UW-Madison Astrophysics REU Program Summer 2018
C.W. Robertson (University of Kansas-Lawrence)
Detecting neutrinos with the Askaryan Radio Array
Mentor: Dr. John Kelley
The Askaryan Radio Array (ARA) is an ultra-high energy neutrino detector at the South Pole. Neutrino properties can be determined by reconstructing the neutrino event from the observed radio pulse using both horizontally and vertically polarized antennas. ARA uses radio interferometry to determine the direction to the neutrino interaction vertex and will use polarization information to determine the neutrino direction. By unfolding the antenna response from both types of antennas, polarization information can be extracted from the measured electric field. This presentation illustrates the process needed to measure the event polarization based on Monte Carlo simulations of neutrino events.
Amy Griffin (University of Oklahoma – Norman)
I Spy With My Little Eye: Building a Catalog of Ghostly Ultra-Diffuse Galaxies in the Coma Cluster
Mentor: Dr. Ralf Kotulla
Ultra-diffuse galaxies (UDGs) are a still poorly understood class of dwarf galaxies with low surface brightnesses. Although the existence of UDGs has been recognized for several decades, their low surface brightnesses and large effective radii make them difficult to detect, and current catalogs are therefore highly incomplete. The overall goal of this project was to determine the number and location of UDGs located in the Coma Galaxy Cluster. To do so, we also needed to identify the parameters that define a UDG. Starting from Subaru HyperSuprimeCam optical images in the R band, we find close to 3 million extended sources. Using a combination of size and brightness selections, followed by two-dimensional model fitting using galfit, we downselect this to a final catalog of ~5000 UDG galaxy candidates, increasing the previously known sample of UDGs in Coma by a factor of ~20x. This large sample now allows us to better study the nature and formation processes of, as well as the stellar populations within, these galaxies.
Taylor Spoo (Angelo State University)
Application of Helmholtz Coils in X-ray Instrumentation
Mentor: Prof. Dan McCammon
Astronomers use different wavelengths of light to study our galaxy and universe. One range of wavelengths produces x-rays and, in that range, good spectral resolution is difficult to achieve. Traditional detectors such as solid state charge-coupled devices (CCDs) measure the charge that is released when a photon is absorbed to estimate the photon energy, and the resolution is fundamentally limited by the statistics of charge formation. The new microcalorimeters can potentially do much better because all the energy from an x-ray photon is transformed into heat, but low temperature operation and very sensitive thermometers are required to measure the tiny temperature rise. One type of thermometer is a superconducting Transition Edge Sensor (TES); these are used right at their superconducting transition point which allows the sensor to be very sensitive to extremely small differences in temperature. However, the TESs are very sensitive to small magnetic fields, so they must be surrounded by superconducting shielding. When the magnetic shielding is cooled into its superconducting state, magnetic fields can be trapped around the TES. To solve this problem, Helmholtz coils can be placed around the cryostat to cancel any fields nearby for the TES to be cooled. Coils, with the correct dimensions for the cryostat, were held in place on the X, Y, and Z axes by a wooden frame. The coils have been characterized and can now be placed around the cryostat to create a zero-field zone near the detectors. The new TES detectors can then be used on x-ray telescopes to obtain data with better resolution.
Samantha Garza (University of Dallas)
Star Formation in the 3 Kiloparsec Arms of the Milky Way
Mentor: Prof. Robert Benjamin
A growing body of evidence suggests that the near and far 3 kpc arms of the Galaxy might be identified as a galactic ring by an outside observer. These arms were discovered kinematically due to their unusual Doppler shifts of -50 km/s and +50 km/s of neutral hydrogen and molecular CO gas in the direction of Galactic center (where circular rotation would predict no Doppler shift.) However, because of the non-circular rotation mapping the 3kpc arms relies on model predictions of how gas would flow in the gravitational field of the barred Milky Way Galaxy. We present progress on mapping these arms using maser parallaxes and photometric distances to stellar clusters associated with star forming regions associated with the 3kpc arms. Starting with a list of HII and maser regions from previous surveys, we visually inspected each direction, measured the angular sizes of associated star forming regions, and cataloged the ones that show stellar cluster candidates for future spectroscopic follow-up. We then re-examined a model for calculating the distance to these objects, refit the model parameters, and compared the distances between the model and our measured distances. We also found that the distribution for the radii of the star forming regions in the Near and Far 3kpc arm are approximately equal while the angular sizes of the Far 3kpc arm regions are smaller than the angular sizes of the Near 3kpc arm regions, as expected. This suggests that the model distances are correct.
