Browsing by Subject "Astrophysics"

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A 3-dimensional analysis of the Cassiopeia a supernova remnant.
(2011-10) Isensee, Karl Andrew
We present a multi-wavelength study of the nearby supernova remnant Cassiopeia A (Cas A). Easily resolvable supernova remnants such as Cas A provide a unique opportunity to test supernova explosion models. Additionally, we can observe key processes in the interstellar medium as the ejecta from the initial explosion encounter Cas A’s powerful shocks. In order to accomplish these science goals, we used the Spitzer Space Telescope’s Infrared Spectrograph to create a high resolution spectral map of select regions of Cas A, allowing us to make a Doppler reconstruction of its 3-dimensional structure structure. In the center of the remnant, we find relatively pristine ejecta that have not yet reached Cas A’s reverse shock or interacted with the circumstellar environment. We observe O, Si, and S emission. These ejecta can form both sheet-like structures as well as filaments. Si and O, which come from different nucleosynthetic layers of the star, are observed to be coincident in some regions, and separated by >500 km s−1 in others. Observed ejecta traveling toward us are, on average, #24;800 km s−1 slower than the material traveling away from us. We compare our observations to recent supernova explosion models and find that no single model can simultaneously reproduce all the observed features. However, models of different supernova explosions can collectively produce the observed geometries and structures of the emission interior to Cas A’s reverse shock. We use the results from the models to address the conditions during the supernova explosion, concentrating on asymmetries in the shock structure. We also predict that the back surface of Cassiopeia A will begin brightening in #24;30 years, and the front surface in #24;100 years. We then used similar observations from 3 regions on Cas A’s reverse shock in order to create more 3-dimensional maps. In these regions, we observe supernova ejecta both immediately before and during the shock-ejecta interaction. We determine that the reverse shock of the remnant is spherical to within 7%, although the center of this sphere is offset from the geometric center of the remnant by 810 km s−1. We determine that the velocity width of the nucleosynthetic layers is #24;1000 km s−1 in a given region, although the velocity width of a layer along any given line of sight is <250 km s−1. Si and O are observed to be coincident in some directions, but segregated by up to #24;500 km s−1 in other directions. We again compare these observations of the nucleosynthetic layers to predictions from supernova explosion models in an attempt to constrain such models. Finally, we observe small-scale velocity structures in the recently shocked ejecta. We determine that this corrugation is likely caused during the supernova explosion itself, rather than hundreds of years later at the remnant’s reverse shock. Finally, we present a detailed multi-epoch X-ray analysis of Cas A using Chandra X-ray Observatory exposures from 2000, 2002, and 2004. We identify the most recently shocked X-ray ejecta with ionization timescales of #24;1010 cm−3 s, nearly an order of magnitude smaller than previously identified shocked ejecta. These ejecta are then used to determine if the original nucleosynthetic layers of the star are arriving at Cas A’s reverse shock at different times. We use recent collisional ionization models that allow us to correlate observed changes in spectrum with a rough estimate of when the Mg and Fe layers reached the reverse shock. We find several regions that have a signature consistent with a separation of #24;200 km s−1 between layers, although we find that most regions show no sign of separation greater than 65 km s−1. This method is able to detect substantially smaller separations between layers than earlier X-ray techniques. We test various supernova explosion models against our observations by comparing our observed velocity separation between layers to predictions from the models. We conclude that any mixing between nucleosynthetic layers is most likely caused by Rayleigh-Taylor filamentation and not partial explosive nucleosynthesis in the layers. Our observations of spectral changes provide feedback for future models which will address important physical issues such as the role of cosmic ray production at a supernova remnant’s reverse shock.
Analysis of Increased Lensing Strength of Substructure in Galaxy Clusters
(2021-08-27) Lasko, Kekoa; Williams, Liliya L.R.
In 2019, an inconsistency between our observations and our simulations was discovered. We were finding that there were more cases of strong lensing appearing around galaxy cluster substructure than was predicted in simulations. This project is born as an effort to provide reasoning for this unexplained discrepancy. We begin by looking for cases of gravitational lensing which include a galaxy cluster potential as well as a single galaxy potential which is contained within the cluster. By looking at the resultant Lens Equation and specifying the cases, we find an analytical expression for the behavior of resulting images. We next apply this expression to both observed and simulated data to look for the best set of parameters to minimize the amount of mass that the single galaxy needs to have the strongest lensing effect. Finally, we propose an explanation for why galaxy cluster substructure has more lensing strength in observations than in simulations. We believe that shallower cluster mass profiles result in stronger lensing potentials for many of the singular galaxies which make up the cluster.
