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24465 results in Cosmology, Relativity and Gravitation

Page 208 of 1224

  • A wide and collimated radio jet in 3C 84

  • Gabriele Giovannini
  • Journal:
    Proceedings of the International Astronomical Union / Volume 14 / Issue S342 / May 2018
    Published online by Cambridge University Press:
    07 April 2020, pp. 55-60
    Print publication:
    May 2018
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  • I report observations of 3C84 (NGC 1275), the central galaxy of the Perseus cluster, made with an interferometric array including the orbiting radio telescope of the RadioAstron mission. The data transversely resolve the edge-brightened jet in 3C84 only 0.03 mas from the core, which is ten times closer to the central engine than was possible in previous ground-based observations. To better understand physical properties of this peculiar source, I will discuss its present properties in comparison to the past and more recent evolution in the radio band of 3C 84.

  • Expanding Radio Lobe associated with 3C 84

  • Keiichi Asada, Makoto Inoue, Hiroshi Nagai, Seiji Kameno
  • Journal:
    Proceedings of the International Astronomical Union / Volume 14 / Issue S342 / May 2018
    Published online by Cambridge University Press:
    07 April 2020, pp. 220-221
    Print publication:
    May 2018
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  • Following the discovery of the expansion of a young radio lobe associated with the bright radio source 3C 84 in the Seyfert galaxy NGC 1275 using VSOP monitoring observation with duration of 3 years, we further investigated its evolution by utilizing VLBA monitoring data taken by MOJAVE project. As the results, we confirmed the expanding motion of both the southern and northern lobes, and detected side-way expanding motions for the first time. In meantime, the total flux density of the lobes are decreasing, while total flux density associated with the nuclei is significantly increasing. This cooling of the radio lobe can be well-explained with the energy loss through the adiabatic cooling.

  • Hints of radio sources evolution

  • Pietro Cassaro
  • Journal:
    Proceedings of the International Astronomical Union / Volume 14 / Issue S342 / May 2018
    Published online by Cambridge University Press:
    07 April 2020, pp. 237-238
    Print publication:
    May 2018
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  • An understanding of the mechanisms that regulate the Active Galactic Nuclei passes through the study of their early life stages, observable in Compact Symmetric Objects. To this purpose, a study was carried out on two compact radio sources, based on data from the VLBA archive at different times and frequencies. The results are compatible with an intermediate scenario between the two main theories about these objects: frustration and youth.

  • Black hole mass measurements in AGN: Polarization in broad emission lines

  • Luka Č. Popović, Victor L. Afanasiev, Djordje Savić
  • Journal:
    Proceedings of the International Astronomical Union / Volume 14 / Issue S342 / May 2018
    Published online by Cambridge University Press:
    07 April 2020, pp. 1-4
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    May 2018
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  • We present a new method for supermassive black hole (SMBH) mass measurements in Type 1 active galactic nuclei (AGN) using polarization angle across broad lines. This method gives measured masses which are in a good agreement with reverberation estimates. Additionally, we explore the possibilities and limits of this method using the STOKES radiative transfer code taking a dominant Keplerian motion in the broad line region (BLR). We found that this method can be used for the direct SMBH mass estimation in the cases when in addition to the Kepler motion, radial inflows or vertical outflows are present in the BLR. Some advantages of the method are discussed.

  • Infrared Diagnostics of the ISM in the Circumnuclear Environments of the Youngest Radio Galaxies

  • E. Kosmaczewski, Ł. Stawarz, A. Wójtowicz
  • Journal:
    Proceedings of the International Astronomical Union / Volume 14 / Issue S342 / May 2018
    Published online by Cambridge University Press:
    07 April 2020, pp. 268-269
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    May 2018
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  • Here we present a systematic analysis of the mid-infrared properties of young radio galaxies, based on lower-resolution data provided by WISE and IRAS satellites. We restrict our analysis to sources in the earliest phase of radio galaxy evolution, with corresponding ages of the radio structures ≤ 3,000 yrs. In our sample of 29 objects, we find a variety of WISE colors, which suggests that the mid-infrared continua of studied sources are not exclusively contributed to by the circumnuclear dust. A comparison of the total mid-infrared and absorption-corrected X-ray luminosities for our sample reveals a clear correlation between the two bands. This favors the scenario in which the observed X-ray emission of young radio galaxies — at least the high-luminosity ones — originates predominantly in accretion disk corona.

  • The correlation between the total jet power and the Poynting flux at the jet base

  • Elena E. Nokhrina
  • Journal:
    Proceedings of the International Astronomical Union / Volume 14 / Issue S342 / May 2018
    Published online by Cambridge University Press:
    07 April 2020, pp. 197-200
    Print publication:
    May 2018
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  • The magneto hydrodynamic models of relativistic jets from active galactic nuclei predict the jet power transported by the Poynting flux at the jet base, setting the correlation between the jet power and the total magnetic flux. For highly collimated jets taking the transversal structure into account allows to rewrite this correlation through the observed jet properties such as spectral flux and core shift. Applying this method we find that, for the sample of 48 sources, their jet power distribution is well peaked at the theoretically predicted level.

