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“Making Normal Galaxies in a Cosmological Setting”. “Making Galaxies Red and Dead Without Feedback”. T.Naab, P. Johansson, K. Nagamine, G.Efstathiou, RY Cen and J.P.O. Princeton, 27 Feb 2009: jpo. Cambridge, 8 May 2008. Cosmological Simulation: Start with WMAP CBR Sky. Hinshaw et al; 2008.

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“Making Normal Galaxies in a Cosmological Setting”

“Making Galaxies Red and Dead Without Feedback”

T.Naab, P. Johansson, K. Nagamine, G.Efstathiou, RY Cen and J.P.O.

Princeton, 27 Feb 2009: jpo

Cambridge, 8 May 2008

wmap spectrum of cosmic perturbations
WMAP Spectrum of Cosmic Perturbations



Spherical Harmonic

cosmic structure formation cartoon version
Cosmic Structure Formation(cartoon version)



linear perturbation theory

nonlinear simulations

Computing the Universe: locally, growth of perturbations computed classically; numerical hydro required to reach the current epoch
  • Transformation to co-moving coordinates x=r/a(t)
  • Co-moving cube, periodic boundary conditions
  • Lbox >>lnl


physics input included
Physics Input Included
  • Newtonian gravity.
  • Standard equations of hydrodynamics.
  • Atomic physics:adiabatic, + cooling, +heating, + non-equilibrium ionization.
  • Radiative transfer: global average, +shielding of sinks, +distribution of sources.
  • Heuristic treatment of star-formation.
  • --------------------------------------------------
  • Maxwell’s equations in MHD form.
physics input missing important on galactic scales
Physics Input Missing(important on galactic scales)
  • Cosmic ray pressure and heating.
  • Dust grain physics (depletion, absorption and catalyzation).
  • Magnetic field generation.
  • Multiphase media.
star formation algorithm
Star Formation Algorithm
  • Consider gas that is dense, cooling and collapsing.
  • Make stellar particle:

DM* = const* x DMgas x dt/Max(Tcool,Tdyn).

(cf R. Kennicutt and M. Kuchner)

  • Label particle with position, mass, metallicity and epoch.
  • Give particle velocity of gas and follow dynamics as if dark matter particle.
  • Allow output of mass, energy and radiation from each particle consistent with a star-cluster of same mass and age: “feedback”.

C* = 0.05


Global Simulations

(better mass resolution)

Naab et al (2007)

(better spatial resolution)

choice of parameters free or otherwise
Choice of parameters (“free” or otherwise)
  • Cosmological parameters:
    • Determined by observational constraints such as WMAP, SDSS etc
  • Star-Formation algorithm:
    • Results nearly independent of algorithm so long as cooling gas made into stars (in agreement w observation)
  • Metallicity:
    • Yield determined by match to cluster IGM
    • ++++++++++++++++++++++++++++++++++++


But what about “feedback” ?

(but feedback is necessary and does cause some moderate variance: NB digression ->)


Bubbles blown by super-winds from forming galaxies heat the ambient medium and retard subsequent gas infall: Cen et al 2004, Dave …

(feedback important for IGM, but relatively unimportant for galaxy properties)


Feedback Increases Number of Small Mass Galaxies and Reduces Number of High Mass Galaxies.

(effects largely compensate and produce little net change in SF rate)

high feedback

no feedback


Star Formation history: Nagamine et al (2005)

TVD Hydro vs Data

SPH Hydro & SAM vs Data


Butcher-Oemler or Gunn-Dressler effect

Blanton, M.; Cen, R.; Ostriker, J. P.; Strauss, M. A.; Tegmark, M.

ApJ.531, 1 (2000)

TVD Hydro Simulation

In clusters, the fall off in star formation since z=1 is much more rapid than in the field.

Cause is simply

C2x > V2gal,esc

Effect of hot gas in suppressing GF

results of global simulations
Results of Global Simulations
  • Reasonably good results on epoch of galaxy formation & mass distribution of galaxies.
  • Reasonably good on spatial distribution of galaxies and environmental effects (eg early “red and dead” in clusters).
  • Good treatment of the IGM.
  • Essentially no information re internal structure of galaxies.

Individual Galaxy Simulations

  • Find and isolate objects of interest in large box.
  • Add small scale power to region(s) of interest.
  • Nest within bigger and bigger boxes (but smaller than the total volume) and add intermediate scale power (for tidal forces).
  • Repeat simulation at higher spatial, temporal and mass resolution in smaller regions.
  • Go back to step # (2) with still higher resolution and repeat steps # (3) and # (4) to convergence.
hydro codes
Hydro Codes
  • SPH (eg Springel & Hernquist, Weinberg & Katz etc)
    • Advantage: good spatial resolution, community standard.
    • Disadvantages: poor mass resolution, too much viscosity, and cooling instabilities.
  • AMR (eg Norman & Bryan, Klypin,Tessier etc )
    • Advantage: more accurate hydro.
    • Disadvantage: technically very costly to resolve many regions simultaneously (communication).

