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Introduction

Star clusters in galaxy mergers – 7 April 2011. Slide 1 of 22. Diederik Kruijssen – Astronomical Institute Utrecht; Leiden Observatory. The formation and destruction of star cluster populations in galaxy mergers . Introduction. Models. Mergers. Diederik Kruijssen

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Introduction

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  1. Star clusters in galaxy mergers – 7 April 2011 Slide 1 of 22 DiederikKruijssen – Astronomical Institute Utrecht; Leiden Observatory The formation and destruction of star cluster populations in galaxy mergers Introduction Models Mergers Diederik Kruijssen Astronomical Institute Utrecht; Leiden Observatory Summer 2011: MPA Garching Collaborators and advisors – HennyLamers (Utrecht), Inti Pelupessy (Leiden),Nate Bastian (Excellence Cluster Munich), Simon PortegiesZwart (Leiden),Vincent Icke (Leiden)Thomas Maschberger (Grenoble), Cathie Clarke (Cambridge), Nick Moeckel (Cambridge), Ian Bonnell (St. Andrews) Antennae

  2. Star clusters in galaxy mergers – 7 April 2011 Slide 2 of 22 DiederikKruijssen – Astronomical Institute Utrecht; Leiden Observatory Galaxy mergers • Crucial in hierarchical cosmology (White & Rees 1978) • Seen to occur at all redshifts • Also affect the Local Group Introduction Models Mergers Antennae McConnachie et al. (2009)

  3. Star clusters in galaxy mergers – 7 April 2011 Slide 3 of 22 DiederikKruijssen – Astronomical Institute Utrecht; Leiden Observatory Star clusters • Are the cradle of at least some stars (Elmegreen, Lada/Lada, Bastian, Gieles, Kroupa, Bressert) • “Can be used to trace galaxy formation/evolution” (Schweizer, Ashman/Zepf, Larsen) • IF we understand the multi-scale physics Introduction Models Mergers log Number • Fall & Zhang 2001 Antennae log Mass

  4. Star clusters in galaxy mergers – 7 April 2011 Slide 4 of 22 DiederikKruijssen – Astronomical Institute Utrecht; Leiden Observatory To trace galaxy evolution with clusters… • We need to understand the impact of the galactic environment on cluster populations • Observations reveal many clusters in mergers(Holtzman, Whitmore) • Born in the starbursts during merger process(Barnes, Hernquist, Mihos) • Are all young, some very massive (> 107 M)(Schweizer, Bastian) • Possibly young globular clusters?(Ashman & Zepf 1992) Introduction Models Mergers Antennae

  5. Star clusters in galaxy mergers – 7 April 2011 Slide 5 of 22 DiederikKruijssen – Astronomical Institute Utrecht; Leiden Observatory Star clusters in changing environments • Star clusters do not live forever • Disrupted by steady tidal field and tidal shocks(Spitzer, Gieles, Gnedin/Ostriker, Heggie, Baumgardt, Portegies Zwart) • High gas densities in mergers  tidal shocks • Mergers should also show enhanced disruption • Does cluster formation or disruption dominate? Introduction Models Mergers Antennae Strength of cluster disruption and formation processes vary in time and space, and depend on the galactic environment

  6. Star clusters in galaxy mergers – 7 April 2011 Slide6of 22 DiederikKruijssen – Astronomical Institute Utrecht; Leiden Observatory Introduction Models Use numerical simulations to model co-evolution of clusters and galaxies Mergers Antennae

  7. Star clusters in galaxy mergers – 7 April 2011 Slide7of 22 DiederikKruijssen – Astronomical Institute Utrecht; Leiden Observatory • Models: galaxy simulation code Stars • We use the N-body/SPH treecode Stars (Pelupessy et al. 2004) • Contains: • Self-gravity • Star formation • Several forms of feedback • Multiphase interstellar medium (explicitly computed) • Cooling curves & cosmic ray heating • Photometry • Radiative transfer & extinction Introduction • Stars • Gas • Dark matter halo Models Mergers Antennae • Pelupessy et al. 2004

