Bulletin of the American Physical Society
2019 Annual Meeting of the APS Far West Section
Volume 64, Number 17
Friday–Saturday, November 1–2, 2019; Stanford, California
Session B01: Poster Session - Astrophysics and Gravitation |
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Chair: Patricia Sparks, Harvey Mudd College Room: Huang Engineering Center Foyer |
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B01.00001: Void Galaxy Properties Classified by Void Identification Algorithms Cynthia olvera Olvera Perez, Kelly Douglass, Segev BenZvi, Regina Demina, Dylan Veyrat The distant universe can be seen as a network of galaxies, a cosmic web, with clusters of galaxies connected together by thinner galaxy filaments, surrounding large, underdense voids. We analyze the Sloan Digital Sky Survey Data Release 7 where we study the properties of the galaxies located in voids. Voids affect galaxy formation because their underdense environments cause galaxies to begin forming stars much later in their life than galaxies in denser environments. There are two leading void identification algorithms: VoidFinder and ZOBOV. VoidFinder filters out faint and isolated galaxies in order to grow spheres in the empty regions. Voids are then defined as the union of these spheres. The filtered galaxies are reintroduced to the classified environments; galaxies located in a void are now considered void galaxies. ZOBOV is a watershed algorithm which filters out faint galaxies and calculates Voronoi cells for each remaining galaxy. The void galaxies are then defined as those with a cell volume greater than some threshold. We are interested in comparing how the two identification algorithms classify a void and how it changes the characteristics of a void galaxy. We compare the distributions of galaxy color, absolute magnitude, star formation rate, specific star formation rate, and inverse concentration index for those galaxies in voids defined by VoidFinder and those defined by ZOBOV. We find that VoidFinder void galaxies are bluer, fainter, have higher star formation rates and have higher inverse concentration indices than galaxies in denser regions. Conversely, void galaxies classified by ZOBOV are no different than galaxies in denser regions. [Preview Abstract] |
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B01.00002: Creating New Triggers for Detecting Gamma-ray Flares from Flat Spectrum Radio Quasars Jamil Batshoun Gamma-ray flares from flat spectrum radio quasars are a challenge to detect due to their rapid nature. A current method utilized to initiate observations of these extreme sources involves an indiscriminate trigger consisting of the observation of three photons of more than 50 GeV in a 24-hour period by the Fermi Large Area Telescope (LAT). We aim to improve on the ability to trigger observations by creating source-specific triggers. Each source is evaluated using three separate but related triggers. The first trigger is a specific energy threshold based on a histogram of the integrated photon energy per day. The second trigger involves spotting three sequential days where the integrated photon energy per day increased every 24 hours for a 72-hour window. The third trigger is a ratio of the integrated photon energy for a particular day above a specific energy threshold over the integrated photon energy for that same day below that threshold. These unique triggers will be displayed on a website to enable rapid follow up of these unique events by the community. [Preview Abstract] |
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B01.00003: Investigating the existence of dynamical chaos in stellar evolution Bjorn Larsen, Nick Nelson Stellar evolution is a highly modeled area of stellar astrophysics with numerous applications. The systems of equations used to effectively model stellar evolution are highly nonlinear, and thus potentially chaotic. Chaotic behavior is an as-yet unquantified source of uncertainty in these models. We test for chaotic behavior by comparing the differences in phase space between two nearly-identical sun-like stellar simulations over their main sequence evolution. We run this test for stars of different initial masses and rotation velocities, with results showing exponential growth across multiple orders of magnitude of phase space separation over time for the majority of rotating models, and a spread of lyapunov times ranging from approximately 100 million to 5 billion years. We also analyze how chaos drives growth in uncertainty in observable star properties including effective temperature and radius with initial uncertainties on the order of 10-9 increasing to as high as 10-3. [Preview Abstract] |
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B01.00004: Probing the Initial Mass Function with the Rate of Supernovae in Voids Kaelyn Dauer, Segev Benzvi, Kelly Douglass, Cynthia Olvera Perez We compare the rate of supernovae in void galaxies to that in galaxies in denser regions to discern if there is an environmental dependence on the initial mass function. We use core collapse supernovae from the Open Supernova Catalog that are within the SDSS DR7 footprint to study the supernova rate in voids and in denser regions using VoidFinder, a leading void identification algorithm that filters out faint and isolated galaxies to grow void spheres in empty regions of the cosmic web. We use the absolute magnitudes from SDSS DR15 to look at void galaxies and galaxies in denser regions of comparable mass to determine the rate of supernovae in both environments. We find that void galaxies have higher supernovae rates than galaxies in denser regions. Possible explanations for this result include a top-heavy IMF in void galaxies and an environmental dependence on cosmic downsizing. [Preview Abstract] |
