Structure of the Galactic Bulge

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The Galactic bulge has been the focus of many researches in the past decade. In 1991 we proposed the non-axisymmetric model of the bulge on the basis of the distribution of IRAS sources around the Galactic center. Subsequently, the global rotation of the bulge was detected by our SiO observation of the bulge IRAS sources.Since the total number of the maser sources in the previous study was only sixteen, systematic survey of the bulge maser source was planned at the Nobeyama Radio Observatory. It took three years to obtain the radial velocities of 124 sources in the bulge area. We determined the bulge rotation rate as 9.3km/sec/deg. which was consistent with those obtained by other methods.Although the number of sources was far from completeness, we still notice some interesting features in the velocity structure of the bulge. First, the mean rotation rate decreses with the distance from the Galactic plane. The only plausible way to compromize the differential rotation with the non-axisymmetric shape of the bulge seems to allow the existence of the internal velocity field inside the bulge. Second, there are areas where few sources were detected on the velocity-longitude diagram. Those 'holes' could be the clues to the full understanding of the velocity structure.Near infrared observation of those bulge sources were repeatedly performed at the Sutherland Observatory in South Africa. More data is needed to determine the precise periods of their light variation.