The Complex Web of Galaxy The Complex Web of Galaxy FormationFormation

Richard BowerRichard BowerAndreea Font, Ian McCarthy, Andrew BensonAndreea Font, Ian McCarthy, Andrew Benson

& the GALFORM team& the GALFORM teamUsing semi-analytic models to create a holistic approach to galaxy ecology.

Nature vs NutureNature vs Nuture

1.1. Are the differences due to evolutionary Are the differences due to evolutionary processes? (nurture)processes? (nurture)

2.2. Or due to intrinsic differences in progenitor’s Or due to intrinsic differences in progenitor’s mass? (nature)mass? (nature)

All most everyone focuses on (1)All most everyone focuses on (1) The universe is hierarchicalThe universe is hierarchical Clusters are formed out of smaller systemsClusters are formed out of smaller systems

3. A new twist - central vs satellite galaxies3. A new twist - central vs satellite galaxies

z = 0.0

The taxonomy of ecological models

The “classical” view

•Galaxies contain lots of cold gas

•They must fight to retain their cold gas

The “modern” view

•Galaxies rapidly deplete their cold gas

•They must continually be refueled

The post-modern view:

Properties only depend on mass and central/satellite

Springel et al 2005

Larson, Tinsley & Caldwell 1984

Gunn & Gott, 1972

Van den Bosch et al 2008

MechanismsMechanisms

Why should galaxy properties depend on the environment?

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

Removal of cold disk gas

Removal of hot gas halo

Current thinking Current thinking (circa 2007):(circa 2007):

Ram-pressureRam-pressure: : only densest regions – most galaxies only densest regions – most galaxies

have lost their gas before this becomes have lost their gas before this becomes importantimportant

Collisions / harassmentCollisions / harassment: : important in shaping morphology, but important in shaping morphology, but

not so important in using up gasnot so important in using up gas““Strangulation”Strangulation”: :

the most important mechanism. In fact the most important mechanism. In fact its often too strong.its often too strong.

What the data say…What the data say…

Constraining models: The Constraining models: The Colour-Magnitude diagramColour-Magnitude diagram

CMR - a good tool for CMR - a good tool for studying clustersstudying clusters Simple, integrated coloursSimple, integrated colours ……but dust obscurred and but dust obscurred and

metal dependentmetal dependent

……or SFR directly?or SFR directly? Direct comparison of Direct comparison of

physical quantitiesphysical quantities Needs high quality Needs high quality

spectra and/or UV dataspectra and/or UV data

This is a theory talk, so I This is a theory talk, so I won’t make an issue of this.won’t make an issue of this.

Baldry et al. 2003 (see also Hogg et al. 2003)

Baldry et al. 2003

(u-r)

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

Faint

(u-r)

Fraction of star forming Fraction of star forming galaxies suppressed in galaxies suppressed in dense environments – dense environments – but it’s a continuous but it’s a continuous trendtrend

Local density is more Local density is more important than halo important than halo mass mass

Luminosity is more Luminosity is more important than important than environmentenvironment

isolated galaxiesisolated galaxies Even isolated regions Even isolated regions

contain “passive” galaxiescontain “passive” galaxiesBalogh et al. 2004

The colour-magnitude diagramThe colour-magnitude diagram

The post-modern view: satellite The post-modern view: satellite colours depend weakly on halocolours depend weakly on halo

Van den Bosch 2008

But clearly the “details” do depend 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 ?

Mean and spread depend only weakly on stellar mass.

Stellar mass

halo mass

the post-modern viewthe post-modern view

Satellite vs central - importantSatellite vs central - importantEnvironment (aka halo mass) unimportantEnvironment (aka halo mass) unimportantNo special processesNo special processesTimescale for SF decay.Timescale for SF decay.

less important!

(updated)

( just one? stripping of halo gas?)