High Resolution Simulation of Massive Galaxy Formation

Naab, Johannson, Ostriker and Eftsatiou

  • Dark Matter Simulation (AP^3M: 50 h-1Mpc)
  • Pick isolated halos (~ 10^12 Msolar)
  • Re-simulate at higher resolution with gas (SPH and GADGET-2)
  • Standard star-formation algorithm
  • Standard cooling
  • No feedback
  • Convergence: how do results change with resolution improvement?
  • Is feedback necessary to make an early formed “red and dead” galaxy?
  • Are the paradigms “monolithic collapse”, “merger”, “dry” or “wet” accretion useful, relevant?
  • What is the physics that matters?
  • What is the expected evolution of an elliptical?
  • How to test the picture presented?

Detailed Hydro Simulations (N,J,O&E : 2007ApJ, 658,710)

Convergence to low and to a flat rotation curve at high resolution:


In Situ Star Formation

Convergence to stellar system formed very early which quickly becomes “red and dead”.


Gas Properties

Gas, at all radii, becomes hotter with time despite fact that the “cooling time”< the Hubble time! Why?


Accreted Stellar Mass

Accreted stellar mass, 45% of total is added late ( z < 1.5), and at larger radii.


Half-Light Radii of In-situ and Accreted Stars

A Normal Elliptical: fits Sersic Profile

(detailed kinematics ok as well)

  • Gas is steadily being heated by in-falling new gas ( -PdV and Tds) and by dynamical friction from infalling lumps of DM and stars (dynamical friction) via viscosity.
  • These effects not included in SAM, but equal - quantitatively - feedback used by the SAM schemes.
  • Of course feedback really exists and must be complementary to effects listed above.
  • Dynamical Friction due to in-falling stellar lumps is very important for evolution of the stellar and DM components.
  • Two phase growth. First in situ star-formation from in-falling cold gas, and then accretion of stellar lumps.
  • DM initially increases in density (adiabatic contraction) and then decreases (dynamical friction)
  • Metal rich component in center from in-situ star-formation and metal poor component in outskirts due to stellar infall of old and small systems.
  • Stellar system grows in size with time and central velocity dispersion actually declines with time
what have we learned
What have we learned?
  • High resolution needed ~ 107 particles.
  • For massive systems the three papers of 1977 (Binney, Silk and Rees & Ostriker) appear to point to the correct physics:

Cooling time of gas becomes longer than the dynamical time and star formation ceases. Systems live in hot bubbles and then grow by accretion of smaller stellar systems.

intermediate mass systems ie normal spirals
Intermediate Mass Systems: ie Normal Spirals
  • Where ?: Filaments - not clusters or voids.
  • When?: Slowly - bulge, then disc and halo.
  • How?: Not understood and complicated (ie cannot simulate), but at least four phases/components:
    • Bulges formed like early ellipticals, smaller scale models of same: “dissipational collapse;
    • Discs form slowly from inflowing cold gas streams;
    • Accretion of satellites adds metal poor stars to halo and gas to disc;
    • Dynamical evolution can produce bars, thick discs, globular cluster in-fall and destruction etc.
technical issues that make this a very hard problem
Technical Issues that make this a VERY hard problem
  • Very high mass space and time resolution is needed, since discs are so thin and relaxation in them is so easy to produce (spuriously).
  • Since things happen slowly, one must have a reasonably good model of star formation (ie if it is short compared to T0 , one can get it wrong and it matters little).
  • Since thin discs are fragile, both feedback and dynamical effects can strongly alter the evolution
  • Etc, etc, etc - a difficult problem.
many papers with beautiful work examples include
Many papers with beautiful work: Examples include

“Simulations of Galaxy Formation in a Λ Cold Dark Matter Universe. I. Dynamical and Photometric Properties of a Simulated Disk Galaxy” & “II. The Fine Structure of Simulated Galactic Disks”

by Abadi, Mario G.; Navarro, Julio F.; Steinmetz, Matthias; Eke, Vincent R. Ap.J;591,499 (2003) & 597, 21


Many other good papers, but typically suffer from same problem, of too bulge dominated and too high a rotation curve, with a recent excellent summary of the issues:

“The formation of disk galaxies in computer simulations”: Mayer, L.; Governato, F.; Kaufmann, T.( astroph0801.3845v )

