with slope η = -0.09±0.01, intercept at log (L [subscript IR] / L [subscript sun symbol]) = 12 of λ [subscript t] = 92±2 µm, and width σ [subscript log] λ [subscript peak] = 0.046±0.003. Our result highlights the importance of accounting for telescope selection effects both while building a galaxy sample and while fitting the relation between the peak wavelengths of dust emission and the IR luminosities of galaxies. Chapter 5 summarizes the key results of this thesis and their impact on our understanding of the physical characteristics of dusty star forming galaxies in the past 10 billion years. Chapter 6 discusses the state of the literature and our prospects for making future progress on each of the topics discussed in chapters 2, 3, and 4.
Physical properties of star-forming regions across the Galaxy
(2010-12) Dunham, Miranda Kay; Evans, Neal J.
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.
Prospects for directly detecting the first supernovae, and their impact on early star formation
(2016-05) Hummel, Jacob Alexander; Bromm, Volker; Milosavljevic, Milos; Wheeler, J. Craig; Finkelstein, Steven; Yoshida, Naoki
The formation of the first stars in the Universe marked a pivotal moment in cosmic history, initiating the transition from the simple initial conditions of the big bang to the complex structures we see today. Ionizing radiation produced by these so-called Population III stars began the process of reionization, and the supernovae marking their deaths initiated the process of chemical enrichment. We assess the prospects for direct detection of the first supernovae should they happen to end their lives as extremely energetic pair-instability supernovae, which should be within the detection limits of the upcoming James Webb Space Telescope. Using a combination of semi-analytic models and cosmological simulations to estimate their source density, we find that the primary obstacle to observing such events is their scarcity, not their faintness. The first supernovae and the compact remnants they leave behind also produce significant amounts of high-energy X-rays and cosmic rays able to travel through the predominantly neutral intergalactic medium and build up a cosmic background. To better understand how these violent explosions impact subsequent episodes of metal-free star formation, we employ ab-initio, cosmological hydrodynamics simulations to model the formation of stars in a minihalo at z = 20-30 under the influence of both an X-ray and cosmic ray background. The presence of an ionizing background---whether X-rays or cosmic rays---serves to expedite the collapse of gas to high densities by enhancing molecular hydrogen cooling, thus allowing stars to form at substantially earlier epochs in strongly irradiated minihalos. The mass of the stars thus formed however appears to be quite robust, maintaining a characteristic mass of order a few tens of solar masses even as the strength of the ionizing background varies by several orders of magnitude. Finally, we describe the novel software developed to enable this research. These tools for manipulating and analyzing simulation data have been released as the open-source GAdget DataFrame Library: gadfly.
Revealing star and planet formation with stellar multiplicity
(2023-08-07) Sullivan, Kendall; Kraus, Adam L.; Herczeg, Gregory; Offner, Stella; Morley, Caroline; Hawkins, Keith
Studies of star and planet formation work to understand the processes that produced the Solar System and the many other systems now known to host exoplanets. Understanding star and planet formation requires measurement of accurate stellar properties at all evolutionary stages of stellar and planetary systems. These stellar properties include age, mass, effective temperature (T [subscript eff]), stellar radius, and stellar multiplicity. Binary stars and higher-order multiples comprise about half of the population of main-sequence solar-type stars, and stellar multiplicity impacts the observed properties of stars across their lifetimes. Because exoplanet and stellar demographics are typically inferred from stellar properties, incorrect stellar characterization because of binaries feeds into biases and errors in stellar populations and exoplanet demographics. In this dissertation, I explored the impact of binary stars in the two scientific contexts of young stellar associations and binary stars that host exoplanets. In my studies of young stellar associations, I developed a simulation suite to perform synthetic spectroscopic surveys. I implemented mass-dependent binary properties to explore the origins of apparent mass-dependent age gradients previously observed in star-forming regions. My subsequent work added starspots to the simulation. I found that although binary stars can explain mass-dependent age gradients, starspots become the dominant contributor to the gradient in populations with Gaia distances. I also explored the nature of the relationship between accretion and circumstellar disks in young stars and found that the inner disks of binaries and single stars are probably similar, and that the inner rim of the dust disk is related to the accretion rate as a result of mass transfer through the disk. These studies demonstrated the importance of considering binary stars when attempting to measure ages or understand star formation histories in young stellar associations. In my studies of main sequence binary star exoplanet hosts, I developed an algorithm to accurately characterize the individual components of binary stars that are unresolved in most observations. As an initial step, I tested this code with an archival sample of M stars. Then, I performed a spectroscopic survey of binary stars from the Kepler sample using the Hobby-Eberly Telescope, and carried out two targeted studies of subsamples from the survey. The first study explored binary stars that supposedly host rocky Earth-analog planets and found that most of them are actually gaseous planets, which has implications for exoplanet demographics and attempts to measure the frequency of Earth analogs. The second study explored the radius distribution of small exoplanets and found that the gap in the radius distribution separating rocky and gaseous exoplanets in single systems was not present in binary stars. This result suggested that the location of the gap may be binary-separation-dependent and therefore “blurred out” by a range of stellar separations in the sample. This series of papers has demonstrated the power of using binary stars that host planets as a laboratory for controlled experiments in planet formation and evolution, because the binary properties leave a record of the planet-forming environment. The work presented in this dissertation has shown the ability of binary stars to influence observations of young stars and exoplanet hosts, and has demonstrated the potential of binary stars to provide a direct link between formation environment and exoplanet properties for the first time.