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B01.00005: Looking for Tidal Disruption Events in Virgo's Globular Clusters William Huang, Arjun Padiyar, Vivian Tang, Piero Madau, Puragra Guhathakurta A great problem in modern astronomy is the apparent absence of Intermediate-Mass Black Holes (IMBHs). One possibility is that they lie in the centers of globular clusters, and if that were the case, one could find IMBHs using tidal disruption events (TDEs). TDEs occur when stars pass near enough to a black hole that they are disrupted, causing an observable, dramatic luminosity flare-up around the black hole. We estimate the rate of TDEs per globular cluster by assuming an empty loss cone in each globular cluster, taking into account the number density of stars and the two-body relaxation timescale. Then, we compute an estimate for the number of TDEs that should be observable based on a three-year observational period for the NGVS survey. Based on these estimates, we find roughly 80 TDEs should occur within the three-year span, but only 5 are likely to be detected. Additionally, we model a TDE's monochromatic luminosity as a superposition of the super-Eddington wind and a multi-layered accretion disk. We confirm that the luminosity generally does not follow the classical $t^{-5/3}$ curve, and in fact may have two peaks, although one is short-lived and hard to observe. These findings should serve as a baseline for future theory and observations on IMBHs. [Preview Abstract] |
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B01.00006: Identifying and Characterizing Radio-Mode Active Galactic Nuclei (AGN) Lark Wang, Emil Noordeh, Ashley King, Rebecca Canning, Steven Allen Supermassive black holes which sit at the centers of galaxies can launch jets of highly relativistic particles which we observe primarily in the radio wavelengths. These jets can carry enough energy to prevent further growth of their host galaxy and even affect the larger-scale environment of galaxies. Key questions remain about how these jets can dissipate energy and under what conditions they are launched. We are studying such jets in the most massive galaxies which reside in galaxy clusters. I will present results from preliminary attempts to identify and characterize radio-bright supermassive black holes using traditional detection techniques and a convolutional neural network. [Preview Abstract] |
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B01.00007: Machine-Based Spectral Classification of Weak CN and Carbon Stars in M31 Suhas Kotha, Rachel Raikar, Puragra GuhaThakurta, Antara Bhattacharya, Allison Chang Stellar evolution has been researched for decades, yet certain phases of the stellar life cycle remain poorly understood. Recent analyses of spectroscopic and photometric data of stars in the Andromeda Galaxy (M31) from the Spectroscopic Landscape of Andromeda's Stellar Halo (SPLASH) and the Panchromatic Hubble Andromeda Treasury (PHAT) surveys have revealed an unusual population of "weak CN" stars that represent such a phase. Their defining feature is a weak double-peaked spectral absorption line around 8000 angstroms indicating the presence of the CN molecule; this same feature appears more strongly in carbon stars. Using data from SPLASH and PHAT, we investigated this association by developing an algorithm that spectrally classified stars as ``carbon'', ``weak CN'', or ``other''. The algorithm performs a clustering analysis comparing spectral features present in the visually classified stars. The results were plotted in color-magnitude diagrams (CMDs) to compare the training set with the algorithm; the algorithm classified more accurately. The results obtained have been instrumental in establishing weak CN stars as a well-defined stage of stellar evolution and will contribute to a greater understanding of the physical properties of carbon and weak CN stars. [Preview Abstract] |
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B01.00008: Anomalous responses of high-ionization lines in Active Galactic Nuclei (AGNs) to continuum variability Iris Xia, Martin Gaskell Despite the predictions of simple reverberation mapping models, the Balmer lines of AGNs often do not follow variations in the optical continuum.~Analysis of long-term H$\beta $ and continuum monitoring of NGC 5548 shows that there are anomalies in the Broad Line Region (BLR) response almost every year.~It has been further shown that these anomalies exist in the BLR H$\beta $ variability of many other AGN objects as well. To attempt to distinguish between various models for the causes of these responses, we have extended the analysis of anomalies to the higher ionization lines which probe higher energies in the continuum.~We also looked at variability in the ultraviolet region. We have studied in particular variations of He II (both in the optical and in the UV) and C IV to investigate the statistics and sizes of anomalous responses to these lines and to compare these anomalies to the anomalous responses of H$\beta $. Our method was to convolve the varying UV and optical continua with response functions to compare to the observed BLR responses.~We present the results of this analysis and of comparisons of the responses of lines to changes in the UV-optical continuum shape. We discuss proposed explanations of BLR anomalies in light of our results. [Preview Abstract] |
