13 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
Dissipation of AGN jets in a clumpy interstellar medium
Accreting supermassive black holes (SMBHs) frequently power jets that
interact with the interstellar/circumgalactic medium (ISM/CGM), regulating
star-formation in the galaxy. Highly supersonic jets launched by active
galactic nuclei (AGN) power a cocoon that confines them and shocks the ambient
medium. We build upon the models of narrow conical jets interacting with a
smooth ambient medium, to include the effect of dense clouds that are an
essential ingredient of a multiphase ISM. The key physical ingredient of this
model is that the clouds along the supersonic jet-beam strongly decelerate the
jet-head, but the subsonic cocoon easily moves around the clouds without much
resistance. We propose scalings for important physical quantities -- cocoon
pressure, head & cocoon speed, and jet radius. We obtain, for the first time,
the analytic condition on clumpiness of the ambient medium for the jet to
dissipate within the cocoon and verify it with numerical simulations of conical
jets interacting with a uniform ISM with embedded spherical clouds. A jet is
defined to be dissipated when the cocoon speed exceeds the speed of the
jet-head. We compare our models to more sophisticated numerical simulations,
direct observations of jet-ISM interaction (e.g., quasar J1316+1753), and
discuss implications for the Fermi/eROSITA bubbles. Our work also motivates
effective subgrid models for AGN jet feedback in a clumpy ISM unresolved by the
present generation of cosmological galaxy formation simulations.Comment: 23 pages, 12 figures, 3 tables; to be submitted; comments are
welcome; accompanying video: http://youtu.be/DUpSwMMrGf
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
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Feedback-free starbursts at cosmic dawn. Observable predictions for JWST
We extend the analysis of a physical model within the standard cosmology that robustly predicts a high star-formation efficiency (SFE) in massive galaxies at cosmic dawn due to feedback-free starbursts (FFBs).
This model implies an excess of bright galaxies at compared to the standard models based on the low SFE at later epochs, an excess that is indicated by JWST observations. Here we provide observable predictions of galaxy properties based on the analytic FFB scenario. These can be compared with simulations and JWST observations.
We use the model to approximate the SFE as a function of redshift and mass,
assuming a maximum SFE of in the FFB regime. From this, we derive the evolution of the galaxy mass and luminosity functions as well as the cosmological evolution of stellar and star-formation densities. We then predict the star-formation history (SFH), galaxy sizes, outflows, gas fractions, metallicities, and dust attenuation, all as functions of mass and redshift in the FFB regime. The major distinguishing feature of the model is the occurrence of FFBs above a mass threshold that declines with redshift.
The luminosities and star formation rates in bright galaxies are predicted to be in excess of
extrapolations of standard empirical models and standard cosmological simulations,
an excess that grows from to higher redshifts. The FFB phase of The stellar systems are compact (z 10z<\!0.1 and low dust attenuation UV 0.5z 10z$). We make tentative comparisons with current JWST observations for initial insights, anticipating more complete and reliable datasets for detailed quantitative comparisons in the future. The FFB predictions are also offered in digital form.</jats:p
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