Remarks

Remarks for writing paper

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  • These studies provide remarkably stringent(tight) constraints on Sgr A*’s properties.
  • This concern is especially pertinent for our case using 2.5D simulations, as these can never truly reach a steady state.
  • The spin of the central black hole and the initial magnetic field configuration of the torus also have important consequences on the dynamics of the system and the resulting spectra.
  • The lack of a priori knowledge of the field configuration introduces an effective error in all simulation conclusions that is comparable to the difference introduced by including cooling.
  • Due to its proximity, Sgr A∗ subtends the largest angle on the sky among all known black holes, and is thus the ideal(excptional) laboratory to study accretion and outflow physics.
  • Supermassive black holes play a crucial role in shaping our Universe: they have symbiotic relationships with their host galaxies and are the cause of extreme environmental changes via accretion, outflows, jets and mergers.
  • our estimates on $r$ are subject to large uncertainties.
  • Modeling of local star formation and feedback processes in galaxy simulations is admittedly rather crude.
  • typical AGN episode duration should be higher in high-redshift galaxies than in the local Universe, but this is almost a corollary of the previous one.
  • feedback from active galactic nuclei is an indispensable element that enables the models to reproduce the stellar mass function (SMF) of the local universe.
  • There is also compelling evidence that quasar activity is preceded and/or accompanied by a period of intense star formation in galactic nuclei
  • Jets from SMBHs that interact with the ICM cover an enormous dynamic range in space and time, being launched at several Schwarzschild radii, and propagating outwards to tens, sometimes even 100 kpc
  • it is clear that before trying to unpick the interaction of different processes and their impact on galaxy formation, it is crucial to understand the numerical consequences of the individual feedback schemes.
  • they are perhaps not up to the challenge of the multidimensional, multicomponent, time-dependent computation involving the necessary range of temporal and spatial scales.
  • Concordance galaxy formation models based on cold dark-matter cosmology widely invoke AGN feedback as a crucial ingredient to self-regulate galaxy and SMBH growth.
  • The study of AGN emission lines is of importance in regard to a number of issues.
  • we must make the problem tractable by forgoing some of the more computationally expensive physics, like a full treatment of the radiation physics and the general relativistic effects which may impact the evolution of the disk in the very inner region close to the black hole.
  • This poses a problem because the defining evolution of the accretion flow can occur on small scales and on long times, which makes a proper simulation of a truncated accretion disk very difficult to do with current computational capabilities.
  • starbursts are sometimes observed to occur before the onset of nuclear activity in galaxies
  • there is ample amount of observational data which probes accreting systems at very different luminosities and accretion rates.
  • In an effort to account for stellar feedback as comprehensively as possible, the (sub-grid) model implemented in this work includes stellar winds, Type II and Type Ia supernovae.
  • its main role is to prevent the gas heated/expelled by SN winds to be re-accreted at a later stage, alongside more pristine material
  • most massive galaxies (M∗ ≥ 3 × 10^11M_sun) are quenched to the extent that their stellar masses decrease by about 80% at z = 0.
  • Further sophistications to the model are possible and can be easily incorporated to fit more specific objects, though at the expense of an increased number of parameters.
  • A sanity check is to retrieve the observed X-ray properties
  • MHD wind is stratified, having a slower velocity at progressively larger launching radii, akin to an entrained outflow
  • The emergence of UFOs and the connection with large-scale warm absorbers has been corroborated by other analytic studies.
  • The necessary, but not sufficient, condition to assess that the cooling flow problem has been solved is the drastic suppression of the cooling rates, below 5–10% of the classic predictions, consistent with the spectroscopic constraints
  • Nevertheless, on kpc scales the observations point toward a feedback process governed by directional mechanical input of energy, with subrelativistic velocities, ∼10^3– 10^4 km s^−1, and substantial mass outflow rates∼10^−1–10^2 M_sun yr^−1
  • The leitmotiv of our previous works is that massive subrelativistic AGN outflows are the key to solv- ing the cooling flow problem.
  • As cooling resumes without a source of heating, another cycle of CCA rain and collisions, mass ejection and entrainment, and restoration is triggered, consistently with X-ray data
  • we present and discuss in-depth the GR-RMHD simulations results, including the mechanical and radiative efficiencies
  • At the present, no simulation is capable of covering simultaneously the 10 dex dynamic range involving SMBH feeding and feedback
  • black holes would not exist without matter feeding them, and cosmic structures would tend to a quick cold death without feedback from SMBHs, thus creating a symbiotic relation.
  • The nature of this turbulence is a matter of intense debate.
  • In parallel to this, high-resolution simulations can indicate what the observational signatures are for various mechanisms of how AGN could transfer energy and momentum into the gas in individual galaxies.
  • great care must be taken when using empirical results to draw conclusions on “smoking gun” evidence for or against the impact of AGN upon star formation. Whilst we may observe the “smoke” (e.g., outflows and/or reduced star formation rates) the “gun” (i.e., the AGN) may no longer be visible (Harrison 2017)
  • insight into the impact of AGN on star-formation may be gained from investigating the star-formation rates as a function of stellar mass.