(needs to be quite long compared to halo lifetime)

Qualitative explanation Qualitative explanation of the trends of the trends

The strong dependence on galaxy massThe strong dependence on galaxy mass Nature? ie., the halo mass before accretionNature? ie., the halo mass before accretion

The subtle dependence on environment?The subtle dependence on environment? Immediately galaxies become satellites?Immediately galaxies become satellites? Or “Pre-processing” in groups?Or “Pre-processing” in groups?

But what fraction of galaxies fall directly into clusters? But what fraction of galaxies fall directly into clusters? ((Berrier et al 2008; McGee et al 2009)

A long timescale for suppression? A long timescale for suppression? …but not too long!

Let’s test the qualitative ideas with semi-analytic models

Semi-analytic modelsSemi-analytic models

And what AGN feedback can And what AGN feedback can do for you…do for you…

Croton et al 2006, Bower et al 2006

Some good things about Some good things about semi-analytic modelssemi-analytic models

Current models Current models seem to do seem to do reasonably well …. reasonably well …. (I’m sure someone will contradict (I’m sure someone will contradict me!)me!)

Present-day Present-day luminosity functionsluminosity functions

The transition massThe transition mass Galaxy down-sizingGalaxy down-sizing

The models achieve The models achieve this by suppressing this by suppressing cooling in high mass cooling in high mass haloeshaloes

See also Croton et al., De Lucia et al.; Kitzbichler et al., Somerville et al 2008

Semi-Analytic models:Semi-Analytic models:The state of the artThe state of the art

AGN + semi-analytic galaxy formation provides AGN + semi-analytic galaxy formation provides a frame work for understanding the “anti-a frame work for understanding the “anti-hierarchical” universehierarchical” universe

B06 is by no means the only model to use AGN: eg., Hatton et al 2003, 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, 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.Cattaneo ‘07, Summerville ‘08 - but be aware of the different flavours.

The model seems to describe the properties of The model seems to describe the properties of central galaxies reasonably well…central galaxies reasonably well…

But..But.. …do they explain the environment-driven trends?

Problems with the Problems with the standard model…standard model…

Satellites are too red. Satellites are too red. Regardless of halo Regardless of halo mass.mass.

The effect of The effect of “strangulation” is “strangulation” is overestimatedoverestimated

A satellite galaxy A satellite galaxy orbiting within another orbiting within another halo is assumed to halo is assumed to loose its own hot gas loose its own hot gas reservoir.reservoir. This is far too simplisticThis is far too simplistic

Weinmann et al 2006; Baldry et al 2006.

Environmental Physics is not Environmental Physics is not correctly handledcorrectly handled

All satellites are red!

All GalaxiesSatellite Galaxies

No blue satellites!

Environmental Physics is not Environmental Physics is not correctly handledcorrectly handled

Is this realistic?

•Mass ratio of haloes

•Gas atmosphere of the main halo

Old Strangulation model

•Remove gas reservoir as galaxy orbits larger halo

Hot gas reservoir

SNe winds quickly exhaust disk gas

Larson, Tinsley & Caldwell 1984

Strangulation = suffocation = starvation

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 & Gott’s formulae re-calibrated for halo gas using numerical simulations)

Solving the problem by adjusting Solving the problem by adjusting the environment physicsthe environment physics

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

Font et al 2008, also Kang & vdBosch 2008; Kimm et al 2009

A closer look at the colour A closer look at the colour distribution of galaxiesdistribution of galaxies

The Field The Clusters

Gilbank & Balogh 2008; Font, McGee, Balogh et al, in prep.

(Distributions shifted to to agree with red-sequence colours)

Too many intermediate colour galaxies, not enough blue objects

Too many faint red galaxies

Towards a holistic Towards a holistic modelmodel

Model improvements:Model improvements:

Including tidal strippingIncluding tidal stripping..needed to remove faint red galaxies..needed to remove faint red galaxies

Towards an accurate model of the intra-group Towards an accurate model of the intra-group mediummedium even the new stripping calculation is too even the new stripping calculation is too

aggressiveaggressive……needed to get ram pressure calculations rightneeded to get ram pressure calculations right

X-ray Emission from X-ray Emission from Groups and ClustersGroups and Clusters

L-T relation : well known L-T relation : well known that the self-similar that the self-similar relation failsrelation fails

AGN: standard model just AGN: standard model just prevents cooling… it prevents cooling… it doesn’t affect the X-ray doesn’t affect the X-ray luminosityluminosity

Data from Horner et al.

