scholarly journals Observations of interstellar Lyman-α absorption

1970 ◽  
Vol 36 ◽  
pp. 281-301 ◽  
Author(s):  
Edward B. Jenkins
Keyword(s):  
Neutral Hydrogen ◽  
Local Region ◽  
Line Of Sight ◽  
Small Scale ◽  
Line Broadening ◽  
Interstellar Gas ◽  
B Stars ◽  
Hydrogen Density ◽  
The Galaxy ◽  
To Come

Absorption at the Lyman-α transition from interstellar neutral hydrogen has been observed in the ultraviolet spectra of 18 nearby O and B stars. Radiation damping is the dominant cause of line broadening, which makes the derived line-of-sight column densities proportional to the square of the observed equivalent widths. An average hydrogen density on the order of 0.1 atom cm−3 has been found for most of the stars observed so far. This is in contrast to the findings from surveys of 21-cm radio emission, which suggest 0.7 atom cm−3 exists in the local region of the Galaxy. Several effects which might introduce uncertainties into the Lyman-α measurements are considered, but none seems to be able to produce enough error to explain the disagreement with the 21-cm data. The possibility that small-scale irregularities in the interstellar gas could give significantly lower values at Lyman-α is explored. However, a quantitative treatment of the factor of ten discrepancy in Orion indicates the only reasonable explanation requires the 21-cm flux to come primarily from small, dense, hot clouds which are well separated from each other. The existence of such clouds, however, poses serious theoretical difficulties.

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.

Variation of the amount of hydrogen along the galactic ridge

1967 ◽  
Vol 31 ◽  
pp. 171-172
Author(s):  
Th. Schmidt-Kaler
Keyword(s):  
Surface Brightness ◽  
Milky Way ◽  
Neutral Hydrogen ◽  
Line Of Sight ◽  
Optical Surface ◽  
Hydrogen Density ◽  
Continuous Radiation ◽  
Galactic Longitude

The integralNHof neutral-hydrogen density along the line of sight is determined from the Kootwijk and Sydney surveys. The run ofNHwith galactic longitude agrees well with that of thermal continuous radiation and that of the optical surface brightness of the Milky Way.

scholarly journals A Distant HII Region In The Galaxy

1965 ◽  
Vol 18 (1) ◽  
pp. 91 ◽  
Author(s):  
K Akabane ◽  
FJ Kerr
Keyword(s):  
Radial Velocity ◽  
Neutral Hydrogen ◽  
Line Of Sight ◽  
Radio Sources ◽  
Velocity Range ◽  
Hii Region ◽  
Continuum Source ◽  
Absorption Studies ◽  
The Galaxy

It has long been realized that 21 cm absorption studies can provide information about the distance of unidentified radio sources. When the radiation from a continuum source is absorbed by neutral hydrogen clouds in the foreground, an observation of the radial velocity range over which the absorption occurs indicates the position of the source in relation to the various hydrogen features along the line of sight. Whenever absorption effects can be seen, we can immediately tell whether the source concerned is galactic or extragalactic; if it is galactic, we can then place limits on its distance.

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 Galactic Surveys in the Gaia Era

2017 ◽  
Vol 12 (S330) ◽  
pp. 136-143
Author(s):  
Rosemary F. G. Wyse
Keyword(s):  
Line Of Sight ◽  
Stellar Content ◽  
Elemental Abundances ◽  
Gaia Dr1 ◽  
The Galaxy ◽  
To Come ◽  
Astrometric Data

AbstractThe final astrometric data from the Gaia mission will transform our view of the stellar content of the Galaxy, particularly when complemented with spectroscopic surveys providing stellar parameters, line-of-sight kinematics and elemental abundances. Analyses with Gaia DR1 are already demonstrating the insight gained and the promise of what is to come with future Gaia releases. I present a brief overview of results and puzzles from recent Galactic Archaeology surveys for context, focusing on the Galactic discs.

