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25644 results in Astronomy: General Interest

Page 211 of 1283

  • 6 - Primordial Magnetic Fields in the Early Universe and Cosmic Microwave Background Anisotropies

  • Edited by Jorge Sánchez Almeida, Instituto de Astrofísica de Canarias, Tenerife, María Jesús Martínez González, Instituto de Astrofísica de Canarias, Tenerife
  • Book:
    Cosmic Magnetic Fields
    Published online:
    21 April 2018
    Print publication:
    12 April 2018, pp 161-190

  • Summary

    Introductory Remarks

    Our aim in the talks given at the XXV Canary Islands Winter School of Astrophysics in 2013 was to present models for the generation of primordial magnetic fields in the early Universe and the imprints that these leave in cosmological observables, with an emphasis on Cosmic Microwave Background (CMB) anisotropies.

    Introduction

    The origin of the large-scale magnetic fields (LSMF) observed in galaxies, clusters of galaxies, with also hints of their presence in cosmic voids and on even larger scales in filaments, is an open issue of great importance in modern astrophysics (seeWidrow, 2002, for reviews). Both large-scale and stochastic components are present in magnetic fields observed in galaxies (usually dependent on the morphology of the host) with magnitudes from a few to several microGauss (μG). In clusters of galaxies, stochastic magnetic fields from a few to several μG strength are present with a correlation scale of the order of magnitude of ten kiloparsec (Clarke et al., 2001; Enslin & Vogt, 2006). More recently, the presence of intergalactic magnetic fields even in cosmic voids of the large-scale structure was proposed as a possible explanation for the gamma-ray observations of a couple of blazars (Neronov & Vovk, 2010; Taylor et al., 2011; Vovk et al., 2012; Tavecchio et al., 2010, 2011; Dolag et al., 2011). As a possible explanation for the lack of TeV photons observed in the high-energy spectrum from these blazars a lower bound of 10−18−10−15 G was derived for such pervasive intergalactic magnetic fields.

    In light of this multitude of observations of LSMF of different magnitudes and different coherence lengths, a primordial hypothesis for generating the seed magnetic fields, which are amplified afterwards by adiabatic compression and dynamo during structure formation, is a viable possibility (Widrow, 2002), also taking in consideration the recent observations of strong magnetic fields in galaxies at high redshift (Bernet et al., 2008; Wolfe et al., 2008). Recently, initial seeds motivated as primordial magnetic fields (PMF) have been used in N-body simulations to reproduce magnetic fields in clusters of galaxies (Govoni et al., 2013; Xu et al., 2012).

  • 4 - The Role of Magnetic Fields in AGN Activity and Feedback

  • Edited by Jorge Sánchez Almeida, Instituto de Astrofísica de Canarias, Tenerife, María Jesús Martínez González, Instituto de Astrofísica de Canarias, Tenerife
  • Book:
    Cosmic Magnetic Fields
    Published online:
    21 April 2018
    Print publication:
    12 April 2018, pp 87-122

  • Summary

    Abstract

    Active galactic nuclei (AGNs), the luminous, compact core regions of galaxies where accretion occurs onto supermassive black holes, can dramatically influence their entire host galaxy evolution by a process referred to as AGN feedback. Energy feedback to the galaxy is the result of combined radiation fields and directed outflows, and especially radio-loud active galaxies show pronounced jets and lobes. Their synchrotron radio emission indicates that dynamically important magnetic fields are at play in AGN jet collimation, stability, energy transfer to the intergalactic medium and their overall morphological appearance. Current knowledge on the launching mechanisms for such highly energetic relativistic jets, as well as the near black-hole accretion processes themselves, all invoke magnetic fields as active agents in angular momentum, mass and energy redistributions. In this review, we cover aspects of AGN feedback and the role played by magnetic fields, almost necessarily studied at vastly different length and timescales. We emphasize how typical large-scale galaxy interaction studies rely on parametric prescriptions for feedback, while detailed dedicated studies for near black-hole dynamics and relativistic jet propagation exist which take full account of magnetic field influences. We discuss representative hydro to magnetohydrodynamic (MHD) numerical simulations that exploit analogies with less energetic X-ray binary sources or even protostellar accretion-ejection systems, emphasize relativistic MHD descriptions, and point out that magnetic fields in accretion disks yield many linear instability routes to turbulence that have scarcely been recognized in the astrophysical community. In combination, they serve to show that magnetic field influences in AGN accretion, jet launch, energy feedback, and overall evolution are still far from completely understood, although many aspects have been disclosed by advanced analytical and numerical relativistic MHD studies.

