Academic literature on the topic 'Non-axisymmetric galactic features'
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Journal articles on the topic "Non-axisymmetric galactic features"
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Lesch, Harald. "Galactic Dynamics and Magnetic Field Amplification." Symposium - International Astronomical Union 157 (1993): 395–401. http://dx.doi.org/10.1017/s0074180900174492.
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Stimulated by recent high frequency radio polarization measurements of M83 and M51, we consider the influence of non-axisymmetric features (bars, spiral arms, etc…) on galactic magnetic fields. The time scale for the field amplification due to the non-axisymmetric velocity field is related to the time scale of angular momentum transport in the disk by the non-axisymmetric features. Due to its dissipational character (cooling and angular momentum transport) the gas plays a major role for the excitation of non-axisymmetric instabilities. Since it is the gaseous component of the interstellar gas in which magnetic field amplification takes place we consider the interplay of gasdynamical processes triggered by gravitational instabilities and magnetic fields. A comparison with the time scale for dynamo action in a disk from numerical models for disk dynamos gives the result that field amplification by non-axisymmetric features is faster in galaxies like M83 (strong bar) and M51 (compagnion and very distinct spiral structure), than amplification by an axisymmetric dynamo. Furthermore, we propose that axisymmetric gravitational instabilities may provide the turbulent magnetic diffusivity ηT. Based on standard galaxy models we obtain a radially dependent diffusivity whose numerical value rises from 1025cm2s−1 to 1027cm2s−1, declining for large radii.
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Donner, Karl Johan, Axel Brandenburg, and Magnus Thomasson. "Galactic Dynamos and Dynamics." Symposium - International Astronomical Union 157 (1993): 333–37. http://dx.doi.org/10.1017/s0074180900174376.
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We discuss some aspects of the interrelationship between the dynamo problem for galaxies and their dynamics. First, we consider the generation of magnetic fields in the presence of fountain flows and galactic winds. Next, we discuss the distortion of a steady magnetic field by tidal effects and other transient spiral features. Finally, we give an expression for the amplitude of density waves generated by large-scale non-axisymmetric fields.
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Ghosh, S., and C. J. Jog. "Effect of finite disk-thickness on swing amplification of non-axisymmetric perturbations in a sheared galactic disk." Astronomy & Astrophysics 617 (September 2018): A47. http://dx.doi.org/10.1051/0004-6361/201832988.
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A typical galactic disk is observed to have a finite thickness. Here, we present the study of the physical effect of introduction of finite thickness on the generation of small-scale spiral arms by swing amplification in a differentially rotating galactic disk. The galactic disk is modelled first as a one-fluid system, and then as a gravitationally-coupled two-fluid (stars and gas) system where each fluid is taken as isothermal, and corotating with each other. We derived the equations governing the evolution of the non-axisymmetric perturbations in a sheared frame of reference while incorporating the effect of finite thickness of a galactic disk. We found that the finite thickness of a galactic disk has a generic trend of suppressing the growth of the non-axisymmetric perturbations via swing amplification. Moreover, even the observed range of disk-thickness values (∼300–500 pc) can lead to a complete suppression of swing amplification for Q ∼ 1.7, whereas for an infinitesimally-thin disk, the corresponding critical value is Q ∼ 2. For a two-fluid (stars and gas) system, the net amplification is shown to be set by the mutual interplay of the effect of interstellar gas in promoting the spiral features and the effect of finite thickness in preventing the spiral arms. The coexistence of these two opposite effects is shown to be capable of giving rise to diverse and complex dynamical behaviour.
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Kakiuchi, Kensuke, Takeru K. Suzuki, Yasuo Fukui, Kazufumi Torii, Mami Machida, and Ryoji Matsumoto. "Vertical flows and structures excited by magnetic activity in the Galactic center region." Proceedings of the International Astronomical Union 11, S322 (July 2016): 220–21. http://dx.doi.org/10.1017/s174392131601190x.
