Asymmetric eROSITA bubbles as the evidence of a circumgalactic medium wind

TL;DR

This paper uses hydrodynamic simulations to show that asymmetric eROSITA bubbles are best explained by a circumgalactic medium wind, indicating our galaxy is both accreting and outflowing gas.

Contribution

It demonstrates that a circumgalactic medium wind model explains the asymmetry of eROSITA bubbles better than other hypotheses.

Findings

A wind from the east-north direction at ~200 km/s explains asymmetries.

The wind redistributes density and metallicity in the halo.

The galaxy is both accreting low-metallicity gas and providing outflow feedback.

Abstract

The eROSITA bubbles are detected via the instrument with the same name. The northern bubble shows noticeable asymmetric features, including distortion to the west and enhancement in the eastern edge, while the southern counterpart is significantly dimmer. Their origins are debated. Here, we performed hydrodynamic simulations showing that asymmetric eROSITA bubbles favor a dynamic, circumgalactic medium wind model, but disfavor other mechanisms such as a non-axisymmetric halo gas or a tilted nuclear outflow. The wind from the east by north direction in Galactic coordinates blows across the northern halo with a velocity of about 200 km s$^{-1}$, and part of it enters the southern halo. This creates a dynamic halo medium and redistributes both density and metallicity within. This naturally explains the asymmetric bubbles in both the morphology and surface brightness. Our results suggest that our Galaxy is accreting low-abundance circumgalactic medium from one side while providing outflow feedback.