Pulsar scintillation as a physical tool

1992 ◽  
Vol 341 (1660) ◽  
pp. 167-176 ◽  
Keyword(s):  
Spatial Coherence ◽  
Line Of Sight ◽  
Interstellar Gas ◽  
Pulsar Emission ◽  
Pulsar Radiation ◽  
Wide Range ◽  
Random Modulation ◽  
Radio Frequency Spectrum ◽  
The Galaxy ◽  
Diffraction Patterns

The interstellar gas contains irregularities of electron density having a wide range of physical scales. Pulsar radiation propagating through this inhomogeneous medium suffers a random modulation of phase which causes the received intensity to scintillate on a variety of timescales. Observations of the radio frequency spectrum and temporal variation of scintillation give information on the form of the irregularity spectrum and the distribution of density structure across the Galaxy. The high spatial coherence of pulsar radiation leads to the formation of extremely fine-scale diffraction patterns which also provide information on the motion of sources across the line of sight and the size of pulsar emission regions. Some uses of scintillation as a means of probing the interstellar gas and elucidating the physical properties of pulsars will be discussed.

scholarly journals 9. A 21-cm. study of the Orion region

1957 ◽  
Vol 4 ◽  
pp. 56-65
Author(s):  
T. K. Menon
Keyword(s):  
Velocity Distribution ◽  
Neutral Hydrogen ◽  
Line Of Sight ◽  
Distinct Advantage ◽  
Galactic Rotation ◽  
Galactic Latitude ◽  
Orion Nebula ◽  
Internal Motions ◽  
Orion Association ◽  
The Galaxy

An analysis of the profiles of the 21-cm. radiation from neutral hydrogen promises to be of great importance for a study of the internal motions of specific regions of the Galaxy. The two factors which influence the shape of the profiles are the velocity distribution and the density distribution of the neutral hydrogen atoms in the line of sight. The velocity distribution is essentially determined by three factors (1) galactic rotation, (2) the random motions of the gases, and (3) local peculiar motions as, for example, expansion. In the plane of the Galaxy in any specific direction the isolation of a region of particular interest is made difficult because of the superposition of the radiation along the entire line of sight. Hence regions at intermediate galactic latitudes are more suitable for study of internal motions than regions on or near the galactic equator. Also, for the study of peculiar motions, regions with small galactic rotation terms have the distinct advantage that any prevailing preferential motion will be clearly indicated by the profiles. The Orion region satisfies most of the above requirements. The galactic latitude of the section under consideration falls between − 10° and − 25°, and at the mean galactic longitude of 170° the galactic rotation term in radial velocity amounts to 7 km./sec. at a distance of 500 parsecs. Moreover the Orion region contains many features of considerable interest like the Orion Nebula, the Orion Association, the great arc of ionized hydrogen and many smaller H II regions. The great arc of Barnard (1895) [1] forms part of an almost elliptical ring of emission nebulosity with dimensions 14 × 12°. At the distance of 500 parsecs for the Orion Association these dimensions are of the order of 120 × 105 parsecs. It is of interest to note that the major axis of this ellipse is parallel to the galactic equator. This ellipticity could presumably be caused by galactic rotation, by a galactic magnetic field with lines of force along the spiral arms, or by the rotation of the whole mass itself. Further investigation is necessary to decide which of the above effects is most important.

scholarly journals Radial velocities of neutral hydrogen in the anticenter region of the Galaxy

1970 ◽  
Vol 38 ◽  
pp. 164-168 ◽  
Author(s):  
L. Velden
Keyword(s):  
Emission Line ◽  
Statistical Method ◽  
Neutral Hydrogen ◽  
Radial Velocities ◽  
Line Profiles ◽  
Interstellar Gas ◽  
Observational Material ◽  
Circular Rotation ◽  
The Galaxy

An observational material of 21-cm H I emission-line profiles is investigated by a statistical method to derive the kinematical properties of the interstellar gas in the region of the galactic anticenter. A description of the method used as well as the results obtained, concerning deviations from a circular rotation, are given.

scholarly journals Interstellar sodium and Galactic structure. A high-resolution survey

1985 ◽  
Vol 106 ◽  
pp. 325-328
Author(s):  
E. Maurice ◽  
A. Ardeberg ◽  
H. Lindgren
Keyword(s):  
High Resolution ◽  
Spectral Resolution ◽  
Absorption Lines ◽  
High Spectral Resolution ◽  
Line Of Sight ◽  
Substantial Part ◽  
Galactic Rotation ◽  
Interstellar Gas ◽  
Interstellar Clouds ◽  
The Galaxy

Observation of absorption lines produced by interstellar gas is a straight-forward way to determine column densities and velocities along the line of sight of interstellar clouds. In practice, peculiar motions often mask galactic rotation and/or cause line blending. We have made a study of absorption lines of interstellar sodium covering a substantial part of the Galaxy at extremely high spectral resolution.

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