Star clusters in galaxy mergers – 7 April 2011
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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 2845519

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


Introduction 2845519

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)


Introduction 2845519

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


Introduction 2845519

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


Introduction 2845519

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


Introduction 2845519

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


Introduction 2845519

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


Introduction 2845519

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


Introduction 2845519

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


Introduction 2845519

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


Introduction 2845519

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)


Introduction 2845519

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)


Introduction 2845519

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


Introduction 2845519

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


Introduction 2845519

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


Introduction 2845519

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


Introduction 2845519

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


Introduction 2845519

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)


Introduction 2845519

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


Introduction 2845519

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


Introduction 2845519

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


Introduction 2845519

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


Introduction 2845519

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)


Introduction 2845519

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


Introduction 2845519

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


Introduction 2845519

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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