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

Domain: physics
Published: 2025-11-26
Editorial outcome: major_revision

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_SN ≈ 0.1 Myr. Together with the Galactic shear timescale t_shear ≈ 30 Myr, these values satisfy t_SN < t_shear < t_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.

Introduction
The structure and kinematics of spiral arms are fundamental to understanding the formation and evolution of the Milky Way1. Spiral arms play a critical role in star formation by concentrating gas and dust into regions of high star-forming activity2–4. Investigating these structures provides key insights into the Galaxy’s overall dynamics and the processes driving its evolution5–8. Both observational studies and theoretical models suggest that spiral arms are dynamic and transient features that significantly influence the spatial distribution and motion of stars and interstellar matter9,10.

Star formation in the Milky Way begins with the collapse of molecular clouds, but this process is strongly regulated by stellar feedback11–13. Stellar feedback disrupts molecular clouds, thereby influencing the efficiency of star formation14,15. It also injects energy into the interstellar medium (ISM), sustaining turbulence and driving large-scale processes within the ISM16–18. The impact of stellar feedback is particularly evident in the widespread presence of bubbles observed throughout galactic disks, as recently highlighted by James Webb Space Telescope (JWST) observations19. The size distribution of the bubbles detected by JWST exhibits a secondary peak and a trailing extension at large radii, with these features becoming more pronounced at greater galactocentric distances20. However, their physical properties remain poorly understood due to observational limitations. Superbubbles in the Milky Way, particularly those in the Solar neighborhood, thus provide rare, kinematically resolved benchmarks for studying superbubble evolution.

The Sun is located within a superbubble known as the Local Bubble, which is associated with the Local Arm of the Milky Way21–24. By analyzing the three-dimensional (3D) distribution and motions of young stars, dust, and gas, we can identify large-scale structures such as superbubbles. These structures are shaped by the combined effects of stellar winds and supernova explosions, offering valuable insights into the feedback mechanisms that sculpt the ISM and influence the evolution of spiral arms. Despite their importance, comprehensive studies of superbubbles in spiral arms beyond the Local Arm remain limited. Recent advancements in observational astronomy, particularly through large-scale surveys such as Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST)25,26 and Gaia27,28, have significantly enhanced the accuracy of stellar position and velocity measurements in the Milky Way. These improved datasets have deepened our understanding of spiral arm structures and underscored the role of young massive stars as key tracers of Galactic dynamics. In particular, the Perseus arm—one of the Milky Way’s major spiral arms and the closest spiral arm in the outer Galaxy—has garnered attention due to its complex interactions and unique kinematic properties.

Results
O-B2 stars in the Perseus arm
We have compiled a comprehensive catalog of 369 very young (age < 20 Myr), massive O- and early B-type (O-B2) stars, using data from the LAMOST and Gaia surveys. These stars are associated with the Perseus arm, located within 0.5 kpc of the arm’s model in the Galactic XY plane. The spatial distribution of these stars exhibit clear clustering, corresponding to the structure of the Perseus arm, as shown in Fig. 1. The 3D positions and velocities of these stars provide significant insights into the kinematic properties of the Perseus arm and the underlying mechanisms that influence the spatial distribution and motion of stars and interstellar matter.

In the outer Galaxy, the Perseus arm is primarily traced by six prominent OB associations, spanning the Cassiopeia, Perseus, Auriga, Gemini, and Orion regions. The average peculiar velocity of stars within the Perseus arm is ~16 km s−1 in the XY plane, consistent with velocities observed in high-mass star-forming regions via maser measurements29. These peculiar motions allow us to explore dynamic trends within each OB association. Our analysis reveals the presence of the largest known superbubble in the Milky Way, referred to as the Giant Oval Cavity30. This structure spans across the Perseus arm, with a diameter exceeding 1 kpc. It is primarily bordered by the Cassiopeia and Auriga OB associations in the Galactic plane, while the Perseus OB association lies below the plane.

The Giant Oval Cavity is a slow-expanding superbubble
The Giant Oval Cavity extends across Galactic longitudes of ~100° to 150°, as shown in Fig. 1. Within this region, at |Z| < 100 pc, the Cassiopeia and Auriga Peninsula groups exhibit coherent XY motions directed outward from the cavity. This pattern strongly suggests an expansion phase for the superbubble. The observed stellar velocities result from a combination of their peculiar velocities and the expansion velocity of the surrounding cavity. To quantify these motions, we calculated the median velocities of the Cassiopeia and Auriga Peninsula groups and decomposed them into two components: a shared peculiar velocity and two oppositely directed components representing expansion from the cavity center. This decomposition is depicted in the right panel of Fig. 1. From our analysis, we derived an expansion velocity of v_expansion ≈ 6.2 ± 2 km s−1 and a shared peculiar velocity of v_parallel ≈ 25.6 ± 3 km s−1. The expansion velocity of the Giant Oval Cavity is consistent with measurements for the Local Bubble, which has an expansion velocity of ~5.5 km s−1 10. However, the Giant Oval Cavity is significantly larger, with a diameter of ~1 kpc compared to the ~200 pc diameter of the Local Bubble23.

In Fig. 2, we provide an edge-on view of the selected O-B2 stars. The Auriga peninsula, Perseus, and Cassiopeia OB groups form a continuous arc-like structure. The Auriga peninsula group is predominantly located near the Galactic plane, at Z ≈ 0, while the Perseus group is situated below the plane, at Z < 0, forming the southern segment of the Giant Oval Cavity. The Cassiopeia group exhibits a vertical distribution rising to the plane. This arc is primarily found on the southern side of the Galactic plane, extending to ~Z ~ −270 pc at its southernmost point, which is significantly higher than the typical scale heights of the Milky Way’s molecular gas disk, estimated to be 40–70 pc31–33.

Additionally, Fig. 2 shows the vertical velocity field associated with this large arc. Our analysis reveals a consistent vertical velocity pattern, with minimal oscillatory behavior. The O-B2 stars in the Auriga Peninsula group exhibit near-zero vertical velocities, while those in the Perseus group have notably negative velocities, indicating a rapid outward movement away from the Galactic plane. The Cassiopeia group shows a clear gradient in vertical velocities, with stars moving away from the plane at decreasing speeds as their distance from the Galactic plane decreases. A comparison of the vertical velocity and positional distributions reveals a striking agreement, with no discernible phase difference, suggesting that the arc is in a stable expansion phase. The estimated vertical expansion velocity of the superbubble is ~8.5 km s−1, which is comparable to its horizontal expansion velocity.

The Giant Oval Cavity, identified as a ~1 kpc scale region with reduced dust density30, is likely the largest known superbubble in the Milky Way disk. It represents an extreme example of stellar feedback-driven structures in disk galaxies. Interest in such large superbubbles has grown considerably following the identification of many similar features in external galaxies like NGC 62820. Several nearby stellar groups—specifically the Cassiopeia, Perseus, and Auriga Peninsula—exhibit positions and motions consistent with expansion driven by feedback from this cavity. In contrast, other groups along the Perseus arm, including Auriga, Gemini, and Outer Orion, show no evidence of such organized expansion. The absence of coherent radial motion in these groups, combined with the precise geometric alignment of expanding stars along the cavity boundary, suggests that these expansion patterns are not the result of random Galactic disk dynamics. Instead, they appear to be a direct consequence of the cavity’s influence. This disparity suggests a direct link between the ISM structure and the dynamics of associated stellar populations.

Observational data reveal a coherent expansion pattern, an organized velocity structure, and spatial undulations along the cavity’s boundary. These features support the interpretation of the cavity as a uniformly expanding superbubble. With an approximate diameter of 1 kpc and a current expansion velocity of ~6.2 km s−1, its estimated dynamical age, based on a simple R/v calculation, is approximately t_expansion ≈ 500 pc / 6.2 km s−1 ≈ 80 Myr. This makes it one of the oldest known superbubbles in the Galaxy, approximately four times older than the Local Bubble.

Theoretical framework for bubble stability
We propose that the Giant Oval Cavity belongs to a class of quasi-stationary bubbles whose boundaries are sustained by a dynamical balance between external gas inflow and energy input from supernovae occurring within the cavity. The cavity is sufficiently large that numerous supernovae are expected to explode inside it before Galactic turbulence and shear can erase the structure. In galactic disks, molecular gas has a typical velocity dispersion of ~4 km s−1 34. This velocity sets the rate at which turbulent gas flows can erode bubble boundaries. The corresponding survival timescale of a bubble is given by
t_survival = l_bubble / σ_v,disk (1)
where l is the characteristic size (i.e., diameter) of the bubble and σ_v,disk is the velocity dispersion of the surrounding medium. Applying this to the Local Bubble and the Giant Oval Cavity yields:
t_survival,Local Bubble ≈ 50 Myr (2)
t_survival,Giant Oval Cavity ≈ 250 Myr (3)

A bubble can retain its structure if a supernova explodes within it before it is eroded by turbulence. To estimate the supernova timescale, we assume a surface density of 5 M⊙ pc−2 35 and a gas depletion time of 2.5 Gyr 36 for the solar neighborhood. This implies a star formation rate of 2 M⊙ pc−2 Gyr−1. Assuming one supernova occurs per 150 M⊙ of star formation37, the local supernova rate is:
n_SN = 0.01 pc−2 Gyr−1 (4)

The time between supernova events within a bubble of size l_bubble is then given by:
t_SN = 1 / (l_bubble^2 * n_SN) (5)

Substituting characteristic sizes gives
t_SN,Local Bubble ≈ 2.5 Myr (6)
t_SN,Giant Oval Cavity ≈ 0.1 Myr (7)

t_SN ≪ t_shear implies feedback-dominated expansion, radial growth might become filled with dense gas before the next supernova can occur. However, larger bubbles—often located in the outer regions of galaxies—remain stable over extended periods.

The stability of the Giant Oval Cavity involves two primary criteria: temporal and energetic. We compare the supernova replenishment time, t_SN, with the timescale for differential rotation to distort the cavity, t_shear ≈ 30 Myr 41, and with the turbulent erosion time, t_survival. Supernovae must recur on a timescale shorter than both competing processes. Our calculated values, t_SN < t_shear < t_survival, meet this condition, indicating that feedback events are sufficiently frequent to prevent the bubble’s disruption. The combined energy and momentum input from supernovae must be sufficient to create the cavity by overcoming the gravitational binding energy and turbulent pressure of the ambient gas, and to offset radiative losses to maintain the cavity’s expansion over time. As detailed in the Methods, the Giant Oval Cavity requires ~400 supernovae to be powered. Our estimated supernova loading of the system satisfies this requirement, confirming that the energy and momentum budgets are ample. Meeting both the temporal and energetic criteria indicates that the Giant Oval Cavity is a long-lived, feedback-driven structure undergoing quasi-stationary expansion. In this regime, Galactic shear deforms the cavity significantly without erasing it completely, while the bubble evolves through concurrent expansion, contraction, and distortion. Its boundary maintains a dynamical equilibrium between shear forces, supernova feedback, and turbulent gas inflow. This explains the irregular boundaries seen in simulations (Fig. 3).

Several additional factors contribute to this picture. The bubble’s low-density interior likely reduces star formation rates compared to Galactic averages, potentially decreasing internal supernova rates. However, triggered star formation at cavity edges counteract this by enhancing stellar birth in compressed shells, which would further shorten t_SN. This aligns with observations showing elevated star formation rates in the Giant Oval Cavity region 42,43, consistent with the feedback-triggered star formation scenario.

Moreover, bubbles exceeding the Milky Way’s gas scale height (~100 pc) develop chimneys that vent hot gas vertically. Although t_SN ≪ t_shear implies feedback-dominated expansion, radial growth might halt when cavities surpass this scale height. For the Giant Oval Cavity, vertical venting converts radial momentum into vertical outflows, limiting lateral expansion while maintaining structural coherence through continuous feedback. Heat conduction44 also contributes to energy loss, but for larger structures, the relevant surface area—approximately r_bubble * h_disk —is proportionally smaller relative to their volume, reducing the overall impact.