    Motivation: Astrophysical Jets

    Radio galaxies confront us with dramatic views on energy redistributions at all scales, as mediated by central massive black holes lurking in their nucleus. A clear example is provided by the elliptical galaxy NGC5532, a nearby (red shift z = 0.0237, type S0) galaxy where the stellar distribution is in sharp contrast with its double-jetted appearance in radio images.

  • Where is Population II?

  • J. Mould, F. Bianchini, Duncan A. Forbes, C. L. Reichardt
  • Journal:
    Publications of the Astronomical Society of Australia / Volume 35 / 2018
    Published online by Cambridge University Press:
    28 March 2018, e016
    • Article
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  • The use of roman numerals for stellar populations represents a classification approach to galaxy formation which is now well behind us. Nevertheless, the concept of a pristine generation of stars, followed by a protogalactic era, and finally the mainstream stellar population is a plausible starting point for testing our physical understanding of early star formation. This will be observationally driven as never before in the coming decade. In this paper, we search out observational tests of an idealised coeval and homogeneous distribution of population II stars. We examine the spatial distribution of quasars, globular clusters, and the integrated free electron density of the intergalactic medium, in order to test the assumption of homogeneity. Any real inhomogeneity implies a population II that is not coeval.

  • A Multi-Frequency Study of the Milky Way-Like Spiral Galaxy NGC 6744

  • Miranda Yew, Miroslav D. Filipović, Quentin Roper, Jordan D. Collier, Evan J. Crawford, Thomas H. Jarrett, Nicholas F. H. Tothill, Andrew N. O’Brien, Marko Z. Pavlović, Thomas G. Pannuti, Timothy J. Galvin, Anna D. Kapińska, Michelle E. Cluver, Julie K. Banfield, Eric M. Schlegel, Nigel Maxted, Kevin R. Grieve
  • Journal:
    Publications of the Astronomical Society of Australia / Volume 35 / 2018
    Published online by Cambridge University Press:
    28 March 2018, e015
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  • We present a multi-frequency study of the intermediate spiral SAB(r)bc type galaxy NGC 6744, using available data from the Chandra X-Ray telescope, radio continuum data from the Australia Telescope Compact Array and Murchison Widefield Array, and Wide-field Infrared Survey Explorer infrared observations. We identify 117 X-ray sources and 280 radio sources. Of these, we find nine sources in common between the X-ray and radio catalogues, one of which is a faint central black hole with a bolometric radio luminosity similar to the Milky Way’s central black hole. We classify 5 objects as supernova remnant (SNR) candidates, 2 objects as likely SNRs, 17 as H ii regions, 1 source as an AGN; the remaining 255 radio sources are categorised as background objects and one X-ray source is classified as a foreground star. We find the star-formation rate (SFR) of NGC 6744 to be in the range 2.8–4.7 M⊙~yr − 1 signifying the galaxy is still actively forming stars. The specific SFR of NGC 6744 is greater than that of late-type spirals such as the Milky Way, but considerably less that that of a typical starburst galaxy.

  • Accuracy of Shack–Hartmann wavefront sensor using a coherent wound fibre image bundle

  • Jessica R. Zheng, Michael Goodwin, Jon Lawrence
  • Journal:
    Publications of the Astronomical Society of Australia / Volume 35 / 2018
    Published online by Cambridge University Press:
    26 March 2018, e014
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  • Shack–Hartmannwavefront sensors using wound fibre image bundles are desired for multi-object adaptive optical systems to provide large multiplex positioned by Starbugs. The use of a large-sized wound fibre image bundle provides the flexibility to use more sub-apertures wavefront sensor for ELTs. These compact wavefront sensors take advantage of large focal surfaces such as the Giant Magellan Telescope. The focus of this paper is to study the wound fibre image bundle structure defects effect on the centroid measurement accuracy of a Shack–Hartmann wavefront sensor. We use the first moment centroid method to estimate the centroid of a focused Gaussian beam sampled by a simulated bundle. Spot estimation accuracy with wound fibre image bundle and its structure impact on wavefront measurement accuracy statistics are addressed. Our results show that when the measurement signal-to-noise ratio is high, the centroid measurement accuracy is dominated by the wound fibre image bundle structure, e.g. tile angle and gap spacing. For the measurement with low signal-to-noise ratio, its accuracy is influenced by the read noise of the detector instead of the wound fibre image bundle structure defects. We demonstrate this both with simulation and experimentally. We provide a statistical model of the centroid and wavefront error of a wound fibre image bundle found through experiment.