An analysis of star formation in M31 using resolved stars and ultraviolet flux
(2014-10) Simones, Jacob Edward
We have used optical observations of resolved stars from the Panchromatic Hubble Andromeda Treasury (PHAT) to measure the recent (< 500 Myr) star formation histories (SFHs) of 33 FUV-bright regions in M31. The region areas ranged from ~ 104 to 106 pc2, which allowed us to test the reliability of FUV flux as a tracer of recent star formation on sub-kpc scales. The star formation rates (SFRs) derived from the extinction-corrected observed FUV fluxes were, on average, consistent with the 100-Myr mean SFRs of the SFHs to within the 1 σ scatter. Overall, the scatter was larger than the uncertainties in the SFRs and particularly evident among the smallest regions. The scatter was consistent with an even combination of discrete sampling of the initial mass function and high variability in the SFHs. This result demonstrates the importance of satisfying both the full-IMF and the constant-SFR assumptions for obtaining precise SFR estimates from FUV flux. Assuming a robust FUV extinction correction, we estimate that a factor of 2.5 uncertainty can be expected in FUV-based SFRs for regions smaller than 105 pc2, or a few hundred pc. We also examined ages and masses derived from UV flux under the common assumption that the regions are simple stellar populations (SSPs). The SFHs showed that most of the regions are not SSPs, and the age and mass estimates were correspondingly discrepant from the SFHs. For those regions with SSP-like SFHs, we found mean discrepancies of 10 Myr in age and a factor of 3 to 4 in mass. It was not possible to distinguish the SSP-like regions from the others based on integrated FUV flux.Starting from SFHs derived from the full PHAT photometric dataset, we have used stellar population synthesis to create maps of synthetic far- and near-ultraviolet (FUV and NUV) flux at sub-kpc resolution for the northeast quadrant of M31. The synthetic maps reproduced all of the main morphological features found in corresponding maps of observed FUV and NUV flux, including rings and large star-forming complexes. Comparing the flux maps pixel-by-pixel, we found the median synthetic-to-observed flux ratios to be 1.02 +0.74/-0.43 in FUV and 0.79 +0.35/-0.24 in NUV. The synthetic fluxes were therefore consistent overall with the observed fluxes in both filters. We used the observed fluxes and standard flux calibrations to derive star formation rate (SFR) maps, which we compared with a map of the mean SFRs over the last 100 Myr of the star formation histories (SFHs). We determined a lower limit of SFR ~ 10-5 M_sun yr-1 below which the commonly assumed linear relationship between UV flux and SFR appears to break down. Above this limit, we found the median ratios of the flux-based SFRs to the mean SFRs to be 0.57 +0.47/-0.26 in FUV and 1.24 +0.88/-0.52 in NUV. Both the FUV and NUV flux-based SFRs were therefore consistent overall with the mean SFRs derived from the SFHs. Integrating over the entire mean SFR map, we found a global SFR of 0.3 M_sun yr-1. The corresponding measurements from the flux-based SFR maps were factors of 0.74 (FUV) and 1.45 (NUV) of the global mean SFR value. It is not yet understood why the SFR ratios in the global case are larger than the median pixel-wise ratios. The primary source of uncertainty in both the synthetic flux maps and the flux-based SFR maps was most likely incomplete IMF sampling due to the small pixel areas. With the exception of the faintest areas of the galaxy, we did not identify any trends for flux or SFR with environment.
Analytical and numerical studies of dark matter halos
(2008-12) Austin, Crystal Gayle
This dissertation focuses on the evolution and structure of dark matter halos of galaxies, groups and clusters of galaxies. I explore the dependence of the final halo’s properties on the initial conditions and the physical processes that guide the halo to equilibrium, with special focus on the power-law nature of the ρ/σ3 profile, where ρ is the density profile and σ is the velocity dispersion profile. As the astronomy community does not yet fully understand these processes, this research expands our understanding of collisionless, gravitationally-interacting systems. In the initial chapters, I study the collisionless semi-analytic halo simulations and show that the final properties are sensitive to the initial conditions, such as the powerspectra filtering scale, the secondary velocities’ magnitudes and directions, and the accretion rate. The general conclusions are that semi-analytic halos are in hydrostatic equilibrium and have a power-law ρ/σ3 profile. If there were discontinuities in the initial conditions, the power-law feature in ρ/σ3 breaks. Because of this, hydrostatic equilibrium is a less restrictive condition than the ρ/σ3 profile. These halos can recover from moderate discontinuities by either correcting a single profile by sacrificing other quantities or by sufficient post-accretion. Finally, I compare collisionless semi-analytic and N-body simulations directly. This novel comparison is useful because these techniques use different physics to collapse the proto-halo. The physical differences between these two methods are used to determine causes of the final halo profiles. Specifically, I find the NFW density profile and power-law ρ/σ3 are due to the slow rate of evolution, which is determined from the initial conditions and cosmology. The density slope-velocity anisotropy relationship is dependent, rather, on the physical processes (notably the radial orbit instability) and three-dimensional evolution used to collapse the proto-halos. We also find that the slow-evolution halos do not undergo violent relaxation (large changes in the global potential). Thus we suggest that slow, collisionless relaxation is responsible for creating the power-law feature ρ/σ3.
Calibration of the E and B EXperiment (EBEX), a balloon-borne cosmic microwave background polarimeter.
(2009-10) Polsgrove, Daniel Edward
We discuss pre-flight calibration of the E and B EXperiment (EBEX), a balloon-borne telescope designed to measure the B-mode polarization anisotropy of the cosmic microwave background (CMB). EBEX will observe the sky with 8' resolution in each of three bands centered on 150, 250 and 410 GHz. Employing over 1,400 detectors and performing polarimetry through a continuously rotating half-wave plate with fixed wire-grid polarizer, we expect to detect the B-mode signal or set a new upper limit one order of magnitude below the current value. In this thesis we describe a set of ground-based experiments devised for calibrating instrumental response to incident millimeter-wave flux with varying spectral and polarization properties. We chronicle the design, construction and execution of these experiments, along with preliminary results from tests executed prior to our North American (NA) test flight which originated at the Columbia Scientific Ballooning Facility, Ft Sumner, NM in June 2009. A brief review of this inaugural flight is provided, as is a synopsis of our current plan for a comprehensive calibration strategy to be implemented in conjunction with a future long duration balloon (LDB) flight over Antarctica.
The E and B EXperiment: implementation and analysis of the 2009 engineering flight.