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B01.00009: The Frequency of Anomalous H Beta Responses During Long-term Reverberation Mapping of NGC 5548 Julia Deffner, Kayla Bartel, C. Martin Gaskell, Iris Xia In the standard model of reverberation mapping for AGNs, broad-line region (BLR) fluxes can be predicted by convolving optical continuum variability (a proxy for ionizing continuum variations) with a response function determined by the geometry of the emitting and reprocessing regions. However, the recent Space Telescope monitoring of NGC 5548 revealed anomalous responses of the BLR in NGC 5548 where BLR variability deviated from continuum predictions. To further investigate this, we analyze H$\beta$ and continuum observations of NGC 5548 covering 13 years. We find that while the integrated H$\beta$ flux generally follows optical continuum variability for the 13-year data set as a whole, there are deviations of the order of +/- 10\% in the total line flux almost every year on timescales of weeks to months and longer. These anomalies show no obvious correlations with continuum variability. Our study shows that anomalous responses are common in NGC 5548. The strong variability at times of a very narrow velocity range of the line favors the off-axis continuum variability model of Gaskell (2008, 2010). [Preview Abstract] |
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B01.00010: Ages, Metallicities, and [alpha/Fe] of Virgo Cluster GCs and dE Nuclei from Coadded Keck/DEIMOS Spectra Arnav Krishnamoorthi, Manik Taneja, Giovanni Gollotti, Kadri Bin Mohamad Nizam, Elisa Toloba Our team attempted to learn about the origin of the nuclei observed in dwarf elliptical galaxies (dEs) and the relation, if any, between the globular clusters (GC; small groups of stars) found orbiting these dEs, orbiting ultra diffuse galaxies (UDGs), free floating in the Virgo cluster of galaxies (large collection of galaxies gravitationally bound to M87, a very massive elliptical galaxy sitting at the cluster center), and orbiting the M87 galaxy. The comparison between the different groups of GCs will tell us whether UDGs can be considered extended dEs, and will provide some information about the cannibalism history of M87 and the Virgo cluster itself. This project will also shed some light into the idea that the dE nuclei might be GCs that migrated to the center of the dwarf galaxy. To distinguish between this scenario for the formation of nuclei and other possibilities that describe nuclei as events of strong star formation in the center of the cluster, and to learn about the properties of dEs, UDGs, M87, and the Virgo cluster, the mentor and SIP interns will analyze the stellar content (ages and chemical composition) of these nuclei and GCs. [Preview Abstract] |
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B01.00011: Novel Tests of Gravity Below Fifty Microns Adam Turk, Alyssa Johnson, Berlin Del Aguila, Darian Karadjov, Emily Ord, Frank Trombetta, Kassandra Weber, C.D. Hoyle Attempts to unify the Standard Model and General Relativity often include features that violate the Weak Equivalence Principle (WEP) and/or the gravitational Inverse-Square Law (ISL). A violation of these would question our fundamental understanding of gravity. To further understand nature, undergraduate researchers and faculty at Humboldt State University are using an experiment to measure gravitational interactions below 50 microns. The experiment uses a torsion pendulum with equal masses of two different materials arranged as a composition dipole. The twist of the torsion pendulum is measured as an attractor mass in a parallel-plate configuration is oscillated nearby. This creates a time dependent torque on the pendulum. The magnitude and size of this torque may lead to deviations in the WEP or ISL at this untested scale. [Preview Abstract] |
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B01.00012: Towards room-temperature optomechanical squeezing in a kilogram-scale suspended interferometer Gautam Venugopalan, Kevin Kuns, Koji Arai, Rana Adhikari The sensitivity of future gravitational wave interferometers is expected to be limited throughout the detection band by quantum vacuum fluctuations, which can be reduced by quantum nondemolition methods such as squeezed vacuum injection. Currently, non-linear crystals are used to generate the aforementioned squeezed vacuum field. An alternative method exploits the radiation-pressure mediated interaction between the mirrors forming the optical cavity and the resonant laser field. This technique has been successfully demonstrated in the field of nanoscale cavity optomechanics, but not at the kilogram mass scale and at audio frequencies where it would be useful for quantum noise evasion in terrestrial kilometer scale gravitational wave detectors. In this work, we describe the experimental design and analysis of fundamental and technical noise sources of a prototype $40\, \mathrm{m}$ interferometer with $250 \, \mathrm{g}$ mirrors that can produce an optomechanically squeezed vacuum field at $\approx 100 \, \mathrm{Hz}$. [Preview Abstract] |
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