tokens: by these telescopes (e.g., Serabyn Lacy 1985 ; Serabyn et al. 1988 ; Lacy et al. 1991b ; Roberts et al. 1996 ; Zhao et al. 2009 , 2010 ). The stretched appearance and kinematics of the GCMS suggest the models in which the streamers are tentative structures with Keplerian orbits around Sgr A* (e.g., Serabyn et al. 1988 ; Zhao et al. 2009 , 2010 ). There are still alternate models to explain these properties (e.g., one-armed spiral; Lacy et al. 1991b ; Irons et al. 2012 ). The tidal force of Sgr must have a serious effect on the interstellar medium (ISM) in the vicinity of Sgr A*, i.e., the GCMS (e.g., Lacy et al. 1982 ). Furthermore, the strong Lyman continuum radiation from the OB and WR stars in the Central cluster ionizes the ISM rapidly in the region (e.g., Genzel et al. 1996 ; Paumard et al. 2006 ). A recent dust observation of the GCMS with ALMA suggested that there is a possible scenario for the formation of the Central cluster: a star-forming molecular cloud is falling from a region somewhat far from Sgr A* and supplies young stars and the ISM to the vicinity of Sgr A* (e.g., Tsuboi et al. 2016 ). However, it is not clearly demonstrated how the gas is fed to Sgr A* because the innermost part of the ionized gas has very weak intensity and complicated structures. We present new observational results on the ionized gas streams in the vicinity of Sgr A* based on the observation of the H42 α recombination line using the Atacama Large Millimeter/submillimeter Array (ALMA). The distance of the Galactic center is assumed to be 8 kpc in this paper. 2. Observation and Data Reduction We have observed the GCMS in the H42 α recombination line (85.6884 GHz) as a part of the ALMA Cy.1 observation (2012.1.00080.S; PI: M.Tsuboi), which is an ionized gas tracer. The entire ALMA observation consists of a 137-pointing mosaic of the 12 m array and a 52-pointing mosaic of the 7 m array (ACA), covering a area including the “Galactic center 50 km s −1 molecular cloud” and the GCMS in CS (97.980953 GHz), SiO (86.846995 GHz), H 13 CO (86.754288 GHz), and H42 α emission lines. The molecular cloud encompasses the most conspicuous star-forming region in the vicinity of Sgr A*. We will present the detailed description of the analysis and the full results in another paper. We have detected the recombination line in the GCMS and the Sgr A East H ii region, and we concentrate on the GCMS in the H42 α recombination line in this paper. The data of the recombination line have angular resolutions of and using “natural weighting” and “Briggs weighting ( R = 0.5)” as u–v sampling, respectively. The frequency channel width is 244 kHz. The velocity resolution is 1.7 km s −1 (488 kHz). J0006−0623, J1517−2422, J717−3342, J1733−1304, J1743−3058, J1744−3116, and J2148+0657 were used as phase calibrators. The flux density scale was determined using Titan, Neptune, and Mars. Because the observation has a large time span of 19 months, the absolute flux density accuracy may be as
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