(2011-06) Milligan, Michael Bryce
The E and B EXperiment (EBEX) is a balloon-borne telescope designed to map the polarization of the cosmic microwave background (CMB) and emission from galactic dust at millimeter wavelengths from 150 to 410 GHz. The primary science objectives of EBEX are to: detect or constrain the primordial B-mode polarization of the CMB predicted by in ationary cosmology; measure the CMB B-mode signal induced by gravitational lensing; and characterize the polarized thermal emission from interstellar dust. EBEX will observe a 420 square degree patch of the sky at high galactic latitude with a telescope and camera that provide an 80 beam at three observing bands (150, 250, and 410 GHz) and a 6:2#14; diffraction limited field of view to two large-format bolometer array focal planes. Polarimetry is achieved via a continuously rotating half-wave plate (HWP), and the optical system is designed from the ground up for control of sidelobe response and polarization systematic errors. EBEX is intended to execute y or more Antarctic long duration balloon campaigns. In June 2009 EBEX completed a North American engineering flight launched from NASA's Columbia Scientific Ballooning Facility (CSBF) in Ft. Sumner, NM and operated in the stratosphere above 30 km altitude for #24; 10 hours. During flight EBEX must be largely autonomous as it conducts pointed, scheduled observations; tunes and operates 1432 TES bolometers via 28 embedded Digital frequency-domain multiplexing (DfMux) computers; logs over 3 GiB/hour of science and housekeeping data to onboard redundant disk storage arrays; manages and dispatches jobs over a fault-tolerant onboard Ethernet network; and feeds a complex real-time data processing infrastructure on the ground via satellite and line-of-sight (LOS) downlinks. In this thesis we review the EBEX instrument, present the optical design and the computational architecture for in-flight control and data handling, and the quick-look software stack. Finally we describe the 2009 North American test flight and present analysis of data collected at the end of that flight that characterizes scan-synchronous signals and the expected response to emission from thermal dust in our galaxy.
Evolution of Weak Magnetic Fields in a Turbulent Plasma
(2013-08-09) Emerick, Andrew;
Magnetic fields play an important role in astrophysical objects, in everything from planet for- mation through large scale structure evolution. The understanding of how these fields evolve and affect the medium in which they are contained has been bolstered through complex computational simulations of both the micro physics involved, and on cosmological scales. I focus on the context of magnetic fields in diffuse plasmas such as that present in the intracluser medium (ICM) of galaxy clusters. In this paper, I examine the evolution of initially weak magnetic fields through solenoidally driven turbulence in a plasma medium. This evolution is expected to be controlled predominantly by the small scale turbulent dynamo until the system reaches equillibrium. The relevant time domain in galaxy clusters, with eddy turnover times on the order of a few to several Myr, corresponds to well before the equillibrium stage. I focus then on the early magnetic field evolution and turbulent amplification under three initial magnetic field conditions utilizing an ideal, isothermal MHD code. I examine the detailed time and power spectra evolution of both the kinetic and magnetic energies, and compare to what would be expected in the small scale dynamo picture. In addition, I examine the prospects of distinguishing initial magnetic field structures within the ICM.
Exploring and modeling high-excitation emission in the ejecta and the Wind of Eta Carinae.
(2011-06) Mehner, Andrea
Eta Carinae (! Car) is the most massive, most luminous star in our region of the Galaxy. It is an evolved massive star system and therefore provides many clues to the fate of the most massive stars. In the 1840s its unstable nature culminated in the Great Eruption when it briefly became the second brightest star in the sky and ejected more than ten solar masses, which today enshroud the surviving star as a bipolar nebula. The “supernova impostor” phenomenon and its aftermath constitute a major gap in the theory of massive stars, and ! Car is the only example that can be studied in detail. Its recovery has been unsteady with unexplained photometric and spectral changes in the 1890s and 1940s. Combining data from HST STIS and Gemini-S GMOS between 1998 and 2010, I analyzed several spectroscopic cycles that occur every 5.54 years. In addition, I used some data from the VLT UVES, Magellan II MIKE, and Ir´en´ee du Pont B&C instruments. Observations with a variety of different slit position angles made it possible to map the emission across the nebula and the complex outer ejecta of ! Car permit to observe the star at different stellar latitudes via reflected light. In order to study the distribution of gas and ionizing radiation around ! Car and their implications for its likely companion star, I examined several high-excitation emission lines. The principal results are: (1) The high-excitation fluxes varied systematically and non-trivially throughout ! Car’s 5.5-year spectroscopic cycle. (2) A brief, strong secondary maximum occurred just before the 2003.5 spectroscopic event. (3) These emission lines are strongly concentrated at the Weigelt knots several hundred AU northwest of the star. With less certainty, [Ne III] appears to be somewhat more concentrated than [Fe III]. (4) A faster, blueshifted component appears concentrated near the star and elongated perpendicular to the system’s bipolar axis. (5) Using the photoionization program Cloudy, I estimated the range of parameters for the hot secondary star that would give satisfactory high-excitation line ratios in the close ejecta; Teff ! 