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 $t_{\mathrm{SN}}\approx 0.1$ Myr. Together with the Galactic shear timescale t_shear ≈ 30 Myr, these values satisfy $t_{\mathrm{SN}}<t_{\mathrm{shear}}<t_{\mathrm{survival}}$ . 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.

Fig. 1. Face-on view of O-B2 stars' kinematics and gas distribution.

Left panel: spatial distribution of O-B2 stars (blue dots) within 0.5 kpc of the Perseus Arm in the Galactic XY plane. The Sun (green pentagram) is positioned at (X, Y) = (−8.34, 0) kpc. OB associations follow the nomenclature of K. Jardine (http://gruze.org/galaxymap/map_2020/). Cyan dashed lines indicate Galactic longitude directions l = 90°, 180°, and 270°. Right panel: A zoomed-in view of the XY velocity vectors of O-B2 stars near the Galactic plane, focusing on the region surrounding the Giant Oval Cavity (cyan arrows). Median velocities of the Cassiopeia and Auriga Peninsula groups (magenta arrows) are decomposed into a common peculiar velocity and two oppositely directed expansion components (red arrows). Velocities are shown for stars within 100 pc of the plane, excluding a few outliers. The black arrow (lower left) denotes a 20 km s⁻¹ reference velocity. Blue ellipses delineate the Cassiopeia and Auriga Peninsula groups. The grayscale map (ranging from 0 to 0.5 mag pc⁻¹) and contours (0.005 and 0.01 mag pc⁻¹) show differential extinction δA₀ at 550 nm from ref. . The cavity center, defined as the centroid of the low-dust region (δA₀ < 0.005 mag pc⁻¹), is marked by a red cross. Source data are provided as a Source Data file.

Fig. 2. Side view of the superbubble.

The upper two panels display the vertical positions (Z) and vertical velocities (V_Z) along the Perseus arm. The $X^{\prime }$ axis originates from (X, Y) = (−8.8, 4.2) kpc and follows the orientation of the Perseus arm. O-B2 stars located within the Perseus arm and the vicinity of the superbubble are shown in blue and red, respectively. Error bars indicate the uncertainties in position and velocity measurements. The orange and cyan curves represent the smoothed profiles of $Z\left(X^{\prime }\right)$ and $V_Z\left(X^{\prime }\right)$, with shaded regions indicating the 1σ confidence intervals. For comparison, the bottom panel shows the normalized smoothed profiles. Gray dashed lines denote Z = 0 and V_Z = 0. Source data are provided as a Source Data file.

Fig. 3. Evolution of superbubbles in galaxy-scale simulations.

Midplane gas density from simulations by refs. and are shown. The results demonstrate that large, stationary bubbles—such as the one marked by the red arrow—maintain their structure through a balance between gas inflow and local stellar feedback, while smaller bubbles (purple arrow) are quickly refilled with gas and dissipate unless a supernova occurs shortly after formation. Source data are provided as a Source Data file.

Fig. 4. Selection of O-B2 stars.

Left panel: Gaia color versus `absolute' magnitude diagram of all LAMOST and Gaia stars with distance uncertainties smaller than 20%. The blue curve represents the PARSEC isochrones for main-sequence stars with an age of 10 Myr. The red and purple lines correspond to the reddening curves for B5V and B3V stars, respectively. Right panel: effective temperature versus surface gravity diagram of the selected OB stars. The vertical line marks T_eff > 19,000 K. Source data are provided as a Source Data file.

Fig. 5. Distribution of O-B2 stars in the XY plane.

Left panel: spatial distribution of the selected O-B2 stars in the Galactic XY plane. The Sun, located at (X, Y) = (−8.34, 0) kpc, is marked by a green pentagram. The background grayscale represents the dust distribution from ref. . Solid blue lines delineate the best-fitting spiral arm models for the Local and Perseus Arms from ref. , with dashed lines showing the 0.5 kpc range around the arm models. Right panel: peculiar velocity vectors of O–B2 stars associated with the Perseus Arm and located within 100 pc of the Galactic midplane. Stars near the Giant Oval Cavity are shown as red arrows, while other stars are shown as blue arrows. Some stars with exceptionally high velocities are not shown for clarity. The black arrow at the lower right represents a reference velocity of 20 km s⁻¹. Source data are provided as a Source Data file.