  • Source Finding in the Era of the SKA (Precursors): Aegean 2.0

  • Paul J. Hancock, Cathryn M. Trott, Natasha Hurley-Walker
  • Journal:
    Publications of the Astronomical Society of Australia / Volume 35 / 2018
    Published online by Cambridge University Press:
    20 March 2018, e011
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  • In the era of the SKA precursors, telescopes are producing deeper, larger images of the sky on increasingly small time-scales. The greater size and volume of images place an increased demand on the software that we use to create catalogues, and so our source finding algorithms need to evolve accordingly. In this paper, we discuss some of the logistical and technical challenges that result from the increased size and volume of images that are to be analysed, and demonstrate how the Aegean source finding package has evolved to address these challenges. In particular, we address the issues of source finding on spatially correlated data, and on images in which the background, noise, and point spread function vary across the sky. We also introduce the concept of forced or prioritised fitting.

  • Pulsars Probe the Low-Frequency Gravitational Sky: Pulsar Timing Arrays Basics and Recent Results

  • Caterina Tiburzi
  • Journal:
    Publications of the Astronomical Society of Australia / Volume 35 / 2018
    Published online by Cambridge University Press:
    20 March 2018, e013
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  • Pulsar Timing Array experiments exploit the clock-like behaviour of an array of millisecond pulsars, with the goal of detecting low-frequency gravitational waves. Pulsar Timing Array experiments have been in operation over the last decade, led by groups in Europe, Australia, and North America. These experiments use the most sensitive radio telescopes in the world, extremely precise pulsar timing models and sophisticated detection algorithms to increase the sensitivity of Pulsar Timing Arrays. No detection of gravitational waves has been made to date with this technique, but Pulsar Timing Array upper limits already contributed to rule out some models of galaxy formation. Moreover, a new generation of radio telescopes, such as the Five hundred metre Aperture Spherical Telescope and, in particular, the Square Kilometre Array, will offer a significant improvement to the Pulsar Timing Array sensitivity. In this article, we review the basic concepts of Pulsar Timing Array experiments, and discuss the latest results from the established Pulsar Timing Array collaborations.

  • Calibration of EFOSC2 Broadband Linear Imaging Polarimetry

  • K. Wiersema, A. B. Higgins, S. Covino, R. L. C. Starling
  • Journal:
    Publications of the Astronomical Society of Australia / Volume 35 / 2018
    Published online by Cambridge University Press:
    20 March 2018, e012
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  • The European Southern Observatory Faint Object Spectrograph and Camera v2 is one of the workhorse instruments on ESO’s New Technology Telescope, and is one of the most popular instruments at La Silla observatory. It is mounted at a Nasmyth focus, and therefore exhibits strong, wavelength and pointing-direction-dependent instrumental polarisation. In this document, we describe our efforts to calibrate the broadband imaging polarimetry mode, and provide a calibration for broadband B, V, and R filters to a level that satisfies most use cases (i.e. polarimetric calibration uncertainty ~0.1%). We make our calibration codes public. This calibration effort can be used to enhance the yield of future polarimetric programmes with the European Southern Observatory Faint Object Spectrograph and Camera v2, by allowing good calibration with a greatly reduced number of standard star observations. Similarly, our calibration model can be combined with archival calibration observations to post-process data taken in past years, to form the European Southern Observatory Faint Object Spectrograph and Camera v2 legacy archive with substantial scientific potential.

  • Resources

  • Yoni Kahn, Princeton University, New Jersey, Adam Anderson
  • Book:
    Conquering the Physics GRE
    Published online:
    16 February 2018
    Print publication:
    01 March 2018, pp xii-xiv

  • Summary

    Here we collect all the texts we recommend and will refer to in the review chapters. If you're wondering why books by Griffiths show up so often, it's likely because he was on the question-writing committee for the Physics GRE several years ago. Anecdotally, we know that questions are recycled very often (which is why so few exams have been released), so it's likely that many of the questions you'll see on your exam were written by Griffiths or consciously modeled after his books.

  • • Classical Mechanics: Whatever book you used for freshman physics should suffice here. For a more in-depth review of advanced topics, try Classical Dynamics of Particles and Systems by S.T. Thornton and J.B. Marion.

  • • Electricity and Magnetism: D.J. Griffiths, Introduction to Electrodynamics. This book covers everything you'll need to know about electricity and magnetism on the GRE, except for circuits. For circuits and a review of the most basic electricity andmagnetism problems, which Griffiths glosses over, consult any standard freshman physics textbook. A good treatment of electromagnetic waves can also be found in R.K. Wangsness, Electromagnetic Fields. E. Purcell, Electricity and Magnetism is an extremely elegant introduction emphasizing physical concepts rather than mathematical formalism, should you need to relearn the basics of any topic. Under no circumstances should you consult Jackson! It's far too advanced for anything you'll need for the GRE.