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AbstractVarious observations show peculiar features in the Galactic Center region, such as loops and filamentary structure. It is still unclear how such characteristic features are formed. Magnetic field is believed to play very important roles in the dynamics of gas in the Galaxy Center. Suzuki et al. (2015) performed a global magneto-hydrodynamical simulation focusing on the Galactic Center with an axisymmetric gravitational potential and claimed that non-radial motion is excited by magnetic activity. We further analyzed their simulation data and found that vertical motion is also excited by magnetic activity. In particular, fast down flows with speed of ~100 km/s are triggered near the footpoint of magnetic loops that are buoyantly risen by Parker instability. These downward flows are accelerated by the vertical component of the gravity, falling along inclined field lines. As a result, the azimuthal and radial components of the velocity are also excited, which are observed as high velocity features in a simulated position-velocity diagram. Depending on the viewing angle, these fast flows will show a huge variety of characteristic features in the position-velocity diagram.
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Beck, Rainer, Luke Chamandy, Ed Elson, and Eric G. Blackman. "Synthesizing Observations and Theory to Understand Galactic Magnetic Fields: Progress and Challenges." Galaxies 8, no. 1 (December 21, 2019): 4. http://dx.doi.org/10.3390/galaxies8010004.
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Constraining dynamo theories of magnetic field origin by observation is indispensable but challenging, in part because the basic quantities measured by observers and predicted by modelers are different. We clarify these differences and sketch out ways to bridge the divide. Based on archival and previously unpublished data, we then compile various important properties of galactic magnetic fields for nearby spiral galaxies. We consistently compute strengths of total, ordered, and regular fields, pitch angles of ordered and regular fields, and we summarize the present knowledge on azimuthal modes, field parities, and the properties of non-axisymmetric spiral features called magnetic arms. We review related aspects of dynamo theory, with a focus on mean-field models and their predictions for large-scale magnetic fields in galactic discs and halos. Furthermore, we measure the velocity dispersion of H i gas in arm and inter-arm regions in three galaxies, M 51, M 74, and NGC 6946, since spiral modulation of the root-mean-square turbulent speed has been proposed as a driver of non-axisymmetry in large-scale dynamos. We find no evidence for such a modulation and place upper limits on its strength, helping to narrow down the list of mechanisms to explain magnetic arms. Successes and remaining challenges of dynamo models with respect to explaining observations are briefly summarized, and possible strategies are suggested. With new instruments like the Square Kilometre Array (SKA), large data sets of magnetic and non-magnetic properties from thousands of galaxies will become available, to be compared with theory.
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Moreno, Edmundo, José G. Fernández-Trincado, William J. Schuster, Angeles Pérez-Villegas, and Leonardo Chaves-Velasquez. "Effect of orbital trapping by bar resonances in the local U–V velocity field." Monthly Notices of the Royal Astronomical Society 506, no. 4 (July 6, 2021): 4687–701. http://dx.doi.org/10.1093/mnras/stab1908.
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ABSTRACT The effects in the local U–V velocity field due to orbital trapping by bar resonances have been studied computing 15 resonant families in a non-axisymmetric Galactic potential, considering the bar’s angular velocity between 35 and 57.5 ${\rm \, km\, s^{-1}{kpc}^{-1}}$. Only cases in the low, 37.5, 40 ${\rm \, km\, s^{-1}{kpc}^{-1}}$, and high, 55, 57.5 ${\rm \, km\, s^{-1}{kpc}^{-1}}$, velocity ranges give trapping structures that have some similarity with observed features in the velocity distribution. The resulting structures in the local U–V plane form resonant bands appearing at various levels in velocity V. Cases with angular velocity 40 and 55 ${\rm \, km\, s^{-1}{kpc}^{-1}}$ show the greatest similarity with observed branches. Our best approximation to the local velocity field by orbital trapping is obtained with a bar angular velocity of 40 ${\rm \, km\, s^{-1}{kpc}^{-1}}$ and a bar angle of 40°. With this solution, three main observed features can be approximated: (i) the Hercules branch at V = $-50\, {\rm \, km\, s^{-1}}$ produced by the resonance 8/1 outside corotation, and the close features produced by resonances 5/1 and 6/1, (ii) the newly detected low-density arch at V ≃ 40 ${\rm \, km\, s^{-1}}$ produced approximately by the resonance 4/3, (iii) the inclined structure below the Hercules branch, also observed in the Gaia DR2 data, produced by tube orbits around Lagrange point L5 at corotation. Some predicted contributions due to orbital trapping in regions of the U–V plane corresponding to the Galactic halo are given, which could help to further restrict the value of the angular velocity of the Galactic bar. No support by orbital trapping is found for the Arcturus stream at V$\approx \, -100\, {\rm \, km\, s^{-1}}$.
