The Formation Histories and Mass
Profiles of Some Elliptical Galaxies
Steve Zepf
Michigan State University
Overview – Three Questions
1. Metal-poor globular clusters and early structure
formation (the biasing of the metal-poor GC
population)
2. When were the major formation epoch(s) of elliptical
galaxies? (ages of their metal-rich GCs)
3. What are the properties of the dark matter halos around
elliptical galaxies? (radial velocities of the globular
clusters)
topics left out – GC formation, dynamical evolution,
low-mass X-ray binary GC conncetion
Why Globular Cluster Systems?
• GCs have ~106 stars within a few pc
- remarkable densities
- readily observable
• some GCs are very old – hope of
constraining very early structure
formation.
• GCs are true simple stellar populations
(to first order), so determining their
age and metallicity is much simpler
than for the composite populations in
integrated light.
• GCs are observed to form in all major
star formation events in galaxies.
• GCs are found out to very large radii
around galaxies – useful dynamical
probes of galaxy halos.
one example – NGC 3256
Zepf et al 1999
Another example – the Antennae, Whitmore et al.
Why Globular Cluster Systems?
• GCs have ~106 stars within a few pc
- remarkable densities
- readily observable
• some GCs are very old – hope of
constraining very early structure
formation.
• GCs are true simple stellar populations
(to first order), so determining their
age and metallicity is much simpler
than for the composite populations in
integrated light.
• GCs are observed to form in all major
star formation events in galaxies.
• GCs are found out to very large radii
around galaxies – useful dynamical
probes of galaxy halos.
Bimodality in GC Systems of Elliptical Galaxies
• Distribution because
individual GC
colors/metallicities can be
determined.
• Early results from Zepf &
Ashman 93 and
subsequent work suggest
bimodality in optical
colors of GC systems of
elliptical galaxies.
• Large samples establish
bimodality as the norm
(>60%) for ellipticals, S0s
consistent but less certain.
e.g. Kundu & Whitmore 01
~30 Ellipticals and
~30 S0s
Color and Metallicity Distributions
• For mostly old systems, color traces
metallicity, use Milky Way and
models to do this (age-met
degeneracy still present).
• Can’t get the two peaks in the
observed colors from a singlepeaked metallicity distribution.
 GCs come in two metallicity
groups .
• Metal-rich GCs - closely tied to bulk
of host galaxy - trace galaxy
formation/assembly.
• Metal-poor GCs - Low metals,
extended spatial distribution, and
old ages in Milky Way halo constraints on early structure
formation?
Note this division in abundance also
seen in Galaxy GC system
Formation Sites of Metal-Poor GCs
Metal-poor GCs include the oldest surviving fossil structures. When
and where they formed constrains early star formation in the
universe.
 Formation redshift can be addressed by determining the
“biasing” in GC numbers vs galaxy mass (Rhode & Zepf 2004,
Rhode, Zepf, & Santos 2005). Basic idea (Santos 2003)…
• More massive galaxies have a greater fraction of their mass
collapsed at a given redshift than lower mass galaxies on
average.
• If metal-poor globular cluster formation has a preferred early
formation epoch, then more massive galaxies will have more
metal-poor GCs per stellar mass. The earlier the formation
epoch, the greater the “bias” in the metal-poor GC population
today.
• Need observational determination of total number of metal-poor
GCs in galaxies with a range of masses  Rhode & Zepf
01,03,04
Observational Requirements
• Both wide-field and deep imaging (many extant data only
cover ~10% of the GC system).
• Need colors to separate blue and red GCs to isolate metalpoor GCs for very early formation.
 Katherine Rhode’s PhD thesis at Yale.
• Utilize large-area CCDs, multiple colors, new analysis
techniques to make both these advances for sample of 4
nearby ellipticals and 9 spirals (ellipticals in Rhode &
Zepf 2004, 2001, spirals in Rhode & Zepf 2003 and in
prep).
