The Complex Web of Galaxy Formation

1. 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.

2. Nature vs Nuture • Are the differences due to evolutionary processes? (nurture) • Or due to intrinsic differences in progenitor’s 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

3. 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

4. Mechanisms • Ram-pressure • Collisions / harassment • "Strangulation" Why should galaxy properties depend on the environment? Removal of cold disk gas Removal of hot gas halo

5. 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.

6. What the data say…

7. 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 won’t make an issue of this. Baldry et al. 2003 (see also Hogg et al. 2003)

8. 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

9. The colour-magnitude diagram • Fraction of star forming galaxies suppressed in dense environments – but it’s 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

10. 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

11. 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)

12. 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! • Let’s test the qualitative ideas with semi-analytic models

13. Semi-analytic models And what AGN feedback can do for you… Croton et al 2006, Bower et al 2006

14. Some good things about semi-analytic models • Current models seem to do reasonably well …. (I’m 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

15. Semi-Analytic models:The 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?

16. 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.

17. Environmental Physics is not correctly handled Satellite Galaxies All Galaxies All satellites are red! No blue satellites!

18. 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 & Gott’s 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

19. 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

20. 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)

21. 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

22. 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 doesn’t affect the X-ray luminosity B06 Model Data from Horner et al. Data from Osmond & Ponman

23. The AGN feedback loop (new version) • A note for pundits: • This is “in-situ” heating • there’s no “pre-heating” • its going to be expensive, but not prohibitively so. AGN fuelling Cooling “radio” mode P=min(Ledd,Mcool) Hydrostatic ? Heating redistribute halo gas Based on the “excess energy” method (Wu et al 1999), plus the hydrostatic criterion

24. 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 - I’ve 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)

25. 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 - that’s all! Conclusions: what you should remember! 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)

26. 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 aren’t there yet… Post-modern view is rather too easily accommodated! Conclusions: what you should remember!

27. 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++?] Improvements and challenges

28. LDSS 3 on Magellan Gilbank, Davies, Li, Balogh, Glazebrook, Hau, Bower, Baldry, Savaglio, McCarthy Redshift One Emission Line Survey

29. Thank you!

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

31. Post-modern view is rather too easy!

32. 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