  8. Star clusters in galaxy mergers – 7 April 2011 Slide 8of 22 DiederikKruijssen – Astronomical Institute Utrecht; Leiden Observatory • Models: star cluster formation and evolution • Sub-grid, simple cluster formation within star particle • Assumes fraction of star formation in clusters Introduction Models Mergers Antennae

  9. Star clusters in galaxy mergers – 7 April 2011 Slide 9of 22 DiederikKruijssen – Astronomical Institute Utrecht; Leiden Observatory • Models: star cluster evolution code SPACE • We use the semi-analytic star cluster model SPACE (Kruijssen & Lamers 2008; Kruijssen 2009) • Contains: • Stellar evolution (Padova/Marigo et al. 2008) • Stellar remnants • Disruption by two-body relaxation . • Disruption by tidal shocks . • Evolution of the stellar mass function . • Model described and validated in Kruijssen et al. 2011 (MNRAS in press) ArXiV:1102.1013 Introduction Related to the tidal fieldConsistent with N-body sims Models Mergers Antennae

  10. Star clusters in galaxy mergers – 7 April 2011 Slide10of 22 DiederikKruijssen – Astronomical Institute Utrecht; Leiden Observatory Introduction Models Does cluster formation or destruction dominate in galaxy mergers? Kruijssen et al. in prep. Mergers Antennae

  11. Star clusters in galaxy mergers – 7 April 2011 Slide 11of 22 DiederikKruijssen – Astronomical Institute Utrecht; Leiden Observatory Does cluster formation or destruction dominate in galaxy mergers? Introduction SFR Models Tidal heating Mergers N Antennae t (Gyr)

  12. Star clusters in galaxy mergers – 7 April 2011 Slide 12of 22 DiederikKruijssen – Astronomical Institute Utrecht; Leiden Observatory • Evolution of the mass function of surviving clusters • Low-mass clusters are more efficiently disrupted than massive clusters • Mass function develops a characteristic mass • Young massive clusters in nearby mergers could be young GCs • Can be generalised to ‘violent birth’(Elmegreen 2010) Introduction Models Mergers dN/dlogM Antennae Log(M)

  13. Star clusters in galaxy mergers – 7 April 2011 Slide13of 22 DiederikKruijssen – Astronomical Institute Utrecht; Leiden Observatory Introduction How is the cluster age distribution affected by a changing environment? Models Observed slope of -1 over the entire age range, independently of mass (Whitmore et al. 2007) Mergers Antennae

  14. Star clusters in galaxy mergers – 7 April 2011 Slide 14of 22 DiederikKruijssen – Astronomical Institute Utrecht; Leiden Observatory Age distribution for a constant disruption rate Introduction Lifetime of lowest-mass cluster Slope is zero Models Slope is steeper than -1 Log(dN/dt) Mergers Log(Age) Boutloukos & Lamers (2003) Lamers et al. (2005) Antennae

  15. Star clusters in galaxy mergers – 7 April 2011 Slide 15of 22 DiederikKruijssen – Astronomical Institute Utrecht; Leiden Observatory • Age distribution for a changing disruption rate • Motion of clusters with respect to primordial region, i.e. “Cluster migration” • Disruption rate decreases with age(Elmegreen & Hunter 2010, Kruijssen et al. 2011) Introduction Models Log(dN/dt) Cluster migration Mergers Log(Age/yr) = 6 Log(Age/yr) = 7 Log(Age/yr) = 7.5 Log(Age) Antennae

  16. Star clusters in galaxy mergers – 7 April 2011 Slide 16of 22 DiederikKruijssen – Astronomical Institute Utrecht; Leiden Observatory • Age distribution for a changing disruption rate • Cluster population with a range of disruption rates is subject to “Natural selection” • Disruption rate decreases with age(Elmegreen & Hunter 2010, Kruijssen et al. 2011) Introduction Models Log(dN/dt) Natural selection Mergers Log(Age/yr) = 6 Log(Age/yr) = 7 Log(Age/yr) = 7.5 Log(Age) Antennae