Observations of young OB stars located beneath the cavity reveal vertical velocities directed away from the Galactic plane, consistent with momentum injection from recent supernovae. This supports a feedback cycle in which massive star explosions initially clear surrounding gas to create the cavity, while subsequent shockwaves compress nearby clouds, triggering new star formation. Later generations of supernovae from these young stars then replenish the system with energy, sustaining its expansion. This process aligns with our discovery of the ~10 Myr-old clusters observed along cavity edges, whose velocities match feedback-driven gas motions. We have conducted a detailed calculation of the number of supernovae required to form and maintain the Giant Oval Cavity. Accounting for the combined effects of energy injection, radiative loss, and ISM interaction, we estimate that ~400 supernovae are necessary. This estimate aligns with predictions from the Kennicutt–Schmidt relation45 for star formation on kiloparsec scales. A complementary estimate, scaling the Local Bubble’s ~20 supernovae24,46 by the cavity’s fivefold larger radius, yields about 500 supernovae—in close agreement with our detailed calculation. These feedback processes inject significant momentum into the ISM, driving large-scale structures such as gas chimneys that redistribute material vertically and may play a key role in shaping the interface between the Milky Way’s disk and halo.

In summary, large bubbles like the Giant Oval Cavity reach a quasi-stationary state where their boundaries are shaped by a dynamical balance between mass inflow, galactic shear, and internal supernovae. Shear continuously deforms the structure, while supernovae within the cavity counteract this by pushing material outward, maintaining a quasi-stationary boundary. These findings match galaxy-scale simulations showing that large cavities become long-lived, dynamically evolving structures capable of migrating within host galaxies. This framework provides a clear explanation for the nature of giant cavities such as the Phantom Bubble recently observed by JWST.

Methods
Data and sample selection
This study utilizes data from LAMOST DR9 v1.0 26, which includes both low- and medium-resolution spectra obtained by LAMOST between October 2011 and June 2021. For this work, we focus on the low-resolution catalog, which contains over 10 million spectra. The low-resolution spectra have a resolution of R ~ 1800 and cover the optical wavelength range from 3800 to 9000 Å. Additionally, we incorporate data from Gaia DR3 28, which provides astrometric solutions and broad-band photometric measurements (G, G_BP, G_RP) for ~1.5 billion sources.

We selected O- and early B-type stars from the combined LAMOST and Gaia datasets. First, we cross-matched these datasets using a 1.5″ search radius. We then filtered stars with parallax uncertainties smaller than 20%. Distances were calculated from Gaia DR3 parallaxes using a Bayesian approach50,51, with the posterior probability given by:
p(d|ϖ) = d^2 exp(−1/2 * (ϖ - ϖ_zp)^2 / σ_ϖ^2) p(d) (8)
where σ_ϖ and ϖ_zp are the uncertainties and global zero-point offset of Gaia parallaxes, respectively, and p(d) is the prior on the space density distribution of the stars. We adopt a zero point of ϖ_zp = −0.026 mas from ref. 52, and the Galactic structure model from ref. 53 as the prior.

In the initial phase of our analysis, we excluded late-type main-sequence stars using the Gaia color-‘absolute’ magnitude diagram. The term ‘absolute’ magnitude here refers to a magnitude that accounts for the distance modulus but does not include corrections for interstellar extinction. It is calculated as G − 5 log d + 5, where d denotes the distance. As shown in the left panel of Fig. 4, we systematically removed late-type stars based on their observed colors and ‘absolute’ magnitudes. We retained only stars located above the B5V extinction curve, refining our sample to 57,375 stars.

A significant portion of the sample consisted of late-type giants. The lack of extinction corrections led to some overlap between early B-type main-sequence stars and redder giants. To further refine the selection, we calculated spectral line indices from the LAMOST spectra for these stars. We computed indices such as Ca II K, Hδ, CN, Ca 4227, G4300, Hγ, Fe 4383, He 4388, Fe 4531, Hβ, Ca 4455, He 4471, Fe 4531, He 4542, Fe 4668, Hβ, Fe 5015, Mg 1, Mg 2, Mgb, Fe 5270, Fe 5335, Fe 5406, and Fe 5709 54,55. To derive spectral line indices, we first identify the line of interest and select two continuum regions on either side of it. A linear fit is then applied to the flux values in these regions to establish the pseudo-continuum level. Finally, the line index (i.e., equivalent width) is computed by integrating the flux difference between the observed spectrum and the pseudo-continuum across the line’s wavelength range. Using machine learning techniques, specifically a random forest classifier, we identified OB stars among the candidates. Cross-referencing with the Simbad database, we selected 7873 stars with identified spectral types, including 3529 OB-type stars and 4344 stars of other types. We then trained a random forest classifier with spectral indices as inputs and spectral types as outputs. The distinct spectral line features of OB stars allowed the classifier to achieve an accuracy of 97%. Applying this classifier to the remaining candidates, we identified a final sample of 19,600 OB-type stars.

The primary objective of our study is to isolate very young (with ages t < 20 Myr) O- and early B-type stars (O-B2) to trace the superbubble structure of the Milky Way. For this purpose, we calculated atmospheric parameters for the 19,600 OB stars identified through spectral line indices using the HOTPAYNE pipeline56. As shown in the right panel of Fig. 4, the majority of the selected OB stars are main-sequence stars with temperatures exceeding 10,000 K. We specifically focused on stars earlier than B3 in spectral type, with effective temperatures T_eff > 19,000 K. This rigorous selection process resulted in a final sample of 1608 unique O-B2 stars.

Three-dimensional position and velocity
To determine the radial velocities of the refined O-B2 star sample, we analyzed their spectra obtained from the LAMOST telescope. Using theoretical template spectra for OB stars provided by TLUSTY57, we performed spectral fitting in the blue wavelength range (4000–5400 Å). This allowed us to accurately derive the line-of-sight velocities for each OB star. To estimate the uncertainty in radial velocities, we used a Monte Carlo (MC) method based on the spectral uncertainties reported by LAMOST. Specifically, we added normally distributed noise to the flux values based on their respective errors, recalculated the radial velocities 100 times, and derived the standard deviation of the results as the radial velocity uncertainty.

By combining the distances calculated from Gaia DR3 parallaxes, the radial velocities obtained from LAMOST, and the positional and proper motion data from Gaia DR3, we derived the three-dimensional positions (X, Y, Z) and velocities (V_x, V_y, V_z) for every star in the sample. In this coordinate system, the X-axis points towards the Galactic center, the Y-axis aligns with the direction of Galactic rotation, and the Z-axis points towards the North Galactic Pole. Similarly, the U-axis is directed towards the Galactic center, the V-axis follows the Galactic rotation, and the W-axis extends towards the North Galactic Pole. For this study, we adopted the following reference values for the solar neighborhood: the Sun’s position is (X_⊙, Y_⊙, Z_⊙) = (−8.34, 0, 0.025) kpc 29,58, with local standard of rest (LSR) velocities of (U_⊙, V_⊙, W_⊙) = (11.69, 10.16, 7.67) km s−1, Oort constants (A, B) = (16.31, −11.99) km s−1 kpc−1, and the Sun’s rotational velocity V_LSR = 231 km s−1 59. Using these values, we calculated the peculiar velocities for each star by subtracting the LSR motion, Galactic rotation, and shear effects10.

The spatial distribution of the selected O-B2 stars in the Galactic X–Y plane is shown in the left panel of Fig. 5. Due to the observational footprint of the LAMOST survey, most of these stars are situated in the outer Galactic disk. The majority of the young O-B2 stars exhibit a clustered distribution, while those at larger distances appear more dispersed, likely due to greater uncertainties in the distance estimates. Among OB associations with well-determined distances within 3 kpc, the spatial arrangement of stars clearly reflects the spiral arm structure21. In particular, we observe distinct concentrations of OB stars aligned with the Orion and Perseus arms of the Milky Way.

O-B2 stars associated with the Perseus arm
We identified a subset of O- and early B-type stars associated with the Perseus arm of the Milky Way. This selection was based on the Perseus arm model described by ref. 21. Specifically, we selected stars located within 0.5 kpc of the Perseus arm in the Galactic X–Y plane. This criterion resulted in a sample of 369 O-B2 stars.

The spatial distribution of these stars, as shown in the left panel of Fig. 5, supports their association with the Perseus arm. A detailed catalog of these stars is available online at the following URL: https://paperdata.china-vo.org/diskec/obsamp/ob2perseus.fits. This catalog offers comprehensive data for each star, including celestial coordinates, the signal-to-noise ratio of the LAMOST spectra, distance estimates, parallax values, proper motions, radial velocities, and associated uncertainties.

The right panel of Fig. 5 displays the peculiar velocity vectors of stars associated with the Perseus arm and located within 100 pc of the Galactic midplane, in the X–Y plane. These velocity patterns provide insight into the local kinematics of OB associations along the arm. A clear contrast emerges: stars near the Giant Oval Cavity—particularly those in the Cassiopeia and Auriga Peninsula regions (marked with red arrows)—exhibit coherent radial expansion away from the cavity center. In comparison, stars in other parts of the Perseus arm, such as Auriga, Gemini, and Outer Orion (blue arrows), display more irregular motions without any systematic radial expansion.

Uncertainties of the expansion and peculiar velocities
To estimate the uncertainties of the expansion and peculiar velocities of the OB stellar groups associated with the Giant Oval Cavity, we employed an MC approach based on the uncertainties of the three-dimensional positions and velocities of the individual O-B2 stars. In this method, we perturbed the measured positions and velocities by adding random noise consistent with their observational errors and recalculated the expansion and peculiar velocities across 300 realizations. The standard deviations of these resultant velocities were adopted as the uncertainty estimates: ~2 km s−1 for the expansion velocity and 3 km s−1 for the peculiar velocity.

Smoothing of vertical position and velocity profiles
To visualize the vertical position (Z) and vertical velocity (V_Z) profiles along the Perseus arm, we used the smoothfit Python package, which applies constrained optimization techniques for smooth curve fitting. Specifically, we performed one-dimensional smoothing spline fitting using smoothfit.fit1d. The fitting was conducted over the relevant range using 1000 evenly spaced evaluation points, linear basis functions (degree=1) to ensure piecewise continuity, and a regularization parameter λ=0.1 to balance between fitting accuracy and smoothness.

To assess the uncertainties of the smoothed profiles, we implemented 1000 nonparametric bootstrap resamplings of the original dataset. Each resampled dataset was smoothed using the same method, and the 1σ confidence intervals were computed from the pointwise standard deviations of the ensemble of smoothed curves.

Number of supernova events
To estimate the number of supernovae needed to create and maintain a superbubble in the Galactic disk, we modeled the bubble as an oblate spheroid with an in-plane radius of R ≈ 500 pc and a vertical height of h ≈ 100 pc, reflecting the disk’s thin structure. The volume V of the bubble is calculated as V = 4πR^2h, yielding V ≈ 3.1 × 10^63 cm^3. Our energy budget includes three components: gravitational binding energy (E_bind), turbulent pressure energy (E_turb), and radiative cooling losses (E_loss). First, E_bind is derived from the standard relation E_bind = 3/5 * G * (Σ_gas * πR^2)^2 / R. Assuming a gas surface density of Σ_g = 10 M⊙ pc−2, resulting in E_bind ≈ 6.3 × 10^51 erg. Second, E_turb is computed as E_turb = E_turb * V. We have E_turb = 1/2 * ρ_0 * δv^2, with number density n_0 = 1 cm−3 and velocity dispersion δv = 10 km s−1, giving E_turb ≈ 2.5 × 10^51 erg. Third, E_loss is calculated as E_loss = n_e^2 * Λ(T) * V * t_exp, using electron density n_e = 0.01 cm−3 (for dilute hot gas at T ~ 10^6 K), cooling function Λ(T) = 10^−22 erg cm^3 s−1, and expansion timescale t_exp = 80 Myr, yielding E_loss ≈ 7.8 × 10^52 erg. The total energy requirement is E_total ≈ 8.7 × 10^52 erg. Assuming each supernova releases E_SN ≈ 10^51 erg with a coupling efficiency of 20%, this leads to a required number of supernovae as N_SN ≈ 435.

