VERA astrometry toward the Perseus arm gap

The Perseus arm has a gap in Galactic longitudes (l) between 50° and 80° (hereafter the Perseus arm gap) where the arm has little star formation activity. To better understand the gap, we conducted astrometric observations with VERA (VLBI Exploration of Radio Astrometry) and analyzed archival H I 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 likely associated with the gap. The measured parallaxes are 0.140 ± 0.018 (mas), 0.726 ± 0.038 (mas), 0.074 ± 0.037 (mas), and 0.118 ± 0.035 (mas) for G050.28−00.39, G053.14+00.07, G070.33+01.59, and G079.08+01.33, respectively. Since the fractional parallax error of G070.33+01.59 is large (0.5), we estimated a 3D kinematic distance of the source of 7.7 ± 1.0 kpc using both the LSR velocity (V LSR ) and the measured proper motion. 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 Perseus arm. We discovered rectangular holes with integrated brightness temperatures of <30 K arcdeg in l vs. V LSR of the H I data. One of the holes is centered near (l, V LSR ) = (47°, −15 km s ⁻¹ ), and G049.41+00.32 is associated with the rim of the hole. However, G050.28−00.39 is not associated with the hole. We found extended H I emission on one side of the Galactic plane when integrating the H I data over the velocity range covering the hole (i.e., V LSR = [−25, −5] km s ⁻¹ ). G049.41+00.32 and G050.28−00.39 are moving toward the emission. The Galactic H I 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.