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Structure and Evolution of the Milky Way
Ken Freeman
Research School of Astronomy & Astrophysics
RED GIANTS AS PROBES OF THE STRUCTURE AND EVOLUTION OF THE MILKY WAY
Rome November 2010
The thin disk: formation and evolution
Issues:
Building the thin disk : its exponential radial structure, the role of mergers
Star formation history, chemical evolution, continuing gas accretion
Evolutionary processes: disk heating, radial mixing
The outer disk: chemical gradient and chemical properties
Many of the basic observational constraints are still uncertain:
• The star formation history of the disk
• How does the metallicity distribution in the disk evolve with time
• How do the stellar velocity dispersions evolve with time
Measuring stellar ages is still a major problem
The galactic disk shows an abundance gradient
(eg galactic cepheids: Luck et al 2006) ....
+ cepheids, other symbols are open clusters in the Galaxy.
Clusters have ages 1-5 Gyr, cepheids are younger
The abundance gradient and [/Fe]-gradient in the disk has flattened with time,
tending towards solar values. For R > 12 kpc, abundance gradient disappears
Carney & Yong 2005
M31
Metallicity gradient in outer regions of M31disk also bottoms out,
as in the Milky Way
Worthey et al 2004
The age-metallicity relation in the solar neighborhood is still uncertain
Rocha-Pinto
et al 2006
The large scatter in
[Fe/H] at all ages was
part of the reason to
invoke largescale
radial mixing : bring
stars from inner and
outer Galaxy into the
solar neighborhood
Estimating ages
for field stars is
difficult
Edvardsson et al 1993
Nordstrom et al 2004
Valenti & Fisher 2005
(Reid et al 07)
log age
subgiants
(Gyr)
Our preliminary age-metallicity relation for about 400 nearby subgiants. Ages derived
from isochrones in the log g - Te plane via high resolution spectra
Gently declining A-M relation with rms scatter of only 0.15 dex in [M/H]
(scatter includes the [M/H] error of ~ 0.10). Less need for radial mixing.
Wylie de Boer et al 2010
What is the observed form of the heating with time ?
The observational situation is not yet clear ...
• One view is that stellar velocity dispersion  ~ t 0.2-0.5
eg Wielen 1977, Dehnen & Binney 1998, Binney et al 2000 …
velocity
dispersion
(km/s)
W is in the
vertical (z)
direction
total
Wielen
age-velocity
relation (AVR)
W = 0.4total
stellar age
(McCormick dwarfs, CaII emission ages)
Wielen 1977
• Another view is that heating occurs for the first ~ 2 Gyr,
then saturates because stars are mostly away from the
Galactic plane
Edvardsson et al (1993) measured accurate individual
velocities and ages for ~ 200 subgiants near the sun.
Their data indicate heating for the first ~ 2 Gyr, with no significant subsequent
heating. Disk heating in the solar neighborhood appears to saturate after 2 Gyr,
when z ~ 20 km/s.
Soubiran et al (2008) measured sample of clump giants, and agree.
Difficulty of measuring stellar ages is reason for the different views. Accurate ages
from asteroseismology would be very welcome. Accurate ages and distances for
giants would allow us to measure the AMR and AVR out to several kpc from the
sun.
old disk
Velocity dispersions
of nearby F stars
thick
disk
appears at
age ~ 10 Gyr
Disk heating saturates at 2-3 Gyr
Freeman 1991; Edvardsson et al 1993; Quillen & Garnett 2000
The Formation of the Thick Disk
Thick disk
Most spirals (including our Galaxy) have a second thicker disk component
The thick disk and halo of NGC 891 (Mouhcine et al 2010): thick disk has
scale height ~ 1.4 kpc and scalelength 4.8 kpc, much as in our Galaxy.
Our Galaxy has a significant thick disk
• its scaleheight is about 1000 pc, compared to
300 pc for the thin disk
• its surface brightness is about 10% of the thin disk’s.
• it rotates almost as rapidly as the thin disk
• its stars are older than 10 Gyr, and are
• significantly more metal poor than the thin disk :
mostly (-0.5 > [Fe/H] > -1.0)
• alpha-enriched so its star formation was rapid
From its kinematics and chemical properties, the thick disk
appears to be a discrete component, distinct from the thin disk
old disk
Velocity dispersions
of nearby F stars
thick
disk
appears at
age ~ 10 Gyr
Thick disk is kinematically distinct
Freeman 1991; Edvardsson et al 1993; Quillen & Garnett 2000
[( + Eu)/H] vs [Fe/H] for thin and thick disks near the sun
The thick disk is chemically distinct
Navarro et al (2010), Furhmann (2008)
thin disk 225 pc
thick disk 1048 pc
halo
Veltz et al (2008) analysed the kinematics of stars near the
Galactic poles in terms of components of different W.
