Clues to the origin of Fermi Bubbles from OVIII/OVII line ratio

We constrain the origin of Fermi Bubbles using 2D hydrodynamical simulations of both star formation driven and black hole accretion driven wind models. We compare our results with recent observations of OVIII to OVII line ratio within and near Fermi Bubbles. Our results suggest that independent of the driving mechanisms, a low luminosity ($\mathcal{L} \sim 0.7-1\times 10^{41}$ erg s$^{-1}$) energy injection best reproduces the observed line ratio for which the shock temperature is $\approx 3\times 10^6$ K. Assuming the Galactic halo temperature to be $2\times 10^6$K, we estimate the shock velocity to be $\sim 300$ km s$^{-1}$ for a weak shock. The corresponding estimated age of the Fermi bubbles is $\sim 15-25$ Myr. Such an event can be produced either by a star formation rate of $\sim 0.5$ M$_\odot$ yr$^{-1}$ at the Galactic centre or a very low luminosity jet/accretion wind arising from the central black hole. Our analysis rules out any activity that generates an average mechanical luminosity $\gtrsim 10^{41}$ \ergps as a possible origin of the Fermi Bubbles.Comment: 14 pages, 9 figures, accepted version (MNRAS); includes updates on the electron-proton equilibrium time scale and its implications for high energy jet

Misaligned jets from Sgr A∗^* and the origin of Fermi/eROSITA bubbles

One of the leading explanations for the origin of Fermi Bubbles is a past jet activity in the Galactic center supermassive black hole Sgr A$^*$. The claimed jets are often assumed to be perpendicular to the Galactic plane. Motivated by the orientation of pc-scale nuclear stellar disk and gas streams, and a low inclination of the accretion disk around Sgr A$^*$ inferred by the Event Horizon Telescope, we perform hydrodynamical simulations of nuclear jets significantly tilted relative to the Galactic rotation axis. The observed axisymmetry and hemisymmetry (north-south symmetry) of Fermi/eROSITA bubbles (FEBs) due to quasi-steady jets in Sgr A$^*$ can be produced if the jet had a super-Eddington power ($\gtrsim 5\times 10^{44}$ erg s$^{-1}$) for a short time (jet active period $\lesssim 6$ kyr) for a reasonable jet opening angle ($\lesssim 10^\circ$). Such powerful explosions are, however, incompatible with the observed O VIII/O VII line ratio towards the bubbles, even after considering electron-proton temperature non-equilibrium. We argue that the only remaining options for producing FEBs are i) a low-luminosity ($\approx 10^{40.5-41}$ erg s$^{-1}$)) magnetically dominated jet or accretion wind from the Sgr A$^*$, and ii) a SNe or TDE driven wind of a similar luminosity from the Galactic center.Comment: 12 pages. Submitted to ApJ. Comments are welcom

The Global Star-Formation Law by Supernova Feedback

We address a simple model where the Kennicutt-Schmidt (KS) relation between the macroscopic densities of star-formation rate (SFR, $\rho_{\rm sfr}$) and gas ($n$) in galactic discs emerges from self-regulation of the SFR via supernova feedback. It arises from the physics of supernova bubbles, insensitive to the microscopic SFR recipe and not explicitly dependent on gravity. The key is that the filling factor of SFR-suppressed supernova bubbles self-regulates to a constant, $f\sim 0.5$. Expressing the bubble fading radius and time in terms of $n$, the filling factor is $f \propto S\,n^{-s}$ with $s\sim 1.5$, where $S$ is the supernova rate density. A constant $f$ thus refers to $\rho_{\rm sfr} \propto n^{1.5}$, with a density-independent SFR efficiency per free-fall time $\sim 0.01$. The self-regulation to $f \sim 0.5$ and the convergence to a KS relation independent of the local SFR recipe are demonstrated in cosmological and isolated-galaxy simulations using different codes and recipes. In parallel, the spherical analysis of bubble evolution is generalized to clustered supernovae, analytically and via simulations, yielding $s \simeq 1.5 \pm 0.5$. An analysis of photo-ionized bubbles about pre-supernova stars yields a range of KS slopes but the KS relation is dominated by the supernova bubbles. Superbubble blowouts may lead to an alternative self-regulation by outflows and recycling. While the model is over-simplified, its simplicity and validity in the simulations may argue that it captures the origin of the KS relation

