scholarly journals Filaments in the Galactic Centre—with Special Reference to the ‘Snake’

2001 ◽  
Vol 18 (4) ◽  
pp. 431-442 ◽  
Author(s):  
Geoffrey V. Bicknell ◽  
Jianke Li
Keyword(s):  
Special Reference ◽  
Observational Data ◽  
Resonant Scattering ◽  
Alfven Waves ◽  
The Other ◽  
Alfvén Waves ◽  
Galactic Centre ◽  
Relativistic Electrons ◽  
The Galaxy ◽  
Centre Region

AbstractThe non-thermal filaments in the Galactic centre constitute one of the great mysteries of this region of the Galaxy. We summarise the observational data on these filaments and critically review the various theories which currently outnumber the observed filaments. We summarise our theory for the longest of these filaments, the Snake, and discuss the relevance of this model for the other filaments in the Galactic centre region. The physics involved in our model for the Snake involves much of the physics that has dominated the career of Professor Don Melrose. In particular, the diffusion of relativistic electrons in the Snake is determined from the theory of resonant scattering by Alfvén waves.

Interstellar gas in the central region of the Galaxy

1967 ◽  
Vol 31 ◽  
pp. 239-251 ◽  
Author(s):  
F. J. Kerr
Keyword(s):  
Central Region ◽  
Galactic Plane ◽  
Neutral Hydrogen ◽  
Continuum Emission ◽  
Galactic Centre ◽  
Interstellar Gas ◽  
Hydrogen Recombination ◽  
The Galaxy ◽  
Centre Region ◽  
The Relationship

A review is given of information on the galactic-centre region obtained from recent observations of the 21-cm line from neutral hydrogen, the 18-cm group of OH lines, a hydrogen recombination line at 6 cm wavelength, and the continuum emission from ionized hydrogen.Both inward and outward motions are important in this region, in addition to rotation. Several types of observation indicate the presence of material in features inclined to the galactic plane. The relationship between the H and OH concentrations is not yet clear, but a rough picture of the central region can be proposed.

scholarly journals Reacceleration of Relativistic Electrons by Turbulent Alfvén Waves in Radio Jets

2000 ◽  
Vol 195 ◽  
pp. 439-441
Author(s):  
D.-Y. Wang ◽  
Y. Ma
Keyword(s):  
Electron Energy ◽  
Power Law ◽  
Diffusion Approximation ◽  
Alfven Waves ◽  
Alfvén Waves ◽  
Relativistic Electrons ◽  
Radio Jets

Relativistic electrons may be effectively accelerated by turbulent Alfvén waves in radio jets. The acceleration spectrum is a power law with the electron energy as high as γ ~ 106, but the spectrum index is ~ 1.2 in the condition of diffusion approximation, which is less than the observation value.

scholarly journals A new distance to the Brick, the dense molecular cloud G0.253+0.016

2021 ◽  
Vol 502 (1) ◽  
pp. 1246-1252
Author(s):  
M Zoccali ◽  
E Valenti ◽  
F Surot ◽  
O A Gonzalez ◽  
A Renzini ◽  
...  
Keyword(s):  
Star Formation ◽  
Molecular Cloud ◽  
Near Infrared ◽  
Formation Rate ◽  
Dynamical Evolution ◽  
Galactic Centre ◽  
The Galaxy ◽  
Centre Region ◽  
Order Of Magnitude ◽  
Red Clump