  • AGN Jets, Bubbles, and Heat Pumps

  • Yi-Hao Chen, Sebastian Heinz
  • Journal:
    Proceedings of the International Astronomical Union / Volume 14 / Issue S342 / May 2018
    Published online by Cambridge University Press:
    07 April 2020, pp. 149-153
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    May 2018
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  • Radio-mode feedback from relativistic jets is one of the prominent heating mechanisms in clusters of galaxies. We present a long-term evolution of high-resolution MHD simulation of jets interacting with an environment modeled to represent the Perseus cluster. We investigate the thermodynamics of the ICM due to the gas motion triggered by the action of the jets and show that low-entropy gas is lifted efficiently in the wake of the inflating radio lobe. We look into the uplift mechanism and estimate the energy budget and the rate of thermal conduction. The redistribution of entropy suggests that heat conduction can play a more significant role in the thermal evolution of the cluster core in the presence of jets, which act effectively as a heat pump, thus heating the ICM more efficiently than jets would by themselves in an isentropic cluster.

  • Numerical methods for General Relativistic particles

  • Fabio Bacchini, Bart Ripperda, Alexander Y. Chen, Lorenzo Sironi
  • Journal:
    Proceedings of the International Astronomical Union / Volume 14 / Issue S342 / May 2018
    Published online by Cambridge University Press:
    07 April 2020, pp. 19-23
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    May 2018
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  • We present recent developments on numerical algorithms for computing photon and particle trajectories in the surrounding of compact objects. Strong gravity around neutron stars or black holes causes relativistic effects on the motion of massive particles and distorts light rays due to gravitational lensing. Efficient numerical methods are required for solving the equations of motion and compute i) the black hole shadow obtained by tracing light rays from the object to a distant observer, and ii) obtain information on the dynamics of the plasma at the microscopic scale. Here, we present generalized algorithms capable of simulating ensembles of photons or massive particles in any spacetime, with the option of including external forces. The coupling of these tools with GRMHD simulations is the key point for obtaining insight on the complex dynamics of accretion disks and jets and for comparing simulations with upcoming observational results from the Event Horizon Telescope.

  • High-dynamic-range 21 cm JVLA observations of the Perseus Cluster

  • Charles L. H. Hull
  • Journal:
    Proceedings of the International Astronomical Union / Volume 14 / Issue S342 / May 2018
    Published online by Cambridge University Press:
    07 April 2020, pp. 53-54
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    May 2018
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  • We present 21 cm (L-band) JVLA observations of the Perseus Cluster, comprising data from all four array configurations. The resulting images are nearly an order of magnitude deeper than any other image made of NGC 1275 and its environs at this frequency.

Cosmic Magnetic Fields
  • Edited by Jorge Sánchez Almeida, María Jesús Martínez González
  • Published online:
    21 April 2018
    Print publication:
    12 April 2018
  • Magnetic fields pervade the universe and play an important role in many astrophysical processes. However, they require specialised observational tools, and are challenging to model and understand. This volume provides a unified view of magnetic fields across astrophysical and cosmological contexts, drawing together disparate topics that are rarely covered together. Written by the lecturers of the XXV Canary Islands Winter School, it offers a self-contained introduction to cosmic magnetic fields on a range of scales. The connections between the behaviours of magnetic fields in these varying contexts are particularly emphasised, from the relatively small and close ranges of the Sun, planets and stars, to galaxies and clusters of galaxies, as well as on cosmological scales. Aimed at young researchers and graduate students, this up-to-date review uniquely brings together a subject often tackled by disconnected communities, conveying the latest advances as well as highlighting the limits of our current understanding.

  • 3 - Stellar Magnetic Fields

  • 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 47-86

  • Summary

    Abstract

    Recent developments in astrophysical spectropolarimetry have significantly expanded our understanding of stellar magnetism. Using modern instrumentation and data analysis methods it is becoming possible to detect and characterise surface magnetic fields not just for a few peculiar objects but for essentially all types of stars, ranging from massive OB stars to brown dwarfs. The intention of this chapter is to give a general overview of the stellar magnetic field diagnostic methods and to present results of the application of these methods to different classes of stars. First, the chapter discusses the physical processes that lead to appearance of magnetic signatures in stellar spectra. Based on this discussion, the most commonly used magnetic observables are introduced for the intensity and polarisation spectra. The methods of interpretation of magnetic measurements, starting with robust field detection techniques and leading to reconstruction of the physically realistic vector maps of surface magnetic fields, are discussed. Finally, the current knowledge about the incidence, geometries and the origin of magnetic fields in stars of different masses and ages is reviewed and the astrophysical significance of magnetism in the context of stellar structure and evolution is discussed.

