scholarly journals The Magellanic Stream as a probe of the galactic halo

1979 ◽  
Vol 84 ◽  
pp. 574-574
Author(s):  
R. J. Cohen ◽  
R. D. Davies ◽  
I. F. Mirabel
Keyword(s):  
Galactic Halo ◽  
Neutral Hydrogen ◽  
The Sun ◽  
Typical Size ◽  
Stellar Component ◽  
Large Velocity ◽  
The Galaxy ◽  
Magellanic Stream

Recent observations of the Magellanic Stream can be used to set limits on a possible hot halo surrounding the Galaxy. The observations are described in detail elsewhere (Mirabel, Cohen & Davies, submitted to Mon. Not. R. astr. Soc.). Briefly, the neutral hydrogen in the northern end of the Magellanic Stream is concentrated in narrow filaments which contain small elongated clouds of typical size 0°.4 × 0°.6. These clouds have a large velocity halfpower width (25 km s−1 and are gravitationally unstable, unless there is a massive low luminosity stellar component. If we consider only the observed gas the expansion age of a typical cloud is 6 × 105 D years, where D is the distance in kpc from the Sun, and this falls at least a factor of ten short of the age of the Stream predicted by current models. This strongly suggests that some containment mechanism is operating.

scholarly journals The Magellanic Stream: Observational Considerations

1979 ◽  
Vol 84 ◽  
pp. 547-556 ◽  
Author(s):  
D. S. Mathewson ◽  
V. L. Ford ◽  
M. P. Schwarz ◽  
J. D. Murray
Keyword(s):  
Local Group ◽  
Galactic Center ◽  
Neutral Hydrogen ◽  
Great Circle ◽  
Magellanic Clouds ◽  
Systematic Variation ◽  
The Sun ◽  
The South ◽  
The Galaxy ◽  
Magellanic Stream

The Magellanic Stream is an arc of neutral hydrogen which nearly follows a great circle and which contains the Magellanic Clouds - hence its name (Mathewson, Cleary and Murray 1974). This great circle passes within a few degrees of the south galactic pole and lies close to the supergalactic plane. Mathewson and Schwarz (1976) argued that this indicates that the Magellanic Stream and Magellanic Clouds are not bound to the Galaxy. To reinforce this argument, they pointed out that around the supergalactic plane there is a similar systematic variation in the velocities of the Local Group and those of the Stream which may be due to the reflection of the motion of the galactic center if the velocity of rotatior of the Sun is 225 km s−1; if it is 290 km s−1 then the grounds for this argument would disappear.

scholarly journals The Kinematics of the Magellanic Clouds

1995 ◽  
Vol 166 ◽  
pp. 273-282
Author(s):  
B.E. Westerlund
Keyword(s):  
Galactic Halo ◽  
Neutral Hydrogen ◽  
Magellanic Clouds ◽  
Great Promise ◽  
Common Envelope ◽  
The Past ◽  
Ram Pressure ◽  
Long Time ◽  
Magellanic Stream ◽  
Magellanic System

It is essential for our understanding of the evolution of the Magellanic System, comprising the Large and the Small Magellanic Cloud, the Intercloud or Bridge region and the Magellanic Stream, to know its motions in the past. The Clouds have a common envelope of neutral hydrogen; this indicates that they have been bound to each others for a long time. The Magellanic System moves in the gravitational potential of our Galaxy; it is exposed to ram pressure through its movement in the galactic halo. Both effects ought to be noticeable in their present structure and kinematics. It is generally assumed, but not definitely proven, that the Clouds have been bound to our Galaxy for at least the last 7 Gyr. Most models assume that the Clouds lead the Magellanic Stream. The interaction between the Clouds has influenced their structure and kinematics severely. The effects should be possible to trace in the motions of their stellar and gaseous components as pronounced disturbances. Recent astrometric contributions in this field show a great promise for the future if still higher accuracy can be achieved.

scholarly journals A Search for Low-Velocity Neutral Hydrogen in the Magellanic Stream

1984 ◽  
Vol 108 ◽  
pp. 139-140
Author(s):  
N. V. Bystrova
Keyword(s):  
Neutral Hydrogen ◽  
Velocity Range ◽  
Low Velocity ◽  
Large Velocity ◽  
Magellanic Stream

The large velocity range of the HI clouds belonging to the Magellanic Stream, as found by Haynes (1979), makes it reasonable to search for some low LSR velocity neutral hydrogen possibly belonging to the Stream. This gas, if any, would be severely blended by the local galactic 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.

Maser effects in the interstellar hydrogen clouds of the galactic halo

1967 ◽  
Vol 31 ◽  
pp. 291-293
Author(s):  
I. S. Šklovskij
Keyword(s):  
High Velocity ◽  
Population Inversion ◽  
Galactic Halo ◽  
Galactic Plane ◽  
Neutral Hydrogen ◽  
Collisional Excitation ◽  
Hydrogen Atoms ◽  
Hydrogen Density ◽  
The Galaxy ◽  
High Velocity Clouds

It is suggested that the neutral hydrogen atoms in clouds with high negative velocities observed at high galactic latitudes may, when moving towards a galactic H 11 region, be excited by radiation in the red wing of the Lyman-α profile. The steepness of this wing may cause a population inversion of the hyperfine-structure levels. Consequently, estimates of the hydrogen density in the high-velocity clouds, and of the flow of matter towards the galactic plane (or into the Galaxy), when based on the assumption of collisional excitation, may be too high by two orders of magnitude.

scholarly journals 32. Theoretical problems of discrete radio sources

1957 ◽  
Vol 4 ◽  
pp. 179-191 ◽  
Author(s):  
J. L. Greenstein
Keyword(s):  
Quantitative Data ◽  
Unit Volume ◽  
Galactic Halo ◽  
Energy Output ◽  
Radio Sources ◽  
The Sun ◽  
Total Luminosity ◽  
The Galaxy ◽  
Discrete Radio Sources ◽  
Galactic Sources

