scholarly journals On the comparison of spiral structure as delineated by gas and by stars

1964 ◽  
Vol 20 ◽  
pp. 158-160
Author(s):  
H. F. Weaver
Keyword(s):  
Large Scale ◽  
Spiral Structure ◽  
Neutral Hydrogen ◽  
Hydrogen Gas ◽  
Young Stars ◽  
Space Distribution ◽  
Galactic Rotation ◽  
Spiral Arms ◽  
Gas Concentration ◽  
The Galaxy

In an earlier article* it was pointed out that the galactic radial motions ΔE (R, l) of the very young stars did not show the uniformity of motion to be expected from a smooth regular expansion of the Galaxy. Instead, the very young stars were found to show large-scale regional peculiar motions; these regional peculiar motions are displayed in Figure 1. In addition to regional peculiar motions and the space distribution of stars, Figure 1 also shows the spiral structure delineated by neutral hydrogen gas. As is customary in such diagrams, the space distribution of gas and the space distribution of the stars are not in good agreement. As various investigators have mentioned, stars and gas appear to define different spiral arms. However, such a conclusion is not warranted by data such as those employed in construction of Figure 1. In Figure 1 (as is invariably the case in earlier published diagrams of the same sort) two distance scales have been employed in the construction of the diagram. The distances of the stars have been derived from photometric data; the distances of concentrations of neutral hydrogen gas have been derived from measured hydrogen gas radial velocities and a galactic rotation curve. It should therefore come as no surprise if there are disagreements between hydrogen spiral arms and star spiral arms. Any regional peculiar motion of a gas concentration directly becomes an error in the inferred distance of the gas concentration.

scholarly journals Non-circular motions in the galaxy as exhibited by very young stars

1964 ◽  
Vol 20 ◽  
pp. 92-99
Author(s):  
H. F. Weaver
Keyword(s):  
Large Scale ◽  
Spiral Structure ◽  
Young Stars ◽  
Significant Fraction ◽  
Fundamental Importance ◽  
General Character ◽  
Gaseous Component ◽  
The Galaxy ◽  
Scale Expansion

I. Expansion of the Gaseous and Stellar Components of the GalaxyIf the gaseous component of the Galaxy is expanding as observed by Rougoor and Oort in the centre of the Galaxy and as postulated by Kerr in his early interpretation of spiral structure, the expansion must represent a phenomenon of fundamental importance in the Galaxy which has, in all probability, been operative for a significant fraction of the age of the Galaxy. Presumably, very young stars formed from this gas and having ages less than 1 % of the age of the Galaxy might be expected to retain in their motions the general character of the large-scale expansion of the gas from which they originated.

scholarly journals Neutral hydrogen gas within and around NGC 1316

2019 ◽  
Vol 628 ◽  
pp. A122 ◽  
Author(s):  
P. Serra ◽  
F. M. Maccagni ◽  
D. Kleiner ◽  
W. J. G. de Blok ◽  
J. H. van Gorkom ◽  
...  
Keyword(s):  
Large Scale ◽  
Molecular Form ◽  
Neutral Hydrogen ◽  
Hydrogen Gas ◽  
Neutral Gas ◽  
Tidal Forces ◽  
The Galaxy ◽  
Long Tails ◽  
Stellar Body ◽  
Fornax Cluster