This supernova number aligns with the expected output from a stellar population with a mass of M_* ~ 10^5 M⊙, based on a standard Salpeter initial mass function (IMF) that assumes an average of one supernova per 150 M⊙ of formed stars37. The Kennicutt–Schmidt (KS) relation45 estimates a star formation rate surface density of: Σ_SFR ≈ 2.5 × 10^−4 * (Σ_g / 1 M⊙ pc−2)^1.4 M⊙ yr−1 kpc−2. For Σ_g = 10 M⊙ pc−2 over a period of 80 Myr, this results in a stellar mass of M_* ~ 4 × 10^5 M⊙, corresponding to approximately 2600 supernovae. This value is slightly higher but remains consistent with our estimate, considering uncertainties in supernova feedback efficiency and the stochastic nature of IMF sampling.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This study utilized astrometric data from Gaia with spectroscopic data from LAMOST to obtain the 3D position and velocity maps of O-B2 stars. They measured transverse and expansion velocity of O-B2 stars near Giant oval cavity along the Perseus Arm in the Milky Way and derived that the expansion timescale of the superbubble is longer than the crossing time and shear time. Based on these estimates, they proposed that the superbubble is expanding in a quasi-stationary configuration, unlike smaller structures such as the Local Bubble.\n\nThis work has combined new astrometric and spectroscopic data to give insights into the velocity distributions of O-B2 stars near the Giant Oval Cavity, which is novel. However, all the estimates quoted lack uncertainties, and thus it is difficult to judge whether the results are significant. Furthermore, the scientific significance and potential impact to the field and related field are insufficiently justified. Finally, the physical picture of quasi-stationary expansion is based on the velocity measurement at the present day and may not be generally applicable (see Major Comments below). The manuscript needs to be improved with more quantitative analyses, and the comments below needs to be addressed before the manuscript could be considered for publication in Nature Communications.\n\nMajor comments:\n1. As mentioned above, all estimated velocities lack measurement uncertainties, making it difficult to assess the significance of the results. Specifically, there are no uncertainty estimates for the expansion velocity and peculiar velocity of the O-B2 stars around the Giant Oval Cavity as well as the estimation of the relevant timescales. In Figure 2, it is unclear how the smoothed velocity profiles in the bottom panel are derived, and no error bars are provided.\n2. The scientific significance of this study and potential impact of this and related fields are not sufficiently discussed. How has the study helped to improve understanding of the shaping of star forming regions by stellar feedback (regarding triggered star formation? filament formation? superbubble formation)? What key questions have the study addressed (what has been validated/invalidated)? Are the results expected or surprising (e.g., by comparing with theoretical works)? The proposed physical picture regarding the quasi-stationary expansion is vague, and it is unclear quantitatively what we could learn from such a system (e.g., estimates of injected energy required? Comparisons with inputs provided by the currently observed O-B2 stars?).\n3. The primary conclusion of this study, which is the slowly expanding nature of the superbubble, is based on the velocity measurements and timescale estimates *at the present day*. However, the expansion velocity of the superbubble is likely not a constant, and the observed value at the present today could significantly underestimate the expansion velocity at earlier times. Assuming the expansion velocity was higher by a factor of a few at early phases, the expansion timescale would be shorter than the crossing and shear timescales, in which case stellar feedback would be dominant and no quasi-stationary expansion is required. In other words, this fashion of slow expansion is probably relevant only at later stages of superbubble evolution but likely not applicable throughout its expansion history. Therefore, there are not necessarily two diverse evolutionary pathways for smaller and larger bubbles; their evolutionary history could be fundamentally similar but just have evolved to different stages.\n\nMinor comments:\n1. In the Abstract, it is said that “supernovae from triggered star formation at the bubble’s edge may provide additional energy to maintain this equilibrium.” However, insufficient discussion and evidences are provided to support this conjecture.\n2. In Figure 1, it is unclear what the grayscale background and contours represent.\n3. In the 2nd to last paragraph in the Main Text, it is mentioned: “Sequentially triggered star formation, driven by successive supernova events, appears to be a key mechanism fueling the cavity’s expansion.” It is insufficiently described why triggered star formation is needed for the cavity’s expansion.", "reviewer_2": "Chen et al. combine data on young stars in the relatively nearby Perseus Arm of the Galaxy contained in two large-scale surveys conducted by the Gaia and LAMOST instruments, respectively. They find evidence for a coherent structure in the stellar phase space data, namely two groups of stars, Cassiopeia and Auriga Peninsula which are both located towards the periphery of the \"Giant oval cavity\", identified in dust data, and which seem to have similar, but oppositely directed systematic expansion velocities away from the centre of this cavity.\n\nI found the work to be somewhat interesting; with some further refinement of the text and some more details (as described below), I may be in a position to recommend the draft article for publication.\n\nA first, overarching point is the following: the authors frame their work as the discovery of a `superbubble'; I find this usage somewhat problematic: it seems to me that the authors have found a structure in the O-B2 star phase space distribution that is, very plausibly, *related* to a superbubble identified in dust data, but it should not be identified with the superbubble. In other words, a superbubble, while related to concentrated and somewhat sustained star formation activity, is an ISM structure, not a stellar structure. In particular, I would generally take a superbubble to imply a region of high temperature, high pressure, and largely ionised gas bounded by cooler and denser, perhaps largely neutral, ISM gas. So for instance, it would sit better with me that the title were something more like \"A large, long-lived, slowly-expanding stellar structure associated with a superbubble...\" or similar.\n\nFollowing from this point, one gets a strong visual impression from figure 1 that the Cassiopeia and Auriga Peninsula stellar groups are located at the edge of the Giant oval cavity, but how much of this apparent association is due to judicious choice of the grey scale of the dust contours? [I think the authors should define the grey scale.]\n\nFurther on this point, if I understand correctly, the stars in the Auriga peninsula and Cassiopeia groups are up to ~20 Myr, similar to the claimed expansion time of the superbubble. To they extent that it's correct to treat the stars as moving ballistically, they should then have been launched close to the centre of the Giant oval cavity and they would have inherited the kinematics of the gas from which they formed. But it does not seem necessary to me that that the superbubble boundary (defined by the gas) should expand at a constant velocity, so the fact that the boundary of the white region lies close to these two stellar groups is a coincidence to be explained. Do the authors agree with that? Are they, in fact, saying that the gas boundary is expanding at a more-or-less constant speed? I would appreciate hearing what the authors have to say about this; maybe I misunderstand the physical interpretation.\n\nIn any case, what, exactly, is meant by the superbubble becomes important in discussion, e.g., about the evolution of the bubble's boundary (in the first paragraph and elsewhere). In this context, do the authors mean the boundary between the hot/low density gas and the surrounding denser, ISM or do they mean the boundary defined by the young star groups?\n\nDetailed comments:\nFigure 1: can the authors show the position of the cavity centre? How is it defined? Please define the greyscale. An important take-away from this figure is the opposing, small red arrows for each group: I found these very hard to see.\nFigure 2: the authors should show representative error bars on a single star datum in each panel. It would be good to include the y = 0 line in each panel (in faint grey) to guide the eye. What is the smoothing scale? The authors should be able to define some sort of statistical error band around the smoothed profile plot lines.\nLine 79: I don't see from fig.1 how we are meant to infer that the Cavity extends from 100 to 150 degrees in l.\nLine 85: \"opposing components\" -- maybe \"oppositely directed components\" for clarity; it took me a while to understand what the authors meant here.\nLine 103: \"...Perseus group have notable negative velocities...\". From the figure, this claim seems to be founded on only a small number of detected stars. Is it statistically robust? Again, it would be good to see some sort of statistical error band on the smoothed curve.\nLine 104-105: the claim about the \"clear gradient\" would be better sustained with an actual plot.\nFigure 3: instead of writing \\approx 80 Myr, etc, can the authors actually quote central values with errors for these various timescales?\nLine 122-124: \"...in a state of dynamical equilibrium...\" I'm not sure I really understand the full argument here. See my point above questioning whether it is (or is not) a coincidence that the edge of the cavity seen in the dust map is currently coincident with the stellar groups. Can the authors rephrase/expand this argument please?\nLine 178 please quantify \"very young\"", "reviewer_3": "This paper uses Gaia-identified O-B2 stars to characterize expansion motions within a large super-bubble across one of the spiral arms of our Galaxy, a structure previously identified by Vergely et al 2022 using dust extinction maps. Based on these motions, the authors argue that this is a long-lived structure capable of surviving erosion through shear or other competing processes. They favor an explanation in which supernovae, themselves the result of star formation triggered at the edges of the super-bubble, contribute to the long-term survival of this cavity.\n\nThe paper adds valuable kinematic information about the velocity of matter in the bubble walls as inferred from the space motions of massive stars. However, we identify several issues with its central assertion.\n\n1. The crux of the argument is that the expansion timescale is much larger than the shearing timescale. The authors estimate an expansion timescale as d/v where d is the size of the bubble and v is the expansion velocity as measured from O and B stars. However, if the current structure is the product of expansion from a significantly smaller size, then it is very likely to have decelerated to its current velocity. To argue this point on physical grounds, it would be necessary to present a physically motivated expansion law that accounts for the current and past state of the structure -- for instance, the fact that the expansion velocity is slow enough for the motion to be highly radiative, and that it is larger than the scale height of the gaseous disk. Any assertion that it is older than the shear timescale should take this history into account.\n\n2. To fully explore the scenario in which the cavity is cleared, and kept clear, by supernovae associated with its formation, it would be necessary to evaluate how many such events are required to create its current expansion, including phenomena like the release of internal pressure when a super-bubble blows out of the gaseous disk. This would need to be compared to the plausible gas and star formation environment of the region, which one expects (on kiloparsec scales) to follow the Kennicutt-Schmidt relation.\n\n3. The paper is incomplete without a fair evaluation of alternative scenarios. For instance, the cavity could be a purely gravitational structure, e.g., an unusually empty inter-arm region. Or, it could be related to the Parker instability. In either case, OB stars would be working within an existing under-density; it would then be easier to clear, and this may have happened more recently. The spatial correlation between OB stars and the cavity walls does not, in itself, implicate them in forming the entire structure.\n\nBecause little evidence is provided for the authors' preferred interpretation, we judge that the paper is not acceptable in its current form.\n\nA few additional points and questions may be useful for a future version:\n4. What are the uncertainties of the quoted velocities and expansion times?\n5. What other signatures should accompany the structure, if it has indeed been created by supernovae? A few possibilities come to mind. There may be a giant chimney structure left behind by the blowout of the driving super-bubble. Other signatures of the driving pressure may also be identifiable. Or, there may exist a noticeable age gradient of stars whose main sequence lifetimes match the past history of the structure. The lack of such signatures may be important future constraints.\n6. Are similar expansion features identifiable in other stellar structures, such as the Auriga Peninsula or the Gemini cluster?", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "Response to Reviewers\nWe express profound gratitude to the reviewers for their expert analysis and substantive\nrecommendations, which have enhanced the rigor and presentation of this study. Each\nscholarly concern raised has been methodically incorporated through systematic\nrevisions to the manuscript. All modifications responding to the reviewers' observations\nappear in blue highlighting to facilitate editorial evaluation, accompanied by detailed\nrationales in this response document.