A large, long-lived, slowly-expanding superbubble across the Perseus arm.

Abstract

Stellar feedback is a crucial mechanism in galactic evolution, as demonstrated by the widespread bubbles observed with JWST. In this study, we combine data from Gaia and LAMOST to obtain a sample of young O-B2 stars with full three-dimensional velocity information. Focusing on the largest known superbubble in the Milky Way, we identify groups of O-B2 stars at its periphery, exhibiting a transverse velocity of 25.8 km s^-1 and an expansion velocity of 6.2 km s^-1. Using these velocities, we calculate a crossing time t_cross ≈ 20 Myr and an expansion timescale t_expansion ≈ 80 Myr. We estimate a survival timescale t_survival ≈ 250 Myr and a supernova interval <math xmlns="http://www.w3.org/1998/Math/MathML"> <msub><mrow><mi>t</mi></mrow> <mrow><mi>SN</mi></mrow> </msub> <mo>≈</mo> <mn>0.1</mn></math> Myr. Together with the Galactic shear timescale t_shear ≈ 30 Myr, these values satisfy <math xmlns="http://www.w3.org/1998/Math/MathML"> <msub><mrow><mi>t</mi></mrow> <mrow><mi>SN</mi></mrow> </msub> <mo><</mo> <msub><mrow><mi>t</mi></mrow> <mrow><mi>shear</mi></mrow> </msub> <mo><</mo> <msub><mrow><mi>t</mi></mrow> <mrow><mi>survival</mi></mrow> </msub> </math> . The energy and momentum from supernovae are sufficient to sustain the bubble's growth against ambient pressure. This indicates that repeated supernovae replenish energy faster than shear and turbulent distort the cavity. Our analysis classifies the Giant Oval Cavity as a large, quasi-stationary superbubble, similar to the Phantom Bubble observed by JWST, stabilised by the interplay between stellar feedback and Galactic disk dynamics.