We review the progress made by numerical simulations carried out on large parallel supercomputers. Recent progress stems from a combination of increased resolution and improved treatment of the astrophysical processes modeled in the simulations, such as the phenomenological description of the interstellar medium and of the process of star formation. High mass and spatial resolution is a necessary condition in order to obtain large disks comparable with observed spiral galaxies avoiding spurious dissipation of angular momentum. A realistic model of the star formation history. gas-to-stars ratio and the morphology of the stellar and gaseous component is instead controlled by the phenomenological description of the non-gravitational energy budget in the galaxy. We show that simulations of gas collapse within cold dark matter halos including a phenomenological description of supernovae blast-waves allow to obtain stellar disks with nearly exponential surface density profiles as those observed in real disk galaxies, counteracting the tendency of gas collapsing in such halos to form cuspy baryonic profiles. However, the ab-initio formation of a realistic rotationally supported disk galaxy with a pure exponential disk in a fully cosmological simulation is still an open problem.

however see solution
However, see solution…
  • Increase resolution to ~ 107 mass elements.
  • Add supernova feedback of types I and II
  • Repeat (Quinn et al)…

Alternate approach: put in cosmological context, but model the components separately without attempting hydro

“A simple model for the evolution of disc galaxies: the Milky Way”

Naab, Thorsten; Ostriker, Jeremiah P. (MNRAS; 366,899;2006)

A simple model for the evolution of disc galaxies is presented. We adopt three numbers from observations of the Milky Way disc, Σd the local surface mass density, rd the stellar scalelength, Vc, the amplitude of the rotation curve, and physically, the local Kennicutt star formation prescription, standard chemical evolution equations assuming a Salpeter initial mass function and a model for spectral evolution of stellar populations. We can determine the detailed evolution of the model with only the addition of standard cosmological scalings with the time of the dimensional parameters. A surprising wealth of detailed specifications follows from this prescription including the gaseous infall rate as a function of radius and time, the distribution of stellar ages and metallicities with time and radius, surface brightness profiles at different wavelengths, colors, etc.


Same but better: Schoenrich and Binney (astroph 0809.3006S):

Models of the chemical evolution of our Galaxy are extended to include radial migration of stars and flow of gas through the disc. The models track the production of both iron and alpha elements. A model is chosen that provides an excellent fit to the metallicity distribution of stars in the Geneva-Copenhagen survey (GCS) of the solar neighbourhood, and an acceptable fit to the local Hess diagram. The model provides a good fit to the distribution of GCS stars in the age-metallicity plane although this plane was not used in the fitting process. Although this model's star-formation rate is monotonic declining, its disc naturally splits into an alpha-enhanced thick disc and a normal thin disc. In particular the model's distribution of stars in the ([O/Fe],[Fe/H]) plane resembles that of Galactic stars in displaying a ridge line for each disc. The thin-disc's ridge line is entirely due to stellar migration and there is the characteristic variation of stellar angular momentum along it that has been noted by Haywood in survey data. Radial mixing of stellar populations with high sigma_z from inner regions of the disc to the solar neighbourhood provides a natural explanation of why measurements yield a steeper increase of sigma_z with age than predicted by theory. The metallicity gradient in the ISM is predicted to be steeper than in earlier models, but appears to be in good agreement with data for both our Galaxy and external galaxies. The absolute magnitude of the disc is given as a function of time in several photometric bands, and radial colour profiles are plotted for representative times.

what have we learned1
What have we learned?
  • Very high resolution (N ~ 107) and SN feedback are both necessary.
  • For lower mass systems cool gas accretion is important at late times (Weinberg, Katz & Dekel, Birnboim).

Cooling time of gas is shorter than the dynamical time and star formation continues via accretion of gas to discs which become the familiar spiral systems.

conclusions high mass systems
Conclusions: High Mass Systems
  • High resolution SPH simulations without feedback produce normal, massive but small elliptical galaxies at early epochs.
  • Accreted smaller systems add, over long times a lower metallicity stellar envelope.
  • Physical basis for cutoff of star-formation is gravitational energy release of infalling matter acting through -PdV and +Tds energy input to the gas (R&O, 1973)
  • Feedback from SN and AGN is a real phenomenon - but secondary and mainly important for clearing out gas at late times.
conclusions lower mass predominantly spiral systems
Conclusions: Lower Mass (predominantly spiral) Systems
  • Preliminary hydro simulations indicate cool gas accretes onto disks (around old bulges) and produces familiar spiral late forming galaxies.
  • Physical basis for transition is cooling time vs in-fall time of gas. input to the gas.
  • Dynamical evolution at late times is important.
  • Major mergers at late times are relatively unimportant (would overly thicken discs if they occurred), but satellite accretion is significant.