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.

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.

A Warp in the HI Distribution at the Extreme NE and SW of M31

1978 ◽  
Vol 77 ◽  
pp. 183-189
Author(s):  
D.T. Emerson ◽  
K. Newton
Keyword(s):  
Radial Velocity ◽  
Angular Resolution ◽  
Neutral Hydrogen ◽  
Aperture Synthesis ◽  
The Galaxy

Aperture synthesis observations of the neutral hydrogen in the extreme NE and SW regions of M31 have been made using the Cambridge Half-Mile Telescope, with an angular resolution of 3′.6 × 5′.5 and a resolution in radial velocity of 16 km/s (Newton and Emerson, 1977). These observations show that a warp in the HI distribution exists in opposite directions at each end of the galaxy.

scholarly journals S266: A Distant HII Region in the Galaxy

1989 ◽  
Vol 120 ◽  
pp. 129-129
Author(s):  
A. Manchado ◽  
C. Esteban ◽  
J.M. Vilchez
Keyword(s):  
High Resolution ◽  
Radial Velocity ◽  
Central Star ◽  
Strong Wind ◽  
Low Resolution ◽  
Galactocentric Distance ◽  
Hii Region ◽  
Narrow Component ◽  
Precise Nature ◽  
The Galaxy

Long slit high and low resolution spectroscopy of S266 was conducted in order to investigate its precise nature. The Hα high resolution profile shows two different components; the narrow one extends all over the nebula while a very broad (ΔVFWHM = 700 km s−1) indicates that a strong wind is associated with the central star. From the narrow component we have worked out radial velocity (VLRS = 23 km s−1) which yields to a kinematic distance of a 9 kpc. This value implies a galactocentric distance of 19 kpc which makes this region one of the most distant in the Galaxy.

HI in the Small Magellanic Cloud Re-examined

1981 ◽  
Vol 4 (2) ◽  
pp. 189-195 ◽  
Author(s):  
R. X. McGee ◽  
Lynette M. Newton
Keyword(s):  
Radial Velocity ◽  
Neutral Hydrogen ◽  
Total Content ◽  
Hii Regions ◽  
Magellanic Cloud ◽  
Hydrogen Gas ◽  
Radial Velocities ◽  
Line Of Sight ◽  
Small Magellanic Cloud

Our re-examination of the neutral hydrogen gas in the Small Magellan Cloud has led to four important results. Firstly, we find that Hindman’s (1967) total content HI map is a satisfactory representation of the gas in the line of sight. Secondly, we find that the HI gas in the SMC exists in four distinct large masses separated from one another in radial velocity by 20 to 30 km s−1. Thirdly, having made this division of the gas we show that there is good correlation between the radial velocities of HII regions, supergiant stars and HI. Finally, we believe that our observations reveal that the SMC is associated with an extremely large trailing halo of HI gas which forms the major component of the inter-cloud bridge region.

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 Twinkle, Twinkle Little Pulsar/Quasar

1999 ◽  
Vol 52 (1) ◽  
pp. 1
Author(s):  
D. B. Melrose
Keyword(s):  
Multipath Propagation ◽  
Source Region ◽  
Low Frequency ◽  
Observation Time ◽  
Line Of Sight ◽  
Short Time Scale ◽  
Radio Sources ◽  
The Galaxy ◽  
Time Variations ◽  
Short Time

The twinkling of stars is a familiar example of scintillations, due to turbulence in the Earth’s atmosphere causing fluctuations in the refractive index of the air along the line of sight. Scintillations lead to time variations in the apparent position of the source, and hence to an angular broadening on integration over an observation time. Scintillations also lead to fluctuations in the intensity of the source. Pointlike astronomical radio sources such as pulsars and (the compact cores of some) quasars scintillate due to fluctuations in the electron density along the line of sight through the interstellar medium. For quasars, low-frequency (100s of MHz) variability over periods of years is a scintillation effect, as are probably more rapid (as short as an hour) intensity variations at higher radio frequencies. Unlike the twinkling of stars, which is due to weak scintillations, the scintillations of radio sources are usually strong. Important qualitative effects associated with strong scattering are multipath propagation and a clear separation into diffractive and refractive scintillations. Quasars exhibit only refractive scintillations. Pulsars are extremely small and bright, and they vary temporally on a very short time scale, making them almost ideal sources on which to test our ideas on scintillations. Pulsars exhibit a variety of scintillation phenomena, due to both refractive and diffractive effects, the latter seen most clearly in dynamic spectra. These data are used to model the distribution of electrons through the Galaxy, to determine the distribution of pulsar velocities, and potentially to resolve the source region in a pulsar magnetosphere. These scintillation phenomena and their interpretation in terms of the theory of strong scintillations are reviewed briefly. The generalisation of the theory to include the birefringence of the plasma (Faraday effect), and its possible implications on the interpretation of circular polarisation, are then outlined. An attempt to generalise the theory to describe scattering by a distribution of discrete scattering objects is also discussed briefly.