40, 000 K, L " 4×105 L!, and Minit " 40–50 M!, for example, would be satisfactory. The allowed region in parameter space is wider and mostly less luminous than some previous authors suggested. Spectra obtained with Gemini GMOS throughout 2007–2010 were used to observe the 2009 spectroscopic event from different stellar latitudes. The He II "4687 emission, only observed during the “events,” was analyzed in spectra in direct view and in reflected polar-on spectra at FOS4. The time-delay of He II at FOS4 is about 18 days and therefore consistent with the predicted additional time-travel time at FOS4. The equivalent width and radial velocity behavior of He II at FOS4 mirrors the behavior observed in direct view. These findings imply a symmetric geometry for the origin of the He II emission and are difficult to reconcile with some proposed He II emitting regions and some orbital models. H I, He I, and Fe II lines, observed at different latitudes, reveal details about the changing wind during the “event.” N II ""5668–5712 emission and absorption lines behave qualitatively like the He I lines. Spectral lines of ! Car’s stellar wind regions can be classified into four physically distinct categories: 1) low-excitation emission such as H I and Fe II, 2) higher excitation He I features, 3) N II lines, and 4) He II emission. These categories have different combinations of radial velocity behavior, excitation processes, and dependences on the secondary star. In this sense the N II features resemble the He I lines, but they represent zones of lower ionization. This combination of attributes appears to be unique in ! Car’s well-observed spectrum. N II probably excludes some proposed models, such as those where He I lines originate in the secondary star’s wind or in an accretion disk. Spectra in 2009 and 2010 showed that major stellar-wind emission features in the spectrum of ! Car have recently decreased by factors of order 2 relative to the continuum. This is unprecedented in the modern observational record. The simplest explanation is a rapid decrease in the wind density. Work presented in this thesis was published in Mehner et al. (2010a,b, 2011). v
Extending the Reach of Gravitational Wave Detectors and Probing the Isotropic Stochastic Background
(2021-08) Ormiston, Rich
Beginning with the first detection of gravitational waves in 2015 by the Advanced Laser Interferometer Gravitational Wave Observatory (aLIGO) a new era of astrophysics emerged. Within 5 years, aLIGO has detected 50 mergers from binary black hole (BBH) and binary neutron star (BNS) systems and kick started the field of multi-messenger astrophysics with the measurement of an electromagnetic counterpart to the BNS merger GW170817. A detection on the horizon for LIGO is that of the stochastic gravitational wave background (SGWB). The detection of such a background would have far reaching consequences in astrophysics and cosmology as these measurements can probe the first fractions of a second after the Big Bang, revealing insights and parameters of proposed and possibly undiscovered cosmological models. Even a SGWB formed by BBH and BNS mergers near to us would provide valuable information about star formation rates, the formation of large scale structure, as well as the populations of these compact objects. There are two main topics in this dissertation: detector characterization/data quality methods and characterization of the SGWB. The first chapter provides a background into General Relativity and derives import dynamics relevant to LIGO that are heavily used throughout the thesis. Chapter 2 delves into detector characterization and understanding the noise which enters into the detector output data stream. This is accomplished through the development of a coherence calculation package, \texttt{STAMP-PEM}, which creates a hash table lookup for quick followup analysis and a user friendly API. In Chapter 3 I discuss the sky-averaged SGWB, search methods and present the most recent results combined over aLIGO's first three observing runs. Chapter 4 will extend the isotropic SGWB search to Cosmic Explorer sensitivities and attempt a novel solution to subtract the foreground compact binary coalescences (CBCs) in the $f-t$ space. This mock data analysis sets a benchmark for future search methodologies and sensitivities. Finally in Chapters 5 and 6 I discuss data quality filtering methods. The former will employ a new deep learning architecture known as \texttt{DeepClean} to identify and subtract noise couplings of arbitrary order without introducing artifacts or phase misalignment of the output signal. The final chapter is dedicated to the construction of analytic filters used for removing linear, nonlinear and non-stationary noise. These analytic filters are useful as they are lightweight and can brute force search through the auxiliary channel combinatorics and aid in identifying relevant physical couplings.
Fitting DARKexp predictions to dark matter halo profiles from simulation
(2014-10-01) Nolting, Christopher
I performed analysis of dark matter halo data from the Millennium II simulation and t the resulting energy and density distributions to the predictions of the DARKexp model. For halos near virial equilibrium, I found DARKexp to t the simulated data well, supporting it as a theoretical model for the distribution of matter in galaxies. I also discuss the methods of data analysis and the Markov Chain Monte Carlo method used for fitting.
Gravitational Waves As Tools For Astrophysics And Cosmology
(2021-05) Banagiri, Narayana Sri Sharan
The discovery of gravitational waves by LIGO and Virgo have unveiled a sector of the Universe previously hidden from us. Gravitational waves allow us to detect and observe the dynamics of phenomena usually hidden from electromagnetism probes like binary black hole mergers, providing new tools to study astrophysics and cosmology in the process. As the number of detections increase, the statistics of the mergers are starting to inform us about their progenitor distributions. This dissertation consists of three parts. First, analyses to detect potential gravitational waves from a post-merger remnant of the binary neutron star merger GW170817 are described, with particular emphasis on the STAMP pipeline-based search targeting gravitational waves from a long-lived remnant. Bayesian parameter estimation techniques for the poorly modeled post-merger signals are then described. A novel likelihood formalism is developed to account for the inaccuracies in models, focusing in particular on the phase evolution of the waveform. In the second part, techniques are developed, using hierarchical Bayesian modeling to measure N-point correlations of the distributions of black hole mergers, with a focus on two-point correlations. These methods allow us to use black hole mergers as a tool to measure the angular distribution and the large-scale structure of the matter in the Universe. The two-point correlation method is validated with simulations for the angular structure of the mergers in the Universe. Finally, Bayesian methods are devised to probe anisotropies in the angular distribution of the stochastic gravitational-wave backgrounds and foregrounds in the LISA band. A novel decomposition using \cg coefficients in the spherical harmonic basis is developed that allows us to infer the anisotropy of arbitrary distributions of gravitational-wave power. This method is employed and tested using different kinds of simulations, including that of the galactic gravitational-wave foreground from galactic white dwarf binaries.
