The VATLY radio telescope has been used to draw the map of atomic hydrogen gas in the disk of the Milky Way. Effects resulting from its differential rotation, its cloud and arm structure and the presence of a dark matter halo have been observed. | Communications in Physics, Vol. 23, No. 2 (2013), pp. 107-119 MAPPING ATOMIC HYDROGEN IN THE DISK OF THE MILKY WAY NGUYEN VAN HIEP, PHAM TUAN ANH, PIERRE DARRIULAT, PHAM NGOC DIEP, PHAM NGOC DONG, DO THI HOAI, PHAM THI TUYET NHUNG AND NGUYEN THI THAO VATLY, Institute for Nuclear Science and Technology, 179 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam Email: dieppn@ Received 19 December 2012; revised manuscript received 28 March 2013 Accepted for publication 08 May 2013 Abstract. The VATLY radio telescope has been used to draw the map of atomic hydrogen gas in the disk of the Milky Way. Effects resulting from its differential rotation, its cloud and arm structure and the presence of a dark matter halo have been observed. I. INTRODUCTION The Milky Way, which hosts the Sun, is a spiral galaxy that contains a black hole of nearly 4 million solar masses in its centre, a large central bulge, a relatively young and thin disk and a halo. The dark matter halo dominates the mass of the Galaxy and the luminous halo includes some 150 globular clusters believed to have formed at the same time as the Galaxy, over ten billion years ago, and very diffuse, hot, highly ionized gas extending out to some hundred kpc (1 pc= lyr). The central bulge hosts old Population II stars with a bar cutting across it. The disk, a flat rotating system, contains atomic (HI) and molecular (H2 ) hydrogen gas, dust, and stars. The Sun, about five billion years old, sits ∼ 2/3 of the way from the centre to the edge of the disk (∼ kpc) and revolves around the centre of the Galaxy once every ∼ 250 million years. The disk has been observed at all wavelengths. It is ∼ 300 pc thick and ∼ 30 kpc in diameter and is structured in spiral arms associated with density waves that trigger the birth of new stars. In addition to intermediate age stars such as the Sun, it contains brighter Population I objects, in the form of young, hot stars, stellar associations, open clusters, diffuse nebulae,