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Solomon, P. M. "Giant Molecular Clouds in the Galaxy: The Massachusetts-Stony Brook CO Galactic Plane Survey." Symposium - International Astronomical Union 115 (1987): 499. http://dx.doi.org/10.1017/s0074180900096297.
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The CO Galactic Plane Survey consists of 40,572 spectral line observations in the region between 1 = 8° to 90° and b = −1°.05 to +1°.05 spaced every 3 arc minutes, carried out with the FCRAO 14-m antenna. The velocity coverage from −100 to +200 km/s includes emission from all galactic radii. This high resolution survey was designed to observe and identify essentially all molecular clouds or cloud components larger than 10 parsecs in the inner galaxy. There are two populations of molecular clouds which separate according to temperature. The warm clouds are closely associated with H II regions, exhibit a non-axisymmetric galactic distribution and are a spiral arm population. The cold clouds are a disk population, are not confined to any patterns in longitude-velocity space and must be widespread in the galaxy both in and out of spiral arms. The correlation between far infrared luminosities from IRAS, and molecular masses from CO is utilized to determine a luminosity to mass ratio for the clouds. A face-on picture of the galaxy locating the warm population is presented, showing ring like or spiral arm features at R ∼ 5, 7.5 and 9 kpc. The cloud size and mass spectrum will be discussed and evidence presented showing the presence of clusters of giant molecular clouds with masses of 106 to 107 M⊙. The two populations of clouds probably have different star forming luminosity functions. The implication of the two populations for star formation mechanisms will be discussed.
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MELIANI, ZAKARIA, RONY KEPPENS, and CHRISTOPHE SAUTY. "TWO-COMPONENT JETS AND THE FANAROFF–RILEY DICHOTOMY." International Journal of Modern Physics D 19, no. 06 (June 2010): 867–72. http://dx.doi.org/10.1142/s0218271810017056.
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Transversely stratified jets are observed in many classes of astrophysical objects, ranging from young stellar objects, μ-quasars, to active galactic nuclei and even in gamma-ray bursts. Theoretical arguments support this transverse stratification of jets with two components induced by intrinsic features of the central engine (accretion disk + black hole). In fact, according to the observations and theoretical models, a typical jet has an inner fast low density jet, surrounded by a slower, denser, extended jet. We elaborate on this model and investigate for the first time this two-component jet evolution with very high resolution in 3D. We demonstrate that two-component jets with a high kinetic energy flux contribution from the inner jet are subject to the development of a relativistically enhanced, rotation-induced Rayleigh–Taylor type non-axisymmetric instability. This instability induces–strong mixing between both components, decelerating the inner jet and leading to overall jet decollimation. This novel scenario of sudden jet deceleration and decollimation can explain the radio source Fanaroff–Riley dichotomy as a consequence of the efficiency of the central engine in launching the inner jet component versus the outer jet component. We infer that the FRII/FRI transition, interpreted in our two-component jet scenario, occurs when the relative kinetic energy flux of the inner to the outer jet exceeds a critical ratio.
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Tucker, M. A., B. J. Shappee, J. T. Hinkle, J. M. M. Neustadt, M. Eracleous, C. S. Kochanek, J. L. Prieto, et al. "An AMUSING look at the host of the periodic nuclear transient ASASSN-14ko reveals a second AGN." Monthly Notices of the Royal Astronomical Society 506, no. 4 (July 22, 2021): 6014–28. http://dx.doi.org/10.1093/mnras/stab2085.
