Recent 21-cm absorption measurements in the direction of the galactic center.

Abstract

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Recent 21-cm absorption measurements in the direction of the galactic center. McClain, Edward F. Abstract The absorption by interstellar neutral hydrogen of continuum radiation from a discrete radio source (Hagen and McClain, 1954; Williams and Davies, 1954; Hagen et al., 1954) permits one to assign relative distance positions to the source and the hydrogen along the line of sight. Using this effect it is possible to determine whether the hydrogen lies in front of or behind the radio source. Lilley (1955) has_introduced the concepts of "Expected Profile" AT(v) and "Observed Profile" AT' (v). The expected profile is that profile which would be obtained for the position containing the radio source if the source were not present. The observed profile is that profile which is actually measured by scanning the receiver in frequency with the antenna pointed at the radio source. He has derived the equation ri(v) = - ln (i AT(v) fA where r,(v) is the optical depth of the hydrogen gas in front of the source and lying within the solid angle subtended by the source. TA is the indicated temperature of the radio source. If within the uncertainty of the measurements AT(v) and AT'(v) are equal for a portion of a profile and unequal for the remainder, one is justified in associating radial velocities and relative distance positions with the gas beyond the source and the gas between the source and observer. This technique has been applied to source IAU I7S2A (NRL No. 5 Sagittarius A, BSS 68) and a curve of T (v) versus wave length obtained. The technique is sensitive to the relative solid angles subtended by the source and the antenna beam and a more precise measurement will require detailed information on the source diameter. The measurements indicate that hydrogen having radial velocities extending from -50 km/ sec to 40 km/sec is present. The value of r, (v) is found to be zero from -15 km/sec to -50 km/sec referred to the local standard of rest indicating that gas responsible for emission in this range of velocities is probably beyond the radio source. Optical depths derived from the absorption occur over the range from -15 km/sec to 40 km/sec indicating that gas having velocities in this range is present between the observer and the source. Optical depths for the gas in front of the source reach peak values as high as 3.5 at a radial velocity of $3 km/sec. Radio source IAU 17S2A has been suggested as being associated with the galactic center. The position 1 = 327?8, b = - I?4 (Lund Pole) is quite close to the predicted center of the galaxy. If one assumes that this source is the galactic center the above results indicate that there is a relative velocity of approach between hydrogen on the far side of the galaxy and the local standard of rest and that a departure from circular motion is present in the galaxy. This conclusion is in agreement with models of the galaxy having a component of velocity toward the galactic center such as proposed in private communications by Edmundson and Rubin. The region containing the center is heavily obscured optically however and radio measurements are confused by an extensive hot background which seems to merge with the source. For these reasons it is impossible to make a definitive statement concerning the distance to the source at this time. Hagen, J. P. and McClain, E. F. `954, Ap. J. 120, 364. Lilley, A. E. and McClain, F. F. `954, Naval Research Lab. Rep. 4448, Oct. i8. Lilley, A. F. `955, A.. J. 6o, I67. Williams, D. R. W. and Davies, R. D. 1954, Nature 173, 1182. Naval Research Laboratory, Washington, D. C. Publication: The Astronomical Journal Pub Date: June 1955 DOI: 10.1086/107198 Bibcode: 1955AJ.....60..169M full text sources ADS |