Open Questions Satellite galaxies II (and other stuff)

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:• 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”?



M*

Mhalo

Mhalo 2

Mhalo 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

Form

ing

Passive

Data rich subject





M*

Mhalo

Mhalo 2

Mhalo 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





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





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?





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 te

st of m

odels

– need to get b

oth galaxy

formation and th

e 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?W

oo e

t al. (2

01

3)

ICBS, zCOSMOS, GEEC/GEEC2, GCLASS

Massive clusters (>1014)

Large Groups (0.5-1 x 1014)

Small groups (<5x1013)

log(Mstar) ~ 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. 2010

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 structureN-body + GALFORM

(semi-analytic)

Galaxy PopulationEAGLE

(Hydro-simulation)

ISM simulation

Blackhole accretion

Sub-grid physics

emulation

1 pc 100 pc 100 Mpc 100 Gpc

Eagle’s HeritageOWLS (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-1MpcBut Gimic did not include AGN feedback

Not e

noug

h re

solu

tion

to

relia

bly s

imul

ate

galax

ies (

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 1013 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?

Documents

Open Questions Satellite galaxies II (and other stuff)