Evidence of Hubble Flow-like Motion of Young Stellar Populations away from the Perseus Arm

In this Letter we present evidence of the coherent outward motion of a sample of young stars (t < 30 Myr) in the Perseus Arm, whose apparent origin is located in the vicinity of the W3/W4/W5 complex. Using astrometric and photometric data from the Gaia Data Release 2 catalog of an 8◦ radius field centered near W4, we selected a sample of young intermediate- to high-mass star candidates. The sample is limited to sources with parallax uncertainties below 20% and Bayesian distance estimates within 1800 and 3100 pc. The selection includes embedded stellar populations as well as young open clusters. Projected velocities derived from perspective-corrected proper motions clearly suggest that the young star population emerged from the Perseus Arm, with a possible convergence zone near W3/W4/W5 region, tracing a front that expands away at a rate of about $15\,\mathrm{km}\,{{\rm{s}}}^{-1}\,{\mathrm{kpc}}^{-1}$.