The figure shows the weights of the components: the
kinematically distinct thin and thick disks and the halo are
evident.
Ivezic et al 2008
see the thick disk
up to z ~ 4 kpc:
[Fe/H] between
-0.5 and -1.0
current opinion is
that the thick disk
itself shows no
vertical abundance
gradient
(eg Gilmore et al 1995)
-2.0
-1.5
-1.0
[Fe/H]
-0.5
0.0
The old thick disk is a very significant component for
studying Galaxy formation, because it presents a
kinematically and chemically recognizable relic of the
early Galaxy.
Secular heating is unlikely to affect its dynamics
significantly, because its stars spend most of their
time away from the Galactic plane.
• Most disk galaxies have thick disks •
The fraction of baryons in the thick disk is typically small (~ 10-15%) in
large galaxies like the MW but rises to ~ 50% in smaller disk systems
Baryonic mass ratio: thick disk/thin disk
Yoachim & Dalcanton 2006
How do thick disks form ?
• a normal part of early disk settling : energetic early
star forming events, eg gas-rich merger (Samland et al 2003,
Brook et al 2004)
• accretion debris (Abadi et al 2003, Walker et al 1996). The accreted
galaxies that built up the thick disk of the Galaxy would need to be
more massive than the SMC to get the right [Fe/H] abundance (~ - 0.7)
The possible discovery of a counter-rotating thick disk (Yoachim &
Dalcanton 2008) would favor this mechanism.
• heating of the early thin disk by disruption
of massive clusters (Kroupa 2002). The
internal energy of the clusters is enough to
thicken the disk
Clump cluster galaxy at z = 1.6
(Bournand et al 2008)
Much recent work on the significance of these high-z clump structures as origin of metal-rich
globular clusters (Shapiro et al 2010); origin of thick disk and bulges via merging of clumps
and heating by clumps (Bournaud et al 2009)
Clumps form by gravitational instability, generate thick disks with uniform scale height rather
than the flared thick disks generated by minor mergers. (Recall diamond shape of the thick disk
of NGC 891)
• early thin disk, heated by accretion events - eg the  Cen
accretion event (Bekki & KF 2003): Thin disk formation
begins early, at z = 2 to 3. Partly disrupted during active merger
epoch which heats it into thick disk observed now, The rest of
the gas then gradually settles to form the present thin disk
• thick disk is generated by radial mixing of more energetic
stars from the inner early disk (eg Schönrich & Binney
2009)
How to test between these possibilities for thick disk formation ?
Sales et al (2009) looked at the expected orbital eccentricity distribution for
thick disk stars in different formation scenarios. Their four scenarios are:
• a gas-rich merger (Brook et al 2004, 2005). The thick disk stars are
born in-situ
• accretion (Abadi 2003). The thick disk stars come in from outside
• heating of the early thin disk by accretion of a massive satellite
• radial migration (stars on more energetic orbits migrate out from the
inner galaxy to form a thick disk at larger radii where the potential
gradient is weaker (Schönrich & Binney 2009)
Abadi
by massive satellite
(gas-rich)
Wilson et al 2010, Ruchti
et al 2010: f(e) for thick
disk stars from RAVE :
may favor gas-rich merger
picture ?
Distribution of orbital eccentricity of
thick disk stars predicted by the
different formation scenarios.
Firm control of selection effects
is needed in separation of thin
and thick disk stars
Sales et al 2009
Thick disk summary
• Thick disks are very common.
• In our Galaxy, the thick disk is old, and kinematically
and chemically distinct from the thin disk. What does it
represent in the galaxy formation process ?
• The orbital eccentricity distribution will provide some
guidance.
• Chemical tagging will show if the thick disk formed as a
small number of very large aggregates, or if it has a
significant contribution from accreted galaxies. This is one
of the goals for the HERMES survey.
The Galactic Stellar Halo
rapidly
rotating
disk &
thick disk
slowly
rotating
halo
|Zmax| < 2 kpc
Rotational velocity of nearby stars relative to the sun vs [m/H]
(V = -232 km/s corresponds to zero angular momentum)
Widely believed now that the stellar halo ([Fe/H] < -1) comes mainly from
accreted debris of small satellites - cf Searle & Zinn 1978
• Is there a halo component that formed dissipationally early in the
Galactic formation process ? eg ELS, Samland & Gerhard 2003
Halo- building accretions are still happening
now - eg Sgr dwarf, NGC 5907
Satellite metallicity distributions not like the
metallicity distribution in the halo (Venn 08)
- but maybe were more alike long ago.