The history of star formation in nearby dwarf galaxies.
(2010-08) Weisz, Daniel Ray
We present detailed analysis of color-magnitude diagrams (CMDs) of resolved stellar populations in nearby dwarf galaxies based on observations taken with the Hubble Space Telescope (HST). From the positions of individual stars on a CMD, we are able to derive the star formation histories (SFHs), i.e., the star formation rate (SFR) as a function of time and metallicity, of the observed stellar populations. Specifically, we apply this technique to a number of nearby dwarf galaxies to better understand the mechanisms driving their evolution. The ACS Nearby Galaxy Survey Treasury program (ANGST) provides multi-color photometry of resolved stars in ∼ 60 nearby dwarf galaxies from images taken with HST, This sample contains 12 dSph, 5 dwarf spiral, 28 dIrr, 12 dSph/dIrr (transition), and 3 tidal dwarf galaxies. The sample spans a range of ∼ 10 in MB and covers a wide range of environments, from highly interacting to truly isolated. From the best fit lifetime SFHs we find three significant results: (1) the average dwarf galaxy formed ∼ 60% of its stars by z ∼ 2 and 70% of its stars by z ∼ 1, regardless of morphological type, (2) the only statistically significant difference between the SFHs of different morphological types is within the most recent 1 Gyr (excluding tidal dwarf galaxies), and (3) the SFHs are complex and the mean values are inconsistent with simple SFH models, e.g., single epoch SF or constant SFH. We find that an exponentially declining star formation model with τ ∼ 6.4 Gyr provides a reasonable representation of the mean SFHs. The dominance of the older stellar populations implies that the typical dwarf galaxy formed most of its stars at times similar to more massive galaxies. The sample shows a strong density-morphology relationship, i.e., the dSphs in the sample are less isolated than dIs. We find that the transition from a gas-rich to gas-poor galaxy cannot be solely due to internal mechanisms, e.g., stellar feedback, and is likely the result of external mechanisms, e.g., ram pressure and tidal stripping and tidal forces. The average transition dwarf galaxy is slightly less isolated and less gas-rich than the typical dwarf irregular. From a comparison between the ANGST and Local Group dwarf galaxy SFHs, we find consistency between the two samples, suggesting that the Local Group dwarf galaxies are a good representation of the broader universe. Both samples show a similarly strong morphology-density relationship, further emphasizing the importance of environment in dwarf galaxy evolution. We then present the recent (. 1 Gyr) SFHs of nine M81 Group Dwarf Galaxies. Comparing the SFHs, birthrate parameters, fraction of stars formed per time interval, and spatial distribution of stellar components as a function of luminosity, we find only minor differences in SF characteristics among the M81 Group dIs despite a wide range of physical properties. We extend our comparison to select dIs in the Local Group (LG), with similar quality photometry, and again find only minor differences in SF parameters. The lack of a clear trend in SF parameters over a wide range of diverse environments suggests that SF in low mass systems may be dominated by stochastic processes. The fraction of stars formed per time interval for an average M81 Group and LG dI is consistent with a constant SFH. However, individual galaxies can show significant departures from a constant SFH. Thus, we find this result underlines the importance of stochastic SF in dIs. In addition to a statistical comparison, we examine possible formation scenarios of the less luminous and candidate tidal dwarfs in the M81 Group. The SFHs and the lack of an overdensity of associated red stars suggest that the Garland and Ho IX are not dIs and are potentially tidal dwarf galaxies. Interestingly, a noteworthy difference between the LG and the M81 Group is the lack of tidal dwarf candidates in the LG. Comparing the recent SFHs and spatial locations of young stars with observations of the neutral interstellar medium (HI), we are able to gain new insight into the physics of stellar ‘feedback’. We first make this type of comparison in IC 2754, a luminous dwarf irregular galaxy in the M81 Group with a ∼ 1 kpc supergiant HI shell. We find two significant episodes of SF inside the SGS from 200 − 300 Myr and ∼ 25 Myr ago. Comparing the timing of the SF events to the dynamic age of the SGS and the energetics from the HI and SF, we find compelling evidence that stellar feedback is responsible for creating the SGS and triggering secondary SF around its rim. We then conduct an extensive analysis of HI holes in M81 Group dwarf irregular galaxy, Holmberg II. From the deep photometry, we construct the CMDs and measure the SFHs for stars contained in HI holes from two independent holes catalogs, as well as select control fields, i.e., similar sized regions that span a range of HI column densities. The CMDs reveal young (< 200 Myr) stellar populations inside all HI holes, which contain very few bright OB stars with ages less than 10 Myr, indicating they are not reliable tracers of HI hole locations while the recent SFHs confirm multiple episodes of star formation within most holes. Converting the recent SFHs into stellar feedback energies, we find that enough energy has been generated to have created all holes. However, the required energy is not always produced over a time scale that is less than the estimated kinematic age of the hole. A similar analysis of stars in the control fields finds that the stellar populations of the control fields and HI holes are statistically indistinguishable. However, because we are only sensitive to holes ∼ 100 pc in diameter, we cannot tell if there are smaller holes inside the control fields. The combination of the CMDs, recent SFHs, and locations of young stars shows that the stellar populations inside HI holes are not coherent, single-aged, stellar clusters, as previously suggested, but rather multi-age populations distributed across each hole. From a comparison of the modeled and observed integrated magnitudes, and the locations and energetics of stars inside of HI holes, we propose a potential new model: a viable mechanism for creating the observed HI holes in Ho II is stellar feedback from multiple generations of SF spread out over tens or hundreds of Myr, and thus, the concept of an age for an HI hole is intrinsically ambiguous. For HI holes in the outer parts of Ho II, located beyond the HST/ACS coverage, we use Monte Carlo simulations of expected stellar populations to show that low level SF could provide the energy necessary to form these holes. Applying the same method to the SMC, we find that the holes that appear to be void of stars could have formed via stellar feedback from low level SF. We further find that Hα and 24μm emission, tracers of the most recent star formation, do not correlate well with the positions of the HI holes. However, UV emission, which traces star formation over roughly the last 100 Myr, shows a much better correlation with the locations of the HI holes.