Andrew Mizener (Macalester College)
The Temperature and Turbulence of Circumgalactic CIV Absorbers at z ~ 0.07
Mentor: Dr. Taesun Kim
Understanding gas flows in the intergalactic and circumgalactic medium (IGM and CGM) around galaxies is an important aspect of galaxy formation and evolution over cosmic time. In particular, the thermal and dynamical properties of the CGM will constrain the feedback mechanisms and outflow properties of these galaxies driven by supernovae and/or OB stars. In this work, we use ~100 archival HST COS/STIS spectra to obtain new measurements of the temperature and turbulence of the CGM, derived from CIV/HI pairs in AGN absorption spectra at z~0.07. We combine our results with measurements made using the same observables at higher redshifts, constraining the thermal/dynamical evolution of the CGM.
Alexis Arsenault (Minnesota State University – Mankato)
Thorium Oxide: The Dirt in Astronomy’s Genie Lamp
Mentors: Prof. James Lawler & Dr. Elizabeth Den Hartog
Thorium Argon hollow cathode lamps provide optical wavelength calibration for exoplanet searching spectrographs on ground-based astronomical telescopes. Thorium is an ideal source for high precision spectroscopy because it has a high atomic weight, and a large density of lines in the visible range. In recent years Thorium has been deemed a nuclear material, forcing manufactures to substitute Thorium Oxide as a cathode material in calibration lamps. Molecular bands caused by this ThO substitution are present in approximately 25% of the spectral orders in the visible range. The molecular bands cause nearby atomic lines to be blended, and weaker atomic lines to be obscured completely. However, if the wavelengths of the ThO features can be measured accurately, it will be possible to use them in addition to the atomic thorium lines for wavelength calibration. Without further characterization, this new generation of Th-Ar hollow cathode lamps will be useless for high-resolution wavelength calibration. Using the echelle spectrograph at the University of Wisconsin-Madison the new generation calibration lamp spectra has been measured. In collaboration with National Institute of Standards and Technology (NIST) personnel, this data will be added to the NIST Th-Ar atlas (https://physics.nist.gov/cgi-bin/TH/site.cgi) where it will provide an important resource for the wavelength calibration of high-resolution spectrographs.
Allison Erena (Smith College)
The Occasional slumbering Balrog in a 100Mpc3 Mine: Examining Simulated AGN Feedback in Galaxy Groups
Mentor: Prof. Eric Wilcots
In looking at the simulated history of active galactic nuclei (AGN) in galaxy groups, we hope to shed light on the following broad areas in the narrative of galactic evolution: the missing baryon problem, heating of the intragroup medium (IGM) and quenching of star formation. I attempt to examine the behavior of AGN in the context of galaxy groups within the confines of Illustris, a hydrodynamical cosmological simulation (made by Vogelsberger et al.). So far I have been able to isolate periods of AGN within subhalos for 40 groups, learning about frequency, duration, and age at which AGN occurred. My results match with current knowledge of AGN behavior, suggesting that for the most part, AGN occurs in the brightest group galaxy, if at all, and that for subhalos exhibiting AGN, it tends to occur fewer than ~10x between the galaxy’s formation and present day. This project is far from complete currently, and I intend to continue using Illustris to examine the heating effects of AGN on the IGM and to compare my results to observational data later.
Audra K. Hernandez, Ph.D.
Director of the UW-Madison Astrophysics REU Program
Department of Astronomy
University of Wisconsin-Madison
Room 3512 Sterling Hall
475 N. Charter Street
Madison, WI 53706
Email: hernande (at) astro.wisc.edu