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Open Questions

Open Questions. Satellite galaxies II (and other stuff). Data rich subject. Lots of data out there: Observers have quantified how satellite properties depend on stellar mass halo mass redshift But what do these trends mean?. Take-away message from yesterday:

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Open Questions

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  1. Open Questions Satellite galaxies II (and other stuff)

  2. Data rich subject • Lots of data out there: • Observers have quantified how satellite properties depend on • stellar mass • halo mass • redshift • But what do these trends mean? • Take-away message from yesterday: • Stellar mass dominates the properties of galaxies • Or is it “velocity dispersion”? • Halo mass determines fraction of galaxies that are passive • But star forming satellites look just like centrals

  3. A working hypothesis • DM halo grows by accretion of smaller haloes • Smaller haloes orbit • Outer halo is stripped along with diffuse gas reservoir • This breaks cycle of gas ejection and recapture • Star formation declines • Sufficiently massive galaxies spiral to center and merge • What’s wrong with this picture? • How does this stack up to the data? Hot gas reservoir SNe winds quickly exhaust disk gas Strangulation = suffocation = starvation

  4. Data rich subject AGN become effective • Stellar mass dominates the properties of galaxies • Or is it “velocity dispersion”? • Halo mass determines fraction of galaxies that are passive • But star forming satellites look just like centrals Passive M* Mhalo0.5 Galaxy Mergers Star Forming Mhalo2 Supernova feedback Mhalo • Open questions • Why do AGN become effective at a particular halo mass scale? • Why are supernovae more effective in low mass haloes?

  5. Data rich subject AGN become effective • Stellar mass dominates the properties of galaxies • Or is it “velocity dispersion”? • Halo mass determines fraction of galaxies that are passive • But star forming satellites look just like centrals M* Mhalo0.5 Galaxy Mergers Lots of scatter – correlated with formation history? Mhalo2 Supernova feedback Mhalo • Open questions (?!): • Why do AGN become effective at a particular halo mass scale? • Why are supernovae more effective in low mass haloes?

  6. How to shape the mass function…. ….including recapture in semi-analytic models (Bower et al 2012) Varying wind speed Varying wind mass loading …allows us to understand how to shape the mass function…. But this doesn’t help match evolution of the mass function any better!

  7. Data rich subject • Stellar mass dominates the properties of galaxies • Or is it “velocity dispersion”? • Halo mass determines fraction of galaxies that are passive • But star forming satellites look just like centrals Satellites of massive haloes have been satellites in smaller haloes Open Question: does the IG(r)M also play a role – denser in larger haloes? (group haloes are well below the cosmic baryon fraction)

  8. The need for a holistic approach • Blowing the gas out of the halo (NEW) • Don’t just blow the ISM out of the galaxy… • Blow the diffuse gas out of the halo • Needs a lot of energy! • But there is lots. • Is the comparison to group binding energy just a coincidence? • For most semi-analytic models the lack of observable halo gas is a problem • For this model it is a prediction • Comparison to the observed gas fractions and MW luminosities Bower et al 2008 SAM McCarthy et al. (2010), OWLS hydro-sim Data from M. Sun et al. (2009)

  9. Data rich subject Modify semi-analytic model to allow satellites haloes to keep their own halo gas (and thus allow them to continue star formation) until “strangulation-pressure” is exceeded • Stellar mass dominates the properties of galaxies • Or is it “velocity dispersion”? • Halo mass determines fraction of galaxies that are passive • But star forming satellites look just like centrals Font et al 2008 Passive/star forming fractions are very encouraging… but…

  10. But why are centrals and satellites so similar? Weinmann et al. (2010) • Stellar mass dominates the properties of galaxies • Or is it “velocity dispersion”? • Halo mass determines fraction of galaxies that are passive • But star forming satellites look just like centrals This is a tough test of models – need to get both galaxy formation and the halo gas correct!! • In the models. passive/star forming fractions are very encouraging… but… colours of satellites are distinct from centrals unless truncation is abrupt • …but if truncation is abrupt why is passive fraction low?? • Need a delay, then sudden decline?? • Open Question: … its even worse… just cutting halo growth makes satellite colours redder than centrals! See Sean McGee’s talk