scholarly journals Intermediate Resolution Spectroscopy of the Radio Galaxy B2 0902+34 at Z ≈ 3.4

1996 ◽  
Vol 168 ◽  
pp. 499-500
Author(s):  
J. L. Sanz ◽  
J. M. Martín-Mirones ◽  
E. Martínez-González ◽  
J. I. González-Serrano
Keyword(s):  
Galaxy Evolution ◽  
Spectral Range ◽  
Spectral Resolution ◽  
Radio Galaxy ◽  
High Redshift ◽  
Neutral Hydrogen ◽  
Radio Continuum ◽  
Continuum Source ◽  
The Galaxy ◽  
William Herschel

We have carried out optical spectroscopic observations at intermediate spectral resolution of the massive high redshift radio galaxy 0902+34 atz≈ 3.39. This source was first identified by Lilly (1988) (from hereafter L88). The study of high redshift radio galaxies is interesting to analyze the physical conditions of the early universe and the galaxy evolution at cosmological redshifts. It has been claimed that some of these systems may be protogalaxies in the process of formation. Indications for this are the flat spectrum and the absence of the 4000 Å break, features which have already been observed in many cases. In particular, observations in the spectral range fromVtoKsuggest that 0902+34 is a young galaxy (Eisenhardt and Dickinson 1992). Recent radio observations of the 21 cm line of neutral hydrogen have detected (Uson et al. 1991) an absorption against the radio continuum source. This absorption could also leave a track in the optical, redwards the Lyα line. Our observations were carried out with the ISIS spectrograph at the 4.2 m William Herschel Telescope (seeing ≈ 1.2–1.6 arcsec). A spectral dispersion of 0.78 Å/pixel (blue arm) and 1.38 Å/pixel (red arm) was obtained. Å long slit of width 3′ was used providing a spectral resolution of ≈ 5.4 Å in the blue arm and of ≈ 9.5 Å in the red one. Both resolutions are a clear improvement over that achieved by L88 of 20 Å, allowing us to resolve the Lyα line (and its possible structure) and any other possible strong features appearing in the spectral range observed (e. g., C iv λ1549, He ii λ1640, …). Six different observations of 2700 s of the radio galaxy 0902+34 were carried out. The slit was rotated to coincide with the parallactic angle at the beginning of each exposure. This will allow us to map spectroscopically different regions of the galaxy (for more details see Martín-Mirones et al. 1994).

The Spin Temperature of the Interstellar Neutral Hydrogen

1969 ◽  
Vol 1 (5) ◽  
pp. 215-216 ◽  
Author(s):  
V. Radhakrishnan ◽  
J. D. Murray
Keyword(s):  
Galactic Plane ◽  
Neutral Hydrogen ◽  
Spin Temperature ◽  
The Past ◽  
Brightness Temperatures ◽  
Direct Measurements ◽  
Absorption Studies ◽  
The Galaxy ◽  
Considerable Controversy ◽  
Emission Studies

The lack of any direct measurements on the spin temperature of the neutral hydrogen in the Galaxy has led to considerable controversy in the past. Estimates of the temperature have depended strongly on whether they are based on emission or absorption studies. The widely accepted value of 125°K based on emission studies dates back to Schmidt. He adopted this figure on the premise that the maximum observed brightness temperatures in the galactic plane were in directions of high optical depth. The brightness temperature was then equated with the spin temperature on the assumption that the temperature did not fluctuate very much in a large region around the Sun.

scholarly journals Spectral and VLBI-structure monitoring of an OH maser flare in W75N

2007 ◽  
Vol 3 (S242) ◽  
pp. 186-187
Author(s):  
A. V. Alakoz ◽  
V. I. Slysh ◽  
V. Migenes
Keyword(s):  
Radial Velocity ◽  
Flux Density ◽  
Linear Polarization ◽  
Proper Motion ◽  
Tangential Velocity ◽  
Spectral Features ◽  
Velocity Range ◽  
Hii Region ◽  
Star Forming ◽  
Structure Monitoring

AbstractWe present VLBI and single dish observations of a strong (1000 Jy) OH maser flare at 1665 MHz in the star-forming region W75N. This new flare appeared in 2003, after a 100 Jy flare in 2000. Three major spectral features N1, N2, and N3 were present at all epochs, with N1 declining and N2 increasing in flux density during observations taken in 2003–2006. All three features have strong linear polarization approaching 90%. VLBI maps show that along with the strong features N1-3, many weaker features have appeared within 100 mas of the ultra compact HII region VLA 2. The proper motion of the flare feature N1 was measured: Δα=1.36±0.3; Δδ=-0.3±0.3 mas in 1.75 years, corresponding to a tangential velocity of about 8 km s−1 relative to the rest of the features, and is comparable to the radial velocity range in the VLA 2 disk.

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.

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