NGC 4472 – 34’ x 34’ field. Ground-based Mosaic image and GC identification
Rhode & Zepf 2001, 2004; WFPC2 images in blue, ACS in red
Results: Early-type Sample
(Rhode & Zepf 2004, AJ, 127, 302)
NGC 4594: 1748 GC candidates
selected in BVR
NGC 4406: 1400 GC candidates
NGC 3379: 321 GC candidates
Mosaic image of NGC 4594
FOV = 38’ x 37’, FWHM ~ 1”
Results: Early-type Sample
(Rhode & Zepf 2004)
Cluster Es
Field E/S0s
Results: Early-type Sample
(Rhode & Zepf 2004)
Color Distributions
• Two (or more) Gaussians fit better than
one at >99.9% confidence
• NGC 4472, NGC 4406, and NGC 4594
have 60% blue, 40% red GCs
• NGC 3379 has 70% blue, 30% red
 Disagrees with collapse+accretion and
multi-phase SF models, which predict
larger proportions of blue GCs in more
luminous galaxies
Observational Results
T  NGC/109MSun
Determined
observationally for
metal poor GCs in
galaxies with a range
of galaxy masses.
 Some biasing
observed
Rhode, Zepf, & Santos
2005 ApJL
Formation Redshift
Hierarchical model with
Greg Bryan
Most systematics tend to
flatten the trend in the
data.
 most likely not
extremely early
formation
zform < ~11
Ages of Formation Epoch(s) of Elliptical Galaxies –
Why “Metal-Rich” GC Systems?
• Metal-rich GC systems trace
elliptical galaxy populations
spatially and in color.
• GC formation observed in all
starbursts in local universe –
trace major formation episodes.
• Each individual GC is a simple
stellar population, most basic
determination of ages and
abundances. GC systems reveal
distribution in age and
metallicity of major formation
events of ellipticals.
Ages of the Formation Epoch(s)
How to get ages of GC populations?
Optical colors good for finding metallicity peaks, but
bad for ages because of age-metallicity degeneracy –
older age and higher metal content have same effect
on optical colors (colors can be red because of old
age or high metal content). Ideas…
• luminosity function (assumes mass function)
• Balmer absorption lines
• optical to near-infrared colors  efficient, effective
Breaking Age-Metallicity Degeneracy with
Near-IR to Optical Photometry
I-H color primarily sensitive to metallicity, g-I color more
sensitive to age. True of any set of optical to near-infrared
color vs optical color – consequence of basic stellar properties.
Difference between 3 Gyr (int. age) and 13 Gyr (old age) is about
0.3 mag.
NGC 3115, NGC 4365 in VIK
First try with this technique –
Puzia, Zepf, Kissler-Patig,
Hilker, & Minniti 2002
NGC 3115 – primarily consistent
with typically old GCs
NGC 4365 – surprise! – many GCs
with VIK colors only explained
by intermediate ages.
Important result indicating some
elliptical galaxies had substantial
formation activity at t <~ 5 Gyr.
Implications
Not without controversy
• Most galaxy light studies suggest older ages for integrated
stellar light of NGC 4365.
• Spectroscopy of GCs in NGC 4365 confused – initial
confirmation of int. age by Larsen et al. 2003, similar
data from same instrument, different result, Brodie et al.
2005.
• Only case of possible inconsistency between optical to
near-ir colors and optical spectroscopy for GC systems
(agree for NGC 1316, NGC 1399, NGC 3115, NGC 4472)
 Get better data!
Three deep NIC3 pointings for NGC 4365, very high S/N
New NICMOS data
(Kundu, Zepf, Hempel et al 2005 ApJ, 634, L41 )
Independent, deep data – data deep enough to separate optical
and near-ir colors.
Same result as before – substantial population of NGC 4365 GCs
with optical/near-ir colors only accounted for with int. age.
Comparison with other archival NIC3 data for NGC 1399 GCs.
Mostly old GCs in NGC 1399, with a few int. ages, same as seen in
previous spectroscopy.
Simple Fraction of Int. Age GCs
Simple test,
adopts two
populations,
one at 13 Gyr,
gets best
fitting age and
fraction of 2nd
population.
NGC 1399 result
of ~10% int.
age population
same as seen
in small
spectroscopy
sample.
NGC 4365
substantial int.
age result
confirmed
Is it age?
• Test colors by getting much better, independent data.
Deep NICMOS H-band imaging of NGC 4365.
Much reduced photometric uncertainties.
 Colors confirmed, intermediate ages show up in
independent, deep data
Is it age?
• Test colors by getting much better, independent data.
Deep NICMOS H-band imaging of NGC 4365.
