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The origin of the Perseus-arm gap revealed with VLBI astrometry

Published online by Cambridge University Press:  07 February 2024

Nobuyuki Sakai*
National Astronomical Research Institute of Thailand (Public Organization).
Hiroyuki Nakanishi
Graduate School of Science and Engineering, Kagoshima University
Kohei Kurahara
National Astronomical Observatory of Japan, Oshu-shi, Iwate 023-0861, Japan
Daisuke Sakai
National Astronomical Observatory of Japan, Oshu-shi, Iwate 023-0861, Japan The Iwate Nippo Co., Ltd.
Kazuya Hachisuka
National Astronomical Observatory of Japan, Oshu-shi, Iwate 023-0861, Japan
Jeong-Sook Kim
Ulsan National Institute of Science and Technology / Chungbuk National University
Osamu Kameya
National Astronomical Observatory of Japan, Oshu-shi, Iwate 023-0861, Japan Oshu Space & Astronomy Museum


The Perseus arm has a gap in Galactic longitudes (l) between 50° and 80° where the arm has little star formation activity. To understand the gap, we conducted VERA (VLBI Exploration of Radio Astrometry) astrometry and analyzed archival H <SC>I</SC> data. We report on parallax and proper motion results from four star-forming regions, of which G050.28–00.39 and G070.33+01.59 are associated with the gap. Perseus-arm sources G049.41+00.32 and G050.28–00.39 lag relative to a Galactic rotation by 77 ± 17 km s-1 and 31 ± 10 km s-1, respectively. The noncircular motion of G049.41+00.32 cannot be explained by the gravitational potential of the arm. We discovered rectangular holes with integrated brightness temperatures less than 30 K arcdeg in l vs. VLSR of the H <SC>I</SC> data. Also, we found extended H <SC>I</SC> emission on one side of the Galactic plane when integrating the H <SC>I</SC> data over the velocity range covering the hole. G049.41+00.32 and G050.28–00.39 are moving toward the emission. The Galactic H <SC>I</SC> disk at the same velocity range showed an arc structure, indicating that the disk was pushed from the lower side of the disk. All the observational results might be explained by a cloud collision with the Galactic disk.

Contributed Paper
© The Author(s), 2024. Published by Cambridge University Press on behalf of International Astronomical Union

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