Stellar winds pump the heart of the Milky Way

Abstract: The central super-massive black hole of the Milky Way, Sgr A*, accretes at a very low rate making it a very underluminous galactic nucleus. Despite the tens of Wolf-Rayet stars present within the inner parsec supplying ${\sim}10^{-3}\rm\ M_{\odot}\ yr^{-1}$ in stellar winds, only a negligible fraction of this material ($<10^{-4}$) ends up being accreted onto Sgr A*. The recent discovery of cold gas (${\sim}10^4\rm\ K$) in its vicinity raised questions about how such material could settle in the hostile (${\sim}10^7\rm\ K$) environment near Sgr A*. In this work we show that the system of mass-losing stars blowing winds can naturally account for both the hot, inefficient accretion flow, as well as the formation of a cold disk-like structure. We run hydrodynamical simulations using the grid-based code Ramses starting as early in the past as possible to observe the state of the system at the present time. Our results show that the system reaches a quasi-steady state in about ${\sim}500\rm\ yr$ with material being captured at a rate of ${\sim}10^{-6}\rm\ M_{\odot}\ yr^{-1}$ at scales of ${\sim}10^{-4}\rm\ pc$, consistent with the observations and previous models. However, on longer timescales ($\gtrsim3000\rm\ yr$) the material accumulates close to the black hole in the form of a disk. Considering the duration of the Wolf-Rayet phase (${\sim}10^5\rm\ yr$), we conclude that this scenario likely has already happened, and could be responsible for the more active past of Sgr A*, and/or its current outflow. We argue that the hypothesis of the mass-losing stars being the main regulator of the activity of the black hole deserves further consideration.

• The paper demonstrates that stellar winds from Wolf–Rayet stars supply roughly 10⁻³ M☉/yr, with only a tiny fraction directly accreted by Sgr A*.
• It reveals that the system reaches a quasi-steady state within 500 years before evolving into a disk phase over approximately 3000 years.
• Hydrodynamical simulations confirm consistency with observational inflow rates and disk properties, shedding light on Sgr A*'s dynamic activity.

Stellar Winds and the Accretion Processes at the Milky Way's Galactic Center

This study, led by Calderón et al., examines accretion processes at the Galactic Center of the Milky Way, specifically focusing on the role of stellar winds from Wolf-Rayet (WR) stars in contributing to both the accretion onto the supermassive black hole, Sgr A*, and the formation of a cold, disk-like structure in its vicinity. The research was motivated by observations of unexpectedly cold gas close to Sgr A*, despite the typically hostile, high-temperature environment near the black hole. The study utilized hydrodynamical simulations to explore how mass-losing stars affect accretion processes and the resulting large-scale gas structures around the black hole.

Key Findings

1. Accretion Rates and Material Flow: The simulations indicate that a complex interplay of stellar winds from the WR stars supplies approximately $10^{-3} \, M_{\odot} \, \text{yr}^{-1}$ of material, yet only a minuscule fraction, less than 10^{-4} of this, is directly accreted by Sgr A*. The simulations establish a capture rate on smaller scales (${\sim}10^{-4} \, \text{pc}$) at approximately $10^{-6} \, M_{\odot} \, \text{yr}^{-1}$.
2. Quasi-Steady and Disk Phases: Over the course of the simulations, which spanned several thousand years, the system exhibited a quasi-steady state within about 500 years but evolved into a disk phase over longer timescales ($\gtrsim 3000 \, \text{yr}$). This disk formation suggests a potential explanation for the presence of colder gas close to the black hole and points to the accumulation of material over prolonged periods as a key driver.
3. Numerical and Observational Consistency: The results are consistent with observed data, where the inflow rate at the Bondi radius matches simulations by other researchers, and the characteristics of the disk align with observations of cold gas through recombination lines. The research points to a scenario where mass-losing stars dynamically influence both current accretion flows and the historical activity of Sgr A*.

Implications

The implications of the research extend to both theoretical and practical understandings of galactic dynamics. The research provides insights into the nature of accretion flows around Sgr A*, particularly in balancing the vast outflows from the WR stars against the minuscule fraction that contributes to active accretion. Additionally, the formation of disk structures over long timescales suggests a mechanism for potential episodic increases in accretion activity, possibly explaining past activity inferred from light echoes and molecular cloud observations.

Speculations and Future Perspectives

While the study sheds light on a possibly overlooked facet of galactic center mechanics, it also invites further investigation into other contributing factors, such as the impact of external gas funneling from structures like Sgr A West, and the role of turbulence or magnetic fields in material shaping at these scales. Future simulations could extend the duration to encapsulate full WR star lifecycles, thus cementing the long-term dynamical elements described.

In conclusion, Calderón et al. argue that the regulation of Sgr A*’s activity by these stellar winds warrants further consideration, especially given the role they may play in both current and past galactic center phenomena. This comprehensive approach to modeling offers a substantial contribution to our understanding of the dynamic environment at the heart of our galaxy.