  11. What has happened to the Butcher-Oemler effect? log(Mstar) ~ 10.5 • MB: Quiescent fractions are high out to z=1 • SF galaxies are much more SF – in both environments! • Halo assembly is much more rapid • Does this make sense? Do models have predictive power? ICBS, zCOSMOS, GEEC/GEEC2, GCLASS Woo et al. (2013) ? ? Field Massive clusters (>1014) Large Groups (0.5-1 x 1014) Small groups (<5x1013) Haynes et al 2008

  12. Two more issues Pasquali et al. 2010 • Metal abundance of satellites vs centrals • Why is the offset so small if satellites are closed boxes? • Satellite conformity • Why are satellites of moderate mass passive galaxies more likely to be passive? Kauffmann et al 2013

  13. Open Questions • Why are there so many blue satellites? • Does this mean a long delay time? • Or is this the result of IG(r)M ? • Why is the environment independent of galaxy mass? • Is it just that processes balance out, or is there more to it? • Why are star forming satellites so like star forming centrals? • Does this mean a short and sudden death? • We have assumed that “quenching” is a one-way trip… • Is “refueling” a possibility? • What creates “conformity” • Is it connected to the spread in halo mass at given stellar mass? • Galaxies being blasted by AGN feedback in the central?

  14. An advert for the EAGLE project • “stop screwing around with semi-analytic models and do it properly. We just need a hydrodynamic simulation that matches the present-day galaxy mass function” • Ian McCarthy, 2008 EAGLE (Evolution and Assembly of GaLaxies and their Environments) constructors: Booth, RGB, Crain, DallaVecchia, Frenk, Furlong, Jenkins, Rosas-Guevara, Schaye, Theuns, McCarthy, Schaller, Springel, +++

  15. A hierarchy of galaxy formation simulations ISM simulation Galaxy Population EAGLE (Hydro-simulation) emulation Sub-grid physics large scale structure N-body + GALFORM (semi-analytic) Blackhole accretion 100 Mpc 100 Gpc 1 pc 100 pc • The big uncertainties in galaxy formation simulations • Star formation • The ISM equations of state • Supernova driven winds • Black hole accretion and feedback

  16. Eagle’s Heritage OWLS (schaye et al 2010) • Suite of 50+ simulations • Varying physics • Gas particle resolution ~108MO (to z=0) ~106MO (to z=2.5) • Boxes of 25 – 100h-1Mpc GIMIC (Crain et al 2010) • 4 simulations to redshift 0 • One physics model • High resolution ~106MO (resolves the Jeans instability in galactic disks) • Radius of ~20h-1Mpc Not enough resolution to reliably simulate galaxies (at z=0) But Gimic did not include AGN feedback

  17. Sub-grid physics in EAGLE • Star formation law from DallaVecchia & Schaye2008 • Implicitly uses Kennicut-Schmidt relation • Metalicity dependent star formation threshold • Effective EOS treatment of the ISM • Thermal feedback in EAGLE • No decoupling!!! • Stochastically heat particles to target temperature (Creasey et al 2010; DallaVecchia & Schaye 2012) • Heating to fixed T ensures particles are hot enough not to cool immediately • Probability of heating ensures that correct energy is input Material heated by feedback Cooling accretion flows ISM: imposed equation of state Primoridal gas

  18. Eagle Ecology Movie shows formation of 1013 Mo group • A key goal of the Eagle project has been to match the stellar mass function • This provides a strong basis for exploring the effect of environment • Look at how complex it is… • Filaments penetrate halo “pursuing” satellites • Widespread impact of AGN feedback

  19. Part II The small satellites of “normal” galaxies

  20. Why so different? • Rapidly increasing efficiency of star formation in the central galaxy. • Much larger gap between central and satellites • Efficiency trend required to match mass function • Do the same physics apply? • Extrapolation of cluster/group trends to lower halo mass • Much weaker gas halo? Picture from Quo? (satellites of other galaxies?) Only 3% of MW mass galaxies have SMC/LMC companions Robotham et al 212

  21. “too big to fail”? • Dwarf satellites may yet prove a problem for L-CDM • Predicted central velosity dispersions are too large • Is this an indication of residual thermal velocities in the DM? • Or a consequence of baryonic feedback

  22. Summary – open questions • Accounting for the properties of satellites • Broad picture exists suggesting: • Trends with halo mass • Trends with redshift • …but why do satellites look so like centrals? • important to match the “details” • Why the abrupt decline? • Why the separability? • MW satellites • Continuation of group/cluster trends? • Or something completely different?

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