\nReviewer #1 (Remarks to the Author):\nMajor comments:\n1. As mentioned above, all estimated velocities lack measurement uncertainties, making it\ndifficult to assess the significance of the results. Specifically, there are no uncertainty estimates\nfor the expansion velocity and peculiar velocity of the O-B2 stars around the Giant Oval Cavity\nas well as the estimation of the relevant timescales. In Figure 2, it is unclear how the smoothed\nvelocity profiles in the bottom panel are derived, and no error bars are provided.\nReply: In our study, both the three-dimensional positions and velocities of the stars\ninclude inherent measurement errors. The uncertainties in distance arise from the\npropagation of Gaia's parallax errors, while the velocity uncertainties are based on Gaia\nproper motion errors combined with line-of-sight velocity errors obtained from\nLAMOST spectra. To estimate the uncertainties in radial velocities, we employed a\nMonte Carlo method. Using the spectral uncertainties reported by LAMOST, we added\nnormally distributed noise to the flux values according to their respective errors,\nrecalculated the radial velocities 100 times, and derived the standard deviation of the\nresults as the radial velocity uncertainty. Building on the full set of three-dimensional\npositions and velocities, along with their associated uncertainties, we then applied a\nMonte Carlo approach to estimate the uncertainties in both the expansion velocity and\nthe peculiar velocity of the OB groups associated with the Giant Oval Cavity. In this\nprocess, the positions and velocities were perturbed with random noise consistent with\ntheir measurement errors, and the expansion and peculiar velocities were recalculated\nover 300 realizations. The standard deviations from these results provided the\nuncertainty estimates, yielding approximately 2 km\\,s$^{-1}$ for the expansion velocity\nand 3 km\\,s$^{-1}$ for the peculiar velocity. These methodological details and results are\nnow explicitly stated in the revised manuscript (Section Methods, Pages 19 - 21).\nRegarding the timescale estimation, we used a characteristic size of 1 kpc for the cavity\nto derive an order-of-magnitude estimate of the expansion timescale. Since this\ncalculation was intended as a rough approximation to assess the general evolutionary\ntimescale, we did not compute a formal uncertainty.\nFor Figure 2, we have updated the Figure to include error bars representing the\nuncertainties in both position and velocity for each data point. Furthermore, we have\nclarified in the revised manuscript (Section Methods, Page 21) how the smoothed velocity\nand position profiles were generated. Specifically, we employed the \\texttt{smoothfit}\nPython package, which uses a constrained optimization method to fit smooth curves to\nthe vertical position ($Z$) and vertical velocity ($V_Z$) as functions of the transformed\ncoordinate $X'$.\n2. The scientific significance of this study and potential impact of this and related fields are not\nsufficiently discussed. How has the study helped to improve understanding of the shaping of\nstar forming regions by stellar feedback (regarding triggered star formation? filament\nformation? superbubble formation)? What key questions have the study addressed (what has\nbeen validated/invalidated)? Are the results expected or surprising (e.g., by comparing with\ntheoretical works)? The proposed physical picture regarding the quasi-stationary expansion is\nvague, and it is unclear quantitatively what we could learn from such a system (e.g., estimates\nof injected energy required? Comparisons with inputs provided by the currently observed O-\nB2 stars?).\nReply: We thank the referee for emphasizing the importance of clearly articulating the\nbroader scientific significance of our study. Our work is motivated by the need to better\nunderstand the origins and evolution of superbubbles in disk galaxies, a topic recently\nhighlighted by JWST observations revealing the widespread presence of superbubbles,\nfor example in NGC 628. The Giant Oval Cavity offers a particularly valuable case study,\nas it likely represents a local analogue of the Phantom bubbles observed in external\ngalaxies. Being one of the most remarkable structures linked to stellar feedback in the\nSolar vicinity, it provides a rare opportunity to investigate superbubble formation and\nevolution in detail, thanks to the availability of precise astrometric and kinematic data\nfrom Gaia and LAMOST.\nOur analysis reveals that the young stellar groups associated with the Giant Oval\nCavity exhibit coherent velocity structures, indicating that their dynamics have been\ninfluenced by feedback-driven outflows. The motions of the Cassiopeia and Auriga\nPeninsula stellar groups suggest that they formed along a large-scale molecular filament,\nwhich was subsequently fragmented and reshaped by stellar feedback. These results\nillustrate how feedback mechanisms not only regulate star formation but also redistribute\ngas and stars on large scales, thereby shaping the structure of the ISM.\nA key finding of our study is the very slow present-day expansion velocity of the\ncavity, measured to be approximately 6.2 km\\,s$^{-1}$. Measuring the expansion velocity\nof a superbubble with full three-dimensional positional and velocity information is\nextremely challenging given the technological limits, and to our knowledge, this is the\nfirst superbubble beyond the Local Bubble for which such a measurement has been\nachieved. Despite being nearly five times larger than the Local Bubble, the Giant Oval\nCavity expands at a similar rate, implying an evolutionary timescale of about 80 Myr.\nThis finding challenges theoretical models that predict rapid disruption of large-scale\nstructures by Galactic shear and supports the idea that feedback-driven superbubbles\ncan maintain coherence over significantly longer periods than previously assumed. These\nobservations provide an important framework for interpreting the Phantom bubbles\nidentified by JWST in external galaxies, where direct kinematic information is not\navailable. Moreover, the size distribution of the JWST-detected bubbles shows a\nsecondary peak and a trailing extension at large radii, features that become more\nprominent at greater galactocentric distances. However, the underlying physical\nprocesses behind these distributions remain poorly constrained due to observational\nlimitations. The Giant Oval Cavity, together with the Local Bubble, serves as a rare,\nkinematically resolved benchmark for advancing our understanding of superbubble\nformation and evolution.\nThe observed slow expansion suggests that the cavity is currently in a late stage of\nevolution, where feedback energy input is balanced by radiative cooling and Galactic\nshear. This finding supports scenarios where superbubbles gradually decelerate over\ntime. To provide a quantitative context, we applied the superbubble model of Weaver et\nal. (1977), which predicts that under constant stellar wind energy injection, a bubble with\na radius of 500 pc would require approximately 130 Myr to form. If supernova energy\ninputs are included, the formation timescale would be shorter. Based on our estimates,\nsustaining the Giant Oval Cavity would require an energy input equivalent to roughly\n400 supernovae, assuming a 20\\% energy coupling efficiency. This is consistent with the\nexpected output of a stellar population with a total mass of order $10^5\\,M_{\\odot}$.\nIn summary, our study addresses a key question in ISM research: how feedback-\ndriven structures can survive against disruptive processes like Galactic shear. By\ndemonstrating that a coherent superbubble can persist for approximately 80 Myr, we\nprovide crucial insights into the timescales and energy requirements of superbubble\nevolution. This has direct relevance for interpreting observations of superbubbles in\nother galaxies, especially where three-dimensional kinematic data are unavailable.\nThe revised manuscript now explicitly discusses these broader implications, incorporates\nenergy budget estimates, and compares our results with theoretical predictions.\n3. The primary conclusion of this study, which is the slowly expanding nature of the\nsuperbubble, is based on the velocity measurements and timescale estimates *at the present\nday*. However, the expansion velocity of the superbubble is likely not a constant, and the\nobserved value at the present today could significantly underestimate the expansion velocity at\nearlier times. Assuming the expansion velocity was higher by a factor of a few at early phases,\nthe expansion timescale would be shorter than the crossing and shear timescales, in which case\nstellar feedback would be dominant and no quasi-stationary expansion is required. In other\nwords, this fashion of slow expansion is probably relevant only at later stages of superbubble\nevolution but likely not applicable throughout its expansion history. Therefore, there are not\nnecessarily two diverse evolutionary pathways for smaller and larger bubbles; their\nevolutionary history could be fundamentally similar but just have evolved to different stages.\nReply: We thank the referee for the thoughtful and constructive comment regarding the\ntime dependence of the superbubble’s expansion velocity. In response, we have revised\nthe manuscript to clearly state that the currently observed expansion velocity (~6.2 km/s)\nrepresents a late-stage value and likely underestimates the velocity during earlier phases\nof evolution.\nSuperbubbles are expected to decelerate over time due to radiative energy losses and\ninteractions with the ambient interstellar medium. We now incorporate in the revised\ntext that the expansion velocity was likely higher in the past—potentially in the range of\n20–30 km/s, as proposed by Weaver et al. (1977). To better estimate the dynamical age,\nwe adopt a time-averaged expansion velocity of approximately 12.5 km/s. Using this value,\nwe derive a dynamical timescale of roughly 40 Myr, which remains longer than the\nestimated Galactic shear timescale of about 30 Myr. Furthermore, we have incorporated\na physically motivated expansion model based on the framework of Weaver et al. (1977).\nApplying this model with representative parameters, we find that a superbubble would\nrequire approximately 130 Myr to reach a radius of 500 pc, slightly longer than our\nearlier simple estimate based on R/v. However, the Weaver solution assumes a continuous\nenergy input from stellar winds, and if multiple supernovae contribute, as likely occurred,\nthe effective energy injection rate would be higher, thereby reducing the timescale\naccordingly.\nWe have also revised the conclusions to emphasize that the present-day dynamics of\nthe Giant Oval Cavity do not exclude a faster early expansion phase. Instead, we interpret\nits current properties as the result of an evolutionary sequence in which feedback-driven\nexpansion gradually slows over time due to radiative cooling and Galactic shear. This\nframework suggests that the Giant Oval Cavity and structures such as the Local Bubble\nrepresent different stages along a continuous evolutionary path, rather than arising from\nfundamentally distinct formation mechanisms. To reflect this understanding, we have\nremoved prior references to “diverse” evolutionary pathways and now present the Giant\nOval Cavity as a mature superbubble where feedback and shear are approximately\nbalanced.\nMinor comments:\n1. In the Abstract, it is said that “supernovae from triggered star formation at the bubble’s edge\nmay provide additional energy to maintain this equilibrium.” However, insufficient discussion\nand evidences are provided to support this conjecture.\nReply: In the revised manuscript, we provide additional context to support the suggestion\nthat triggered star formation may contribute to sustaining the cavity’s expansion. Using\nthe superbubble evolution framework from Weaver et al. (1977), we estimate that a\nbubble driven solely by stellar winds would require ~130 Myr to reach its current radius\nof 500 pc. This timescale exceeds our earlier 80 Myr estimate based on the simple ratio of\nradius to velocity (𝑅/𝑣). However, the Weaver model assumes continuous energy input\nfrom winds, whereas the inclusion of supernova energy from subsequent stellar\ngenerations would enhance the total energy injection rate. For instance, we calculate that\nthe cavity’s energy budget corresponds to ~400 SNe (assuming 20\\% energy coupling\nefficiency).\n2. In Figure 1, it is unclear what the grayscale background and contours represent.\nReply: We thank the referee for noting the ambiguity. The caption of Figure 1 has been\nrevised to explicitly define the grayscale background and contours.\n3. In the 2nd to last paragraph in the Main Text, it is mentioned: “Sequentially triggered star\nformation, driven by successive supernova events, appears to be a key mechanism fueling the\ncavity’s expansion.” It is insufficiently described why triggered star formation is needed for\nthe cavity’s expansion.\nReply: We have expanded our discussion to clarify the importance of triggered star\nformation. The revised text explains that the cavity’s sustained growth relies on a\nfeedback cycle: Early supernovae from the progenitor OB stars evacuate gas to form the\ncavity, while subsequent shockwaves compress nearby molecular clouds, triggering new\nstar formation. Later supernovae from these younger stars replenish energy losses,\nprolonging the expansion. This cycle aligns with the observed correlation between the\ncavity’s edges and ~10 Myr-old stellar clusters, which trace dense filaments along the\ncavity walls and exhibit coherent velocities indicative of feedback-driven dynamics.\nReviewer #2 (Remarks to the Author):\nA first, overarching point is the following: the authors frame their work as the discovery of a\n`superbubble'; I find this usage somewhat problematic: it seems to me that the authors have\nfound a structure in the O-B2 star phase space distribution that is, very plausibly, *related* to\na superbubble identified in dust data, but it should not be identified with the superbubble. In\nother words, a superbubble, while related to concentrated and somewhat sustained star\nformation activity, is an ISM structure, not a stellar structure. In particular, I would generally\ntake a superbubble to imply a region of high temperature, high pressure, and largely ionised\ngas bounded by cooler and denser, perhaps largely neutral, ISM gas. So for instance, it would\nsit better with me that the title were something more like \"A large, long-lived, slowly-\nexpanding stellar structure associated with a superbubble...