The role of gas in galaxy evolution : infall, star formation, and internal structure
(2013-05) Barentine, John Caleb; Kormendy, John
The story of a typical spiral galaxy like the Milky Way is a tale of the transformation of metal-poor hydrogen gas to heavier elements through nuclear burning in stars. This gas is thought to arrive in early times during the assembly phase of a galaxy and at late times through a combination of hot and cold “flows” representing external evolutionary processes that continue to the present. Through a somewhat still unclear mechanism, the atomic hydrogen is converted to molecules that collect into clouds, cool, condense, and form stars. At the end of these stars’ lives, much of their constituent gas is returned to the galaxy to participate in subsequent generations of star formation. In earlier times in the history of the universe, frequent and large galaxy mergers brought additional gas to further fuel this process. However, major merger activity began an ongoing decline several Gyr ago and star formation is now diminishing; the universe is in transitioning to an era in which the structural evolution of disk galaxies is dominated by slow, internal (“secular”) processes. In this evolutionary regime, stars and the gas from which they are formed participate in resonant gravitational interactions within disks to build ephemeral structures such as bars, rings, and small scale-height central bulges. This regime is expected to last far into the future in a galaxy like the Milky Way, punctuated by the periodic accretion of dwarf satellite galaxies but lacking in the “major” mergers that kinematically scramble disks into ellipticals. This thesis examines details of the story of gas from infall to structure-building in three major parts. The High- and Intermediate-Velocity Clouds (HVCs/IVCs) are clouds of H i gas at velocities incompatible with simple models of differential Galactic rotation. Proposed ideas explaining their observed properties and origins include (1) the infall of low-metallicity material from the Halo, possibly as cold flows along filaments of a putative “Cosmic Web”; (2) gas removed from dwarf satellite galaxies orbiting the Milky Way via some combination of ram pressure stripping and tidal disruption; and (3) the supply and return feeds of a “Galactic Fountain” cycling gas between the Disk and Halo. Numerical values of their observed properties depend strongly on the Clouds’ distances. In Chapter 2, we summarize results of an ongoing effort to obtain meaningful distances to a selection of HVCs and IVCs using the absorption-line bracketing method. We find the Clouds are not at cosmological distances, and with the exception of the Magellanic Stream, they are generally situated within a few kiloparsecs of the Disk. The strongest discriminator of the above origin scenarios are the heavy element abundances of the Clouds, but to date few reliable Cloud metal- licities have been published. We used archival UV spectroscopy, supplemented by new observations with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope and H I 21 cm emission spectroscopy from a variety of sources to compute elemental abundances relative to hydrogen for 39 HVC/IVC components along 15 lines of sight. Many of these are previously unpublished. We find support for all three origin scenarios enumerated above while more than doubling the number of robust measurements of HVCs/IVCs in existence. The results of this work are detailed in Chapter 3. In Chapter 4 we present the results of a spectroscopic study of the high-mass protostellar object NGC 7538 IRS 9 made with the Texas Echelon Cross Echelle Spectrograph (TEXES), a sensitive, high spectral resolution, mid-infrared grating spectrometer and compare our observations to published data on the nearby object NGC 7538 IRS 1. Forty-six individual lines in vibrational modes of the molecules C₂H₂, CH₄, HCN, NH₃ and CO were detected, including two isotopologues (¹³CO, ¹²C¹⁸O) and one combination mode ([nu]₄+[nu]₅ C₂H₂). Fitting synthetic spectra to the data yielded the Doppler shift, excitation temperature, Doppler b parameter, column density and covering factor for each molecule observed; we also computed column density upper limits for lines and species not detected, such as HNCO and OCS. We find differences among spectra of the two objects likely attributable to their differing radiation and thermal environments. Temperatures and column densities for the two objects are generally consistent, while the larger line widths toward IRS 9 result in less saturated lines than those toward IRS 1. Finally, we compute an upper limit on the size of the continuum-emitting region (~2000 AU) and use this constraint and our spectroscopy results to construct a schematic model of IRS 9. In Chapters 5 and 6, we describe studies of the bright, nearby, edge-on spiral galaxies NGC 4565 and NGC 5746, both previously classified as type Sb spirals with measured bulge-to-total luminosity ratios B/T ≃ 0.4. These ratios indicate merger-built, “classical” bulges but in reality represent the photometric signatures of bars seen end-on. We performed 1-D photometric decompositions of archival Hubble Space Telescope, Spitzer Space Telescope, and Sloan Digital Sky Survey images spanning a range of wavelengths from the optical to near-infrared that penetrate the thick midplane dust in each galaxy. In both, we find high surface brightness, central stellar components that are clearly distinct from the boxy bar and from the disk; we interpret these structures as small scale height “pseudobulges” built from disk material via internal, resonant gravitational interactions among disk material − not classical bulges. The brightness profiles of the innermost component of each galaxy is well fitted by a Sersic function with major/minor axis Sersic indices of n = 1.55±0.07 and 1.33±0.12 for NGC 4565 and n = 0.99±0.08 and 1.17 ± 0.24 for NGC 5746. The true “bulge-to-total” ratios of these galaxies are considerably smaller than once believed: 0.061+0.009 and 0.136 ± 0.019, −0.008, respectively. Therefore, more galaxies than we thought contain little or no evidence of a merger-built classical bulge. We argue further that a classical bulge cannot hide behind the dust lane of either galaxy and that other structures built exclusively through secular evolution processes such as inner rings, both revealed through the infrared imagery, argue strongly against any merger violence in the recent past history of these objects. From a formation point of view, NGC 4565 and NGC 5746 are giant, pure-disk galaxies, and we do not understand how such galaxies form in a ΛCDM universe. This presents a challenge to our picture of galaxy formation by hierarchical clustering because it is difficult to grow galaxies as large as these without making big, classical bulges. We summarize the work presented in this thesis in Chapter 7 and conclude with speculations about the future direction of research in this field.