Nested Fermi and eROSITA bubbles require very similar $\sim10^{55}$ erg collimated Galactic-center outbursts; their asymmetry indicates an eastern density gradient

TL;DR

The paper addresses the origin of two nested galactic bubbles, the FBs and RBs, by developing a stratified 1D analytic model for jet-driven outflows and validating it with 2D/3D hydrodynamic simulations. It shows that both bubble pairs can be produced by GC outbursts with similar parameters, specifically $E_j\sim10^{55}$ erg, half-opening angles $\theta_j\sim4^\circ$, and speeds $v_j\sim2000$ km s$^{-1}$, with an eastern ambient-density gradient explaining their observed asymmetry. The RBs are at the onset of slowdown while the FBs remain ballistic due to RB-induced rarefaction of the CGM; the nested-outburst scenario can reproduce the observed edge morphologies and projected shapes, even when the two events are identically parameterized but separated in time by $\Delta\mathbb{T}_j\sim7$–$18$ Myr. Overall, the work links high-energy GC activity to the current high-latitude bubble structures and provides tight constraints on CGM structure and past SMBH activity.

Abstract

Observations indicate two nested pairs of extended bipolar bubbles emanating from the Milky-Way center - the $|b|\sim80^\circ$ latitude eROSITA bubbles (RBs), encompassing the smaller, $|b|\sim 50^{\circ}$ Fermi bubbles (FBs) - and classify the edges of both bubble pairs as strong forward shocks. Identifying each bubble pair as driven by a distinct, collimated outburst, we evolve these bubbles and constrain their origin using a stratified 1D model verified by a suite of 2D and 3D hydrodynamic simulations which reproduce X-ray observations. While the RBs are at the onset of slowdown, the FBs are still expanding ballistically into the RB-shocked medium. Observational constraints indicate that both RB and FB outbursts had (up to factor $\sim2$-$3$ uncertainties) $\sim4^\circ$ half-opening angles and $\sim 2000$ km s$^{-1}$ velocities $100$ pc from their base, carrying $\sim10^{55}$ erg. The FBs and RBs could thus arise from identical outbursts separated by $\sim10$ Myr; their longitudinal asymmetry favors an eastern ambient-density gradient over western wind suggestions.

Figures (15)

• Figure 1: Coarse-grained edge-detector edges (dot-dashed) of FBs Keshetgurwich18 and RBs KeshetGhosh26, and their analytically projected ballistic (solid) and slowdown (dashed) models, which are based on maximal $|l|$ and $|b|$ values in each sector: north (short dashing) vs. south (long dashing), east (thick curves) vs. west (thin). Ballistic bubbles with a westward tilt are also shown (dotted white curves) for the FB.
• Figure 2: Thermal structure (top row) and projected X-ray brightness (bottom) for RB-only, nominal simulations $\mathcal{B}_0$ (left column; $t\simeq 11.2 \hbox{Myr}$) and $\mathcal{S}_0$ (right column; $t\simeq 22.3\hbox{Myr}$), shown with identical spatial scales. Thermal images combine density ($\mathrm{cm}^{-3}$ units, left half) and temperature (keV, right half), whereas surface brightness is shown for $2\text{--}10\hbox{keV}$ bremsstrahlung (erg$~\mathrm{s}^{-1}\hbox{cm}^{-2}\,\mathrm{sr}^{-1}$); color scales are base $10$ logarithmic. RB edges extracted from gradient filters KeshetGhosh26 are overlaid on the projections as in \ref{['fig:edges1']}, for north (short dot-dashed cyan curves) vs. south (long dot-dashed yellow), east (thick curves) vs. west (thin) sectors.
• Figure 3: RB evolution in the nominal $\mathcal{B}_0$ (top) and $\mathcal{S}_0$ (bottom) simulations: bubble head height (teal circles), half-width at half-height (blue squares), and aspect ratio (red diamonds with right axis); the time of crossing $|b|\simeq 84^\circ$ is highlighted (vertical dash-dotted line). Also shown are power-law slopes (where well-fitted; dashed lines), the ballistic-model aspect ratio (for $\mathcal{B}_0$; solid red), and for $\mathcal{S}_0$, the estimated $t_s$ (labeled vertical solid line; \ref{['eq:Tc']}) and $t_\circ$ (vertical dashed line; \ref{['eq:slowLifeTime']}) transitions based on the viscosity-modified parameters (see text), and the isotropic $z_H$ expansion (solid teal; \ref{['eq:selfSimEvol']}).
• Figure 4: RB age (disks with left axis) and aspect ratio (diamonds with right axis) in simulations similar to $\mathcal{B}_0$ (error bars, showing convergence with respect to numerical resolution) but with one parameter (abscissa) varied (symbols): outburst energy (top left), velocity (top right), opening angle (bottom left), and duration (bottom right). Circles show the age of the bubbles when crossing the FB latitude. Shaded regions provide numerical error estimates (see \ref{['appendix:transition']}). Curves indicate the aspect ratio (dashed) and RB age (solid) in the ballistic model.
• Figure 5: Same as \ref{['fig:nomAgeVaried']}, but for $\mathcal{S}_0$ instead of $\mathcal{B}_0$.