Open Questions Satellite galaxies II (and other stuff)

Open questions Some problems that have come up Many have been raised before – Collect them here – Discuss – Figure out the answer? My bias is to “sensible” galaxies

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

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

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 M* M halo M halo 2 M halo 0.5 Supernova feedback AGN become effective Galaxy Mergers Open questions Why do AGN become effective at a particular halo mass scale? Why are supernovae more effective in low mass haloes? Star Forming Passive

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 M* M halo M halo 2 M halo 0.5 Supernova feedback AGN become effective Galaxy Mergers Open questions (?!): Why do AGN become effective at a particular halo mass scale? Why are supernovae more effective in low mass haloes? Lots of scatter – correlated with formation history?

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

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)

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)

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 Font et al 2008 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 Passive/star forming fractions are very encouraging… but…

But why are centrals and satellites so similar? 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 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 Weinmann et al. (2010) This is a tough test of models – need to get both galaxy formation and the halo gas correct!!

What has happened to the Butcher-Oemler effect? 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? Woo et al. (2013) ICBS, zCOSMOS, GEEC/GEEC2, GCLASS Massive clusters (>10 14 ) Large Groups (0.5-1 x ) Small groups (<5x10 13 ) log(M star ) ~ 10.5 Field Haynes et al 2008 ?

Two more issues 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? Pasquali et al Kauffmann et al 2013

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?

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 constructors: Booth, RGB, Crain, Dalla Vecchia, Frenk, Furlong, Jenkins, Rosas-Guevara, Schaye, Theuns, McCarthy, Schaller, Springel, +++ EAGLE (Evolution and Assembly of GaLaxies and their Environments)

A hierarchy of galaxy formation simulations The big uncertainties in galaxy formation simulations Star formation The ISM equations of state Supernova driven winds Black hole accretion and feedback large scale structure N-body + GALFORM (semi-analytic) large scale structure N-body + GALFORM (semi-analytic) Galaxy Population EAGLE (Hydro-simulation) Galaxy Population EAGLE (Hydro-simulation) ISM simulation Blackhole accretion Sub-grid physics emulation 1 pc100 pc 100 Mpc100 Gpc

Eagle’s Heritage OWLS (schaye et al 2010) Suite of 50+ simulations Varying physics Gas particle resolution ~10 8 M O (to z=0) ~10 6 M O (to z=2.5) Boxes of 25 – 100h -1 Mpc GIMIC (Crain et al 2010) 4 simulations to redshift 0 One physics model High resolution ~10 6 M O (resolves the Jeans instability in galactic disks) Radius of ~20h -1 Mpc But Gimic did not include AGN feedback Not enough resolution to reliably simulate galaxies (at z=0)

Sub-grid physics in EAGLE Star formation law from Dalla Vecchia & Schaye 2008 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; Dalla Vecchia & 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 Primoridal gas Cooling accretion flows ISM: imposed equation of state

Eagle Ecology 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 Movie shows formation of Mo group

Part II The small satellites of “normal” galaxies

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

“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

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?