The interstellar medium and star formation of nearby, low-mass galaxies.
(2012-08) Warren, Steven Ray
This thesis presents four different studies of the interstellar medium (ISM) and stellar content of [approximately] 40 nearby (D [approximately]< 4 Mpc), low-mass galaxies. We aim to address two fundamental questions: “How do stellar processes effect the ISM in low-mass galaxies?” and “What are the local gas conditions which lead to molecular cloud formation?” Much of the data presented here come from our survey the “Very Large Array - Advanced Camera for Surveys Nearby Galaxy Survey Treasury” (VLA-ANGST). VLA-ANGST is a targeted atomic hydrogen (H I) emission line survey directed towards 35 low-mass galaxies selected from the ANGST Hubble Space Telescope (HST) galaxy sample of the nearby universe. The VLA-ANGST project is the largest survey of its kind, demanding nearly 600 hours of VLA observing time. This unprecedented amount of observing time gives us data which has long lasting legacy value for its wealth of high resolution and high sensitivity information on the H I gas content and dynamics in a large sample of nearby, low-mass galaxies. H I data from the VLA-ANGST project will be used to explore the interactions between the gas and stellar content as well as trace the underlying dark matter distribution. Combining the H I and HST data with other tracers of recent star formation (e.g., emission processes from far ultraviolet star light, dust in the infrared, and carbon monoxide in the submillimeter) provides a comprehensive census of each galaxy, useful for understanding their evolution. We investigate the role of multiple generations of star formation in the formation of large, kiloparsec scale cavities observed in the global H I distributions of five nearby, low mass galaxies. The small gravitational potential wells of some low-mass galaxies allow the outflow of energy from stellar processes (e.g., winds, supernovae, etc.) to help shape their gas distributions. We find that stellar processes produce ample energy (at least an order of magnitude or more) to have been the dominant creation source for the observed cavities. The molecular gas responsible for the formation of stars remains elusive in many of the low-mass galaxies. We present a novel new technique to trace the immediate precursor of the molecular gas: cold H I. We apply our technique to a large sample of 31 nearby, low-mass galaxies and detect cold H I in [approximately] 85% of the final sample (23/27) after quality control cuts are applied. The cold H I discoveries presented here represent a significant step forward in our ability to study the precursory gas to star formation where standard techniques fail. We find that the cold H I occupies only a small fraction of the total H I content in each galaxy, consistent with both theory and other observational techniques in the literature. The cold Hi is typically found in higher density gas, but is markedly absent from the highest density peaks where current star formation is presumably heating the gas. Observations targeting the areas rich in cold H I gas may be the only way to study the conditions of star formation in some low-mass galaxies. Finally, we present direct observations of the molecular hydrogen content in one of the only low-mass galaxies with a molecular gas detection, NGC 4214. We use the Infrared Spectrograph onboard the Spitzer Space Telescope to measure pure rotational lines of the ground state of molecular hydrogen (H[subscript]2). These observations are some of the only direct H[subscript]2 detections in a low-mass galaxy to date. They confirm the association on the carbon monoxide (CO) molecule with the H[subscript]2 molecule assumed in the literature. We provide limits to the gas phase temperatures and column densities of the warm H[subscript]2 along the lines-of-sight of three distinct CO clouds, two of which are actively forming stars. The results presented here add to the growing understanding of how these low-mass systems form stars. This knowledge may be applicable to galaxy evolution in the early universe, which may have had similar star forming conditions.
