The Complex Web of Galaxy Formation

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The Complex Web of Galaxy Formation

• Richard Bower
• Andreea Font, Ian McCarthy, Andrew Benson
• the GALFORM team

Using semi-analytic models to create a holistic
approach to galaxy ecology.
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Galaxy Ecology

• Galaxy Ecology - modeling how galaxies interact
  with their environment
• Role in shaping the star formation history of the
  universe

• Talk overview
• Mechanisms
• Observationsthe challenges
• Problems with semi-analytic models
• Towards a model that works
• The need for a holistic model

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Nature vs Nuture

• Are the differences due to evolutionary
  processes? (nurture)
• Or due to intrinsic differences in progenitors
  mass? (nature)
• All most everyone focuses on (1)
• The universe is hierarchical
• Clusters are formed out of smaller systems
• 3. A new twist - central vs satellite galaxies

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The taxonomy of ecological models

• The modern view
• Galaxies rapidly deplete their cold gas
• They must continually be refueled

• The classical view
• Galaxies contain lots of cold gas
• They must fight to retain their cold gas

Larson, Tinsley Caldwell 1984
Gunn Gott, 1972
The post-modern view Properties only depend on
mass and central/satellite
z 0.0
Springel et al 2005
Van den Bosch et al 2008
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• Post-modern "of, relating to, or being a theory
  that involves a radical reappraisal of modern
  assumptions about culture, identity, history, or
  language
• (merriam-webster on-line dictionary)

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Mechanisms
Why should galaxy properties depend on the
environment?
Removal of cold disk gas

• Ram-pressure
• Collisions / harassment
• "Strangulation"

Removal of hot gas halo
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Current thinking (circa 2007)

• Ram-pressure
• only densest regions most galaxies have lost
  their gas before this becomes important
• Collisions / harassment
• important in shaping morphology, but not so
  important in using up gas
• Strangulation
• the most important mechanism. In fact its often
  too strong.

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What the data say

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Constraining models The Colour-Magnitude diagram

• CMR - a good tool for studying clusters
• Simple, integrated colours
• but dust obscurred and metal dependent
• or SFR directly?
• Direct comparison of physical quantities
• Needs high quality spectra and/or UV data
• This is a theory talk, so I wont make an issue
  of this.

Baldry et al. 2003 (see also Hogg et al. 2003)
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• For a better analysis, take slices through the
  CMR
• Colour distribution in 0.5 mag bins can be fit
  with two Gaussians
• Mean and dispersion of each distribution depends
  strongly on luminosity
• Dispersion includes variation in dust,
  metallicity, SF history, and photometric errors

Bright
(u-r)
(u-r)
Faint
Baldry et al. 2003
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The colour-magnitude diagram

• Fraction of star forming galaxies suppressed in
  dense environments but its a continuous trend
• Local density is more important than halo mass
• Luminosity is more important than environment
• isolated galaxies
• Even isolated regions contain passive galaxies

Balogh et al. 2004
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The post-modern view satellite colours depend
weakly on halo
But clearly the details do depend on stellar
mass.
halo mass
Stellar mass
Mean and spread depend only weakly on stellar
mass.

• Even at fixed mass, there are
• more red-sequence galaxies in higher mass haloes
• Radial gradients within systems (Smith et al
  2008)
• Can this be explained by tidal stripping, or
  reduced or more extended suppression ?

Van den Bosch 2008
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Back to the the modern view
???but the differences between panels are
largely due to the properties of central galaxies
Weinmann et al 2006 same data as Van dev Bosch
et al 2008
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the post-modern view
(updated)

• Satellite vs central - important
• Environment (aka halo mass) unimportant
• No special processes
• Timescale for SF decay.

less important!
( just one? stripping of halo gas?)
(needs to be quite long compared to halo
lifetime)
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Qualitative explanation of the trends

• The strong dependence on galaxy mass
• Nature? ie., the halo mass before accretion
• The subtle dependence on environment?
• Immediately galaxies become satellites?
• Or Pre-processing in groups?
• But what fraction of galaxies fall directly into
  clusters? (Berrier et al 2008 McGee et al
  2009)
• A long timescale for suppression?
• but not too long!
• Lets test the qualitative ideas with
  semi-analytic models

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Semi-analytic models

• And what AGN feedback can do for you

Croton et al 2006, Bower et al 2006
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Some good things about semi-analytic models

• Current models seem to do reasonably well . (Im
  sure someone will contradict me!)
• Present-day luminosity functions
• The transition mass
• Galaxy down-sizing
• The models achieve this by suppressing cooling in
  high mass haloes

See also Croton et al., De Lucia et al.
Kitzbichler et al., Somerville et al 2008
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Semi-Analytic modelsThe state of the art

• AGN semi-analytic galaxy formation provides a
  frame work for understanding the
  anti-hierarchical universe
• B06 is by no means the only model to use AGN
  eg., Hatton et al 2003, Granato et al 04,
  Croton et al. 06, De Lucia et al 07, Menci et
  al 07, Cattaneo 07, Summerville 08 - but be
  aware of the different flavours.
• The model seems to describe the properties of
  central galaxies reasonably well
• But..