\" or similar.\nFollowing from this point, one gets a strong visual impression from figure 1 that the Cassiopeia\nand Auriga Peninsula stellar groups are located at the edge of the Giant oval cavity, but how\nmuch of this apparent association is due to judicious choice of the grey scale of the dust\ncontours? [I think the authors should define the grey scale.]\nFurther on this point, if I understand correctly, the stars in the Auriga peninsula and Cassiopeia\ngroups are up to ~20 Myr, similar to the claimed expansion time of the superbubble. To they\nextent that it's correct to treat the stars as moving ballistically, they should then have been\nlaunched close to the centre of the Giant oval cavity and they would have inherited the\nkinematics of the gas from which they formed. But it does not seem necessary to me that that\nthe superbubble boundary (defined by the gas) should expand at a constant velocity, so the fact\nthat the boundary of the white region lies close to these two stellar groups is a coincidence to\nbe explained. Do the authors agree with that? Are they, in fact, saying that the gas boundary is\nexpanding at a more-or-less constant speed? I would appreciate hearing what the authors have\nto say about this; maybe I misunderstand the physical interpretation.\nIn any case, what, exactly, is meant by the superbubble becomes important in discussion, e.g.,\nabout the evolution of the bubble's boundary (in the first paragraph and elsewhere). In this\ncontext, do the authors mean the boundary between the hot/low density gas and the surrounding\ndenser, ISM or do they mean the boundary defined by the young star groups?\nReply：We thank the referee for the thoughtful and constructive comments, which have\nsignificantly helped improve the clarity of our manuscript. We fully agree that,\ntraditionally, the term \"superbubble\" refers to a structure within the ISM—specifically,\na cavity filled with hot, low-density gas bounded by cooler, denser material—rather than\nto the associated stellar populations. In response, we have revised the manuscript to\nclearly distinguish between these elements. Throughout the text, we now define the Giant\nOval Cavity as an ISM structure, originally identified through three-dimensional dust\nmaps by Vergely et al. (2022), with its boundary determined by enhanced dust density.\nIn contrast, the Cassiopeia and Auriga Peninsula stellar groups are described as young\nstellar populations whose formation and kinematics have been shaped by feedback from\nthis cavity. It is generally very challenging to obtain the full three-dimensional position\nand velocity information for gas and dust in the ISM. Therefore, young stars and stellar\nassociations are often used as tracers of the gas structure and kinematics, as in many\nstudies such as Alves et al. (2020) and Zucker et al. (2022), etc.. In our case, we first\nobserved that, in the XY plane, the Cassiopeia and Auriga Peninsula groups are spatially\ndistributed along the boundary (with larger dust density) of the Giant Oval Cavity.\nFurthermore, they share a common transverse (peculiar) motion relative to the cavity,\nand exhibit outward radial velocities indicative of expansion. In the vertical direction, we\nobserve that the Cassiopeia, Perseus, and Auriga Peninsula groups form an arc-like\nstructure with clear signatures of vertical expansion. In contrast, other OB groups located\nin the Perseus arm, such as Auriga, Gemini, and Outer Orion, do not exhibit similar\nkinematic trends. Based on these observations, we conclude that the Cassiopeia, Perseus,\nand Auriga Peninsula stellar groups exhibit positions and kinematics consistent with\nfeedback-driven expansion from the Giant Oval Cavity, suggesting a physical link\nbetween the ISM structure and the stellar populations. Thus, using the kinematic\nproperties of these stellar groups to trace the dynamics of the Giant Oval Cavity is, in our\nview, a reasonable and justifiable approach. To address the referee’s concern about\nFigure 1, we have updated its caption to explicitly define the grayscale and contour levels.\nThe boundary of the cavity was determined using a low extinction threshold (0.005 to 0.01\nmag/pc), highlighting a region of exceptionally low dust content. While the apparent\nspatial and kinematical coincidence between the stellar groups and the cavity boundary\nis striking, the boundary reflects a real physical structure characterized by significant\ndust depletion, supporting the association between the ISM cavity and the young stellar\ngroups.\nRegarding the dynamics of the stellar groups, we clarify that we are not proposing\nthat the Cassiopeia and Auriga Peninsula stars were ejected from the center of the cavity.\nThe O-B2 stars we selected are generally younger than ~10 Myr. Our working hypothesis\nis that a pre-existing, elongated dense gas filament passed through the expanding\nsuperbubble, where star formation was triggered along its path. The resulting stellar\nkinematics were then influenced by local feedback processes associated with the\nsuperbubble’s expansion, rather than by a ballistic launch from the cavity center.\nWe also agree that the expansion of the superbubble boundary is not expected to\nhave occurred at a constant velocity. In response to this and similar concerns raised by\nother referees, we have revised the manuscript to clarify that the currently observed\nexpansion velocity of approximately 6.2 km\\,s$^{-1}$ likely reflects a late-stage value,\nand underestimates the velocity at earlier phases of evolution. Superbubbles are expected\nto decelerate over time due to radiative energy losses and interactions with the\nsurrounding ISM. We now incorporate estimates of earlier expansion velocities (e.g., 20–\n30 km\\,s$^{-1}$; Weaver et al. 1977) and demonstrate that, even when adopting a time-\naveraged velocity, the inferred dynamical age of approximately 40 Myr remains\nconsistent with our conclusions. Accordingly, we have revised the manuscript to present\nthe Giant Oval Cavity as a mature superbubble in a late evolutionary phase, where its\nexpansion is now regulated by Galactic shear and radiative cooling. Although our study\nprimarily relies on optical observations of stars, the scientific focus is on the connection\nbetween stellar feedback and superbubble dynamics. Recent JWST observations\nrevealing the widespread presence of superbubbles in galaxies such as NGC 628 have\nrenewed interest in understanding the formation and evolution of these structures.\nHowever, in external galaxies, the absence of detailed kinematic information limits our\nability to constrain their physical properties. In contrast, the Solar neighborhood, with\nthe precise astrometry provided by Gaia, offers a unique opportunity to study these\nprocesses in detail. To our knowledge, this work represents the first measurement of the\nexpansion velocity for a superbubble beyond the Local Bubble. This forms the primary\nmotivation for our study. Our contribution is to identify the Giant Oval Cavity as a slowly\nexpanding structure and to establish its connection to the broader framework of\nfeedback-regulated ISM evolution in disk galaxies. Understanding how superbubbles\nform, evolve, and interact with their environment is a critical component of the larger\npicture of galaxy evolution across cosmic time.\nDetailed comments:\nFigure 1: can the authors show the position of the cavity centre? How is it defined? Please\ndefine the greyscale. An important take-away from this figure is the opposing, small red arrows\nfor each group: I found these very hard to see.\nReply: In the updated Figure 1, the cavity center is now marked with a red cross, defined\nas the centroid of the low-dust region where differential extinction (\\delta A_0) falls below\n0.005 mag/pc. The grayscale background represents local dust density from Vergely et al.\n(2022), with darker shading corresponding to higher dust levels (range: 0–0.5 mag/pc).\nTo improve visibility, we increased the size of the red velocity arrows and changed the\nblue color to magenta to enhance contrast against the background.\nFigure 2: the authors should show representative error bars on a single star datum in each panel.\nIt would be good to include the y = 0 line in each panel (in faint grey) to guide the eye. What\nis the smoothing scale? The authors should be able to define some sort of statistical error band\naround the smoothed profile plot lines.\nReply: Representative error bars have been added to individual star data points in each\npanel, and faint grey dashed lines at y=0 now guide interpretation of velocity offsets.\nThe smoothed velocity profiles were generated using a constrained optimization approach\n(via the Python package smoothfit), employing linear basis functions and a regularization\nparameter \\lambda=0.1 to balance smoothness and fidelity to the data. To quantify\nuncertainties, we performed 1,000 nonparametric bootstrap resamplings of the dataset,\nderiving 1\\sigma confidence intervals from the standard deviation of the resulting\nsmoothed curves. This method is now introduced in the Method section of the revised\nmanuscript.\nLine 79: I don't see from fig.1 how we are meant to infer that the Cavity extends from 100 to\n150 degrees in l.\nReply: Cyan dashed lines marking Galactic longitudes l = 90, 180 and 270 degree have\nbeen added to Figure 1. These reference lines allow readers to verify that the Giant Oval\nCavity spans approximately l ≈ 100 to 150 degree, aligning with the low-dust region’s\nboundaries in the dust extinction map.\nLine 85: \"opposing components\" -- maybe \"oppositely directed components\" for clarity; it took\na while to understand what the authors meant here.\nReply: Thanks for the suggestion. The phrase “opposing components” has been revised\nto “oppositely directed components” for clarity.\nLine 103: \"...Perseus group have notable negative velocities...\". From the figure, this claim\nseems to be founded on only a small number of detected stars. Is it statistically robust? Again,\nit would be good to see some sort of statistical error band on the smoothed curve.\nLine 104-105: the claim about the \"clear gradient\" would be better sustained with an actual\nplot.\nReply: While the Perseus group contains only ~10 O-B2 stars in our sample, 9 of these\nexhibit statistically consistent negative vertical velocities (V_Z), supporting the\nrobustness of this feature. The revised figure includes error bars on individual\nmeasurements and a shaded confidence band around the smoothed velocity profile,\nreinforcing the significance of the gradient.\nFigure 3: instead of writing \\approx 80 Myr, etc, can the authors actually quote central values with\nerrors for these various timescales?\nReply: The timescales in Figure 3 (e.g., ~80 Myr) are approximate, reflecting the inherent\nuncertainty in defining superbubble boundaries. As the cavity’s edges are diffuse and\nobservationally uncertain, we present these values as order-of-magnitude estimates\nrather than precise measurements with formal errors.\nLine 122-124: \"...in a state of dynamical equilibrium...\" I'm not sure I really understand the full\nargument here. See my point above questioning whether it is (or is not) a coincidence that the\nedge of the cavity seen in the dust map is currently coincident with the stellar groups. Can the\nauthors rephrase/expand this argument please?\nReply: The spatial and kinematic alignment between the stellar groups and cavity\nboundary is unlikely to arise by chance. We propose that the cavity exists in a quasi-\nstationary state, where feedback energy from sequential supernovae balances radiative\nlosses and shear forces. This equilibrium is consistent with the cavity’s slow present-day\nexpansion velocity (~6.2 km s⁻¹) and its survival over timescales exceeding typical Galactic\nshear disruption periods (t_shear∼30 Myr). The dust morphology and coherent stellar\nmotions jointly support this interpretation.\nLine 178 please quantify \"very young\"\nReply: The term “very young” has been clarified to specify that the O–B2 stars analyzed\nhave ages <20 Myr.\nReviewer #3 (Remarks to the Author):\n1. The crux of the argument is that the expansion timescale is much larger than the shearing\ntimescale. The authors estimate an expansion timescale as d/v where d is the size of the bubble\nand v is the expansion velocity as measured from O and B stars. However, if the current\nstructure is the product of expansion from a significantly smaller size, then it is very likely to\nhave decelerated to its current velocity. To argue this point on physical grounds, it would be\nnecessary to present a physically motivated expansion law that accounts for the current and\npast state of the structure -- for instance, the fact that the expansion velocity is slow enough for\nthe motion to be highly radiative, and that it is larger than the scale height of the gaseous disk.\nAny assertion that it is older than the shear timescale should take this history into account.\nReply: We thank the referee for highlighting this important point. We agree that\nassuming a constant expansion velocity oversimplifies the bubble’s dynamical history,\nespecially given the likelihood of deceleration due to radiative cooling and interaction\nwith the ambient interstellar medium. To address this, we have revised the manuscript\nto incorporate a physically motivated expansion model. Specifically, we adopt the\nframework of Weaver et al. (1977), which describes the evolution of a superbubble driven\nby stellar winds. This model provides a time-dependent scaling for the bubble’s radius\nduring the adiabatic phase:\nR(t) = 27 n_0^{-1/5} L_{36}^{1/5} t_6^{3/5} pc,\nwhere n_0 is the ambient density in cm^{-3}, L_{36} is the wind luminosity in units of\n10^{36} erg s^{-1}, and t_6 is the age in Myr. Applying this relation with representative\nparameters (n_0 = 1 cm^{-3}, L_w = 10^{36} erg s^{-1}), we find that a superbubble\nwould require approximately 130 Myr to reach a radius of 500 pc—longer than our\nearlier estimate of 80 Myr based on the simpler R/v approximation. However, the\nWeaver solution assumes continuous energy input from stellar winds. If multiple\nsupernovae contribute, as is likely, the effective energy input increases, thus reducing the\nrequired expansion time. For the vertical extent of the cavity relative to the Galactic disk\nscale height, the structure reaches |Z| \\sim 270 pc, well above the molecular disk scale\nheight of H_Z sim 40–70 pc (Li et al. 2024). This suggests weak vertical confinement.