Download Formation of Globular Clusters: In and Out of Dwarf

Formation of Globular Clusters
in Hierarchical Cosmology:
ART and Science
Oleg Gnedin
Ohio State University
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
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.
Pushing the limit of current hydrodynamic simulations
14 kpc
Milky Way-type
system
300 kpc (physical)
?
Kravtsov & OG (2005)
20 pc
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
Zhang & Fall (1999)
Wilson et al. (2000)
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 > 104 M pc-3
density
threshold density
for star cluster
formation
space
Star clusters in spiral arms of high-redshift disks
14 kpc
Milky Way-type
system
300 kpc (physical)
20 pc
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
Half-mass radii of model GCs match
those of the Galactic globular clusters
observed
Metallicities of model GCs at z > 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)
Clusters with different metallicity are forming at
the same epoch in progenitors of different mass
stellar mass M* correlates with star formation rate
SFR
Supergiant molecular clouds form after gas-rich mergers
Rate of galaxy mergers declines steadily from high to low
z
z=9
z=1
Kravtsov, OG, Klypin
Does reionization matter?
Vc = 10 km/s
km/s
HI
100
HeII
Yes!
No
H2
(figure from Barkana & Loeb 2001)
The mass function of young clusters deviates from
the mass function of globular clusters at low masses
characteristic mass
Zhang & Fall (1999)
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 twobody relaxation and tidal
shocks.
And in the 21st century:
INFANT MORTALITY
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
Different types of orbits of globular clusters
Not all initial conditions and evolutionary scenarios
are consistent with the observed mass function
Rh(0) = Rh(t) = const
Rh(t)  M(t)
final/initial mass = 0.29
final/initial number = 0.54
Rh(0)  M(0)1/3,
final/initial mass = 0.54
final/initial number =
0.09
Mergers of progenitor galaxies ensure
spheroidal distribution of GC system now
z=12
z=0
Moore et al. (2006)
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
X
Z
X
Y
Kinematics
eccentricity
e = (Ra– Rp)/(Ra+
R p)
radial
perigalactic distance
velocity anisotropy
 = 1 – Vt2/ 2 Vr2
tangential
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 selfconsistently in simulations of galaxy formation. Theoretical
predictions will be much less dependent on initial conditions.
Direct detection of young globular clusters at z
~4
Milky Way
1 h-1 Mpc comoving (41)
Metallicity bimodality: decide what we should explain
Yoon, Yi, Lee (2006) astro-