Formation of Globular Clusters in  CDM Cosmology

1. Formation of Globular Clusters in CDM Cosmology Oleg Gnedin (University of Michigan)

2. What we knew before HST: globular clusters are old, dense, compact – a distinct type of stellar spheroids Kormendy (1985)

3. Over 20 years the Hubble Space Telescope has revealed young massive star clusters in interacting and gas-rich galaxies. Example: the Antennae galaxies show recently formed star clusters and left-over molecular gas. Wilson et al. (2000)

4. The mass function of young massive clusters is a power law, while the mass function of old globular clusters is peaked characteristic mass Zhang & Fall (1999)

5. HST also measured old globular cluster systems in the Virgo and Fornax clusters • Luminosity function is effectively universal • Half-light radii are independent of cluster or galaxy mass Masters et al. (2010) Jordan et al. (2007)

6. Color and Metallicity Bimodality • Found in most galaxies • Usual interpretation: red clusters are associated with host galaxy, blue clusters formed somehow independently Peng et al. (2006) – ACS Virgo Cluster Survey

7. How to understand globular clusters in the context of galaxy formation? Not easy. Assuming that GCs follow galactic star formation rate produces too many red/metal-rich clusters with a unimodalmetallicity distribution. Globular clusters formed earlier than the majority of field stars in host galaxy. Beasley et al. (2002)

8. Additional constraint: spatial distribution Simple hypothesis: if one globular cluster formed per dark matter halo at high redshift, spatial distribution of blue GCs requires zform ~ 12 However, there is a problem! Moore et al. (2006)

9. Stellar density in globular clusters: av~ 102  105 M pc-3 The gas in early halos is not dense enough to form the observed globular clusters In addition, the cosmic time is less than 0.4 Gyr Moore et al. (2006) z=12 z=0

10. More observational clues: globular clusters have a spread of ages and not too low metallicity – must form over an extended period age spread increases with metallicity and distance from the Galactic center Dotter et al. (2010) Marín-Franch et al. (2009)

11. 300 kpc (physical) 14 kpc 20 pc Hydrodynamic cosmological simulations can now resolve molecular clouds that could host dense and massive star clusters gas dark matter A. Kravtsov& OG (2005)

12. 14 kpc 20 pc These molecular clouds lie in the disks of high-redshift galaxies but the spatial distribution is similar to nearby disk galaxies M33 If star clusters form from the gas above a single density threshold in the cloud clump, 104Mpc-3 their initial masses and sizes are in excellent agreement with the observations of young clusters

13. Initial mass function of model GCs is a power law as observedSize distribution is consistent, independent of redshift observed observed MGC 10-4Mhost

14. Globular cluster formation efficiency Georgiev et al. (2010) Spitler & Forbes (2009) MGC 10-4Mhost

15. peak of global SF • Cluster density is key to when they can form! • Mergers may be another not hereGCs here

16. The globular cluster system is gradually built up by the contributions of main disk and satellite galaxies main disk (thick disk clusters) surviving satellite galaxy (galaxy in red) disrupted satellite galaxy J. Prieto & OG (2008)

17. Can a single formation mechanism produce bimodality? YesModel: GC formation is triggered by gas-rich mergers begin with cosmological simulations of halo formation supplement halos with cold gas mass based on observations use MGC - Mgas relation from hydro simulations metallicity from observed M*-Z relation for host galaxies, include evolution with time A. Muratov & OG (2010)arXiv:1002.1325

18. What about dynamical evolution? OG & Ostriker (1997) Fall & Rees (1977) Spitzer (1987) + collaborators Chernoff & Weinberg (1990) Murali & Weinberg (1997) Vesperini & Heggie (1997) Ostriker & OG (1997) OG, Lee & Ostriker (1999) Fall & Zhang (2001) Baumgardt & Makino (2003) DYNAMICAL EVOLUTION: Low-mass and low-density clusters are disrupted over the Hubble time by two-body relaxation and tidal shocks And in the 21st century: INFANT MORTALITY

19. Dynamical evolution is partly responsible for bimodality: it removes most low-mass clusters • Evolution of the cluster mass function: • competition between formation and disruption Only massive clusters survive, therefore need to follow only mergers of massive protogalaxies. They are rare at low redshift.

20. The number of massive mergers declines with cosmic time,results in a spread of ages of red clusters of several Gyr (64 random realizations of each cluster) surviving GCs disrupted GCs

21. Most of globular clusters are old but they form in a variety of systems

22. Predicted trend matches the ACS data Dotter et al. (2010) Marín-Franch et al. (2009)

23. Globular clusters were a more dominant component of galactic star formation at z>3 than in the last 10 Gyr

24. Globular clusters may form in giant molecular clouds in progenitor galaxies at intermediate redshifts • Model explainsobserved sizes, masses, ages, metallicities • Dynamical evolution explains the present mass function and may be important for metallicity bimodality • Red clusters in the Galaxy are due to massive late gas-rich mergers • Blue clusters are due to early continuous mergers and later massive mergers • Break between populations is due to few late massive mergers • Massive mergers produce both red and blue clusters in almost equal amounts: in large elliptical galaxies expect red fraction of about 50% (Peng et al. 2008) Summary

25. Globular cluster vs. field star metallicity