\nHowever, the current low vertical velocity (V_z \\sim 8.5 km/s) indicates that vertical\nmotion has likely been halted by the gravitational potential of the Gaiactic disk, resulting\nin a transition to lateral, momentum-driven expansion within the plane. In general, for a\nstructure whose expansion have slowed down, through its history, the majority of the\nevolution time still belongs to the stage where the expansion is the slowest, and the\ntimescale we drive represents the expected lifetime of such object.\n2. To fully explore the scenario in which the cavity is cleared, and kept clear, by supernovae\nassociated with its formation, it would be necessary to evaluate how many such events are\nrequired to create its current expansion, including phenomena like the release of internal\npressure when a super-bubble blows out of the gaseous disk. This would need to be compared\nto the plausible gas and star formation environment of the region, which one expects (on\nkiloparsec scales) to follow the Kennicutt-Schmidt relation.\nReply: We thank the referee for raising the important point regarding the energy budget\nrequired to form and maintain the cavity. In the revised manuscript (Section Method,\nPage 22-23), we present a detailed calculation of the number of supernovae necessary.\nSpecifically, we estimate the total energy required to: (i) overcome the gravitational\nbinding energy of the gas, (ii) offset turbulent support within the ISM, and (iii)\ncompensate for radiative cooling losses. Our analysis shows that approximately 400\nsupernovae are needed to form and sustain the cavity, considering these combined effects.\nThese energy requirements are consistent with expectations from the Kennicutt–Schmidt\nrelation for star formation on kiloparsec scales. Additionally, we provide a simpler,\norder-of-magnitude estimate based on the bubble’s size relative to the Local Bubble. The\ncavity’s radius is about five times larger than that of the Local Bubble. Given that the\nLocal Bubble was formed by approximately 20 supernovae (Berghofer & Breitschwerdt\n2002; Zucker et al. 2022), we estimate that the formation of the larger cavity would\nrequire roughly 25 times more supernovae, or approximately 500 events. This\nindependent estimate is in good agreement with our detailed calculation.\n3. The paper is incomplete without a fair evaluation of alternative scenarios. For instance, the\ncavity could be a purely gravitational structure, e.g., an unusually empty inter-arm region. Or,\nit could be related to the Parker instability. In either case, OB stars would be working within\nan existing under-density; it would then be easier to clear, and this may have happened more\nrecently. The spatial correlation between OB stars and the cavity walls does not, in itself,\nimplicate them in forming the entire structure.\nBecause little evidence is provided for the authors' preferred interpretation, we judge that the\npaper is not acceptable in its current form.\nReply: We thank the referee for highlighting the importance of evaluating alternative\nscenarios for the origin of the cavity. In the revised manuscript, we have carefully\nconsidered the two hypotheses suggested—namely, a pre-existing inter-arm underdensity\nand structures formed by Parker instability—and have provided a detailed comparison\nwith our preferred interpretation involving stellar feedback.\nIf the cavity were simply a quiescent inter-arm region, we would expect its\nkinematics to follow Galactic rotation and shear patterns. However, the Cassiopeia and\nAuriga Peninsula groups display radially outward velocities (v_exp ≈ 6.2 km/s) that\ndeviate significantly from the expected motions in a passive shear environment. Moreover,\nthe dust column density within the cavity (\\delta A_0 < 0.005 mag/pc) is an order of\nmagnitude lower than that typically observed in inter-arm regions (~0.2 mag/pc; Vergely\net al. 2022), implying an initial underdensity that would be unusually extreme. The stellar\npopulation also differs markedly: stars associated with the cavity walls are young (ages ≤\ndiscrepancies in both kinematics and stellar ages strongly argue against a passive, pre-\nexisting underdensity origin.\nRegarding the Parker instability scenario, theoretical models predict gas-dominated\nstructures characterized by undular (wavelength λ ∼ 1 kpc; Parker 1979; Rodrigues et\nal. 2016). In contrast, the Giant Oval Cavity shows a smooth, elliptical morphology that\ndoes not match these predictions. Furthermore, the cavity is defined by significant dust\nevacuation and coherent vertical velocity gradients—features not typically associated\nwith Parker instability, which primarily organizes gas without significant feedback\nsignatures from young stars.\nAdditionally, most O-B2 stars in the Cassiopeia and Auriga Peninsula groups exhibit\nradially outward motions relative to the cavity center. Such kinematic coherence is\nunlikely to arise from random star formation within a pre-existing void, which would be\nexpected to produce more isotropic velocity distributions. In comparison, other nearby\nOB groups (e.g., Auriga, Gemini, and Outer Orion) do not show similar alignment. Taken\ntogether, the extremely low dust density, the young stellar population, the departure from\nshear-driven kinematics, and the organized feedback-driven morphology collectively\nargue against the alternatives of a pre-existing underdensity or Parker instability origin.\nThe spatial and kinematic correlation between the OB stars and the cavity walls provides\nstrong evidence that stellar feedback is the primary driver of the cavity’s formation and\nongoing evolution.\nWe have expanded the discussion in the revised manuscript to explicitly contrast\nthese alternative hypotheses and to clarify why stellar feedback remains the most\nconsistent explanation.\nA few additional points and questions may be useful for a future version:\n4. What are the uncertainties of the quoted velocities and expansion times?\nReply: We have added a detailed uncertainty analysis for the kinematic measurements in\nthe revised manuscript. Using the full three-dimensional positions and velocities of stars\nnear the Giant Oval Cavity, we applied a Monte Carlo method to quantify the\nuncertainties. Specifically, each star’s position and velocity were perturbed 300 times\nwith Gaussian noise based on their observational errors (e.g., parallax and proper motion\nuncertainties from Gaia DR3, and radial velocity uncertainties from LAMOST). The\nstandard deviation of these realizations gives an uncertainty of approximately 2 km/s for\nthe cavity’s expansion velocity and about 3 km/s for the peculiar motions of OB stars.\nThe expansion timescale is an order-of-magnitude calculation based on the current size\nof the cavity (R∼1 kpc). We did not compute a formal error for this timescale.\n5. What other signatures should accompany the structure, if it has indeed been created by\nsupernovae? A few possibilities come to mind. There may be a giant chimney structure left\nbehind by the blowout of the driving super-bubble. Other signatures of the driving pressure\nmay also be identifiable. Or, there may exist a noticeable age gradient of stars whose main\nsequence lifetimes match the past history of the structure. The lack of such signatures may be\nimportant future constraints.\nReply: We fully agree with the referee that, if the cavity was formed by supernova-driven\nfeedback, additional signatures such as giant chimneys, thermal pressure gradients, or\nstellar age gradients might be expected. However, current observational limitations make\nsystematic verification of these features challenging.\nFor vertical gas channels (chimney structures), detection is difficult. Identifying soft\nX-ray emission or high-velocity H$\\alpha$ filaments requires highly sensitive\nspectroscopic imaging. Since the cavity lies within the Galactic disk, separating\nforeground and background emissions introduces significant systematic uncertainties,\npotentially obscuring such features.\nQuantitative analysis of thermal pressure is also limited. Accurate determination of the\ninternal pressure would require multi-wavelength diagnostics combining X-ray spectral\nfitting (for electron temperatures and densities) and ultraviolet absorption-line\nmeasurements (to assess non-thermal turbulence). Investigating stellar age gradients\npresents further technical challenges. Identifying feedback-triggered star formation\nsequences would require precise photometric measurements and reliable extinction\ncorrections, typically using isochrone fitting methods. However, in high-extinction\nregions of the Galactic plane, the uncertainties are significant, limiting the accuracy of\nage estimates.\nDespite these difficulties, we recognize that detecting these additional signatures\nwould provide important constraints, and we view this as a valuable direction for future\nwork.\n6. Are similar expansion features identifiable in other stellar structures, such as the Auriga\nPeninsula or the Gemini cluster?\nReply: No coherent expansion signatures are observed in other OB associations such as\nthe Auriga Peninsula or the Gemini groups. The absence of similar kinematic patterns in\nthese groups suggests that the Giant Oval Cavity is unique in its feedback-driven origin.\nDear Referees,\nWe sincerely thank you for your thoughtful and constructive feedback, which has been\ninstrumental in refining our understanding of the Giant Oval Cavity. In response to your\ncomments, we have undertaken a substantial revision of the manuscript. As part of this\nprocess, we consulted with experts in the field of superbubble simulations to enhance the\nrobustness of our interpretation. This collaboration led to the integration of new insights\ninto the nature and evolution of the structure. Accordingly, the author list has been\nupdated to include Prof. Martin Krause and Dr. Ashley Coombes (University of\nHertfordshire), who made significant contributions to the hydrodynamic simulations and\ntheir analysis.\nEach concern raised in your reviews has been carefully considered and addressed\nthrough systematic revisions to the manuscript. To assist with your evaluation, all\nchanges made in response to your comments are highlighted in blue within the revised\nmanuscript. Additionally, this response document provides a detailed, point-by-point\nexplanation of how each comment was addressed.\nA key outcome of the revision is a shift in focus—from the historical evolution of the\nbubble to its physical nature and role within the interstellar medium. We believe this\nchange better aligns with both the strengths of our dataset and the broader scientific\nsignificance of the Giant Oval Cavity. Our primary goal is now to clarify the fundamental\ncharacteristics and long-term stability of this structure, as well as its implications for the\ndynamics of the galactic environment in which it resides.\nTo support this new focus, we have incorporated results from a simulation published by\nProf. Krause in 2019. This model had predicted the existence of large, stable bubbles—\nsimilar to what we now identify as the Giant Oval Cavity—prior to the recent surge of\ninterest in such features (i.e. the Phantom Bubbles as seeem by the JWST telescope).\nBuilding on this, we propose a simple theoretical framework involving two characteristic\ntimescales: the bubble survival time, t_survival, and the average interval between\nsupernovae occurring within the bubble, t_SN. We find that a necessary condition for a\nbubble to remain stable and stationary is t_SN ≪ t_survival. This condition appears to\nbe well satisfied in the case of the Giant Oval Cavity, lending support to our interpretation\nof its long-term persistence.\nThis insight has allowed us to simplify our discussion significantly. By incorporating\nselected simulation results and focusing on the essential physics, we now present a more\nconcise and coherent picture of the formation and stability of large, stationary\nsuperbubbles like the Giant Oval Cavity. We hope this clearer narrative will make the\nrevised manuscript more accessible and impactful.\nWe recognize that the scope of these revisions may exceed what the referees initially\nanticipated. In particular, the historical aspects of the bubble's development, which\nfeatured more prominently in the earlier draft, have been deemphasized in light of our\nimproved understanding. However, we view this change as a direct and positive\nconsequence of the dialogue with the referees, for which we are deeply grateful. We hope\nthis evolution in the manuscript is understood as a reflection of genuine scientific progress.\nSincerely,\nThe Authors\nREVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\n1. After considering possible deceleration during the evolution using the model of Weaver et\nal., the estimated expansion time is adjusted to ~40 Myr. This is in fact not much longer than\nthe shear timescale of ~30 Myr, or could be considered comparable given that these numbers\nare order-of-magnitude estimates. Since these timescales are not significantly different, this\nchallenges the physical picture proposed by the authors. In other words, there does not need to\nbe a balance between stellar feedback and shear forces, but it could well be that we are\nobserving the Giant Oval Cavity currently undergoing deformation/disruption due to the\nGalactic shear, i.e., the Cavity does not need to be long lived.\n2. There appears to be some inconsistency between the power of energy injection assumed in\nthe timescale estimate based on the model of Weaver et al. and that used to justify the formation\nof the Giant Oval Cavity by SN explosions. The former assumes cavity formation due to\nconstant energy injections with a stellar wind luminosity of 10^36 erg/s. On the other hand, the\nlatter requires 400-500 SN explosions, which corresponds to a kinetic luminosity of ~400 SNe\nx 1e51 erg / 80 Myr x 20\\% ~ 2.53e39 erg/s, which is much greater than the stellar wind\nluminosity assumed in the timescale estimate. Since the adopted luminosity would significantly\ninfluence the timescale estimate, could the authors please clarify the adopted numbers and the\nassumed source of energy injection for cavity formation?