SiO maser survey of the galactic bulge IRAS sources. II. The 4° < |b| < 5° strips

We have surveyed Galactic bulge IRAS sources in the strips between - 15° < / < 15° and 4° < |b| < 5° in the SiO J = 1-0 v = 1 and 2 transitions. The sources were selected from the IRAS Point Source Catalog based on the IRAS 25/12 μm flux density ratio, extracting dust-enshrouded objects at a distance of about 8 kpc. The SiO masers were detected in 62 of 93 sources surveyed in these strips. The JHKL photometric data (1.2, 1.6, 2.2, and 3.4 μm) were also obtained for 85 sources. Because distances to the IRAS sources in these strips are not known, we have evaluated the distance based on the observed luminosity. The probability of the objects being a bulge source is calculated assuming that the luminosity function of these infrared sources is the same as that of previously observed |b| = 7°-8° sources. The contamination due to foreground sources in the sample is minimized by excluding sources with the bulge probability less than 50%. The SiO maser properties of the |b| = 4°-5° sources are found to be very similar to those of |b| = 7°-8° sources. No systematic difference between bulge and disk sources is found for the SiO J = 1-0 v = 2/1 intensity ratio. The average rotation rate of the bulge stellar system for the |b| = 4°-5° sources is found to be 10.8 (±2.6) kms-1 deg-1, indicating no sign of increase of the rotation rate at this strip compared with the rate obtained for the |b| = 7°-8° sources. On the contrary, the velocity dispersions of stars. 91.0 ±9.5 km s-1, for the |b| = 4°-5° sources seems appreciably larger than the dispersion, 72.5 ± 8.9 km s-1 obtained for the |b| = 7°-8° sources. A small, systematic velocity shift of the radial velocities is found between the south and north strip sources. If this shift is interpreted as due to a tilt of the bulge, the tilt angle of the bulge rotation axis to the Galactic circle would be 8-31+27 deg, which is slightly smaller than the value obtained for the |b| = 7°-8° sources.