Much reduced photometric uncertainties.
 Colors confirmed, intermediate ages show up in
independent, deep data
• What about the stellar populations models?
M87 – deep NIC3 GO and AR data
Kundu, Zepf & Hempel 2006
New NIC3 data in center, archival NIC data in outer field
with new ACS data.
 Models lines appear to be mostly correct.
Dominant old age in agreement with optical color bimodality
and spectroscopic results for M87
Is it age?
• Test colors by getting much better, independent data.
Deep NICMOS H-band imaging of NGC 4365.
Much reduced photometric uncertainties.
 Colors confirmed, intermediate show up in
independent, deep data
• What about the stellar populations models?
 Other galaxies match old model prediction lines very
well. Can’t shift model lines.
More NICMOS Results
(Kundu, Zepf, Hempel 2006 in prep)
Comparison of same NIC data for NGC 4365 and M87 reveals
clear difference – only known way to account for this is
intermediate age for substantial fraction of NGC 4365 GCs.
Is it age?
• Test colors by getting much better, independent data.
Deep NICMOS H-band imaging of NGC 4365.
Much reduced photometric uncertainties.
 Colors confirmed, intermediate show up in
independent, deep data
• What about the stellar populations models?
 Other galaxies match old model prediction lines very
well. Can’t shift model lines.
only extant explanation that works for
NGC 4365 optical to near-ir is intermediate age.
• Optical to near-ir color technique is efficient and
effective. What about a galaxy sample?
Optical to near-ir to date (Hempel, Kundu, Puzia, K-P, Geisler, Zepf)
Sample in words
NGC 4472 – old
NGC 3115, NGC 7192, M87 – old with possible
“frosting”
NGC 1399 – old with definite ~10% intermediate age
NGC 5846 – probable substantial int. age component
NGC 4365 – clear intermediate age component
Data in hand – Cen A, NGC 3379, NGC 4594
Notes: old for colors and spectra just means t > ~8-10 Gyr, z >~1
Tests higher metallicity GCs, not lower metallicity ones.
Expect higher fraction of younger ones at brighter limits.
Stochastic effects from finite number of stars come into play.
Hempel, Kundu, Puzia, Kissler-Patig, Geisler, Zepf, et al.
Overview of Ages
• Current data indicate a modest but significant fraction of
Es have substantial intermediate-age GC population in
metal-rich population (major formation even at z < 1).
Majority have major formation events at z > 1.
Of this majority many have small population of
intermediate-age GCs (e.g. NGC 1399).
• Overall seems consistent with hierarchical merging –
some merging today, increasing significantly to the past.
• Large optical, near-ir GCS survey enable much more
detailed tests (NIC3, SOAR,...). Combine with targeted
optical spectroscopy.
Kinematics of Globular Cluster Systems
Address two questions
1. Dark matter around elliptical galaxies – how
much, spatial distribution? Globular clusters
one of best probes way out into the halos.
2. Angular momentum in elliptical galaxies. Why
don’t they have any (in their inner regions
anyway). Search for angular momentum
transported outwards predicted by models.
The Utility of GCs for Halo Kinematics
Globular Clusters
provide observable
individual objects at
very large radii for
which the sky
dominates the
integrated light.
e.g. NGC4472 giant
Virgo E Photometry KPNO Mosaic
Camera (Rhode &
Zepf 2001, 2004)
Obviously useful as
dynamical probes of
halo mass distribution
Do Lower Luminosity Ellipticals have
Expected Dark Matter Halos?
• A few recent results suggest maybe lower luminosity
ellipticals are lacking dark matter halos or have
halos with either low concentration or modest M/L
(e.g. Romanowsky et al).
• Test this with VLT multi-fiber observations of
globular clusters over a very wide field around best
case L* elliptical galaxy, NGC 3379.
1. Confirm or reject previous result.
2. Extend kinematical information to larger radii.
Results for NGC 3379
Bergond, Zepf, Romanowsky,
Rhode, & Sharples 2006,
A&A, 448, 155
• Red points and error bars GC
dispersions with radius.
• Models: CDM-like halos
with standard
concentrations in blue
hatched region. Blue
dashed lines region covered
by halo occupation models.
Purple line best fit from
outer HI ring and CDM
halo with most likely
concentration. Bottom
dashed line is mass traces
light.