ABSTRACT We analyse the near-infrared colour–magnitude diagram of a field including the giant molecular cloud G0.253+0.016 (a.k.a. The Brick) observed at high spatial resolution, with HAWK-I@VLT. The distribution of red clump stars in a line of sight crossing the cloud, compared with that in a direction just beside it, and not crossing it, allow us to measure the distance of the cloud from the Sun to be 7.20, with a statistical uncertainty of ±0.16 and a systematic error of ±0.20 kpc. This is significantly closer than what is generally assumed, i.e. that the cloud belongs to the near side of the central molecular zone, at 60 pc from the Galactic centre. This assumption was based on dynamical models of the central molecular zone, observationally constrained uniquely by the radial velocity of this and other clouds. Determining the true position of the Brick cloud is relevant because this is the densest cloud of the Galaxy not showing any ongoing star formation. This puts the cloud off by one order of magnitude from the Kennicutt–Schmidt relation between the density of the dense gas and the star formation rate. Several explanations have been proposed for this absence of star formation, most of them based on the dynamical evolution of this and other clouds, within the Galactic centre region. Our result emphasizes the need to include constraints coming from stellar observations in the interpretation of our Galaxy’s central molecular zone.

scholarly journals Spectral Line Observations of the Galactic Centre Region

1977 ◽  
Vol 45 ◽  
pp. 119-120
Author(s):  
R. D. Davies ◽  
R. J. Cohen
Keyword(s):  
Velocity Field ◽  
Spectral Line ◽  
Angular Resolution ◽  
Spectral Lines ◽  
Galactic Centre ◽  
Radio Telescopes ◽  
Gas Distribution ◽  
The Galaxy ◽  
Central Regions ◽  
Centre Region

An investigation of the central regions of the Galaxy has been made with an angular resolution of ~10 arcmin with the radio telescopes at Jodrell Bank using the spectral lines of HI (λ21 cm), OH (λ18 cm) and H2CO (λ6 cm). Observations of radio recombination lines in the range (λ21 to 125 cm) have also been taken. These data taken together provide information on the velocity field and gas distribution in the galactic centre region. A continuing programme of spectral line observations of the galactic centre is being pursued at Jodrell Bank.

scholarly journals Structure and Kinematical Properties of the Galaxy at Intermediate Galactic Latitudes

1998 ◽  
Vol 179 ◽  
pp. 221-222
Author(s):  
D. K. Ojha ◽  
O. Bienaymé ◽  
A. C. Robin
Keyword(s):  
Vertical Structure ◽  
Local Density ◽  
Thick Disk ◽  
Sample Survey ◽  
Galactic Centre ◽  
Proper Motions ◽  
Perfect Agreement ◽  
The Galaxy ◽  
Centre Region ◽  
Disk Component

We have carried out a sample survey in UBVR photometry and proper motions in various directions in the Galaxy. Three fields in the direction of galactic anticentre, centre, and antirotation have been surveyed. Using our new data together with wide-area surveys in other fields available to date, we discuss the radial and vertical structure of the Galaxy. Our results confirm that the thick disk population is distinct from other populations based on their kinematical and spatial distribution. The most probable value of scale height for the thick disk component is determined to be hz≃760±50 pc and a local density of ≃7.4+2.5−1.5% relative to the thin disk. The ratio of the number of thick disk stars in our galactic centre region to that in anticentre region yield hR≃3±1 kpc for the scale length of thick disk. These values are in perfect agreement with the recent determination given by Robin et al. (1996).

scholarly journals A Survey of OH in the Galactic Centre Region

1996 ◽  
Vol 169 ◽  
pp. 311-316
Author(s):  
P.J. Boyce ◽  
R. J. Cohen
Keyword(s):  
Central Region ◽  
Molecular Clouds ◽  
Radio Continuum ◽  
Continuum Emission ◽  
Galactic Centre ◽  
Molecular Gas ◽  
The Galaxy ◽  
Centre Region ◽  
Line Survey ◽  
The Continuum

