SiO Maser Survey of the Galactic Bulge IRAS Sources. III. Kinematics of the Bulge Stars

Abstract

We have surveyed IRAS sources toward the Galactic bulge in the SiO J = 1-0, υ = 1 and 2 transitions. The sources were selected from the IRAS Point Source Catalog on the basis of the IRAS 25/12 microns flux density ratio, extracting dust-enshrouded objects at distances of 8 kpc. SiO masers were detected in 79 of the 129 sources we surveyed in the region of |l| &lt; 15° and 3° &lt; |b| &lt; 15° (excluding the strips of 4° &lt; |b| &lt; 5° and 7 &lt; |b| &lt; 8°). The sources are divided into disk and bulge members based on the observed infrared luminosity. The SiO maser properties of the present samples are quite similar to the maser properties of the SiO sources that were found in the strips of |b| = 4°-5° and 7°-8° in previous studies. A full discussion of the radial velocities of the entire bulge SiO maser sources is given. Rotation curve analysis shows that the radial velocities of the northern sources are systematically shifted from the radial velocities of the southern sources. If this shift is interpreted as being the result of a tilt of the rotation axis of the bulge, the tilt angle is derived to be about 7° from the Galactic longitudinal circle. After correcting the effect of the tilt on the radial velocities, we obtain an average shift of the radial velocity of -17.7±7.6 km s<SUP>-1</SUP> at zero Galactic longitude for the bulge sources. For the disk sources, in contrast, we get an average shift of the radial velocity nearly equal to zero. This fact indicates that (1) the local standard of rest is moving toward the Galactic center direction, and/or (2) a streaming motion of stars in the Galactic bulge is observed. The velocity dispersion of the bulge sources in the rectangular area of |l| &lt; 4° and |b| &lt; 5° is found to be about 110 km s<SUP>-1</SUP> The dispersion of the bulge sources in the remaining area is about 70 km s<SUP>-1</SUP>, which is close to the dispersion of 60-70 km s<SUP>-1</SUP> of disk stars. Several forbidden regions (holes) are recognizable in the l - υ map of sampled stars; for particular ranges of radial velocities, no star can be found at some Galactic longitudes. The positions of such holes in the l - υ map correspond to the contour dips in the H I l - υ map taken at low Galactic latitude (|b| &lt; 3°). This may be evidence for complex streaming motion of stars resulting from a bar structure in the Galactic bulge.