do they explain the environment-driven trends?
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Problems with the standard model

• Satellites are too red. Regardless of halo mass.
• The effect of strangulation is overestimated
• A satellite galaxy orbiting within another halo
  is assumed to loose its own hot gas reservoir.
• This is far too simplistic

Weinmann et al 2006 Baldry et al 2006.
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Environmental Physics is not correctly handled
Satellite Galaxies
All Galaxies
All satellites are red!
No blue satellites!
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Environmental Physics is not correctly handled

• Old Strangulation model
• Remove gas reservoir as galaxy orbits larger halo

Larson, Tinsley Caldwell 1984
McCarthy et al 2007 an improved model for halo
stripping depends on the orbit of the satellite
and the gas content of the satellite and main
halo. (Actually, Gunn Gotts formulae
re-calibrated for halo gas using numerical
simulations)
Hot gas reservoir

• Is this realistic?
• Mass ratio of haloes
• Gas atmosphere of the main halo

SNe winds quickly exhaust disk gas
Strangulation suffocation starvation
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Controlled SPH and FLASH simulations
McCarthy et al. 2007
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Implementation in GALFORM
- Hot gas modelled with a beta- model / NFW
Satellite velocities are sampled with a 2D
distribution (Benson 2005)
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Environmental Physics is not correctly handled
New Strangulation Physics
Old Strangulation model
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Solving the problem by adjusting the environment
physics

• Dramatic improvement in the environmental
  dependence of satellite properties from reducing
  the effects of starvation.

Font et al 2008, also Kang vdBosch 2008 Kimm
et al 2009
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A closer look at the colour distribution of
galaxies
The Field
The Clusters
Too many intermediate colour galaxies, not enough
blue objects
Too many faint red galaxies
Gilbank Balogh 2008 Font, McGee, Balogh et al,
in prep.
(Distributions shifted to to agree with
red-sequence colours)
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A closer look at the luminosity function
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Towards a holistic model

• Model improvements
• Including tidal stripping
• ..needed to remove faint red galaxies
• Towards an accurate model of the intra-group
  medium
• even the new stripping calculation is too
  aggressive
• needed to get ram pressure calculations right

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X-ray Emission from Groups and Clusters

• L-T relation well known that the self-similar
  relation fails
• AGN standard model just prevents cooling it
  doesnt affect the X-ray luminosity

B06 Model
Data from Horner et al.
Data from Osmond Ponman
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The AGN feedback loop (new version)

• A note for pundits
• This is in-situ heating
• theres no pre-heating
• its going to be expensive, but not prohibitively
  so.

AGN fuelling
Cooling
radio mode Pmin(?Ledd,?Mcool)
Hydrostatic ?
Heating
redistribute halo gas
Based on the excess energy method (Wu et al
1999), plus the hydrostatic criterion
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X-ray Emission from Groups and Clusters

• L-T relation well known that the self-similar
  relation fails
• AGN standard model just prevents cooling
• Revised model, AGN feedback redistributes halo
  gas until the cooling rate drops and AGN power is
  cut off

AGN redistributes halo gas
A huge step forward - Ive been trying to achieve
this for ten years!
Scatter driven by diverse assembly history
Voit Bryan 2001 Bower et al 2008, MN, in press
(astro-ph/0808.2994)
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The baryon content of haloes - where are all
those baryons?
Ejected gas
T (keV)
Hot X-ray emitting gas
Stars and cold gas
Also Conroy Ostriker 2008 Puchwein et al
2008 Dave et al 2008
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Conclusions what you should remember!

• The classical view Galaxies contain lots of cold
  gas and fight to retain their cold gas
• The modern view Galaxies rapidly deplete their
  cold gas and must continually be refueled
• Post-modern view central or satellite - thats
  all!

Lengthens the timescale of starvation
The properties of (faint) satellites are
somewhat dependent on their parent halo
nature wins (the mass of the halo before
accretion)
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Conclusions what you should remember!

• Galaxy formation models
• 06 models a reasonable description of centrals
• need to be modified to calculate environmental
  effects properly
• 08 models match some satellite properties
  reasonably well
• but the details still arent there yet
• Post-modern view is rather too easily
  accommodated!

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Improvements and challenges

• Model Improvements
• Tidal stripping - removing faint red galaxies
  reproducing the intra-cluster light
• Better modeling the intra-group medium,
  understanding the X-ray haloes of groups and
  galaxies

• Observational challenges
• Pushing low-redshift measurements to fainter
  galaxies GAMA, a wide-field redshift survey 2mag
  deeper than SDSS (pi Norberg)
• Establishing robust measurements for low mass
  galaxies at higher redshift. ROLES, Gilbank et
  al 2009 zCOSMOS?

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Redshift One Emission Line Survey

• LDSS 3 on Magellan
• Gilbank, Davies, Li, Balogh, Glazebrook, Hau,
  Bower, Baldry, Savaglio, McCarthy

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Thank you!
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How many parameters do semi-analytic models have?
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Post-modern view is rather too easy!
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Beyond the modern view
but the differences between panels are largely
due to the properties of central galaxies
Weinmann et al 2006 Van dev Bosch et al 2008