Mass distribution of the Galaxy with VERA

Abstract

We aim to reveal the mass distribution of the Galaxy based on a precise rotation curve constructed using VERA observations. We have been observing Galactic H²O masers with VERA. We here report one of the results of VERA for IRAS 05168+3634. The parallax is 0.532 ± 0.053 mas which corresponds to a distance of 1.88^+0.21_−0.17 kpc, and the proper motions are (μ_αcosδ, μ_δ) = (0.23 ± 1.07, −3.14 ± 0.28) mas yr⁻¹. The distance is significantly smaller than the previous distance estimate of 6 kpc based on a kinematic distance. This drastic change places the source in the Perseus arm rather than in the Outer arm. Combination of the distance and the proper motions with the systemic velocity provides a rotation velocity of 227⁺⁹₋₁₁ km s⁻¹ at the source assuming Θ₀ = 240 km s⁻¹. The result is marginally slower than the rotation velocity at LSR with ~ 1−σ significance, but consistent with previous VLBI results for six sources in the Perseus arm. We also show the averaged disk peculiar motion over the seven sources in the Perseus arm as (U_mean, V_mean) = (11 ± 3, −17 ± 3) km s⁻¹. It suggests that the seven sources in the Perseus arm are systematically moving toward the Galactic center, and lag behind the Galactic rotation with more than 3-σ significance.