Formation of Globular Clusters in Hierarchical Cosmology: ART and Science

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Formation of Globular Clusters in Hierarchical
Cosmology ART and Science

• Oleg Gnedin
• Ohio State University

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The outcome of models of GC formation
depends largely on the initial conditions

• Cosmological objects
• Jeans mass after recombination (Peebles Dicke
  1968), DM halos before reionization (Bromm
  Clarke 2002), triggered by ionization fronts
  during reionization (Cen 2001) or by galaxy
  outflows (Scannapiesco et al. 2004)
• Hierarchical dissipational models
  Searle-Zinn fragments
• supergiant molecular clouds 108 M? (Harris
  Pudritz 1994), agglomeration of gas clouds at z
  5 ?1 (Weil Pudritz 2001), multi-phase collapse
  (Forbes, Brodie, Grillmair 1997), semi-analytical
  galaxy formation (Beasley et al. 2002)
• Hierarchical dissipationless models
• accretion of dwarf galaxies (Côté, Marzke
  West 1998)
• Thermal instability (Fall Rees 1985)
• warm 104 K clouds in pressure
  equilibrium with hot 106 K gas
• Mergers of gas-rich spirals (Ashman Zepf
  1992)
• massive young star clusters observed in
  mergers, high-pressure environment

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In hierarchical cosmology, initial conditions are
Art
ART
Structures in the Universe grow hierarchically,
starting from primordial density fluctuations.
CMB anisotropies provide, in principle, complete
initial conditions to simulate the
formation of galaxies and star clusters.
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Pushing the limit of current hydrodynamic
simulations
Milky Way-type system
Kravtsov OG (2005)
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Clues about star cluster formation from local
galaxies
The Antennae and other nearby interacting
galaxies show plenty of molecular gas and
recently-formed globular clusters.
Can incorporate these local physical conditions
in the simulations, on the (unresolved) scale of
parsecs
Wilson et al. (2000)
Zhang Fall (1999)
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Use simulations of galaxy formation to predict
the properties (masses, sizes, turbulent
velocities, metallicities) of giant molecular
clouds Following arguments of Larson and
Harris Pudritz, imagine that massive star
clusters form in the same way as smaller open
clusters, i.e. in the self-gravitating cores
of molecular clouds. The cluster is only 1 of
the H2 mass ? globular clusters require
supergiant molecular clouds (107 M?). Elmegreen
(2002) young star clusters in the Galaxy form
whenever ?gas gt 104 M? pc-3
threshold density for star cluster formation
density
space
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Star clusters in spiral arms of high-redshift
disks
Milky Way-type system
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Zero-age mass function of model GCs is in
excellent agreement with the mass function of
young clusters
Cumulative mass function accumulated over all
previous epochs
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Half-mass radii of model GCs match those of the
Galactic globular clusters
observed
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Metallicities of model GCs at z gt 3
ART
GGCS
large range of metallicities of GCs formed at the
same epoch up to two orders of
magnitude (absolute metallicity scale in the
simulation is somewhat uncertain)
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Clusters with different metallicity are forming
at the same epoch in progenitors of different mass
stellar mass M correlates with star formation
rate SFR
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Supergiant molecular clouds form after gas-rich
mergers
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Rate of galaxy mergers declines steadily from
high to low z
z9
z1
Kravtsov, OG, Klypin 2004
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Does reionization matter?
Vc 10 km/s 100 km/s
HI
HeII
Yes!
No
H2
(figure from Barkana Loeb 2001)
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The mass function of young clusters deviates from
the mass function of globular clusters at
low masses
characteristic mass
Zhang Fall (1999)
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Dynamical disruption of star clusters
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
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Evolution of the GC mass function in a Milky
Way-sized galaxy
Jose Prieto OG, 2006
Stellar evolution
relaxation
tidal shocks
Rh(0) ? M(0)1/3 Rh(t) ? M(t)1/3 average density
is constant
final/initial mass 0.46 final/initial
number 0.16
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Different types of orbits of globular clusters
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Not all initial conditions and evolutionary
scenarios are consistent with the observed
mass function
Rh(0) Rh(t) const
Rh(0) ? M(0)1/3, Rh(t) ? M(t)
final/initial mass 0.29
final/initial mass 0.54 final/initial
number 0.54
final/initial number 0.09
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Mergers of progenitor galaxies ensure spheroidal
distribution of GC system now
z12
z0
Moore et al. (2006)
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Spatial distribution
Space density is consistent with a power-law,
slope 2.6 to 2.8 Azimuthal distribution is
isotropic
150 kpc
50 kpc
Y
Z
Z
Y
X
X
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Kinematics
perigalactic distance
radial
eccentricity e (Ra Rp)/(Ra Rp)
velocity anisotropy ? 1 Vt2/ 2 Vr2
tangential
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Summary

• Globular clusters can form in giant molecular
  clouds within the disks of high-redshift
  galaxies, resolved by hydrodynamical simulations
• same microphysics as for young clusters in
  interacting galaxies
• model explains observed ages, sizes, masses
• metallicities correspond to blue/metal-poor
  clusters
• dynamical evolution explains the present mass
  function, but not all initial
  conditions or evolutionary scenarios work
• spatial distribution isotropic, power-law as
  observed
• velocity distribution isotropic at the center,
  radial at large radii
• Formation of massive star clusters will soon be
  included self-consistently in simulations of
  galaxy formation. Theoretical predictions will
  be much less dependent on initial conditions.

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Direct detection of young globular clusters at z
4
Milky Way
1 h-1 Mpc comoving (41?)
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Metallicity bimodality decide what we should
explain
Yoon, Yi, Lee (2006) astro-ph/0601526