 GC velocity dispersions are
in agreement with CDMlike halos.
Angular Momentum in Galaxies
• General idea – protogalaxies torqued up by tidal interactions with
other protogalaxies, have “spin parameter”   0.05 ( = J E^1/2 /
G M^1/2), goes back to Hoyle, confirmed in modern cosmological
simulations, mostly independent of mass and environment.
• Halos to galaxies – Baryons that make up the parts of the galaxies we
see cool, collapse, and spin up.
• Similar densities of ellipticals and spirals suggest ellipticals and
spirals collapsed by roughly the same amount. But ellipticals have
an order of magnitude or more less angular momentum.
What happened to ellipticals?
• Need to transfer angular momentum. This has long been realized, as
has the utility of hierarchical structure formation and merging to
transfer ang momentum.
• Can we find the angular momentum at large radii around ellipticals?
Kinematics of GC Systems
NGC 4472
• Our new VLT Flames data for NGC 4472, up to ~350 velocities out to
~ 75 kpc, builds on 144 GCs (Zepf et al 2000, Sharples et al 1998) to
~ 40 kpc, and another 100 over same area from Cote et al. (2003).
Extant results (Z2000 et al)
• not much evidence for rotation anywhere (to ~50 kpc).
• Metal-poor GC population may have modest rotation. Higher  than
more spatially concentrated metal-rich GCs.
• No evidence for rotation in metal-rich GC population about any axis - Upper limit on v/ < 0.34 (99%)
 Metal-rich GC pop not rotationally supported. Significant angular
momentum transport required. Implicates mergers, But where did
it go?
• M87 consistent with 4472, evidence for ang mom in outer regions,
M104 may have counter-rotating outer GCs.
Kinematics of GC Systems
NGC 4472
• Our new VLT Flames data for NGC 4472, up to ~350 velocities out to
~ 75 kpc, builds on 144 GCs (Zepf et al 2000, Sharples et al 1998) to
~ 40 kpc, and another 100 over same area from Cote et al. (2003).
Extant results (Z2000 et al)
• not much evidence for rotation anywhere (to ~50 kpc).
• Metal-poor GC population may have modest rotation. Higher  than
more spatially concentrated metal-rich GCs.
• No evidence for rotation in metal-rich GC population about any axis - Upper limit on v/ < 0.34 (99%)
 Metal-rich GC pop not rotationally supported. Significant angular
momentum transport required. Implicates mergers, But where did
it go?
• M87 consistent with 4472, evidence for ang mom in outer regions,
M104 may have counter-rotating outer GCs.
SUMMARY
• Globular Cluster Systems show the episodic formation
history of their host elliptical galaxies.
• Not all of these episodes are old (intermediate-age GC
populations in normal ellipticals). Broad agreement with
hierarchical merging models.
• Optical to near-infrared colors of GC systems efficient,
straightforward way to constrain ages of major formation
episodes.
• Metal-poor GCs are “biased” with galaxy mass, potential to
constrain their formation sites/epochs.
• Dark matter halos ubiquitous around elliptical galaxies.
• Angular momentum transport needed for elliptical galaxy
formation  mergers. Out to large radii in at least one wellstudied case.
NGC 4472 – new data
Hempel, Zepf,
Kundu, &
Geisler 2006
Ongoing survey,
NGC 4472
results show
old ages in one
of strongest
bimodal cases,
as expected.
Insensitivity to Models
Models on previous page Bruzual-Charlot, here Anders & Fritze v.
Alvensleben and Maraston. Basically same answers. Optical to near-ir
approach is astrophysically straightforward.
NGC 4365 – photo and spect.
Kundu, Zepf et al
optical to
near-ir colors
of objects with
spectroscopy.
Note optical to
near-ir data
around re ,
B05 at large r .
Mass Profile of NGC 4472
Derived from
• Spherical Jeans eqn and
isotropic orbits
• smoothed  distribution
• Observed GCS profile
Compared to
• Mass profile from hot gas
observed in X-rays
(Irwin & Sarazin 1997)
 Hydrostatic equilibrium
for gas and isotropic
orbits for stars give the
same answer within the
errors.
(M/L)B  12 at 34 kpc,
increases roughly as
sqrt (r) to larger radii.