\nReply (combing 1 and 2): We thank the reviewer for the insightful comments, which have\nprompted us to critically reassess our interpretation of the timescales involved in the\nevolution of the Giant Oval Cavity. The point regarding the similarity between the\nestimated expansion timescale (~40 Myr) and the galactic shear timescale (~30 Myr) is\nwell taken, particularly given the approximations inherent in both estimates. We agree\nthat this proximity in values complicates the interpretation of the cavity’s longevity and\nchallenges the assumption of a clear distinction between a dynamically balanced and a\ntransient structure.\nAs the reviewer rightly notes, the expansion timescale derived using the Weaver et al.\n(1977) model assumes a constant energy injection rate and simplified conditions, which\nmay not capture the complexity of the actual environment. Given the sensitivity of such\nmodels to assumed parameters (e.g., ambient density, efficiency of energy coupling, and\nthe nature of energy injection), we recognize that relying on this analytical framework to\nestablish the cavity’s age or stability introduces considerable uncertainty. In light of this,\nwe have removed the Weaver model analysis from the revised manuscript.\nTo provide a more physically grounded picture, we now base our interpretation on\nresults from numerical simulations provided by Prof. Martin Krause. These simulations\ndemonstrate that large cavities can enter a state of dynamical equilibrium, where their\nboundaries are shaped and maintained by ongoing interactions between supernova-\ndriven feedback and inflowing interstellar gas. While such structures may deform or\nmigrate over time, the simulations show they can remain coherent and long-lived if\nenergy injection is sustained. This view supports the idea that the Giant Oval Cavity may\npersist not because it is “young” (or “old”) but because it exists in a quasi-stationary state\nmaintained by feedback processes.\nIn the revised manuscript, we have moved away from age-based arguments and instead\ncharacterize the cavity’s stability through a comparison of relevant dynamical timescales.\nWe introduce the concept of a survival timescale, defined as: t_survival = l_bubble /\n\\sigma_v,disk ≈ 250 Myr (Eq. 3), where l_bubble is the characteristic size of the cavity and\n\\sigma_v,disk is the velocity dispersion of the surrounding interstellar medium. This estimate\nsignificantly exceeds the shear timescale (t_shear ≈ 30 Myr), indicating that the cavity is\nresilient against turbulent disruption. Furthermore, the characteristic timescale between\nsupernova events is: t_SN ≈ 0.5 Myr (Eq. 6), which is orders of magnitude shorter than\nt_survival. This ensures a continuous supply of energy capable of offsetting the effects of\nshear and maintaining structural coherence.\nThis framework is also supported by recent simulations of Milky Way-like disks, which\nshow that superbubbles with t_SN ≪ t_survival can maintain quasi-stationary\nboundaries for extended periods through a balance between feedback and inflow. We\nhave included these results and relevant figures (e.g., Figure 3 in the revised manuscript)\nto illustrate this mechanism more clearly.\nWe hope this revised analysis, grounded in simulation-based evidence and dynamical\nreasoning, addresses the concerns raised and provides a more robust physical\ninterpretation of the Giant Oval Cavity.\nReviewer #2 (Remarks to the Author):\nThe authors have systematically and adequately addressed the concerns I raised in my first\nreport except for a single, remaining point: could they please generate a greyscale numerical\nbar for Figure 1? Besides this minor point, I am happy to recommend the paper as suitable for\npublication.\nReply: We acknowledge the reviewer's suggestion to include a greyscale numerical bar\nin Figure 1 to quantify the dust density scale in the background map. This addition\nenhances the interpretability of the figure by providing a clear reference for the dust\ndistribution values. In response, we have modified the figure accordingly. The bar is\npositioned adjacent to the right panel of Figure 1.\nReviewer #3 (Remarks to the Author):\n1. It is not sufficient to introduce the Weaver model and state the bubble age for some choice\nof parameters (e.g., L_36 = 1) without exploring which parameters match the structure as it is\ncurrently observed -- in other words, finding which values of luminosity and age are consistent\nwith both the current radius and the current expansion velocity.\n(Note: in analytical models for superbubbles, it is appropriate to include supernovae as part of\nthe effective wind luminosity.)\n2. Using the Weaver model, the authors report an even older age (130 Myr) than the simple\nratio of radius to velocity (80 Myr) stressed elsewhere. This is obviously an overestimate,\nbecause the radial evolution within the Weaver model is R ~ t^eta with eta = 3/5. This law is\nconsistent with t = eta R/v = 48 Myr.\nReply (combining 1 and 2): We thank the reviewer for their careful reading of the revised\nmanuscript and for the detailed comments, which have led us to reconsider and refine\nboth our methodology and the structure of our argument. Regarding the application of\nthe Weaver et al. (1977) model, we acknowledge the limitations inherent in using a\nsimplified analytic framework without a thorough exploration of parameter space.\nSpecifically, as the reviewer notes, it is insufficient to adopt a fixed wind luminosity.\nMoreover, the use of the standard expansion law R \\propto t^{3/5} yields an age closer to\ndiscrepancy undermines the utility of the Weaver model in this context. In response, we\nhave significantly revised the manuscript. We no longer rely on the Weaver framework\nto infer the age of the cavity. Instead, we have reoriented the analysis toward a more\nphysically grounded and comprehensive picture based on numerical simulations. With\nthe assistance of Prof. Martin Krause, we now incorporate simulation results that\nemphasize the role of large-scale feedback-driven structures in establishing long-lived\ncavities. These simulations clarify that the size of the cavity itself plays a central role in\nregulating its evolution, enabling efficient energy conversion from successive supernovae\nwithin the low-density interior.\nThe revised manuscript now focuses on the dynamical balance between feedback and\ngalactic shear, rather than on absolute age estimates. In this new framework, we compare\nthree key timescales: the shear timescale (t_shear≈30 Myr), the supernova interval\n(t_SN≈0.5 Myr), and the survival timescale of the cavity, defined as t_survival=\nl_bubble/\\sigma_v,disk ≈250 Myr, where l_bubble is the characteristic size of the cavity and\n\\sigma_v,disk is the turbulent velocity dispersion of the disk. This formulation avoids the need\nfor model-dependent assumptions about the expansion history and instead emphasizes\nthe long-term stability of the cavity through continuous energy injection. These\narguments are now supported by recent simulations, which show that cavities with t_SN\n≪ t_survival can remain coherent and quasi-stationary over long periods.\n3. Moreover, the bubble cannot reach its current size without blowing out of the Galactic disk.\nThis transition relieves the pressure of the interior, leading to a stage of momentum-conserving\n\"snowplow\" expansion. The appropriate value of eta, for a circular shell undergoing\nsnowplow expansion within a disk, is 1/3, which suggests an age as low as 27 Myr -- comparable to the\nshear timescale.\nReply: We fully agree that the cavity must have blown out of the Galactic disk. This\ninterpretation is supported by the simulations we now include in the revised manuscript,\nparticularly those from Krause et al. (2019), which explicitly show blowout phases in the\nevolution of similar large cavities. We have updated the text to reflect this physical\ntransition and its implications for the bubble’s expansion dynamics. As a result, we no\nlonger rely on age estimates derived from spherical or pressure-driven expansion models,\nwhich are not appropriate once blowout has occurred.\n4. In the authors' estimates of the energy required to open the cavity, the dominant term comes\nfrom cooling in the bubble interior. However, spherical models of superbubbles often find that\nheat lost by conduction is greater than cooling in the interior: see for instance El Badry et al\n2019. When blowout has occurred, the energy lost is even greater.\nReply: We appreciate the reviewer’s observation regarding energy losses in large cavities.\nWe agree that energy loss through thermal conduction and radiative cooling can\nsignificantly affect the internal energy budget of the bubble—particularly in the post-\nblowout phase. However, as the reviewer notes, the impact of thermal conduction scales\nwith the surface area of the interface between the hot bubble and the cooler surroundings.\nFor large cavities, this area is effectively set by r_bubble*h_disk, and the relative\nimportance of conduction diminishes as the cavity grows. Conversely, energy loss into the\ncircumgalactic medium becomes more significant after blowout. This mechanism has\nbeen addressed in the simulations by Krause et al. (2019), which we now cite and discuss\nin the revised manuscript. We have also clarified that our energy budget estimates are\nintended as order-of-magnitude assessments rather than precise calculations.\n5. The authors say that do not find any signatures in other OB associations, but it is important\nto provide specifics in the text. Showing that the velocity signatures of expansion are unique\nto the associations lining this cavity, and not characteristic of random motions elsewhere in the\ndisk, would strengthen the case for its existence.\nReply: We thank the reviewer for underscoring the importance of explicitly establishing\nthe uniqueness of the kinematic expansion signatures associated with the Giant Oval\nCavity. We have strengthened our argument by added a new panel of Figure 5, which\nvisually and unequivocally demonstrates the spatial correlation between coherent radial\nmotions and the cavity boundary. In the revised manuscript, we now explicitly describe\nthe velocity patterns revealed by the figure: stars in the Cassiopeia and Auriga Peninsula\ngroups (marked as red arrows) exhibit coherent radially outward motions aligned with\nthe periphery of the Giant Oval Cavity. This geometric consistency—where velocity\nvectors point away from the cavity center—supports their expansion origin. Critically,\nthis pattern is absent in other Perseus arm groups (Auriga, Gemini, and Outer Orion;\nblue arrows), whose velocity vectors show random orientations without systematic radial\nalignment relative to any cavity."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Thanks to the authors for carefully addressing the comments. The added text as well as estimates of the uncertainties have substantially improved the clarity of the manuscript. However, I still have some questions regarding the calculation of relevant timescales and the proposed physical picture, which may affect the robustness of the conclusions. These questions should to be adequately addressed before the manuscript could be considered for publication.\n\n1. After considering possible deceleration during the evolution using the model of Weaver et al., the estimated expansion time is adjusted to ~40 Myr. This is in fact not much longer than the shear timescale of ~30 Myr, or could be considered comparable given that these numbers are order-of-magnitude estimates. Since these timescales are not significantly different, this challenges the physical picture proposed by the authors. In other words, there does not need to be a balance between stellar feedback and shear forces, but it could well be that we are observing the Giant Oval Cavity currently undergoing deformation/disruption due to the Galactic shear, i.e., the Cavity does not need to be long lived.\n\n2. There appears to be some inconsistency between the power of energy injection assumed in the timescale estimate based on the model of Weaver et al. and that used to justify the formation of the Giant Oval Cavity by SN explosions. The former assumes cavity formation due to constant energy injections with a stellar wind luminosity of 10^36 erg/s. On the other hand, the latter requires 400-500 SN explosions, which corresponds to a kinetic luminosity of ~400 SNe x 1e51 erg / 80 Myr x 20% ~ 2.53e39 erg/s, which is much greater than the stellar wind luminosity assumed in the timescale estimate. Since the adopted luminosity would significantly influence the timescale estimate, could the authors please clarify the adopted numbers and the assumed source of energy injection for cavity formation?", "reviewer_2": "The authors have systematically and adequately addressed the concerns I raised in my first report except for a single, remaining point: could they please generate a greyscale numerical bar for Figure 1?\n\nBesides this minor point, I am happy to recommend the paper as suitable for publication.", "reviewer_3": "The authors have improved the manuscript by, among other things, introducing the Weaver et al. model for a stellar wind bubble, and conducting a rough comparison between the energetics required to open the putative cavity, and the energy provided by the supernovae expected in that region. These efforts are appreciated. However the manuscript is still not acceptable in its current form, for the following reasons.\n\n1. It is not sufficient to introduce the Weaver model and state the bubble age for some choice of parameters (e.g., L_36 = 1) without exploring which parameters match the structure as it is currently observed -- in other words, finding which values of luminosity and age are consistent with both the current radius and the current expansion velocity.\n\n(Note: in analytical models for superbubbles, it is appropriate to include supernovae as part of the effective wind luminosity.)\n\n2. Using the Weaver model, the authors report an even older age (130 Myr) than the simple ratio of radius to velocity (80 Myr) stressed elsewhere. This is obviously an overestimate, because the radial evolution within the Weaver model is R ~ t^eta with eta = 3/5. This law is consistent with t = eta R/v = 48 Myr.