We present MeerKAT observations of neutral hydrogen gas (H I) in the nearby merger remnant NGC 1316 (Fornax A), the brightest member of a galaxy group which is falling into the Fornax cluster. We find H I on a variety of scales, from the galaxy centre to its large-scale environment. For the first time we detect H I at large radii (70–150 kpc in projection), mostly distributed on two long tails associated with the galaxy. Gas in the tails dominates the H I mass of NGC 1316: 7 × 108 M⊙– 14 times more than in previous observations. The total H I mass is comparable to the amount of neutral gas found inside the stellar body, mostly in molecular form. The H I tails are associated with faint optical tidal features thought to be the remnant of a galaxy merger occurred a few billion years ago. They demonstrate that the merger was gas-rich. During the merger, tidal forces pulled some gas and stars out to large radii, where we now detect them in the form of optical tails and, thanks to our new data, H I tails; while torques caused the remaining gas to flow towards the centre of the remnant, where it was converted into molecular gas and fuelled the starburst revealed by the galaxy’s stellar populations. Several of the observed properties of NGC 1316 can be reproduced by a ∼10:1 merger between a dominant, gas-poor early-type galaxy and a smaller, gas-rich spiral occurred 1–3 Gyr ago, likely followed by subsequent accretion of satellite galaxies.

scholarly journals High Velocity Neutral Hydrogen Clouds

1974 ◽  
Vol 60 ◽  
pp. 599-616
Author(s):  
R. D. Davies
Keyword(s):  
High Velocity ◽  
Spiral Structure ◽  
Neutral Hydrogen ◽  
Magellanic Clouds ◽  
Galactic Rotation ◽  
Velocity Pattern ◽  
Tidal Interactions ◽  
The Galaxy ◽  
High Velocity Clouds

A review is given of the observations of neutral hydrogen high velocity clouds (|ν| > 80 km s−1) in and near the Galaxy. The positive and negative clouds are seen to have different distributions in the sky, following roughly the velocity pattern of galactic rotation. A characteristic of the majority of the clouds is their distribution in elongated bands or strings. The various theories of origin of HVCs are discussed; the possible role of the tidal interactions between the Magellanic Clouds and the Galaxy is emphasized. Tests are suggested to distinguish between the Oort theory of the infall of intergalactic material and theories which envisage the HVCs as originating in the outermost spiral structure.

scholarly journals Sydney Work on 21-cm Observations of the HI Galactic Disk

1958 ◽  
Vol 8 ◽  
pp. 924-925
Author(s):  
F. J. Kerr
Keyword(s):  
Large Scale ◽  
Spiral Structure ◽  
Galactic Disk ◽  
Beam Width ◽  
Central Plane ◽  
Rotational Model ◽  
General Picture ◽  
Spiral Arms ◽  
The Galaxy ◽  
Half Power

This paper reports briefly on the provisional results of the 21-cm galactic work at the Radiophysics Laboratory, Sydney. Observations were made with a 36-ft, transit-type radio-telescope whose beam width at half-power is 1°.5 and a 4-channel receiver whose band width, 40 kcps, is equivalent to 8 km/sec.The large scale structure of the galaxy has been studied by Kerr, Hindman, and Carpenter. They derive a general picture of the spiral structure in the southern hemisphere extending the Leiden results (see Fig. 1) and, in addition, a “relief map” of the galaxy showing the location of the densest parts of the hydrogen layer with respect to a central plane (see Fig. 2). Analysis of the spiral structure is for the present limited to regions beyond 3 kpc from the center. The Sydney and Leiden data agree well where they overlap, but location of spiral arms is sensitive to the choice of rotational model. Four or five spiral arms, or sections of arms, are indicated in the longitude interval l= 170° to l= 320°. The arms are clearly trailing in the rotation.

scholarly journals Current Problems in 21 cm Line Studies of the Large-Scale Structure of the Galaxy

1974 ◽  
Vol 60 ◽  
pp. 551-572 ◽  
Author(s):  
W. B. Burton
Keyword(s):  
Shock Front ◽  
Large Scale ◽  
Spiral Structure ◽  
Density Wave ◽  
Model Fitting ◽  
Neutral Hydrogen ◽  
Wave Theory ◽  
Large Scale Structure ◽  
The Galaxy ◽  
Density Wave Theory