The range of theoretical problems connected with the discrete sources is very large. It is convenient to distinguish between the normal and abnormal radio-frequency emitters that have quite different energy outputs per unit volume. Since the estimates by Minkowski and Greenstein [1] there has been considerable progress in the identification of these objects and in the provision of quantitative data about them. Table 1 includes revised estimates of the total luminosity, L, the emitting volume, V, and the luminosity per cubic parsec, J. Here L has been obtained by multiplying the observed power at 100 Mc./s. by an effective band-width of 500 Mc./s. and using the newly estimated distances. J is expressed in units of the total energy output of the sun (= 3·82 × 1026 watt) per cubic parsec. The figure for the Galaxy and M31 is considerably higher than that given by Baldwin in this symposium, since I have not revised the estimate of the emitting volume from the original paper by Minkowski and Greenstein. If most of the emission comes from a larger galactic halo the specific luminosity, J, for the Galaxy and M31 should be considerably reduced. For certain of the extra-galactic sources it is not certain whether the volume of the whole nebula, or only of the parts now in collision (or peculiar in nature, like the jet in M87) should be used in computing the specific emissivity, J. In Table 1, two values are then given.

scholarly journals Warm gas accretion onto the Galaxy

2008 ◽  
Vol 4 (S254) ◽  
pp. 241-254 ◽  
Author(s):  
J. Bland-Hawthorn
Keyword(s):  
Galactic Halo ◽  
Galactic Disk ◽  
Constant Rate ◽  
Diffuse Ionized Gas ◽  
The Galaxy ◽  
Hα Emission ◽  
The Stability ◽  
Gas Accretion ◽  
Halo Gas ◽  
Magellanic Stream

AbstractWe present evidence that the accretion of warm gas onto the Galaxy today is at least as important as cold gas accretion. For more than a decade, the source of the bright Hα emission (up to 750 mR†) along the Magellanic Stream has remained a mystery. We present a hydrodynamical model that explains the known properties of the Hα emission and provides new insights on the lifetime of the Stream clouds. The upstream clouds are gradually disrupted due to their interaction with the hot halo gas. The clouds that follow plough into gas ablated from the upstream clouds, leading to shock ionisation at the leading edges of the downstream clouds. Since the following clouds also experience ablation, and weaker Hα (100–200 mR) is quite extensive, a disruptive cascade must be operating along much of the Stream. In order to light up much of the Stream as observed, it must have a small angle of attack (≈ 20°) to the halo, and this may already find support in new Hi observations. Another prediction is that the Balmer ratio (Hα/Hβ) will be substantially enhanced due to the slow shock; this will soon be tested by upcoming WHAM observations in Chile. We find that the clouds are evolving on timescales of 100–200 Myr, such that the Stream must be replenished by the Magellanic Clouds at a fairly constant rate (≳ 0.1 M⊙ yr−1). The ablated material falls onto the Galaxy as a warm drizzle; diffuse ionized gas at 104 K is an important constituent of galactic accretion. The observed Hα emission provides a new constraint on the rate of disruption of the Stream and, consequently, the infall rate of metal-poor gas onto the Galaxy. We consider the stability of Hi clouds falling towards the Galactic disk and show that most of these must break down into smaller fragments that become partially ionized. The Galactic halo is expected to have huge numbers of smaller neutral and ionized fragments. When the ionized component of the infalling gas is accounted for, the rate of gas accretion is ~0.4 M⊙ yr−1, roughly twice the rate deduced from Hi observations alone.

scholarly journals Models for the dynamical evolution of the Magellanic System

2008 ◽  
Vol 4 (S256) ◽  
pp. 105-116
Author(s):  
Kenji Bekki
Keyword(s):  
Galactic Halo ◽  
Dynamical Evolution ◽  
Magellanic Clouds ◽  
Young Stars ◽  
Proper Motions ◽  
Self Consistent ◽  
The Galaxy ◽  
Formation And Evolution ◽  
Magellanic Stream ◽  
Magellanic System

AbstractI discuss the following five selected topics on formation and evolution of the LMC and the SMC based on fully self-consistent chemodynamical simulations of the Magellanic Clouds (MCs): (1) formation of bifurcated gaseous structures and young stars in the Magellanic bridge (MB), (2) formation of the Magellanic stream (MS) due to the tidal interaction between the LMC, the SMC, and the Galaxy within the last 2 Gyrs, (3) origin of the observed kinematical differences between H i gas and stars in the SMC, (4) formation of stellar structures dependent on their ages and metallicities in the LMC, and (5) a new common halo model explaining both the latest HST ACS observations on the proper motions of the LMC and the SMC and the presence of the MS in the Galactic halo. I focus exclusively on the latest developments in numerical simulations on formation and evolution of the Magellanic system.

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.

Galactic orbits of stars

1966 ◽  
Vol 25 ◽  
pp. 93-97
Author(s):  
Richard Woolley
Keyword(s):  
Globular Cluster ◽  
Potential Field ◽  
High Velocity ◽  
Velocity Vector ◽  
Variable Stars ◽  
The Sun ◽  
Proper Motions ◽  
Rr Lyrae ◽  
The Galaxy

It is now possible to determine proper motions of high-velocity objects in such a way as to obtain with some accuracy the velocity vector relevant to the Sun. If a potential field of the Galaxy is assumed, one can compute an actual orbit. A determination of the velocity of the globular clusterωCentauri has recently been completed at Greenwich, and it is found that the orbit is strongly retrograde in the Galaxy. Similar calculations may be made, though with less certainty, in the case of RR Lyrae variable stars.

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