Download What Globular Clusters Teach Us About Oleg Gnedin

What Globular Clusters Teach Us About
The Assembly Of Their Host Galaxies
Oleg Gnedin
(University of Michigan)
modeling
formation
The Galaxy – Halo Connection
• In the first two days of this meeting, we talked about matching galaxies
to halos, in the full range from faint galaxies to massive galaxy clusters
• While the distribution of dark matter is approximately self-similar on
different mass scales, the baryon physics is not:
Inside galaxies, the gas has higher density and shorter cooling time;
significant fraction of baryons is in stars
• We can trace the assembly of galaxies by following their stellar
components. These are important observational constraints because:
Modeling star formation, and feedback of young stars and black holes
on the interstellar medium, is complex (many processes and huge
dynamic range, from > 10 Mpc to < 1 pc)
Globular clusters are unique tracers of early star formation
Spherical clusters of stars, from ∼104 M to 107 M
Very high stellar density, up to 105 M pc-3 , half-mass
radius 1-5 pc
Remain self-gravitating for a Hubble time and are easy to
pick up in photometric surveys
Single-age stellar populations (can measure stellar IMF)
Ages 8 – 13 Gyr, formation redshift > 1
GC metallicity reflects the conditions of
galactic ISM at the time of formation:
window into early star formation
Li & OG 2014
Amazingly simple connection to halos
fraction of galaxy mass in the globular cluster system is similar in halos
of all type and environment over 5 orders of magnitude in mass
group
environment
Spitler & Forbes
(2009)
Georgiev at al.
(2010)
Harris et al.
(2013)
Hudson et al.
(2014)
single
galaxy
environment
dwarfs are
problem
MGC ≈ (1-5)×10-3 M* ≈ 10-4 Mhost
In contrast, fairly complex metallicity distributions
blue
red
[Fe/H]≈−0.5
[Fe/H]≈−1.5
Peng et al. 2006 – HST Virgo Cluster Survey
Li & OG 2014
Color for old stellar population is a good proxy for metallicity
Metallicity distribution of GCs in most galaxies is multimodal
Each metallicity group tells us about different episodes of cluster formation
Build-up of the globular cluster
system from gas-rich mergers
and accretion of galaxies:
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, including
evolution with time
Muratov & OG 2010
Li & OG 2014
Hui Li
Formation of massive star clusters may be triggered/enhanced by
major mergers of gas-rich galaxies
• Star clusters form in giant molecular clouds (GMC)
• Higher density of globular clusters requires higher external pressure on GMC
• Interstellar medium is over-pressured during galaxy mergers
• Extreme masses of proto-globular GMC require very gas-rich galaxies
In addition, accreted satellites
bring their populations of GCs
outer GCs in M31 are associated with tidal
streams (D. Mackey)
Basic idea:
• Globular clusters form in galaxies throughout cosmic time
• Galaxies merge into bigger galaxies
• Their globular cluster systems combine
• Can we reconstruct the assembly history of the host galaxy
from observations of its globular cluster system?
Need better understanding of the conditions leading to the
formation of massive star clusters
Important for modeling galaxy formation in general:
young star clusters are dominant components of
active star formation
Adamo et al. 2015
Fraction of all young stars contained in massive star clusters
increases with the intensity of star formation, up to 50-60%
Initial Mass Function of Young Clusters is almost invariant
z≈5
Cosmological simulation of a Milky Way sized-galaxy (Li et al. 2016):
• MF is a power law as observed for young star clusters
• Unless star formation is too disruptive
z ≈ 3.3
Massive clusters form within galaxy disks,
but the disks are perturbed by frequent
mergers and accretion of satellites
Simulations with the ART code.
Novel treatment of subgrid-scale
turbulence (see poster by Kravtsov)
z≈4
Feedback of own stars terminates star formation within
a few Myr and sets the final cluster mass
ρ > 103 cm-3
εff = 10-100%
also:
ρ > 10 cm-3
εff from SGS
turbulence model
(Semenov et al.
2016)
• spatial resolution 5 pc at z=5
• typical timestep 103 yr – resolved cluster growth
• clusters grow from gas mass inside fixed-sized
sphere of 10 pc (typical GMC core/clump)
In the simulations, cluster mass function remains a
stable power-law
Different epochs within the
same central galaxy
Different galaxies at the same
epoch (z ≈ 3.3)
Cluster formation efficiency
fraction of total mass of young stars contained in star clusters
more massive than 104 M
observations
--- central galaxy
--- first satellite
--- other satellites
Maximum cluster mass scales
with the star formation rate
Do mergers trigger cluster formation? Maybe
(Mi-Mi-1)/Mi
M2/M1
no merger
major merger
Cluster MF is truncated between mergers
Summary
• Globular clusters could be used to trace major episodes of
mass assembly of their host galaxies
• Red clusters are due to intermediate-z gas-rich mergers, blue
clusters are due to early mergers and later massive mergers
• At the time of formation, GC mass function is a power law,
both in observations and galaxy formation simulations
• Mergers of gas-rich galaxies may trigger/enhance formation
rate of massive star clusters (M > 105 M)