a bit of (my) history
DESCRIPTION
A bit of (my) history. My main PhD simulations were performed on COSMOS Mk I in 1998-99! 32 R10000, 8 GB of memory, $2,000,000 0.5×10 6 particles, only 4,000 timesteps Simulations I’ll talk about today, 32 core servers, with 64 GB, $20,000 2.5×10 6 particles, but × 10 timesteps. - PowerPoint PPT PresentationTRANSCRIPT
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A bit of (my) history My main PhD
simulations were performed on COSMOS Mk I in 1998-99!
32 R10000, 8 GB of memory, $2,000,000 0.5×106 particles, only
4,000 timesteps Simulations I’ll talk
about today, 32 core servers, with 64 GB, $20,000 2.5×106 particles, but
×10 timesteps
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AGN feedback modelling: a
comparison of methods
(a work in progress) Rob ThackerAssociate Professor
& Canada Research Chair
Saint Mary’s University, Canada
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Credit where a lot of credit is due
This work is part of PhD student James Wurster’s thesis
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Outline Motivation
Physics issues, obs vs theory Methods
Difficult choices to make, complicating factors
Problem(s) and resolution(s) Our results Conclusions
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In a PhD thesis, far, far away….
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Motivation Obs. evidence of AGN feedback has
been noted for years Is the observational case compelling?
Schawinski et al 2007, Fabian review (arXiv:1204.4114)
Large ellipticals case is pretty good Radio mode commonly observed
Still need to understand situation in intermediate masses, plus redshifts
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Feedback Terminology Radio mode Accreting hot
gas Sub-Eddington
luminosity Radiatively
inefficient accretion
Radio jets provide heat source
Quasar mode Accreting cold
gas Up to Eddington
luminosity Radiatively
efficient accretion disk
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Why compare? Comparison studies:
1999 Santa Barbara cluster comparison 2006 Radiative transfer comparison 2011 Aquila galaxy formation
comparison Don’t give any real “answers”
But do provide estimates of variation between methods
=> “Be careful” about results until 3 groups agree on it
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Remember…
“The 9 orders of magnitude in physical scale means that all such simulations include subgrid assumptions and approximations.”
- Andy Fabian
The Optimistic Numericists view:
Can we be “unwrong” enough to give good insight?
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Some thoughts to ponder…
Timescale between onset of nuclear inflow and AGN activity ~ 108 yrs
Many dynamical signatures evolve signifcantly on that time scale
ALMA + JWST will be an enormous help Simultaneous SFRs, mass inflow rates,
understanding radiative behaviour Good reasons to be optimistic
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Prototype merger
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Merger movie
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Four base models + one extra
Springel, di Matteo, Hernquist 2005
(SDH05)
Okamato, Nemmen & Bower 2008
(ONB08)
Booth & Schaye 2009 (BS09, slightly
odd one out)
De Buhr, Quataret, & Ma 2011(DQM11)
+WT2012
But plenty of other work is related:
High res simulations of individual BH evolution/small scale
accretion
e.g. Levine et al 2008, 2010Alvarez, Wise & Abel 2009
Kim et al 2011Hopkins & Quateart 2010
Other “collision” work
e.g. Johansson, Naab & Burkert 2009
Halo evolutione.g. Sijacki et al 2009
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Five key components Model for BH accretion rate
(Feedback) energy return
algorithm
SPH particle accretion algorithm
Black hole advection algorithm
Black hole merger
algorithm
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Accretion physics Accretion of gas on to point in 1d:
Bondi-Hoyle-Lyttleton (1939,1944,1952)
- Gas density & sound speed at infinity
- Velocity of BH wrt to (distant) gas
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Accretion physics II Maximal symmetric accretion rate is
limited by the Eddington rate
- Proton mass and Thompson X-section
- Efficiency of mass to energy conversion
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Problems with BHL Physics:
2d problem is known to produce unstable flow
Material inflow not radial – what about angular momentum?
Radiative, magnetic effects etc Numerics:
How to relate physical variables to simulation ones?
What additional variables to introduce for this?
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What about angular momentum?
Is the key physics actually how material reaches the black hole? Gravitational
torques & viscosity keys?