    Introduction

    This chapter presents an introduction to the studies of stellar magnetic fields. We explore the questions of how magnetic fields can be detected on the stellar surfaces, what information about magnetic field topologies can be extracted from different observations, and what are the observed properties of magnetic fields in stars of different masses and ages. The origin of magnetism in different types of stars and its relation to other physical processes operating in stellar interiors and atmospheres are briefly discussed.

    The following sections start by presenting the essential theoretical foundations of the stellar magnetic field diagnostic and modelling. We consider the Zeeman effect in spectral lines and information contained in the polarisation characteristics of stellar radiation. A connection between the local magnetic field parameters and morphology of the stellar polarisation profiles is established using both detailed numerical calculations and simplified analytical considerations. Based on this discussion, we introduce different methodologies suitable for the detection and characterisation of stellar magnetic fields.

  • Preface

  • 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 xiii-xiii

  • Summary

    Magnetic fields play an important role in many astrophysical processes. They are difficult to detect and characterize because often their properties have to be inferred through interpreting the polarization of the light. Magnetic fields are also challenging to model and understand. Magnetized plasmas behave following highly non-linear differential equations having no general solution, so that every astrophysical problem represents a special case to be studied independently. Hence, magnetic fields are often an inconvenient subject that is overlooked or simply neglected (the elephant in the room, as they are dubbed on posters in the XXV Canary Islands Winter School of Astrophysics). Such a difficulty burdens the research on magnetic fields, which has evolved to become a very technical subject, with many small disconnected communities studying specific aspects and details. The school tried to amend the situation by providing a unifying view of the subject. The students had a chance to understand the behavior of magnetic fields in all astrophysical contexts, from cosmology to the Sun, and from starbursts to AGNs. The school was planned to present a balanced yet complete review of our knowledge, with excursions into the unknown to point out present and future lines of research.

    The subject of Cosmic Magnetic Fields was split into seven different topics: cosmic magnetic field essentials, solar magnetic fields, stellar magnetic fields, the role of magnetic fields on AGN feedback, magnetic fields in galaxies, magnetic fields in galaxy clusters and at larger scales, and primordial magnetic fields and magnetic fields in the early Universe. The corresponding lectures were delivered by seven well known and experienced scientists that have played key roles in the major advances of the field during the last years: F. Cattaneo, P. Judge, O. Kochukhov, R. Keppens, R. Beck, K. Dolag, and F. Finelli. Their lectures were recorded and are freely available at the IAC website: http://iactalks.iac.es/talks/serie/19. Together with the reviews included in the present volume, they form a unique resource for both students and professional researchers. They provide a global view of this very compartmentalized, yet fundamental, field of research.

  • 5 - Magnetic Fields in Galaxies

    • By Rainer Beck, Max Planck Institut für Radioastronomie, Germany
  • 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 123-160

  • Summary

    Introduction

    Magnetism is a fundamental force with special properties.

  • • Most baryonic matter is ionized. Magnetic fields are easy to generate.

  • • Magnetic monopoles do not exist or are extremely rare. Magnetic fields are hard to destroy.

  • • Magnetic fields need illumination. Magnetic fields are difficult to observe.

    • The scarceness of data leaves many questions on cosmic magnetic fields.

  • • When and how were the first fields generated?

  • • Did significant fields exist before galaxies formed?

  • • How and how fast were the fields amplified?

  • • How did fields affect the evolution of stars, planets, galaxies and galaxy clusters?

  • • How strongly is intergalactic space magnetized?

    • Magnetic fields are often ignored in astrophysics, in particular in models of galaxies and the interstellar medium (ISM), although they are a major agent in the ISM and in galaxy halos, and important for the structure and evolution of galaxies.

  • • Magnetic fields contribute significantly to the total pressure which balances the gas disk of galaxies against gravitation (Fletcher & Shukurov, 2001).

  • • Magnetic turbulence distributes energy from supernova explosions within the ISM (Subramanian, 1998).

  • • Magnetic reconnection is a possible heating source for the ISM and halo gas (Birk et al., 1998).

  • • Magnetic fields increase angular momentum transport and hence the gas inflow rate in barred galaxies (Beck et al., 2005; Kim & Stone, 2012).

  • • Magnetic fields affect the dynamics of the turbulent ISM (de Avillez & Breitschwerdt, 2005) and the gas flows in spiral arms (Gómez & Cox, 2002).

  • • Magnetic fields make the gaseous spiral arms more patchy and drive gas outflows into the halo (Pakmor & Springel, 2013).

  • • The shock strength in spiral density waves is decreased and structure formation is reduced in the presence of strong fields (Dobbs & Price, 2008; Fletcher et al., 2011).

  • • Magnetic fields stabilize gas clouds and reduce the star-formation efficiency to the observed low values (Vázquez-Semadeni et al., 2005; Price & Bate, 2008).

  • • Magnetic fields are essential for the onset of star formation as they enable the removal of angular momentum from the protostellar cloud via ambipolar diffusion (Heitsch et al., 2004).

  • 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.

Page 208 of 1224