Potential of near-IR photometry
V-I colors have more age
sensitivity than V-K
colors  V-I, V-K can
break age-metallicity
degeneracy.
Plot on the right shows
the Milky Way and
M31 GCs and various
models for 12 Gyr and
a range of [Fe/H]
Younger ages will have
blue V-I for a given VK.
Note good agreement of
models with CMD
ages.
Low-Mass X-ray Binary Globular Cluster Connection
• A LMXB is a compact object accreting from a
companion low-mass star.
• LMXBs are X-ray bright – typical L ~ 1037 ergs/s
Extragalactic obs are more typically 1038
• In Milky Way, only ~13 LMXBs in Galactic GCs.
This is 10% of total LMXB population, even
though less than 0.1% of the stars are in GCs.
1) Need more numbers for stats and
2)
LMXBs are clearly over-represented in GCs –
dynamical formation in dense environment.
 Great progress in extragalactic studies
 Much of the X-ray emission from early-type galaxies is from
LMXBs
 Many (~50%) of the LMXBs are in globular clusters
NGC 4472 as an
example
Chandra image in
Grayscale
HST-WFPC2 dark
outlines
Chandra sources
circled, black circles are
matches with GCs
found with HST.
Kundu, Maccarone, &
Zepf 2002, ApJL
What observables determine LMXB presence?
Kundu, Maccarone, & Zepf 2002, Sarazin et al. 2003, MKZ03
KMZP03, Jordan et al. 2004, KMZ 2004…
• Globular cluster luminosity (mass) – probability of a LMXB
scales roughly linearly with GC mass
• Globular cluster color (metallicity) – probability of a LMXB
roughly 3x larger for red (metal-rich) GCs relative to
blue (metal-poor) GCs
Statistically marginal correlations with globular cluster size
and distance from galaxy center.
Metallicity effect unexpected, what’s the evidence?
NGC 4472 – KMZ02
Small points globular clusters
Dark Points –
LMXB – GC
matches
Shows LMXBs
preferentially in
higher L,redder
GCs.
Metallicity effect
at very high
significance.
KMZ 2002
Another look and another galaxy (KMZ02, KMZP03)
More galaxies…. (KMZ04)
And another one…. (KMZ04)
What About Age?
- No Strong Age Effect in NGC 4365
Key is intermediate age
globular cluster
population in NGC 4365
(discovered using optical
to near-ir photometry, will
go deeper with NIC3).
Known broad age range
allows test of effect of age.
GCs open circles, red dots
GCs with LMXbs
Result is no significant
age effect!
Kundu, Maccarone, Zepf,
& Puzia 2003, ApJL
Possible Explanation –
Irradiation Induced Winds
(Maccarone, Kundu, & Zepf 2004)
• X-rays heat mass donor star. Energy can be dissipated by
line cooling or winds.
• Line cooling dominates for metal-rich stars, winds for
metal-poor stars (Iben, Tutukov, & Federova 1997).
• Lifetime then varies with metallicity - more winds means
less mass to accrete and shorter lifetime, so fewer X-ray
sources in metal poor systems.
• Also consistent with observed spectral differences with
metallicity (e.g. Maccarone, Kundu, & Zepf 2003, Irwin
& Bregman 1999).
LMXB- Globular Cluster Review
1. Much of the X-ray emission in early-type galaxies comes
from Low-Mass X-ray Binaries (LMXBs), and many
(~50%) of the LMXBs are in globular clusters.
2. Metal-rich globular clusters are 3x as likely as otherwise
similar metal-poor clusters to host LMXBs (Kundu,
Maccarone, & Zepf 2002, MKZ2004).
3. Age is not a significant parameter determining whether or
not a LMXB is found in a globular cluster (Kundu,
Maccarone, Zepf, & Puzia 2003).
4. An irradiation induced wind model can account for the
strong metallicity dependence and lack of an age
dependence (Maccarone, Kundu, & Zepf 2004).
An extragalactic globular cluster
Spatial Distribution of GCs by Color
Because of dissipation and enrichment in progenitor disks
and during the merger itself, the metal-rich red GCs
should be more concentrated towards the center of the
galaxy than the metal-poor blue GCs. Confirmed by
Geisler et al in 96, many other cases now.
red
blue
M87, Kundu & Zepf 2004
Dynamical Information
Velocities for 144 GCs
around NGC 4472
(Zepf et al. 2000, Beasley et al.