Berkeley group (Hopkins et al) pursuing this aggressively
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Accreting SPH particles on to the BH
wi
wi
wi
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Generic feedback physics E=mc2 makes life
easily parameterizable, εr
Factor in efficiency of energy coupling, εf
But is the impact better modelled as heating or momentum?
+How to decide on sphereof influence?
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Heating approach (example)
wi
wi
Note ONB08 apply heating tohalo gas directly!
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Momentum approach
Sphere of influence 4sft
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Black hole advection Black hole advection
is trickier than you might think Very important for
accretion calculation N-body integrators
subject to 2-body effects
Want smooth advection Ideally toward
potential well bottom
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Black hole advection – SDH05
For low mass BH (<10Mgas)
Find gas part. with lowest PE
Relocate to that position if vrel<0.25 cs
If BH starts to carve void – can get problems
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Black hole advection – ONB08
Calculate local stellar density Follows local potential
well Move toward density
maximum Step distance
determined by both velocity and softening limit
Avoids significant 2-body issues
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Black hole merger algorithm Can give BH it’s own
smoothing length Or use grav softening
Merge when within certain distance + When grav bound (e.g.
ONB08) Or, when relative
velocity less than circ (e.g. BS09)
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Summary of implemented modelsModel Accretio
n modelSPH
accretion
Feedback model
BH advectio
n
BH merger
SDH05 BHL Classic probabilit
y
Heating Lowest local PE
Sound speed
criterionBS09 BHL+alph
a modProb
based on mass
Heating Lowest local PE
Circular vel
criterionDQM11 Viscous
timescaleProb
based on mass limit
Wind Massive tracer
Distance only
ONB08 Drag based
Prob based on
mass
Halo heating
Toward max
density
Grav bound
WT12 BHL Local particles
first
Heating Toward max
density
Sound speed
criterion
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Numerical issues Some of these processes involve
very small cross-sections => numerically sensitive
Non-associativity of floating point has an impact Worse in parallel comps –
accumulations come in different orders
We’re still quantifying the impact
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Difficult decisions To vary star formation model or not
to vary?
We’ve kept things the same – “classical” model that’s pseudo-multiphase Modified cooling based upon pressure
eqlb between phases Heated regions obvious in plots/movies Can introduce some differences
compared to other researcher’s models (ask me at end)
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Simulation models Classic two spiral
merger (very close to Springel et al 2005 model)
End state: red & dead elliptical
Low (~200k particles per galaxy) and mid (~1m) resolution models
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Movie 2
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SFRs can be numerically sensitive
SFRs are very numerically sensitive, from Springel et al 2005:
Multiphase models suppress passage peak
If the star formation rate is tied togas density, the amplitudes of merger-induced starbursts dependon the compressibility of the gas, which is influencedby both the stiffness of the EOS, as well as dynamic range inresolution of the numerical algorithm.
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Results – SFRs
Initial peak fromdisc response
SDH05BS09DQMeDQMONB08WT12
Mid res
Low res
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Notice barmode lessstrong
Disk morphology at apoapsis
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Movie 3
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Results – black hole mass growth
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M-σ for mid res final states
ONB08
BS09
DQMe
DQM, SDH05, WT12
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Densities & temps “similar”
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Results – time stepSDH05BS09ONB08WT12DQMDQMe
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Conclusions Very different behaviours – model
assumptions have enormous range Interaction with SF very important
Need to quantify degeneracies between model parameters!
BH tracking is also quite resolution dependent
AGN impact is far harder to model than SF
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Thanks for the invite! Acknowledgements:
NSERC Canada Research Chairs Program Canada Foundation for Innovation Nova Scotia Research & Innovation Trust
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Observational hope Duty cycle of AGN
activity remains big unknown
Transverse proximity effect (TPE) can measure it
Problems finding enough
background sources
30m class problem?
ForegroundAGN
Backgroundsources
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SF & AGN interaction Starburst-AGN connection well known
Obs -> AGN peak activity about 0.5 Gyr after starburst
SF impacts ISM around BH significantly Impacts temperature & accretion rates
How do these factors interplay? Not that well studied in simulations Likely degeneracies between models