The massive Star Population in M101
(2014-08) Grammer, Skyler
An increasing number of non-terminal giant eruptions are being observed by modern supernova and transient surveys. Very little is known about the origin of these giant eruptions and their progenitors which are presumably very-massive, evolved stars such as luminous blue variables, hypergiants, and supergiants. Motivated by the small number of progenitors positively associated with these giant eruptions, we have begun a survey of the luminous and evolved massive star populations in several nearby galaxies. We aim to identify the likely progenitors of the giant eruptions, study the spatial variations in the stellar populations, and examine the relationship between massive star populations and their environment.The work presented here is focused on stellar populations in the relatively nearby, giant, spiral galaxy M101 from sixteen archival BVI HST/ACS images. We create a catalog of stars in the direction to M101 with photometric errors < 10% for V < 24.5 and 50% completeness down to V = 26.5 even in regions of high stellar crowding. Using color and magnitude criteria we have identified candidate luminous OB type stars and blue supergiants, yellow supergiants, and red supergiants for future observation. We examine their spatial distributions across the face of M101 and find that the ratio of blue to red supergiants decreases by two orders of magnitude over the radial extent.From our catalog, we derive the the star formation history (SFH) for the stellar populations in five 2' wide annuli by fitting the color-magnitude diagrams. Binning the SFH into time frames corresponding to populations traced by Halpha, far ultraviolet (FUV), and near ultraviolet (NUV) emission, we show that the fraction of stellar populations young enough to contribute in Halpha is 15% " 35% in the inner regions, compared to less than 5% in the outer regions. This provides a sufficient explanation for the lack of Halpha emission at large radii. We also model the blue to red supergiant ratio in our five annuli, examine the effects that a metallicity gradient and variable SFH have on the predicted ratios, and compare to the observed values. We find that the radial behavior of our modeled blue to red supergiant ratios is highly sensitive to both spatial variations in the SFH and metallicity. Incorporating the derived SFH into the modeled ratios, we are able to reproduce the observed values at large radii (low metallicity), but at small radii (high metallicity) the modeled and observed ratios are discrepant. Though photometry has proven to be a powerful tool to identify candidate evolved massive stars and their effects on their host galaxy, spectroscopy is necessary to study the physical properties of individual stars. We observed moderate-resolution optical spectra for 56 of the brightest stars in the direction to M101 using the Multiple Mirror Telescope. We also created light curves for each target using multi-epoch UBV R images from the Large Binocular Telescope. We separate the spectroscopially confirmed members of M101 into four groups: hot supergiants, intermediate supergiants, emission-line stars, and LBVs. Several stars in each group are discussed in detail. Of the spectroscopically confirmed members, we find that eight meet our criterion for variability. We present light curves for the known LBV candidates, V2, V4, and V9, and introduce a new candidate: 9492 14 11998. Additionally, we identify 20 new variables in M101. Lacking spectra, we separated the variables, by their photometric properties, into three groups: hot, intermediate, and cool. We find two hot stars with V -band variability of ±1 magnitude; we flag these stars as LBV candidates. Of the intermediate and cool variables, we identify several stars with low- to moderate-amplitude variability (0.1"0.5 magnitudes).
Modeling and Removing Contamination in WISP Grism Data
(2016) Garon, Avery;
I develop and present an improved method for modeling and removing the contamination from WFC3 Infrared Spectroscopic Parallel (WISP) Survey grism data. WISP takes slitless spectroscopy data, which means that spectra of di erent objects may overlap in the observations. The current cleaning algorithm assumes a multi-Gaussian intensity pro le for modeling the overlapping objects. The new algorithm I develop calculates the pro les from the direct image of the galaxy. I discuss current shortcomings of my method and where it can be improved. This algorithm will be made publicly available and eventually incorporated into the WISP pipeline.
Multi-wavelength observations of pulsar wind nebulae and composite supernova remnants.
(2009-10) Temim, Tea
Multi-wavelength studies of pulsar wind nebulae (PWNe) and supernova remnants (SNRs) lead to a better understanding of their evolutionary development, the interaction of supernovae (SNe) and pulsar winds with their surroundings, and nucleosynthesis and production and processing of dust grains by SNe. PWNe and composite supernova remnants, in particular, are unique laboratories for the study of the energetic pulsar winds, particle injection processes, and the impact of PWNe on the evolving SNR. They provide information on SNR shock properties, densities and temperatures, and the chemical composition and the ionization state of the material ejected by SNe. SNRs also serve as laboratories for the study of dust production and processing in SNe. While X-ray observations yield important information about the SN progenitor, hot gas properties, SN explosion energy, and the surrounding interstellar medium (ISM), the IR can provide crucial information about the faint non-thermal emission, continuum emission from dust, and forbidden line emission from SN ejecta. Combining observations at a wide range of wavelengths provides a more complete picture of the SNR development and helps better constrain current models describing a SNR's evolution and its impact on the surrounding medium. This thesis focuses on a multi-wavelength study of PWNe in various stages of their evolution and investigates their interaction with the expanding SN ejecta and dust and the SNR reverse shock. The study of these interactions can provide important information on the SNR properties that may otherwise be unobservable.
Multidimensional diffusive shock acceleration in the Winds from massive stars
(2010-07) Edmon, Paul Pretzer
Nonthermal radio emission has been seen in the winds around a quarter of all O-stars. The emission is attributed to shock accelerated cosmic rays. The shocks thought to be causing the acceleration are either wind-embedded shocks due to radiative line driving instabilities in the stellar wind, or shocks due to the colliding winds in a binary system. Very few numerical Diffusive Shock Acceleration (DSA) simulations exist for these systems due to the complicated, multidimensional nature of the winds. We present the first 2-D magnetohydrodynamic DSA (MHD-DSA) simulations of massive stellar winds using the Multidimensional Adaptive Subcycling Tridiagonal solver (MAST), which has been incorporated into the WOMBAT (sWift Objects for Mhd BAsed on Tvd) code to solve diffusive shock acceleration for cosmic rays. Shock modification due to cosmic ray pressure is shown to be important in describing the shock dynamics of the colliding wind binary scenario. With 0.01% of the gas particles passing through the shocks being injected as cosmic rays, about 15 % of the wind ram pressure is converted into cosmic ray pressure. In the wind-embedded shock scenario, the isothermal conditions in the wind, due to radiative heating and cooling, precluded inclusion of cosmic ray feedback. Future 1-D simulations of cosmic ray modified radiative shocks are suggested, as the combined effects of radiative line cooling and cosmic ray feedback dramatically change the shock dynamics from adiabatic analogues. Both cases show efficient cosmic ray acceleration. In the case of the wind-embedded shocks, the isothermal nature of the wind creates shocks capable of accelerating electrons up to 100 MeV and protons up to 1 GeV with a spectral slope of 4. The colliding wind binary scenario produces very strong shocks which are capable of accelerating electrons up 1 GeV and protons up to 1 TeV with a spectral slope of 4. While full radiation models will be performed in the future, preliminary estimates indicate that the radio emission from the wind-embedded shock scenario may be extinguished due to free-free absorption. This would exclude the wind-embedded shock scenario from being able to explain the observed radio emission.
