Gaia DR1 Evidence of Disrupting the Perseus Arm

Junichi Baba,Daisuke Kawata,Jason A S Hunt,Robert J J Grand,Noriyuki Matsunaga

doi:10.3847/2041-8213/aaa839

Abstract

Abstract We have discovered a clear sign of the disruption phase of the Perseus arm in the Milky Way using Cepheid variables, taking advantage of the accurately measured distances of Cepheids and the proper motions from Gaia Data Release 1. Both the Galactocentric radial and rotation velocities of 77 Cepheids within 1.5 kpc of the Perseus arm are correlated with their distances from the locus of the Perseus arm, as the trailing side is rotating faster and moving inward compared to the leading side. We also found a negative vertex deviation for the Cepheids on the trailing side, −27.°6 ± 2.°4, in contrast to the positive vertex deviation in the solar neighborhood. This is, to our knowledge, the first direct evidence that the vertex deviation around the Perseus arm is affected by the spiral arm. We compared these observational trends with our N-body/hydrodynamics simulations based on a static density-wave spiral scenario and with those based on a transient dynamic spiral scenario. Although our comparisons are limited to qualitative trends, they strongly favor the conclusion that the Perseus arm is in the disruption phase of a transient arm.

Highlights

How spiral arms in disk galaxies are created and maintained has been a long-standing question in galactic astronomy
We have discovered a clear sign of the disruption phase of the Perseus arm in the Milky Way using Cepheid variables, taking advantage of the accurately measured distances of Cepheids and the proper motions from Gaia Data Release 1
This is, to our knowledge, the first direct evidence that the vertex deviation around the Perseus arm is affected by the spiral arm

Summary

INTRODUCTION

How spiral arms in disk galaxies are created and maintained has been a long-standing question in galactic astronomy. Because a DYN arm is almost co-rotating with the stars at every radii, N -body simulation studies show that there should be a characteristic gas and stellar motion affected by the spiral arms (Grand et al 2012b, 2016; Baba et al 2013). Cepheids are young stars whose ages are expected to be around 20 − 300 Myr (Bono et al 2005). Such young stars are expected to have small velocity dispersion, and it is easier to find a systematic motion due to dynamical effects if it exists. The age range of Cepheids is comparable to the lifetime of the DYN arm, and they are expected to be sensitive to the dynamical state of the spiral arms.

PECULIAR MOTIONS OF CEPHEIDS

Findings

DYNAMICAL NATURE OF THE PERSEUS ARM