Is the Galaxy a barred spiral?

Abstract

view Abstract Citations (2) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Is the Galaxy a barred spiral? Johnson, Hugh M. Abstract If the Galaxy is actually a barred spiral, should we certainly know it now? The question is not so radical if one thinks of short-barred spirals such as NGC 1398. This note suggests that some features of a barred spiral may be observed in the Galaxy. Let us first consider the inclination of the spiral arms to the radius vector. It is about 650 according to the Yerkes optical observations, but is nearly 900 according to the Leiden 21-cm observations. The discrepancy may be at least partly removed by various dynamical hypotheses for the interpretation of the radio data. On the other hand, barred spirals often show nearly circular rings superposed on a bar, e.g., NGC 4725, or branches that bend sharply from the ends of the bar, e.g., NGC 1300. The optical arms of the Galaxy are observed as segments too short by a factor of 2 or 3 for a good check as to whether they represent branches near the end of a bar whose axis slightly precedes the sun. The second observation is of the neutral hydrogen about the direction of the galactic center (Kwee, Muller, and Westerhout 1954). The Leiden group proposes a central disk of mostly ionized hydrogen that extends 3 kpc from the center in highly turbulent motion. This is surely not like the M 31 nucleus, which is free of Ha emission and which shows spiral arms that become finer, not more turbulent-looking, as they enter it. The disk does not explain asymmetries in the 21-cm line profiles, such as the wings of positive and negative velocity over 3210 < 1 ~ 3290 and the single positive wing of greater intensity over 3290 ~ 1 ~ 3430 However, a slightly inclined and foreshortened bar would be observed over unsymmetrical parts and distances on either side of center, perhaps as described. The third observation is of longitudinal asymmetries of the continuous radio radiation in the Galaxy (Pawsey and Bracewell 1955). Isophotes for a wide range of frequencies show that the preceding hemisphere (pole 1 = ~~0, b = 00) radiates more than the following hemisphere at all latitudes. Some particulars of this imbalance are the north and south bulges roughly along the circle 1 = 3500 and the saddle minimum near 1 = 2000, b = 00, or 500 past the anti-center. Spiral arms and an axially symmetric disk fail to explain these asymmetries, but a radiating bar may suit them if the sun lies within its trailing edge. Kwee, K. K., Muller, C. A. and Westerhout, G. 1954, B.A. N., No.458. Pawsey, J. L. and Bracewell, R. N. 1955, Radio Astronomy, (Oxford: at the Clarendon Press), Chap. 7. State University of Iowa, Iowa City, Iowa. Publication: The Astronomical Journal Pub Date: February 1957 DOI: 10.1086/107441 Bibcode: 1957AJ.....62...19J full text sources ADS |