\n\n3. Moreover, the bubble cannot reach its current size without blowing out of the Galactic disk. This transition relieves the pressure of the interior, leading to a stage of momentum-conserving \"snowplow\" expansion. The appropriate value of eta, for a circular shell undergoing snowplow expansion within a disk, is 1/3, which suggests an age as low as 27 Myr -- comparable to the shear timescale.\n\n4. In the authors' estimates of the energy required to open the cavity, the dominant term comes from cooling in the bubble interior. However, spherical models of superbubbles often find that heat lost by conduction is greater than cooling in the interior: see for instance El Badry et al 2019. When blowout has occurred, the energy lost is even greater.\n\n5. The authors say that do not find any signatures in other OB associations, but it is important to provide specifics in the text. Showing that the velocity signatures of expansion are unique to the associations lining this cavity, and not characteristic of random motions elsewhere in the disk, would strengthen the case for its existence.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "REVIEWER COMMENTS\nReviewer #1 (Remarks to the Author):\n1. After considering possible deceleration during the evolution using the model of Weaver et\nal., the estimated expansion time is adjusted to ~40 Myr. This is in fact not much longer than\nthe shear timescale of ~30 Myr, or could be considered comparable given that these numbers\nare order-of-magnitude estimates. Since these timescales are not significantly different, this\nchallenges the physical picture proposed by the authors. In other words, there does not need to\nbe a balance between stellar feedback and shear forces, but it could well be that we are\nobserving the Giant Oval Cavity currently undergoing deformation/disruption due to the\nGalactic shear, i.e., the Cavity does not need to be long lived.\n2. There appears to be some inconsistency between the power of energy injection assumed in\nthe timescale estimate based on the model of Weaver et al. and that used to justify the formation\nof the Giant Oval Cavity by SN explosions. The former assumes cavity formation due to\nconstant energy injections with a stellar wind luminosity of 10^36 erg/s. On the other hand, the\nlatter requires 400-500 SN explosions, which corresponds to a kinetic luminosity of ~400 SNe\nx 1e51 erg / 80 Myr x 20% ~ 2.53e39 erg/s, which is much greater than the stellar wind\nluminosity assumed in the timescale estimate. Since the adopted luminosity would significantly\ninfluence the timescale estimate, could the authors please clarify the adopted numbers and the\nassumed source of energy injection for cavity formation?\nReply (combing 1 and 2): We thank the reviewer for the insightful comments, which have\nprompted us to critically reassess our interpretation of the timescales involved in the\nevolution of the Giant Oval Cavity. The point regarding the similarity between the\nestimated expansion timescale (~40 Myr) and the galactic shear timescale (~30 Myr) is\nwell taken, particularly given the approximations inherent in both estimates. We agree\nthat this proximity in values complicates the interpretation of the cavity’s longevity and\nchallenges the assumption of a clear distinction between a dynamically balanced and a\ntransient structure.\nAs the reviewer rightly notes, the expansion timescale derived using the Weaver et al.\n(1977) model assumes a constant energy injection rate and simplified conditions, which\nmay not capture the complexity of the actual environment. Given the sensitivity of such\nmodels to assumed parameters (e.g., ambient density, efficiency of energy coupling, and\nthe nature of energy injection), we recognize that relying on this analytical framework to\nestablish the cavity’s age or stability introduces considerable uncertainty. In light of this,\nwe have removed the Weaver model analysis from the revised manuscript.\nTo provide a more physically grounded picture, we now base our interpretation on\nresults from numerical simulations provided by Prof. Martin Krause. These simulations\ndemonstrate that large cavities can enter a state of dynamical equilibrium, where their\nboundaries are shaped and maintained by ongoing interactions between supernova-\ndriven feedback and inflowing interstellar gas. While such structures may deform or\nmigrate over time, the simulations show they can remain coherent and long-lived if\nenergy injection is sustained. This view supports the idea that the Giant Oval Cavity may\npersist not because it is “young” (or “old”) but because it exists in a quasi-stationary state\nmaintained by feedback processes.\nIn the revised manuscript, we have moved away from age-based arguments and instead\ncharacterize the cavity’s stability through a comparison of relevant dynamical timescales.\nWe introduce the concept of a survival timescale, defined as: t_survival = l_bubble /\n\\sigma_v,disk ≈ 250 Myr (Eq. 3), where l_bubble is the characteristic size of the cavity and\n\\sigma_v,disk is the velocity dispersion of the surrounding interstellar medium. This estimate\nsignificantly exceeds the shear timescale (t_shear ≈ 30 Myr), indicating that the cavity is\nresilient against turbulent disruption. Furthermore, the characteristic timescale between\nsupernova events is: t_SN ≈ 0.5 Myr (Eq. 6), which is orders of magnitude shorter than\nt_survival. This ensures a continuous supply of energy capable of offsetting the effects of\nshear and maintaining structural coherence.\nThis framework is also supported by recent simulations of Milky Way-like disks, which\nshow that superbubbles with t_SN ≪ t_survival can maintain quasi-stationary\nboundaries for extended periods through a balance between feedback and inflow. We\nhave included these results and relevant figures (e.g., Figure 3 in the revised manuscript)\nto illustrate this mechanism more clearly.\nWe hope this revised analysis, grounded in simulation-based evidence and dynamical\nreasoning, addresses the concerns raised and provides a more robust physical\ninterpretation of the Giant Oval Cavity.\nReviewer #2 (Remarks to the Author):\nThe authors have systematically and adequately addressed the concerns I raised in my first\nreport except for a single, remaining point: could they please generate a greyscale numerical\nbar for Figure 1? Besides this minor point, I am happy to recommend the paper as suitable for\npublication.\nReply: We acknowledge the reviewer's suggestion to include a greyscale numerical bar\nin Figure 1 to quantify the dust density scale in the background map. This addition\nenhances the interpretability of the figure by providing a clear reference for the dust\ndistribution values. In response, we have modified the figure accordingly. The bar is\npositioned adjacent to the right panel of Figure 1.\nReviewer #3 (Remarks to the Author):\n1. It is not sufficient to introduce the Weaver model and state the bubble age for some choice\nof parameters (e.g., L_36 = 1) without exploring which parameters match the structure as it is\ncurrently observed -- in other words, finding which values of luminosity and age are consistent\nwith both the current radius and the current expansion velocity.\n(Note: in analytical models for superbubbles, it is appropriate to include supernovae as part of\nthe effective wind luminosity.)\n2. Using the Weaver model, the authors report an even older age (130 Myr) than the simple\nratio of radius to velocity (80 Myr) stressed elsewhere. This is obviously an overestimate,\nbecause the radial evolution within the Weaver model is R ~ t^eta with eta = 3/5. This law is\nconsistent with t = eta R/v = 48 Myr.\nReply (combining 1 and 2): We thank the reviewer for their careful reading of the revised\nmanuscript and for the detailed comments, which have led us to reconsider and refine\nboth our methodology and the structure of our argument. Regarding the application of\nthe Weaver et al. (1977) model, we acknowledge the limitations inherent in using a\nsimplified analytic framework without a thorough exploration of parameter space.\nSpecifically, as the reviewer notes, it is insufficient to adopt a fixed wind luminosity.\nMoreover, the use of the standard expansion law R \\propto t^{3/5} yields an age closer to\ndiscrepancy undermines the utility of the Weaver model in this context. In response, we\nhave significantly revised the manuscript. We no longer rely on the Weaver framework\nto infer the age of the cavity. Instead, we have reoriented the analysis toward a more\nphysically grounded and comprehensive picture based on numerical simulations. With\nthe assistance of Prof. Martin Krause, we now incorporate simulation results that\nemphasize the role of large-scale feedback-driven structures in establishing long-lived\ncavities. These simulations clarify that the size of the cavity itself plays a central role in\nregulating its evolution, enabling efficient energy conversion from successive supernovae\nwithin the low-density interior.\nThe revised manuscript now focuses on the dynamical balance between feedback and\ngalactic shear, rather than on absolute age estimates. In this new framework, we compare\nthree key timescales: the shear timescale (t_shear≈30 Myr), the supernova interval\n(t_SN≈0.5 Myr), and the survival timescale of the cavity, defined as t_survival=\nl_bubble/\\sigma_v,disk ≈250 Myr, where l_bubble is the characteristic size of the cavity and\n\\sigma_v,disk is the turbulent velocity dispersion of the disk. This formulation avoids the need\nfor model-dependent assumptions about the expansion history and instead emphasizes\nthe long-term stability of the cavity through continuous energy injection. These\narguments are now supported by recent simulations, which show that cavities with t_SN\n≪ t_survival can remain coherent and quasi-stationary over long periods.\n3. Moreover, the bubble cannot reach its current size without blowing out of the Galactic disk.\nThis transition relieves the pressure of the interior, leading to a stage of momentum-conserving\n\"snowplow\" expansion. The appropriate value of eta, for a circular shell undergoing\nsnowplow expansion within a disk, is 1/3, which suggests an age as low as 27 Myr -- comparable to the\nshear timescale.\nReply: We fully agree that the cavity must have blown out of the Galactic disk. This\ninterpretation is supported by the simulations we now include in the revised manuscript,\nparticularly those from Krause et al. (2019), which explicitly show blowout phases in the\nevolution of similar large cavities. We have updated the text to reflect this physical\ntransition and its implications for the bubble’s expansion dynamics. As a result, we no\nlonger rely on age estimates derived from spherical or pressure-driven expansion models,\nwhich are not appropriate once blowout has occurred.\n4. In the authors' estimates of the energy required to open the cavity, the dominant term comes\nfrom cooling in the bubble interior. However, spherical models of superbubbles often find that\nheat lost by conduction is greater than cooling in the interior: see for instance El Badry et al\n2019. When blowout has occurred, the energy lost is even greater.\nReply: We appreciate the reviewer’s observation regarding energy losses in large cavities.\nWe agree that energy loss through thermal conduction and radiative cooling can\nsignificantly affect the internal energy budget of the bubble—particularly in the post-\nblowout phase. However, as the reviewer notes, the impact of thermal conduction scales\nwith the surface area of the interface between the hot bubble and the cooler surroundings.\nFor large cavities, this area is effectively set by r_bubble*h_disk, and the relative\nimportance of conduction diminishes as the cavity grows. Conversely, energy loss into the\ncircumgalactic medium becomes more significant after blowout. This mechanism has\nbeen addressed in the simulations by Krause et al. (2019), which we now cite and discuss\nin the revised manuscript. We have also clarified that our energy budget estimates are\nintended as order-of-magnitude assessments rather than precise calculations.\n5. The authors say that do not find any signatures in other OB associations, but it is important\nto provide specifics in the text. Showing that the velocity signatures of expansion are unique\nto the associations lining this cavity, and not characteristic of random motions elsewhere in the\ndisk, would strengthen the case for its existence.\nReply: We thank the reviewer for underscoring the importance of explicitly establishing\nthe uniqueness of the kinematic expansion signatures associated with the Giant Oval\nCavity. We have strengthened our argument by added a new panel of Figure 5, which\nvisually and unequivocally demonstrates the spatial correlation between coherent radial\nmotions and the cavity boundary. In the revised manuscript, we now explicitly describe\nthe velocity patterns revealed by the figure: stars in the Cassiopeia and Auriga Peninsula\ngroups (marked as red arrows) exhibit coherent radially outward motions aligned with\nthe periphery of the Giant Oval Cavity. This geometric consistency—where velocity\nvectors point away from the cavity center—supports their expansion origin. Critically,\nthis pattern is absent in other Perseus arm groups (Auriga, Gemini, and Outer Orion;\nblue arrows), whose velocity vectors show random orientations without systematic radial\nalignment relative to any cavity."}, {"round": 3, "reviewer_comments": {"reviewer_1": "Thanks to the authors for the revised manuscript and detailed replies. All my questions have been addressed.", "reviewer_3": "Two final requests.\n\n1. Please distinguish more clearly between two criteria for a bubble to be stable or expanding: first, that one or more supernova explodes per t_survival, and second, that these explosions provide enough energy and momentum. The latter criterion is discussed in the Methods section but not, for instance, in the Abstract.\n\n2. Please specify more precisely what is meant by l_bubble. Is this the diameter, radius, or effective size defined by area = l_bubble^2? These definitions differ by up to a factor of 4 when the area is evaluated.", "reviewer_4": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": ""}]

Decision letter