B06 Model

Data from Osmond & Ponman

The AGN feedback loop The AGN feedback loop (new version)(new version)

CoolingAGN

fuelling

Hydrostatic ?

Heating

redistribute halo gas Based on the “excess energy”

method (Wu et al 1999), plus the hydrostatic criterion

“radio” mode

P=min(Ledd,Mcool)

A note for pundits:

•This is “in-situ” heating

• there’s no “pre-heating”

•its going to be expensive, but not prohibitively so.

X-ray Emission from X-ray Emission from Groups and ClustersGroups and Clusters

L-T relation : well L-T relation : well known that the self-known that the self-similar relation failssimilar relation fails

AGN: standard model AGN: standard model just prevents coolingjust prevents cooling

Revised model, AGN Revised model, AGN feedback redistributes feedback redistributes halo gas until the halo gas until the cooling rate drops and cooling rate drops and AGN power is cut offAGN power is cut off

AGN redistributes halo gas

Scatter driven by diverse assembly history

Voit & Bryan 2001; Bower et al 2008, MN, in press (astro-ph/0808.2994)

A huge step forward - I’ve been trying to achieve this

for ten years!

Conclusions: what you should Conclusions: what you should remember!remember!

The classical view: The classical view: Galaxies contain lots of cold gas and fight to retain their cold gas

The modern view: The modern view: Galaxies rapidly deplete their cold gas and must continually be refueled

Post-modern view: Post-modern view: central or satellite - that’s 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)

Conclusions: what you should Conclusions: what you should remember!remember!

Galaxy formation models:Galaxy formation models: ‘‘06 models: a reasonable 06 models: a reasonable

description of centralsdescription of centrals need to be modified to calculate need to be modified to calculate

environmental effects properlyenvironmental effects properly ‘‘08 models match some 08 models match some

satellite properties reasonably satellite properties reasonably wellwell……but the details still aren’t there but the details still aren’t there

yet…yet…

Post-modern view is rather too Post-modern view is rather too easily accommodated!easily accommodated!

Improvements and Improvements and challengeschallenges

Model Improvements:Model Improvements: Tidal stripping - Tidal stripping -

removing faint red removing faint red galaxies; reproducing galaxies; reproducing the intra-cluster lightthe intra-cluster light

Better modeling the Better modeling the intra-group medium, intra-group medium, understanding the X-understanding the X-ray haloes of groups ray haloes of groups and galaxiesand galaxies

Observational Observational challenges:challenges: Pushing low-redshift Pushing low-redshift

measurements to fainter measurements to fainter galaxies [GAMA, galaxies [GAMA, a wide-field a wide-field redshift survey 2mag deeper than redshift survey 2mag deeper than

SDSS (pi Norberg)SDSS (pi Norberg)]] Establishing robust Establishing robust

measurements for low measurements for low mass galaxies at higher mass galaxies at higher redshift. [ROLES, redshift. [ROLES, Gilbank et Gilbank et

al 2009;al 2009; zCOSMOS++?] zCOSMOS++?]

Redshift One Redshift One Emission Line SurveyEmission Line Survey

LDSS 3 on MagellanLDSS 3 on Magellan

Gilbank, Davies, Li, Balogh, Glazebrook, Hau, Bower, Gilbank, Davies, Li, Balogh, Glazebrook, Hau, Bower, Baldry, Savaglio, McCarthyBaldry, Savaglio, McCarthy

Thank you!Thank you!

How many parameters do How many parameters do semi-analytic models have?semi-analytic models have?

Post-modern view is rather Post-modern view is rather too easy!too easy!

Beyond the “modern” viewBeyond the “modern” view

Weinmann et al 2006; Van dev Bosch et al 2008

…but the differences between panels are largely due to the properties of central galaxies