2. Observational results
The BNM survey was carried out between 1989 and 1992 with the 15-m SEST (Swedish-ESO Submillimeter Telescope) at La Silla, Chile, and with the OSO (Onsala Space Observatory) 20-m Telescope, Sweden, in May 1993.The angular resolution of the telescopes at 99 GHz, the CS(2-1) line frequency, is for SEST and for OSO (main beam FWHM), similar to the IRAS survey position accuracy. The instruments, observing techniques, and data calibration are described by BNM. The observed sources were selected from the IRAS PSC (Version 1), using exactly the same criterion as in WC89. They are all located near the galactic plane, they have a flux ratio larger than 3.7 and a flux ratio larger than 19.3, and are not identified as stars or galaxies. A total of 1427 sources were selected. The survey covered for longitudes corresponding to the inner Galaxy, and . For the rest of the galactic plane the latitude coverage was .
Of the 1427 sources observed, the survey detected 843 (59%) above a level of K peak T (BNM), with a few detections below this level, particularly in the far outer Galaxy (Table 1). Of the total number of sources detected, 37% are in the I quadrant; 10% in the II quadrant; 11% in the III quadrant, and 42% in the IV quadrant. The fraction of detected sources is 0.58 toward the outer Galaxy (II and III quadrants), and 0.60 toward the inner Galaxy (I and IV quadrants); toward certain longitudes in the direction of the molecular annulus, however, the number of detected sources more than doubles that of undetected ones.
Table 1. Distribution in galactocentric radius of the sources included in the analysis
The distribution in longitude of the detected sources, integrated over latitude, shows two prominent maxima around and , toward lines of sight tangent to a galactocentric circle in radius (Fig. 2a). These maxima, symmetric with respect to , are strong direct observational evidence for the existence of an annulus of massive star formation in the Milky Way that is coincident with the molecular annulus, as traced by CO. The center of the distribution in galactic latitude of the detected sources (Fig. 2b) is very close to , with an average of and a FWHM of . The IRAS point-like sources not detected in CS, in comparison, are more evenly distributed in the galactic disk, and their latitude distribution is about 20% wider.
© European Southern Observatory (ESO) 2000
Online publication: June 8, 2000