Transcript Formation of Globular Clusters: In and Out of Dwarf Galaxies

Formation of Globular Clusters
in CDM Cosmology
Oleg Gnedin
(University of Michigan)
What we knew before HST: globular clusters are old, dense,
compact – a distinct type of stellar spheroids
Kormendy (1985)
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)
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)
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)
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
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 unimodal metallicity
distribution.
Globular clusters formed
earlier than the majority of
field stars in host galaxy.
Beasley et al. (2002)
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)
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
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)
Hydrodynamic cosmological simulations can now resolve
molecular clouds that could host dense and massive star clusters
14 kpc
gas
dark matter
300 kpc (physical)
A. Kravtsov & OG (2005)
20 pc
These molecular clouds lie in the disks of high-redshift galaxies
but the spatial distribution is similar to nearby disk galaxies
14 kpc
M33
If star clusters form from the gas above a single density
threshold in the cloud clump, 104 M pc-3
their initial masses and sizes are in excellent
agreement with the observations of young clusters
20 pc
Initial mass function of model GCs is a power law as observed
Size distribution is consistent, independent of redshift
observed
MGC  10-4 Mhost
Globular cluster formation efficiency
Georgiev et al. (2010)
Spitler & Forbes (2009)
MGC  10-4 Mhost
peak of
global SF
• Cluster
density
is key to
when they
can form!
• Mergers
may be
another
not here
GCs here
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)
Can a single formation mechanism produce bimodality? Yes
Model: 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
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 twobody relaxation and tidal
shocks
And in the 21st century:
INFANT MORTALITY
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.
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
Most of globular clusters are old
but they form in a variety of systems
Predicted trend matches
the ACS data
Dotter et al. (2010)
Marín-Franch et al. (2009)
Globular clusters were a more dominant component of
galactic star formation at z>3 than in the last 10 Gyr
Summary
• Globular clusters may form in giant molecular clouds in
progenitor galaxies at intermediate redshifts
• Model explains observed 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)
Globular cluster vs. field star metallicity