view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS An approximate distance determination for radio source Sagittarius A. McClain, Edward F. Abstract The absorption of continuum radiation from a discrete radio source by interstellar neutral hydrogen (Hagen and McClain 1954; Williams and Davies 1954, 1955; Hagen, Lilley and McClain 1955) permits one to assign relative distance positions to the source and the hydrogen gas along the line of sight. The absorption effect does not of itself yield an absolute distance scale. The ability of the absorption technique to reveal structure~in distance is based on the assumption that the radial velocity along the line of sight is a function of distance. This is generally conceded to be true and to be a function of differential galactic rotation for all galactic longitudes with the exception of the galactic center and anticenter. Here the classical assumption of pure circular motion would preclude the existence of a radial velocity other than zero. An approximate distance scale has been derived in the direction of the galactic center from a study of the hydrogen as a function of radial velocity. If one assumes that the galaxy is a flat disk of thickness 1, an observer inside the disk can relate the angular width ~ of the disk, normal to the plane, to the distance d at which the angle was measured by the relation (p t tan - = - 2 2d The angular extent of the gas was measured in the velocity range -40 to +40 km/sec. By assuming a thickness of 240 pc for the galaxy a relation has been found between random velocity and distance. These measurements indicate that the random velocity of the gas probably increases rather rapidly beyond about 2 kpc from the sun. The absorption effect for source I 7S2A (Sagittarius A) occurs roughly within the velocity range -10 to + 15 km/sec corresponding to a distance of 3 or 4 kpc. A study of the uncertainties involved in the measurements places extreme limits on the distance of from 2 to 6 kpc with the most probable distance being between 3 and 4 kpc. Using an independent method, Williams and Davies (1955) have found a distance of about 3 kpc, in good agreement with the present result. A consideration of the measurements reported herein leads to the conclusion that the radio source responsible for the intense emission peakati = ~~7?8andb =- ~?~isprobablynot associated with the galactic center but rather is superimposed on the broad central galactic background. It is interesting to note that Haddock and McCullough (1955) have suggested that this source is thermal and possibly associated with an HIl region in the group of OB stars studied by Hiltner (1954). Haddock, F. T. and McCullough T. P., 1955, A. J. 6o, i6i. Hagen, J. P. and McClain, E. F. 1954, Ap. J. 120, 364. Hagen, J. P., Lilley, A. E. and McClain, E. F. 1955, Ap. J. 122, 361. Hiltner, W. A. 1954, Ap. J. 120, 41. Williams, D. R. W. and Davies, R. D. 1954, Nature `73, 1182. 1955, Ibid. 175, 1079. Naval Research Laboratory, Washington, D. C. Publication: The Astronomical Journal Pub Date: February 1956 DOI: 10.1086/107275 Bibcode: 1956AJ.....61....8M full text sources ADS |
Read full abstract