The galactic centre contains the largest concentration of molecular clouds in the Galaxy. The clouds in the central region are unusual in having large linewidths and masses, and large non-circular motions. Previous surveys of their distribution in the central region have been carried out in OH (Robinson & McGee 1970; Cohen & Few 1976), H2CO (Whiteoak & Gardner 1979; Cohen & Few 1981), CO (Bania 1977; Dame et al. 1987; Bally et al. 1987, 1988) and CS (Bally et al. 1987, 1988). The OH groundstate lines at 18cm wavelength have certain advantages for such a survey. The OH lines appear in absorption against the galactic centre continuum sources, and against the continuum emission from the disk of the Galaxy. The absorption spectra are sensitive to relatively small molecular column densities. In addition they can give information on the relative positions of the molecular gas and the radio continuum sources. This paper describes results from an absorption line survey of the galactic centre region in the OH main lines at 1667.359 MHz and 1665.402 MHz (Boyce & Cohen 1994).

scholarly journals Possible role of magnetosphere-ionosphere coupling in auroral arc generation

2000 ◽  
Vol 18 (9) ◽  
pp. 1108-1117 ◽  
Author(s):  
W. Lyatsky ◽  
A. M. Hamza
Keyword(s):  
Growth Rate ◽  
Plasma Sheet ◽  
Alfven Waves ◽  
The Other ◽  
Alfven Wave ◽  
Alfvén Waves ◽  
Ionospheric Conductivity ◽  
Model Based ◽  
Field Aligned Current ◽  
Auroral Arcs

Abstract. Three models for the magnetosphere-ionosphere coupling feedback instability are considered. The first model is based on demagnetization of hot ions in the plasma sheet. The instability takes place in the global magnetosphere-ionosphere system when magnetospheric electrons drift through a spatial gradient of hot magnetospheric ion population. Such a situation exists on the inner and outer edges of the plasma sheet where relatively cold magnetospheric electrons move earthward through a radial gradient of hot ions. This leads to the formation of field-aligned currents. The effect of upward field-aligned current on particle precipitation and the magnitude of ionospheric conductivity leads to the instability of this earthward convection and to its division into convection streams oriented at some angle with respect to the initial convection direction. The growth rate of the instability is maximum for structures with sizes less than the ion Larmor radius in the equatorial plane. This may lead to formation of auroral arcs with widths about 10 km. This instability explains many features of such arcs, including their conjugacy in opposite hemispheres. However, it cannot explain the very high growth rates of some auroral arcs and very narrow arcs. For such arcs another type of instability must be considered. In the other two models the instability arises because of the generation of Alfven waves from growing arc-like structures in the ionospheric conductivity. One model is based on the modulation of precipitating electrons by field-aligned currents of the upward moving Alfven wave. The other model takes into consideration the reflection of Alfven waves from a maximum in the Alfven velocity at an altitude of about 3000 km. The growth of structures in both models takes place when the ionization function associated with upward field-aligned current is shifted from the edges of enhanced conductivity structures toward their centers. Such a shift arises because the structures move at a velocity different from the E×B drift. Although both models may work, the growth rate for the model, based on the modulation of the precipitating accelerated electrons, is significantly larger than that of the model based on the Alfven wave reflection. This mechanism is suitable for generation of auroral arcs with widths of about 1 km and less. The growth rate of the instability can be as large as 1 s-1, and this mechanism enables us to justify the development of auroral arcs only in one ionosphere. It is hardly suitable for excitation of wide and conjugate auroral arcs, but it may be responsible for the formation of small-scale structures inside a wide arc.Key words: Ionosphere (auroral ionosphere) - Magnetospheric physics (auroral phenomena; magnetosphere-ionosphere interactions)  

scholarly journals Interpretation of velocity distribution of the inner regions of the Galaxy

1964 ◽  
Vol 20 ◽  
pp. 195-199 ◽  
Author(s):  
G. de Vaucouleurs
Keyword(s):  
Velocity Distribution ◽  
Spiral Structure ◽  
The Other ◽  
Galactic Centre ◽  
Line Profiles ◽  
Spiral Pattern ◽  
Other Hand ◽  
The Galaxy

I. The large positive and negative velocities in the 21-cm line profiles near the galactic centre have indicated the presence of substantial departures from circular motions in the central parts of the Galaxy. The Leiden astronomers (Oort and Rougoor 1958; Rougoor and Oort 1960) have interpreted these observations in terms of an “expanding arm” at a mean distance of about 3 kpc from the centre. It is not clear how these arms or arcs are related to the regular spiral structure, if the Galaxy is an ordinary spiral similar to M31 as commonly assumed. If, on the other hand, the Galaxy is similar to the SAB(r) or SAB(rs) systems, as suggested by the multiplicity of the spiral pattern discussed in another communication, a different interpretation of the velocity distribution is possible.

