11 research outputs found
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 ( erg
s) energy injection best reproduces the observed line ratio for which
the shock temperature is K. Assuming the Galactic halo
temperature to be K, we estimate the shock velocity to be km s for a weak shock. The corresponding estimated age of the Fermi
bubbles is Myr. Such an event can be produced either by a star
formation rate of M yr 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
\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 ( erg s) for a short time
(jet active period kyr) for a reasonable jet opening angle
(). 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 ( erg s)) 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, ) and
gas () 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, . Expressing the bubble fading radius
and time in terms of , the filling factor is with
, where is the supernova rate density. A constant thus
refers to , with a density-independent SFR
efficiency per free-fall time . The self-regulation to
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
. 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 pc heights in our model) with a half-opening angle
, a normalized velocity , and an energy erg, launched
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
, and a comparable , and follow a
ballistic stage that must reach a height 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