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ABSTRACT We present Multi-Unit Spectroscopic Explorer (MUSE) integral-field spectroscopy of ESO 253−G003, which hosts a known active galactic nucleus (AGN) and the periodic nuclear transient ASASSN-14ko, observed as part of the All-weather MUse Supernova Integral-field of Nearby Galaxies survey. The MUSE observations reveal that the inner region hosts two AGN separated by $1.4\pm 0.1~\rm {kpc}$ (≈1${_{.}^{\prime\prime}}$7). The brighter nucleus has asymmetric broad permitted emission-line profiles and is associated with the archival AGN designation. The fainter nucleus does not have a broad emission-line component but exhibits other AGN characteristics, including $\hbox{$v_{\rm {FWHM}}$} \approx 700~\hbox{km~s$^{-1}$}$ forbidden line emission, $\rm{\log _{10}(\rm{[O\,\small {III}]}/\rm{H\beta})} \approx 1.1$, and high-excitation potential emission lines, such as [Fe vii] λ6086 and He ii λ4686. The host galaxy exhibits a disturbed morphology with large kpc-scale tidal features, potential outflows from both nuclei, and a likely superbubble. A circular relativistic disc model cannot reproduce the asymmetric broad emission-line profiles in the brighter nucleus, but two non-axisymmetric disc models provide good fits to the broad emission-line profiles: an elliptical disc model and a circular disc + spiral arm model. Implications for the periodic nuclear transient ASASSN-14ko are discussed.
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Masters, Karen L., Coleman Krawczyk, Shoaib Shamsi, Alexander Todd, Daniel Finnegan, Matthew Bershady, Kevin Bundy, et al. "Galaxy Zoo: 3D – crowdsourced bar, spiral, and foreground star masks for MaNGA target galaxies." Monthly Notices of the Royal Astronomical Society 507, no. 3 (August 10, 2021): 3923–35. http://dx.doi.org/10.1093/mnras/stab2282.
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ABSTRACT The challenge of consistent identification of internal structure in galaxies – in particular disc galaxy components like spiral arms, bars, and bulges – has hindered our ability to study the physical impact of such structure across large samples. In this paper we present Galaxy Zoo: 3D (GZ:3D) a crowdsourcing project built on the Zooniverse platform that we used to create spatial pixel (spaxel) maps that identify galaxy centres, foreground stars, galactic bars, and spiral arms for 29 831 galaxies that were potential targets of the MaNGA survey (Mapping Nearby Galaxies at Apache Point Observatory, part of the fourth phase of the Sloan Digital Sky Surveys or SDSS-IV), including nearly all of the 10 010 galaxies ultimately observed. Our crowdsourced visual identification of asymmetric internal structures provides valuable insight on the evolutionary role of non-axisymmetric processes that is otherwise lost when MaNGA data cubes are azimuthally averaged. We present the publicly available GZ:3D catalogue alongside validation tests and example use cases. These data may in the future provide a useful training set for automated identification of spiral arm features. As an illustration, we use the spiral masks in a sample of 825 galaxies to measure the enhancement of star formation spatially linked to spiral arms, which we measure to be a factor of three over the background disc, and how this enhancement increases with radius.
Dissertations / Theses on the topic "Non-axisymmetric galactic features"
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Chakrabarty, Dalia. "The solar neighbourhood and centre of the Milky Way." Thesis, University of Oxford, 2000. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.246400.
Full textConference papers on the topic "Non-axisymmetric galactic features"
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Zozulia, V. D. "Galactic bars: a look at the point of view of action variables." In Всероссийская с международным участием научная конференция студентов и молодых ученых, посвященная памяти Полины Евгеньевны Захаровой «Астрономия и исследование космического пространства». Ural University Press, 2021. http://dx.doi.org/10.15826/b978-5-7996-3229-8.05.
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We have translated the results of bar modeling in N-body simulations into the language of action variables JR, Lz and Jz. We used the AGAMA package and the formalism associated with the Steckel potential. Action variables were calculated in the changing non axisymmetric potential of the model at different moments of the bar evolution. We found that at the later stages of the bar evolution, the maximum values of JR are reached at its ends, after which, upon going to the disk, JR decreases monotonically. As for Jz, the maximum values of Jz actually outline the B/PS bulge, the thickest part of the bar. Such a behavior of the action variables can be used for tests to identify the features of the bar of our Galaxy based on GAIA data.