The Velocities of 6cm Recombination Line Emission originating near the Galactic Nucleus

1977 ◽  
Vol 3 (2) ◽  
pp. 150-152 ◽  
Author(s):  
F. F. Gardner ◽  
J. B. Whiteoak
Keyword(s):  
Radio Source ◽  
Strong Evidence ◽  
Line Emission ◽  
Hii Regions ◽  
Galactic Centre ◽  
Recombination Line ◽  
The Galaxy ◽  
Centre Region ◽  
Sgr A

Although it is well known that HII regions are present in the innermost regions of the Galaxy their kinematics are still not fully understood. In one study Pauls et al. (1976) surveyed with a beamwidth of 3′ arc the 10 GHz recombination line emission in directions within 15′ arc of the nuclear radio source Sgr A. They found that the emission velocities varied from position to position within the range -50 to + 50 km s-1but appeared to lack any overall pattern. In contrast, we have recently observed the recombination line emission from the galactic centre region with a beamwidth of 4′.5 arc, and find strong evidence of ordered motions near the galactic nucleus.

scholarly journals Nonlinear coupling of Alfvén and slow magnetoacoustic waves in partially ionized solar plasmas

2020 ◽  
Vol 641 ◽  
pp. A48
Author(s):  
J. L. Ballester ◽  
R. Soler ◽  
J. Terradas ◽  
M. Carbonell
Keyword(s):  
Solar Atmosphere ◽  
Alfven Waves ◽  
Ambipolar Diffusion ◽  
Second Order ◽  
Slow Waves ◽  
The Other ◽  
Alfvén Waves ◽  
Temporal Behavior ◽  
Other Hand ◽  
Partially Ionized

Context. Partially ionized plasmas constitute an essential ingredient of the solar atmosphere since layers such as the chromosphere and the photosphere and structures such as prominences and spicules are made of this plasma. On the other hand, ground- and space-based observations have indicated the presence of oscillations in partially ionized layers and structures of the solar atmosphere, which have been interpreted in terms of magnetohydrodynamic (MHD) waves. Aims. Our aim is to study the temporal behavior of nonlinear Alfvén waves, and the subsequent excitation of field-aligned motions and perturbations, in a partially ionized plasma when dissipative mechanisms such as ambipolar diffusion, radiative losses, and thermal conduction are taken into account. Methods. First, we applied the regular perturbations method for small-amplitude initial perturbations to obtain the temporal behavior of perturbations. Then we solved the full set of nonlinear MHD equations for larger values of the initial amplitude. Results. We obtain analytical and numerical solutions to first-, second-, and third-order systems of equations and study the effects produced by ambipolar diffusion and thermal mechanisms on the temporal behavior of Alfvén and slow waves. We also study how the majority of the energy is transferred from the Alfvén waves to plasma internal energy. After numerically solving the full nonlinear equations when a large amplitude is assumed, the profile of the perturbations displays the typical sawtooth profile characteristic of associated shocks. Conclusions. When ambipolar diffusion is taken into account, first-order Alfvén waves are damped in time, while second-order perturbations are undamped. However, due to the release of heat produced by ambipolar diffusion, other physical effects that modify the physical conditions in the spatial domain under consideration appear. On the other hand, the second-order perturbations are damped by thermal effects with a damping time that can be longer or shorter than that of Afvén waves. Therefore, after the initial excitation, Alfvén waves can be quickly damped, while slow waves remain in the plasma for a longer time, and vice versa.

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