Stellar Overdensity in the Local Arm in Gaia DR2

Using the cross-matched data of Gaia DR2 and the 2MASS Point Source Catalog, we investigated the surface density distribution of stars aged ~1 Gyr in the thin disk in the range of 90° ≤ l ≤ 270°. We selected 4654 stars above the turnoff corresponding to the age ~1 Gyr, that fall within a small box region in the color–magnitude diagram, (J − K s )0 versus M(K s ), for which the distance and reddening are corrected. The selected sample shows an arm-like overdensity at 90° ≤ l ≤ 190°. 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 explain using the dynamic arm scenario. On the other hand, the pitch angle of the stellar Local Arm, if confirmed, is larger than that of the Perseus arm, and is difficult to explain using the classical density-wave scenario. The dynamic arm scenario can explain the pitch angle of the stellar Local Arm, if the Local Arm is in a growing up phase, while the Perseus arm is in a disrupting phase. Our result provide a new and complex picture of the Galactic spiral arms, and encourages further studies