Fermi bubbles: the collimated outburst needed to explain forward-shock edges

The bipolar, nonthermal, high-latitude lobes known as the Fermi bubbles (FBs) are thought to originate from a massive energy release near the Galactic centre (GC). We constrain the FB engine and the circumgalactic medium (CGM) by analytically and numerically modeling the FB edges as strong forward shocks, as inferred from recent observations. A non-directed energy release produces shocks too spherical to account for observations even for a maximally massive Galactic disc, critical CGM rotation, or injection effectively offset from the GC. In contrast, collimated injection nearly perpendicular to the disc can account for observations in both ballistic (free expansion) and slowdown regimes, as we show using a simple stratified evolution model verified by hydrodynamic simulations. FBs still in their ballistic regime require injection (at $z\simeq100$ pc heights in our model) with a half-opening angle $\theta\simeq4^\circ$, a normalized velocity $\beta_{-2}\equiv v/(0.01c)\gtrsim 0.4$, and an energy $E\gtrsim2\beta_{-2}^2\times 10^{55}$ erg, launched $\mathbb{T}\simeq 3.3\beta_{-2}^{-1}$ Myr ago, showing a distinctive low-pressure region behind the bubble head. Slowing-down (mass accumulated) FBs require a faster injection, a thinner jet, a smaller $E/(\beta_{-2}\theta)^{2}$, and a comparable $\mathbb{T}$, and follow a ballistic stage that must reach a height $z_{s}\gtrsim 5$ kpc.Comment: 18 pages, 17 figures, 3 tables; comments welcom

Review 1994

SIGLEAvailable from British Library Document Supply Centre- DSC:7768.388(1994) / BLDSC - British Library Document Supply CentreGBUnited Kingdo

The global star formation law by supernova feedback

We address a simple model where the Kennicutt-Schmidt (KS) relation between the macroscopic densities of star formation rate (SFR, ρsfr) and gas (n) in galactic discs emerges from self-regulation of the SFR via supernova feedback. It arises from the physics of supernova bubbles, insensitive to the microscopic SFR recipe and not explicitly dependent on gravity. The key is that the filling factor of SFR-suppressed supernova bubbles self-regulates to a constant, f ∼ 0.5. Expressing the bubble fading radius and time in terms of n, the filling factor is f∝Sn−s with s ≃ 1.5, where S is the supernova rate density. A constant f thus refers to ρsfr ∝ n1.5, with a density-independent SFR efficiency per free-fall time ∼0.01. The self-regulation to f ∼ 0.5 and the convergence to a KS relation independent of the local SFR recipe are demonstrated in cosmological and isolated-galaxy simulations using different codes and recipes. In parallel, the spherical analysis of bubble evolution is generalized to clustered supernovae, analytically and via simulations, yielding s ≃ 1.5 ± 0.5. An analysis of photoionized bubbles about pre-supernova stars yields a range of KS slopes but the KS relation is dominated by the supernova bubbles. Superbubble blowouts may lead to an alternative self-regulation by outflows and recycling. While the model is oversimplified, its simplicity and validity in the simulations may argue that it captures the origin of the KS relation.This work was partly supported by the grants France-Israel PICS, GermanyIsrael GIF I-1341-303.7/2016, Germany-Israel DIP STE1869/2- 1 GE625/17-1, I-CORE Program of the PBC/ISF 1829/12, ISF 857/14, US-Israel BSF 2014-273, and NSF AST-1405962. The isolated-galaxy simulations were performed using the HPC resources of CINES and TGCC under the allocations A0030402192 and A0050402192 made by GENCI