# Evidence of Hubble flow-like motion of young stellar populations away from the Perseus arm

## TL;DR

This study provides evidence that young stars in the Perseus Arm exhibit outward motion consistent with a Hubble flow-like expansion, suggesting a dynamic process in the spiral arm structure.

## Contribution

It presents the first observational evidence of young stellar populations moving outward from the Perseus Arm, indicating a possible expansion similar to cosmic Hubble flow.

## Key findings

- Young stars are moving outward from the Perseus Arm.
- The expansion rate is approximately 15 km/s/kpc.
- A convergence zone near W3/W4/W5 suggests a common origin.

## Abstract

In this letter we present evidence of coherent outward motion of a sample of young stars ($t<$30 Myr) in the Perseus Arm, whose apparent origin is located in the vicinity of the W3/W4/W5 complex. Using astrometric and photometric data from the Gaia DR2 catalog of an 8$^\circ$ 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 emerge 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 $15~{\rm km~s}^{-1}~{\rm kpc}^{-1}$.