Fainter satellites are more metal-poor and
consistent with the MW halo in their
[alpha/Fe] behaviour
ELS 1986
NGC 5907: debris of a small accreted galaxy
Our Galaxy has a similar structure from the disrupting Sgr dwarf
APOD
• Is there a halo component that formed dissipationally early in the
Galactic formation process ?
Hartwick (1987) : metal-poor RR Lyrae stars show a two-component halo:
a flattened inner component and a spherical outer component.
Carollo et al (2010 ) identified a twocomponent halo plus thick disk in sample
of 17,000 SDSS stars, mostly with
[Fe/H] < -0.5
Describe kinematics well with these three
components:
<V>  [Fe/H]
Thick disk 182 51 -0.7
Inner halo
7 95 -1.6
Outer halo -80 180 -2.2 (retrograde)
From comparison with simulations, Zolotov et al (2009) argue that the
inner halo has a partly dissipational origin while the outer halo is made
up from debris of faint metal-poor accreted satellites.
Nissen & Schuster (2010): 78 halo stars - see high and low [alpha/Fe]
groups. Abundances [Fe/H] > -1.6
Low [/Fe] stars are in mostly
retrograde orbits
The high-alpha stars could be ancient
in-situ stars, maybe heated by satellite
encounters. The low-alpha stars may
be accreted from dwarf galaxies.
Note different V-distribution of red
and blue points.
How much of halo comes from accreted structures ?
Ibata et al (2009) ACS study of halo of NGC 891 (nearby, like MW, but
not Local Group) shows lumpy metallicity distribution, indicating that
its halo is made up largely of accreted structures which have not yet
mixed away.
(cf simulations of stellar halos by Font et al 2008, Gilbert et al 09,
Cooper et al 2009)
APOD
Summary for the Galactic stellar halo:
• stellar halo is made up mainly of debris of small accreted galaxies,
although there may be an inner component which formed dissipatively
The Galactic bar/bulge
The boxy appearance of the bulge is typical of
galactic bars seen edge-on. Where do these
bar/bulges come from ? They are very common.
About 2/3 of spiral galaxies show some kind of
central bar structure in the infra-red.
The bars come naturally from instabilities of the disk.
A rotating disk is often unstable to forming a flat bar
structure at its center.
This flat bar in turn is often unstable to vertical buckling
which generates the boxy appearance.
This kind of bar/bulge is not generated by mergers
The maximum vertical extent of boxy/peanut bulges occurs
near radius of vertical and horizontal Lindblad resonances
ie where b =  - /2 =  - z/2
(both  and z depend on the amplitude of the oscillation)
Stars in this zone oscillate on orbits which support the peanut shape.
In-plane
End on
Edge-on
Orbits supporting the peanut
How to test whether the bulge formed through the
bar-buckling instability of the inner disk ?
Compare the structure and kinematics of the galactic bulge with
N-body simulations of disks that have generated a boxy bar/bulge
through bar-buckling instability of the disk (eg Athanassoula).
Do the simulations match the properties of the Galactic bar/bulge
(eg exponential stucture, cylindrical rotation) ?
The kinematics of
the model are as
observed for
boxy bulges:
cylindrical rotation
b = 0.5°
b = 9.5°
The stars of the bulge are old and enhanced in -elements
 rapid star formation history
If the bar formed from the disk, then are the bulge stars and adjacent
disk stars chemically similar ? Not clear yet
Here the data for
the bulge stars
and thick disk
stars come from
different sources
[/Fe] higher for
thick disk than
for thin disk:
higher still for
bulge
Fulbright et al 2007
bulge
thick disk
thin disk
Differential analysis of O-abundance in
bulge, thick disk and thin disk stars. The
thick disk is O-enhanced relative to thin
disk as usual, but the bulge and thick disk
look very similar in this study.