A number of current problems in 21 cm line studies of the Galaxy as a whole are discussed. Because of the difficulties involved with straightforward mapping, it is important to isolate integrated and other properties of the hydrogen profiles, the interpretation of which does not require accurate distance determinations. In addition, methods of analysis are necessary which either account for or exploit the sensitivity of hydrogen profiles to velocity irregularities and to geometrical configurations. The model-fitting approach to the interpretation of the hydrogen profiles is useful in this respect. Extragalactic hydrogen studies which show the relative ordering of the various components of spiral structure can inspire research in our own Galaxy. Such investigations are necessary for an understanding of the forces governing the spiral structure. It seems that the neutral hydrogen is primarily a tracer of locations where the overall distribution of stars is producing a gravitational sink. Other spiral tracers, in particular the molecules, are better considered as tracers of regions where the gas has been compressed, perhaps (at least on a large scale) by the shock front predicted by the density-wave theory.

Evidence for Large Scale Cosmic Ray Electron Gradients in the Galaxy

1976 ◽  
Vol 3 (1) ◽  
pp. 1-6 ◽  
Author(s):  
W. R. Webber
Keyword(s):  
Large Scale ◽  
Spiral Structure ◽  
Cosmic Ray ◽  
Point Sources ◽  
Interstellar Matter ◽  
Interstellar Gas ◽  
The Galaxy ◽  
Ray Density ◽  
Γ Rays ◽  
Γ Ray

In recent years observations of γ-ray emission from the disk of the galaxy have provided a new opportunity for research into the structure of the spiral arms of our own galaxy. In Figure 1 we show a map of the structure of the disk of the galaxy as observed for γ-rays of energy > 100 MeV by the SAS-2 satellite (Fichtel et al. 1975). The angular resolution of these measurements is ~ 3°, and besides two point sources at l = 190° and 265° several features related to the spiral structure of the galaxy are evident in the data. Most of these γ-rays are believed to arise from the decay of π° mesons produced by the nuclear interactions of cosmic rays (mostly protons) with the ambient interstellar gas. As a result, the γ-ray fluxes represent a measure of the line of sight integral of the product of the cosmic ray density NCR and the interstellar matter density N1

scholarly journals Analysis of Low-and High-Resolution Observations of High-Velocity Clouds

1989 ◽  
Vol 120 ◽  
pp. 416-423
Author(s):  
Bart P. Wakker
Keyword(s):  
High Velocity ◽  
Galactic Center ◽  
Neutral Hydrogen ◽  
Current Status ◽  
Galactic Rotation ◽  
Single Model ◽  
Neutral Gas ◽  
The Galaxy ◽  
High Velocity Clouds ◽  
Distance Problem

For almost three decades neutral hydrogen moving at velocities unexplicable by galactic rotation has been observed. These so-called high-velocity clouds (HVCs) have been invoked as evidence for infall of neutral gas to the galaxy, as manifestations of a galactic fountain, as energy source for the formation of supershells, etc. No general consensus about their origin has presently been reached. However, it is becoming clear that no single model will suffice to explain all HVCs. A number of clouds may consist of material streaming toward the galactic center, as Mirabel (this conference) has advocated for several years, though their origin still remains unclear. A better understanding is mainly hampered by the fact that the distance remains unknown. An overview of the current status of the distance problem is given by van Woerden elsewhere in this volume.

scholarly journals Filamentary Structures Near the Galactic Center

1989 ◽  
Vol 136 ◽  
pp. 243-263 ◽  
Author(s):  
F. Yusef-Zadeh
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Galactic Center ◽  
Galactic Plane ◽  
Galactic Rotation ◽  
Physical Interaction ◽  
Magnetic Structures ◽  
The Galaxy ◽  
Field Lines ◽  
The Magnetic Field