  • Further work using detailed spectra to assess the star formation histories of AGN host galaxies, in tandem with specific model predictions on how AGN and star formation interact, will also provide insight into the observable signatures of the impact of AGN
  • There are many uncertainties in the evolution of AGN, apart from the role of mergers.
  • the growth of the central black hole by accretion can have a profound effect on its host galaxy.
  • the synchrotron emission from the SN blast wave depends on the external density and the shock micro-physics, which are unknown quantities a priori.
  • A significant effort is being invested into further searches for dark matter in the GC.
  • The Fermi discovery of luminous shocks in coincidence with the optical peak unambiguously establishes their importance to the qualitative picture of nova eruptions.
  • Corroborating this idea that the planes examine sources with different accretion modes, Merloni et al. (2003) find a suggestion of higher scatter above logLX/LEdd = −3.
  • Follow-up work by Kording et al. (2006) reconciles the two competing results by asserting that Merloni et al. (2003) include a sample which spans a wider range in mass-accretion rates, whereas their own work focuses on very low-Eddington sources.
  • Conversely, Falcke et al. (2004) preform a similar analysis and come to the conclusion that the X- ray emission is dominated by optically thin synchrotron emission from the jet.
  • The changes in jet luminosity are ascribed to changes in the accretion flow.
  • Understanding the symbiosis between accretion and outflows in the vicinity of black holes is vital to our understanding of how black holes impact their environments.
  • the impact on the host galaxy and the precise mechanism of the communication of the AGN with the galaxy’s interstellar medium is far from being understood.
  • observationally ruling out Compton shock cooling is so far inconclusive.
  • The 600-ks XMM-Newton observation of the Seyfert 1 galaxy NGC 4051 in 2009, extending over 6 weeks and 15 spacecraft orbits, broke new ground by finding an unusually rich absorption spectrum with multiple outflow velocities, in both RGS and EPIC spectra, up to ∼9,000 km s−1.
  • Some doubts remained as to how common high-velocity outflows were, as the majority of detections were of a single absorption line that had moderate statistical significance, raising concerns of publication bias
  • The dearth of X-ray and radio shock signatures at times coincident with the gamma-rays is unsurprising in retrospect due to the high bound-free and free–free optical depths created.
  • models of nova outbursts remain plagued by uncertainties such as the efficiency of convective mixing and the assumed prescription for mass-loss.
  • Leptonic models for the gamma-ray emission are disfavoured given the low electron acceleration efficiency.
  • The DES is subject to ionizing UV and X-ray radiation from the forward shock.
  • The ionization state of the DES and the post-shock cooling layer are of great relevance to calculating the observed X-ray and radio emission from the forward shock.
  • Evidence for shocks in novae is present at other wavelengths.
  • Although the standard model provides a relatively satisfactory picture of the late radio emission from nova, deviations from this simple picture are observed at early times.
  • Unfortunately, a complete theoretical picture is still missing so that modeling coupled with a close comparison with observations is crucial in this field
  • Particular attention is paid to the appearance and detectability of central hot bubbles
  • we stress that the models describe an isolated galaxy.
  • Since in the simulations the treatment of feedback is physically based, not tuned to reproduce observations, any agreement or discrepancy of the resulting model properties with X-ray observations is relevant to improving our understanding of the SMBH-ISM coevolution, putting further constraints on the input ingredients, and possibly telling us what additional physics may be important in the problem.
  • the black hole has to gain more mass before its luminosity is capable of expelling the gas from the galaxy and quenching star formation and AGN activity
  • Going to larger scales includes more of the galactic environment at the cost of lower resolution which potentially compromise elements of the regulation mechanism
  • Note that, for the sake of consistency, all previously published FIRE simulations (see references in § 1) were run with the identical source code, using GIZMO’s “P-SPH” hydrodynamic solver.
  • We also make use of a new, more accurate hydrodynamic solver, needed to properly treat MHD and anisotropic diffusion.
  • Properly treating this anisotropy requires MHD simulations and is numerically non-trivial.
  • There is also ample observational evidence of AGN-driven massive molecular outflows in relatively distant sources
  • We find excellent correspondence between our numerical and the analytic solution up to a time of roughly 0.8 orbits, beyond which effects associated with the multidimensional nature of the flow in our simulation begin to become important.
  • A detailed comparison of the properties of convection in a rapidly rotating flow such as that begin studied here with the known properties of stellar convection would be fruitful, but will not be discussed further here.
  • This result seems to be a consequence of strong convection in the inner regions.
  • The discrepancy can be attributed to the fact that the density at the equator is about 2/3 the value in Run A.
  • We must express a strong word of caution at this point.
  • There is at present no analytic theory that accurately predicts the inflow rates of gas in mixed stellar-plus-gas systems, despite the rich history of analytic studies of gravitational instabilities in discs