# Stellar Overdensity in the Local Arm in Gaia DR2

## TL;DR

This study identifies a stellar overdensity in the Local arm using Gaia DR2 and 2MASS data, revealing complex spiral arm structures that challenge traditional models of galactic spiral arm formation.

## Contribution

It provides new observational evidence of stellar overdensity in the Local arm and discusses implications for spiral arm theories, highlighting the need for further investigation.

## Key findings

- Detected an arm-like stellar overdensity near the Local arm
- The overdensity's pitch angle is larger than that of high-mass star-forming regions
- Results suggest complex spiral arm dynamics not fully explained by existing models

## Abstract

Using the cross-matched data of Gaia DR2 and 2MASS Point Source Catalog, we investigated the surface density distribution of stars aged ~1 Gyr in the thin disk in the range of 90{\deg} <= l <= 270{\deg}. We selected 4,654 stars above the turnoff corresponding to the age ~1 Gyr, that fall within a small box region in the color-magnitude diagram, (J-Ks)0 versus M(Ks), for which the distance and reddening are corrected. The selected sample shows an arm-like overdensity at 90{\deg} <= l <= 190{\deg}.This overdensity is located close to the Local arm traced by high-mass star forming regions (HMSFRs), but its pitch angle is slightly larger than that of the HMSFR-defined arm. Although the significance of the overdensity we report is marginal, its structure poses questions concerning both of the competing scenarios of spiral arms, the density-wave theory and the dynamic spiral arm model. The offset between the arms traced by stars and HMSFRs, i.e., gas, is difficult to be explained by the dynamic arm scenario. On the other hand, the pitch angle of the stellar Local arm, if confirmed, larger than that of the Perseus arm is difficult to be explained with the classical density-wave scenario. The dynamic arm scenario can explain it if the Local arm is in a growing up phase, while the Perseus arm is in a disrupting phase. Our result provides a new and complex picture of the Galactic spiral arms, and encourages further studies.