  17. Star clusters in galaxy mergers – 7 April 2011 Slide 17of 22 DiederikKruijssen – Astronomical Institute Utrecht; Leiden Observatory Changing environment with high density contrast in Antennae galaxies Introduction Models Mergers Antennae

  18. Star clusters in galaxy mergers – 7 April 2011 Slide 18of 22 DiederikKruijssen – Astronomical Institute Utrecht; Leiden Observatory • Simulating the Antennae galaxies • …is interesting for two reasons • We can test the validity of the model • We can possibly explain observed cluster age distribution • Use the initial conditions from Karl et al. 2010 Introduction Models Mergers Antennae • (Karl et al. 2010)

  19. Star clusters in galaxy mergers – 7 April 2011 Slide 19of 22 DiederikKruijssen – Astronomical Institute Utrecht; Leiden Observatory • Morphological comparison Introduction Models Mergers Kruijssen & Bastian in prep. Antennae

  20. Star clusters in galaxy mergers – 7 April 2011 Slide 20of 22 DiederikKruijssen – Astronomical Institute Utrecht; Leiden Observatory • Age distributions Introduction Models Log(Ambient gas density) dN/dt Mergers Log(Age) Log(Age) Kruijssen & Bastian in prep. Infant mortality??? Antennae

  21. Star clusters in galaxy mergers – 7 April 2011 Slide 21of 22 DiederikKruijssen – Astronomical Institute Utrecht; Leiden Observatory • Virial ratios in star formation simulations(Bonnell et al. 2003; 2008) Introduction Models N y (pc) Mergers Virial ratio x (pc) Kruijssen et al. 2011 to be submitted to MNRAS Infant mortality “Cruel cradle effect”? Antennae

  22. Star clusters in galaxy mergers – 7 April 2011 Slide 22of 22 DiederikKruijssen – Astronomical Institute Utrecht; Leiden Observatory • Conclusions • Dense starburst environments may yield a net destruction of star clusters • Disruption rate decreases with age: cluster migration + natural selection • These processes explain the age distribution of clusters in the Antennae • Accounting for the variation of the disruption rate in time and space enables the tracing of galaxy evolution with star clusters • Instead of ‘infant mortality’, young clusters may be disrupted by the ‘cruel cradle effect’ Introduction Models Mergers Antennae

  23. Star clusters in galaxy mergers – 7 April 2011 Slide 23of 22 DiederikKruijssen – Astronomical Institute Utrecht; Leiden Observatory • Models: star cluster evolution code SPACE • Disruption by shocks is derived from tidal field tensor (Prieto & Gnedin 2008, Kruijssen et al. in prep. 2010) • Shock disruption time depend on the density • Need mass-radius relation: Introduction Models no shocks including shocks including shocks Mergers Theory lifetimes (Gyr) Antennae N-body lifetimes (Gyr)

  24. Star clusters in galaxy mergers – 7 April 2011 Slide 24of 22 DiederikKruijssen – Astronomical Institute Utrecht; Leiden Observatory • Models: star cluster evolution code SPACE • Evolution of stellar mass function from energy considerations(Kruijssen 2009) Introduction Models Mergers Antennae

  25. Star clusters in galaxy mergers – 7 April 2011 Slide 25of 22 DiederikKruijssen – Astronomical Institute Utrecht; Leiden Observatory Proof of concept: follow individual clusters in galaxy interaction Introduction Models Mergers Antennae

  26. Star clusters in galaxy mergers – 7 April 2011 Slide 26of 22 DiederikKruijssen – Astronomical Institute Utrecht; Leiden Observatory • Mergers • Other simulations while varying: • Galaxy mass ratio • Virial mass • Disc scale length • Gas fraction • Bulge fraction • Orbital geometry • Always net destruction, typical number of survivors 2–20% of pre-merger • Number of survivors decreases with increases starburst intensity Introduction Models Mergers Antennae

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