Water maser distributions and their internal motions in the Sagittarius B2 complex

Abstract

Abstract We conducted a multi-epoch very long baseline interferometry (VLBI) observation of 22 GHz H2O maser sources, which are associated with the Sagittarius B2 (Sgr B2) complex, one of the most active star-forming regions in the Galactic center. Using the VLBI exploration of radio astrometry (VERA) array, we detected 29, 90, and seven maser features in Sgr B2(North), (Main), and (South), respectively. We also discovered two H2O maser features in Sgr B2 (Mid-North). Moreover, we successfully measured spatial distributions and internal motions of the H2O maser sources in Sgr B2(M). We then discovered that the H2O maser features in Sgr B2(M) are located at the outer edge of H ii regions detected by using the centimeter radio continuum emission, and observationally demonstrated that they have outward motions from the two H ii regions separated by 1′. We obtained an absolute proper motion of (μαcos δ, μδ) = (−2.17 ± 0.03, −2.63 ± 0.06) mas yr−1 and marginally detected a parallax of Sgr B2, which is consistent with other measurements. The absolute proper motion corrected by the internal motions of the H2O maser features implies that the Sgr B2 complex is located at the near side of the Galactic center and is moving toward the positive galactic longitude. We estimate that Sgr B2 is at 160 ± 10 pc in front of Sgr A* by assuming a simple circular orbit of the central molecular zone (CMZ).