Nature of Energy Release and Transfer for Solar and Stellar Flares Using High-Sensitivity Hard X-Ray Instrumentation
(2019-08) Vievering, Juliana
Sensitive measurements of solar and stellar flares in the hard X-ray regime are necessary for investigating energy release and transfer during flaring events, as hard X-rays provide insight into the acceleration of electrons and emission of high-temperature plasmas. The research presented here seeks to develop and harness the powerful capabilities of hard X-ray focusing optics to probe faint events that have previously been elusive, ranging from small-scale solar flares to bright X-ray flares on distant stars. In exploring these uncharted regimes, this work probes some of the most intriguing mysteries of the stars, from coronal heating to the formation of planetary systems. Due to previous technological challenges with focusing hard X-rays, the recent state-of-the-art solar-dedicated instrument in the hard X-ray regime, RHESSI, utilized an indirect imaging technique, which is fundamentally limited in sensitivity and dynamic range. By instead using a direct imaging technique, the structure and evolution of small-scale solar events can be investigated in greater depth. The Focusing Optics X-ray Solar Imager (FOXSI), a hard X-ray photon counting instrument flown on three sounding rocket campaigns, seeks to achieve these improved capabilities by using focusing optics for solar observations in the 4-20 keV range. In this thesis, the FOXSI technological approach and the development of the instrument through these campaigns is outlined, with an emphasis on the most recent campaign, FOXSI-3. Along with novel hard X-ray focusing technology, the FOXSI instrument utilizes fine-pitch silicon (Si) and cadmium telluride (CdTe) semiconductor strip detectors to measure the energy and position of each incident photon. CdTe detectors offer improved capabilities for detecting faint high-energy emission compared to Si due to a higher quantum efficiency above 10 keV. The characterization of the FOXSI-3 CdTe detector response, including gain, efficiency, and energy resolution, is presented here. During the FOXSI-2 rocket flight, two sub-A class solar microflares were observed. With the direct imaging technique of FOXSI, detailed imaging and spectral analyses could be performed on microflares over an order of magnitude fainter than the faintest microflares observed by RHESSI. Through this work, the energy transfer for these sub-A class microflares was found to be consistent with that of the standard model for solar flares. Additionally, observed spatial and temporal complexity indicate that flares of this small size more closely resemble the structure and dynamics of large flares than the single energy release of a nanoflare. In addition to faint solar microflares, observations of extreme flares on distant young stellar objects, observed by the Nuclear Spectroscopic Telescope Array (NuSTAR), were analyzed. NuSTAR is the first astrophysical satellite to use focusing optics for the hard X-ray regime and offers unprecedented sensitivity >10 keV, making these observations the first of their kind. Through spectral analysis and a study of flare energetics, the energy transfer for these bright flares was also found to be consistent with the standard model for solar and stellar flares. Additionally, an emission feature at 6.4 keV offers evidence of interaction between flare radiation and circumstellar material, which could have implications for planet formation. With advances in hard X-ray instrumentation, we move one step closer to answering some of the biggest questions in solar and stellar physics.
On the nature of starbursts.
(2010-05) McQuinn, Kristen Brookes W.
Starbursts are a fascinating phenomenon that can significantly impact the host galaxy and the surrounding intergalactic medium. Understanding the nature of a starburst requires a detailed analysis of the resolved stellar populations and the recent star formation history (SFH) of the galaxy. Using a CMD fitting technique and stellar evolution models, we derive the SFHs of twenty nearby dwarf starburst galaxies from Hubble Space Telescope V and I band images. The star formation rates (SFRs) from this diverse sample of dwarf galaxies span three orders of magnitude but all show elevated levels of star formation (SF) in their recent past when viewed in the context of the host galaxy’s past SFH. Fifteen of the twenty galaxies show currently bursting SF and five galaxies show “fossil” bursts. From our reconstructed SFHs, it is evident that the elevated SFRs of a burst are sustained for hundreds of Myr. The SF migrates around the host galaxies in many cases as derived from the temporally and spatially resolved stellar populations and is a cumulation of SF not only in star clusters but also in field regions of low surface brightness in the galaxies. Contrary to the shorter time of 3-10 Myr often cited, the starburst durations we measure range from 450 − 600 Myr in fifteen of the dwarf galaxies and up to 1.3 Gyr in four galaxies; comparable to or longer than the dynamical timescales for each system. The same feedback loop from massive stars that may quench flickering SF does not disrupt the overall burst event in this sample of galaxies. In the fifteen galaxies that show ongoing bursts, the final durations may be longer than we report here. One galaxy shows a burst that has been ongoing for only 20 Myr; we are likely seeing the beginning of a burst event in this system. Using the duration of the starbursts, we calculate that the bursts deposited 1053.9 − 1057.2 erg of energy into the interstellar medium through stellar winds and supernovae and produced 3.2%−26% of the host galaxy’s mass. We also explore two other metrics for identifying starbursts: the gas consumption timescale and the strength of H#11; emission produced by the burst. Interestingly, four galaxies classified as starbursts in our most recent time bin of 4-10 Myr show non-starburst levels of H#11; emission from the last ∼5 Myr indicating that, while the bursts are long-lasting events, the SFR can change on timescales of only a few Myr.
A Photographic Study of the Region of the Great Nebula in Orion
(1910-07) Burns, Keivin

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