2000, Sharples et al 1998)
• From CFHT and WHT – note
wide-field is valuable, GCs
available to very large radii
• These data extend to ~ 8´, factor
of 4 larger than previous
velocity data.
(half-light radius of galaxy light is
1’ ~4 kpc, so 8’ ~32 kpc)
How do Globular Clusters Form?
First: PRESSURE – High pressure in the interstellar medium of
starbursts can account for the formation of dense star clusters
(Elmegreen & Efremov, Ashman & Zepf 2001).
r  M1/2 P-1/4
Molecular clouds that form in the high pressure environment of
starbursts have to be very dense in order to exist.
Second: SIZES – Zepf et al. 99 found little or no correlation
between size (radius) and mass (r  M0.1±0.1) for young clusters
in NGC 3256 . But virialized clouds have r  M1/2 P-1/4
This is observed for molecular clouds in the galaxy, and
hard to avoid for virialized clouds.
So how do you erase the r  M1/2 ?
Not easy! Best hope seems to be a star formation efficiency that
is dependent on the Binding Energy of the parent cloud. In this
case, low mass clouds are inefficient, lose mass, and expand.
Fine line to keep numbers of low mass GCs around. See AZ01
for details!
Angular Momentum in Galaxies
• Where is the angular momentum in elliptical
galaxies?
• General idea – protogalaxies torqued up by tidal
interactions with other protogalaxies, have a
dimensionless spin parameter   0.05 .
Goes back to Hoyle, confirmed in modern
cosmological simulations, mostly independent of
mass and environment.
• Halos to galaxies – Baryons that make up the parts
of the galaxies we see cool, collapse, and spin up.
Spin-up works to zeroth order for spirals.
What about Balmer Lines?
Natural first step - carried out for
- M87 GCs by Cohen et al
- NGC 1399 GCs Kissler-Patig et al, Forbes et al.
- NGC 4472 GCs by Beasley et al
BUT
Challenges with calibration for Galactic GCs - when the
same techniques used for extragalactic GCs are applied
to metal-rich Galactic GCs, get ages ~25% older than
CMD ages (e.g. 47 Tuc, Gibson et al, Vazdekis).
Also requires a long time on a 8/10-m telescope to get
reasonable signal, so even when systematics are sorted
out it is a painful way to go.
Dynamical Information
Velocities for 144 GCs around
NGC 4472
(Zepf et al. 2000, also
Beasley et al. 2000, Sharples
et al 1998, Cote et al. 2003)
• These data extend to ~ 8´,
factor of 4 larger than
previous velocity data.
(half-light radius of galaxy
light is 1’ ~4 kpc, so 8’ ~32
kpc)
• New FLAMES data, another
100 GCs to ~ 15’
Which is the bimodal Milky Way?
• One plot is the color
distribution of the Galaxy
GCS, the other is NGC
4406, a modestly bimodal
elliptical galaxy GCS –
which one is which?
• Milky Way GCS known
to be bimodal directly in
metallicity.
• Given bimodal Milky
Way metallicity
distribution, can NGC
4406 and most other
ellipticals be so different?
Plot from Katherine Rhode, data Rhode & Zepf 2004
Which is the bimodal Milky Way?
• One plot is the color
distribution of the Galaxy
GCS, the other is NGC
4406, a modestly bimodal
elliptical galaxy GCS –
which one is which?
• Milky Way GCS known
to be bimodal directly in
metallicity.
• Given bimodal Milky
Way metallicity
distribution, can NGC
4406 and most other
ellipticals not be similar?
Plot from Katherine Rhode, data Rhode & Zepf 2004
Color and Metallicity Distributions
• For mostly old systems, color traces
metallicity, use Milky Way and
models to do this (age-met
degeneracy still present).
• Can’t get the two peaks in the
observed colors from a singlepeaked metallicity distribution.
 GCs come in two metallicity
groups .
• Metal-rich GCs - closely tied to bulk
of host galaxy - trace galaxy
formation/assembly.
• Metal-poor GCs - Low metals,
extended spatial distribution, and
old ages in Milky Way halo constraints on early structure
formation?
Note this division in abundance also
seen in Galaxy GC system