Recent studies of the Galactic center environment have revealed a wealth of new thermal and nonthermal features with unusual characteristics. A system of nonthermal filamentary structures tracing magnetic field lines are found to extend over 200pc in the direction perpendicular to the Galactic plane. Ionized structures, like nonthermal features, appear filamentary and show forbidden velocity fields in the sense of Galactic rotation and large line widths. Faraday rotation characteristics and the flat spectral index distributions of the nonthermal filaments suggest a mixture of thermal and nonthermal gas. Furthermore, the relative spatial distributions of the magnetic structures with respect to those of the ionized and molecular gas suggest a physical interaction between these two systems. In spite of numerous questions concerning the origin of the large-scale organized magnetic structures, the mechanism by which particles are accelerated to relativistic energies, and the source or sources of heating the dust and gas, recent studies have been able to distinguish the inner 200pc of the nucleus from the disk of the Galaxy in at least two more respects: (1) the recognition that the magnetic field has a large-scale structure and is strong, uniform and dynamically important; and (2) the physics of interstellar matter may be dominated by the poloidal component of the magnetic field.

scholarly journals Interarm islands in the Milky Way – the one near the Cygnus spiral arm

2020 ◽  
Vol 494 (1) ◽  
pp. 1134-1142
Author(s):  
Jacques P Vallée
Keyword(s):  
Observational Data ◽  
Milky Way ◽  
Young Stars ◽  
H Ii Regions ◽  
Spiral Arms ◽  
The Galaxy ◽  
The One ◽  
Loop 2 ◽  
Perseus Arm ◽  
Spiral Arm

ABSTRACT This study extends to the structure of the Galaxy. Our main goal is to focus on the first spiral arm beyond the Perseus arm, often called the Cygnus arm or the ‘Outer Norma’ arm, by appraising the distributions of the masers near the Cygnus arm. The method is to employ masers whose trigonometric distances were measured with accuracy. The maser data come from published literature – see column 8 in Table 1 here, having been obtained via the existing networks (US VLBA, the Japanese VERA, the European VLBI, and the Australian LBA). The new results for Cygnus are split in two groups: those located near a recent CO-fitted global model spiral arm and those congregating within an ‘interarm island’ located halfway between the Perseus arm and the Cygnus arm. Next, we compare this island with other similar interarm objects near other spiral arms. Thus, we delineate an interarm island (6 × 2 kpc) located between the two long spiral arms (Cygnus and Perseus arms); this is reminiscent of the small ‘Local Orion arm’ (4 × 2 kpc) found earlier between the Perseus and Sagittarius arms and of the old ‘Loop’ (2 × 0.5 kpc) found earlier between the Sagittarius and Scutum arms. Various arm models are compared, based on observational data (masers, H II regions, H I gas, young stars, CO 1–0 gas).

scholarly journals Spiral structure of the Galaxy Derived from the Hat Creek survey of neutral hydrogen

1970 ◽  
Vol 38 ◽  
pp. 126-139 ◽  
Author(s):  
H. Weaver
Keyword(s):  
Pitch Angle ◽  
Spiral Structure ◽  
Neutral Hydrogen ◽  
The Sun ◽  
General Picture ◽  
Contour Maps ◽  
The Galaxy ◽  
External Galaxies ◽  
Perseus Arm ◽  
Spiral Arm

The extensive Hat Creek survey of neutral hydrogen combined with southern observations provides the basis for a new discussion of the spiral structure of the galaxy. The purpose of this investigation is to provide a general picture of the galaxy. It is found that the pitch of the spiral arms is approximately 12°.5 and that there are many spurs and interarm features as we observe in external galaxies.The sun is not located in a major spiral arm, but rather in a spur or offshoot originating near or at the Sagittarius arm, which is a major structure in the galaxy. The young stars in the general vicinity of the sun delineate this spur, not a major arm structure. The stars and the gas are in agreement in indicating a large pitch angle (20°–25°) for this local structure, which differs from the smaller pitch angle for the arms which form the system as a whole.In the presentation a computer-produced movie of the galaxy based on Hat Creek hydrogen contour maps similar to those in Figure 1 was shown. It was used to illustrate generally the complexity of the gas structure and, in particular, to show (i) observational aspects of the spur in which the sun is located and (ii) the point of origin of the so-called Perseus arm.

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