  • • Optics and Waves: Like classical mechanics, nearly all the relevant information is covered in your freshman physics textbook. Anything you're missing can be found in the relevant chapters of Introduction to Electrodynamics by Griffiths.

  • • Thermodynamics and Statistical Mechanics: No overwhelming recommendation here. Thermal Physics and Elementary Statistical Physics by C. Kittel, or Fundamentals of Statistical and Thermal Physics by F. Reif, are decent. Statistical Physics, by F. Mandl has some decent pedagogy and the nice feature of many problems with worked solutions. Fermi's Thermodynamics is a classic for the most basic aspects of the subject.

  • Preface

  • Yoni Kahn, Princeton University, New Jersey, Adam Anderson
  • Book:
    Conquering the Physics GRE
    Published online:
    16 February 2018
    Print publication:
    01 March 2018, pp ix-x

  • Summary

    Conquering the Physics GRE represents the combined efforts of two MIT graduate students frustrated with the lack of decent preparation materials for the Physics GRE subject test. When we took the exams, in 2007 and 2009, we did what any student in the internet age would do – searched the various online bookstores for “physics GRE prep,” “physics GRE practice tests,” and so on. We were puzzled when the only results were physics practice problems that had nothing to do with the GRE specifically or, worse, GRE practice books having nothing to do with physics. Undeterred, we headed to our local brick-and-mortar bookstores, where we found a similar situation. There were practice books for every single GRE subject exam, except physics. Further web searches unearthed www.grephysics.net, containing every problem and solution from every practice test released up to that point, and www.physicsgre.com, a web forum devoted to discussing problems and strategies for the test. We discovered these sites had sprung up thanks to other frustrated physicists just like us: there was no review material available, so students did the best they could with the meager material that did exist. This situation is particularly acute for students in smaller departments, who have fewer classmates with whom to study and share the “war stories” of the GRE.

    This book endeavors to fix that situation. Its main contribution is a set of three full-length practice tests and fully worked solutions, designed to be as close as possible in style, difficulty, content distribution, and format to the actual GRE exam. We have also included review material for all of the nine content areas on the Physics GRE exam: classical mechanics, electricity and magnetism, optics and waves, thermodynamics and statistical mechanics, quantum mechanics, atomic physics, special relativity, laboratory methods, and specialized topics. To our knowledge, this is the first time that reviews of standard undergraduate subjects such as classical mechanics and thermodynamics have been paired with less standard it material such as laboratory methods in the same text, specifically focused on aspects of these subjects relevant for the GRE. Exam-style practice problems and worked solutions are included for each review chapter, giving over 150 additional GRE-style practice problems in addition to the 300 from the exams. The shorter chapters have review problems at the very end, while the longer ones have review problems distributed throughout the chapter.

  • 8 - Specialized Topics

  • Yoni Kahn, Princeton University, New Jersey, Adam Anderson
  • Book:
    Conquering the Physics GRE
    Published online:
    16 February 2018
    Print publication:
    01 March 2018, pp 146-158

  • Summary

    The Specialized Topics questions on the Physics GRE are probably the most unique aspect of the test. It's hard to think of any other test (other than TV game shows) in which a full 10% is random assorted knowledge. This may seem daunting, but with smart preparation, these questions actually offer a huge advantage.

    The special topics questions are almost entirely pure knowledge recall, otherwise known as fact regurgitation. This is the kind of knee-jerk memorization you probably hated in high-school chemistry or biology. When confronted by a special topics question, you'll either know it or you won't. If you know it, that's one question down in under 10 seconds, which gives you a huge bonus on time for the more difficult calculational questions. If you don't know it, you probably won't be able to figure out the answer just by reasoning through it, and you may waste 5 or more minutes second-guessing yourself when stuck between two equally appealing answer choices. The optimum strategy, then, is to amass a basic knowledge of as many areas of cutting-edge physics as possible, just enough to make the associations between “buzzwords” and concepts that will allow you to recall the required knowledge.

    Luckily, this kind of studying is dead easy. Every couple days, take a break from your normal Physics GRE practice and just read. Pick up a basic textbook in an advanced subject you're unfamiliar with (for example, if you're aiming towards high-energy, choose an introductory solid-state physics or electrical engineering textbook), and don't bother working any problems; just read the book as if it were a novel. You might learn something new and interesting, but that's not really the point: by reading this way, you'll be forming connections and associations in your memory that you might not even be aware of. It's likely you won't be able to remember exactly what you read, but if prompted by a keyword that shows up on the GRE, your memory will spring into action with that feeling of “I've seen this somewhere before.” That's really all you need for these kinds of questions.

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