Stellar-wind Fueled Accretion onto Sagittarius A* in the Presence of a Nuclear Star Cluster

TL;DR

The paper investigates whether the gravitational potential of the Galactic Center nuclear star cluster (NSC) alters wind-fed accretion from Wolf-Rayet winds onto Sgr A* on parsec scales. Using Athena++ hydrodynamics with wind source terms and NSC gravity, the authors compare three models that toggle NSC effects on gas and stars, informed by NSC potential parameters from Chatz2014 and a population of 31 WR stars. They find only minor, time-averaged differences across models; early transient variations linked to WR orbital evolution fade, and the present-day accretion structure and rates converge, indicating the NSC gravity is negligible for parsec-scale accretion. The results validate prior BH-only simulations of the wind-fed flow and imply that NSC gravity does not substantially modify the feeding of Sgr A* at these scales, though magnetic fields may still play a larger role in shaping the flow.

Abstract

The Milky Way's Galactic Center hosts the black hole Sagittarius A* (Sgr A*), which provides us with a close-up view into supermassive black hole accretion and feedback. Recent works have shown that the winds from $\sim 30$ Wolf-Rayet (WR) stars orbiting Sgr A* at about 4 arcsec are important contributors to feeding the supermassive black hole. A nuclear star cluster (NSC) with a mass of several $10^6 \, \text{M}_\odot$, of which $10^6 \, \text{M}_\odot$ is within 1 pc, also surrounds Sgr A*. The NSC contributes to the gravitational potential in the Galactic Center, affecting the orbits of the WR stars and their stellar winds. In this work, we examine the effects that the NSC has on the accretion of these stellar winds onto Sgr A* which have previously been neglected. We find that, on the parsec scale, the effect from the gravitational potential of the NSC is negligible on the wind-fed accretion flow, validating the existing simulations used in the literature.

Figures (4)

• Figure 1: Comparison of WR star orbits under the influence of the gravitational potential of Sgr A* only ($\Phi_\mathrm{BH}$, dashed gray line) and the combination of the Sgr A* and NSC potentials ($\Phi_\mathrm{BH}+\Phi_\mathrm{NSC}$, solid magenta line). The orbits of two typical stars (E23 and E60 from Paumard2006) are plotted in their orbital planes with periapses directed towards the right. The yellow dots show the stars' current day position ($t=0$). Positions for the E60 star at the start and at the end of the simulation is marked. E23 star completes several orbits during the simulation so we only mark it's position at $t=0$.
• Figure 2: Comparison of the density and temperatures within 1 pc and 0.1 pc distance from Sgr A* for all three models. The name of the models are marked at the top. Details of the models are in Section \ref{['sec: models']}. The density and temperatures are calculated on the slice through the simulations volume passing through Sgr A* and parallel to the plane of the sky. Sgr A* is positioned at the center of each plot. The difference in resolution in the all plots are due to the changes in the level of mesh refinement.
• Figure 3: The accretion rate at r = $0.5$ mpc from Sgr A* during the full duration of the simulation for all three models considered. The solid magenta line represents this accretion rate for $\Phi_{\text{BH}} + \Phi_{\text{NSC}}$, the dashed orange represents the accretion rate for $\Phi_{\text{BH}} + \Phi_{\text{NSC}}(\mathrm{gas})$, and the dotted grey represents the accretion rate for $\Phi_{\mathrm{BH}}$. The accretion rates are spatially averaged over solid angle and time-averaged over 50 years in simulation time. A detailed description of the models is given in Section \ref{['sec: models']}.
• Figure 4: Spatially-averaged radial profiles for simulations (a), (b), and (c) measured around the current day. The solid, dashed, and dotted lines represent said profiles for different hydrodynamic variables of simulations (c), (b), and (a) respectively. Purple lines represent mass density ($\rho$) in units $M_\odot/\text{pc}^3$, magenta lines represent radial velocity ($|v_r|$) in units $\text{pc}/\text{kyr}$, orange lines represent sound speed ($c_s$) in units $\text{pc}/\text{kyr}$, sky blue lines represent specific angular momentum ($l$) in units $M_\odot \, \text{pc}/\text{kyr}$, and the green and yellow lines represent the total accretion rate ($\dot{M}$) in units $M_\odot / \text{kyr}$. The green (yellow) lines show a net inwards (outwards) accretion flow. Top: the radial profiles from the current day. Bottom: The same radial profiles time averaged over $-50 \text{ yr}\leq t \leq 50 \text{ yr}$.