Detection of Anisotropies in the Circumgalactic Medium of Disk Galaxies: Supermassive Black Hole Activity or Star Formation-driven Outflows?

Abstract

Gamma and X-ray observatories have revealed spectacular structures in the emission of the tenuous hot gas surrounding the Milky Way (MW), known as the Fermi and extended ROentgen Survey with an Imaging Telescope Array (eROSITA) bubbles. Galaxy formation simulations suggest that MW-like bubbles could be ubiquitous, but their emission may be too faint to detect with today’s instruments in individual external galaxies. In this paper, we present an analysis of stacked Chandra observations of 93 nearby galaxies. We detected soft, diffuse X-rays from the circumgalactic medium (CGM), extending up to 14 kpc, with a luminosity of (4.2 ± 0.7) × 1039 erg s−1 in the 0.3−2 keV band. To probe its spatial distribution, we constructed an azimuthal profile and found a significant enhancement along the galactic minor axis. When dividing our sample by stellar mass, central supermassive black hole (SMBH) mass, and star formation rate (SFR), we found that only high-SFR galaxies exhibit significant anisotropies in the CGM emission. To investigate whether the observed anisotropies could be attributed to MW-like bubbles, we compared our results with TNG50 simulations. In these simulations, X-ray bubbles are strongly correlated with mass of the central SMBH and typically extend to much larger, ∼50 kpc, scales. We conclude that the observed anisotropies are either caused by active galactic nucleus–driven MW-like bubbles confined to smaller, ∼10 kpc, scales or by star formation– or starburst-driven bubbles/outflows.