Observations of the Spatial Structre of Interstellar Hydrogen. I. High-Resolution Observations of a Small Region

Abstract

view Abstract Citations (99) References (51) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Observations of the Spatial Structre of Interstellar Hydrogen. I. High-Resolution Observations of a Small Region Heiles, Carl Abstract The 21<:m line radiation emitted by low-velocity neutral hydrogen has been mapped in detail in the region bounded by 1" , b" using the 300-foot telescope and the 100-channel autocorrelation receiver at the National Radio Astronomy Observatory in Green Bank. The beam width of the antenna at this wavelength is 10', the velocity resolution about 1 km/sec, the total range of velocities observed about 50 km/sec, and the antenna temperature sensitivity about 1 Several considerations lead to the adoption of a common distance of 500 pc to all of the hydrogen as a reasonable approximation. The gas can be resolved into three physical components: a diffuse, smooth background, contributing about of the integrated 21-cm emission; and two large sheets of gas having relative motion in which the hydrogen density is probably about two atoms/cm3, which contain structure. In part of the region the sheets coexist in space and velocity. The velocity of each sheet is highly ordered over distances of tens of degrees on the sky; linear structures characterized by no hydrogen ("rifts") run across each sheet. The disturbance responsible for the rifts probably exists in sheetlike form, too. The present observations do not correspond at all with the predictions of the "standard cloud model." Concentrations of gas which one might call clouds are present in the sheets, but usually their density is no larger than twice the surrounding density; about ten of these objects exist, some of which are so large that the tidal force of the galactic gravitational field must affect their equilibrium. Especially prominent are the "group clouds" which are sirallar to classical "interstellar clouds" but which are clumped together into two groups; much of the large velocity dispersion of these objects may result from systematic radial motion. No enhanced 21-cm emission is observed at positions of dense dust clouds; this is attributed to the presence of cold or molecular hydrogen in these objects, which are bound by self-gravitation if the optical extinction is from classical dielectric spheres. Small concentrations of near stellar mass, distinctly different from the above concentrations, are observed in profusion; but since they are small they contain only about 10 per cent of the mass of the sheets. Typical radii of these "cloudlets" are a few parsecs; excess densities are not large, about a factor 2. The upper limit to the kinetic temperature implied by their velocity width is about 100 K. The highest brightness temperature observed in the region is 130 , equal to the commonly accepted value for the kinetic temperature in the galactic plane; the line profile does not appear saturated. This, together with previous determinations of kinetic temperature, implies the existence of large temperature variations in the interstellar gas. The decrease of the number of hydrogen atoms/cm2 in the line of sight with increasing galactic latitude is much faster than 1/sin b; the decrease near latitude 28 is almost discontinuous. The conclusion follows that in this region little hydrogen exists in the neighborhood of the Sun. Publication: The Astrophysical Journal Supplement Series Pub Date: December 1967 DOI: 10.1086/190164 Bibcode: 1967ApJS...15...97H full text sources ADS | data products SIMBAD (6)