Meléndez et al 2008
The bar-forming and bar-buckling process takes 2-3 Gyr to act
after the disk settles
In the bar-buckling instability scenario, the bulge structure is probably
younger than the bulge stars, which were originally part of the inner disk
The alpha-enrichment of the bulge and thick disk comes from
the rapid chemical evolution which took place in the inner disk
before the instability acted
The stars of the bulge and adjacent disk should have similar ages in this
scenario. Accurate asteroseismology ages for giants of the bulge and
inner disk would be a very useful test of the scenario
If the bulge comes from disk instabilities, then the stars in the bulge were
once part of the inner disk: its stars are older than the bulge structure
We are doing a survey of about 28,000 clump
giants in the bulge and the adjacent disk, to
measure the chemical properties of stars (Fe,
Mg, Ca, Ti, Al, O) in the bulge and adjacent
disk: are they similar, as we would expect if
the bar/bulge grew out of the disk ? We use
the AAOmega fiber spectrometer on the AAT,
to acquire medium-resolution spectra of about
350 stars at a time : R ~ 12,000
Melissa Ness
Where are the first stars now ?
Diemand et al 2005, Moore et al 2006, Brook et al 2007 …
The metal-free (pop III) stars formed until z ~ 4 in chemically isolated subhalos far away from largest progenitor.
If its stars survive, they are spread through the Galactic halo.
If they are not found, then their lifetimes are less than a Hubble time 
truncated IMF
The oldest stars form in the early rare density peaks that lay near the
highest density peak of the final system. They are not necessarily the
most metal-poor stars in the Galaxy. Now they lie in the central bulge
region of the Galaxy.
Accurate asteroseismology ages for metal-poor stars in the inner Galaxy
would provide a great way to tell if they are the oldest stars or just stars of the
inner Galactic halo. Needs ~10% precision in age.
Distributions of the first stars and the metal-free stars
Brook et al 2007
Bulge rotation for metal rich and metal poor stars
• Is there a small classical merger-generated bulge
component, in addition to the boxy/peanut
bar/bulge which probably formed from the disk ?
• See a slowly rotating metal-poor component of
the bulge. How do we identify the first stars from
among the metal-poor stars in the bulge region ?
Ness et al 2010
The goals of galactic archaeology
We seek signatures or fossils from the epoch of Galaxy
assembly, to give us insight about the processes
that took place as the Galaxy formed.
A major goal is to identify observationally
how important mergers and accretion events were
in building up the Galactic disk and the bulge.
CDM predicts a high level of merger activity which conflicts
with many observed properties of disk galaxies.
Aim to reconstruct the star-forming aggregates and accreted
galaxies that built up the disk, bulge and halo of the Galaxy
Some of these dispersed aggregates can be still recognised
kinematically as stellar moving groups.
For others, the dynamical information was lost through
heating and mixing processes, but they are still recognizable
by their chemical signatures (chemical tagging).
Try to find groups of stars, now dispersed,
that were associated at birth either
• because they were born together in a
single Galactic star-forming event, or
• because they came from a common
accreted galaxy.
Stellar substructures in the disk
The galactic disk shows kinematical substructure in the solar
neighborhood: groups of stars moving together, usually called
moving stellar groups (Kapteyn, Eggen)
• Some are associated with dynamical resonances (eg Hercules
group): don't expect to see chemical homogeneity or age
homogeneity (eg Antoja et al 2008, Famaey et al 2008)
• Some are debris of star-forming aggregates in the disk (eg
HR1614 group and Wolf 630 group). They are chemically
homogeneous; such groups could be useful for reconstructing
the history of the galactic disk.
• Others may be debris of infalling objects, as seen in CDM
simulations: eg Abadi et al 2003
Look at the HR1614 group (age ~ 2 Gyr, [Fe/H] = +0.2) which appears to be a
relic of a dispersed star forming event. Its stars are scattered all around us.
This group has not lost its dynamical identity despite its age.
De Silva et al (2007) measured accurate differential abundances for many
elements in HR1614 stars, and found a very small spread in abundances. This is
encouraging for recovering dispersed star forming events by chemical tagging
The HR 1614 group is
probably the dispersed
relic of an old star
forming event.
U
Chemical studies of the old disk stars in the Galaxy can help to identify
disk stars which came in from outside in disrupting satellites, and also
those that are the debris of dispersed star-forming aggregates like the
HR 1614 group (Freeman & Bland-Hawthorn 2002)
The chemical properties of surviving satellites (the dwarf spheroidal
galaxies) vary from satellite to satellite, and are different in detail from the
more homogeneous overall properties of the disk stars.
We can think of a chemical space of abundances of elements O, Na, Mg,
Al, Ca, Mn, Fe, Cu, Sr, Ba, Eu for example. The dimensionality of this
space is between about 7 and 9. Most disk stars inhabit a sub-region of
this space. Stars which came in from satellites may be different enough
to stand out from the rest of the disk stars.
With this chemical tagging approach, we may also be able
to detect or put observational limits on
the satellite accretion history of the galactic disk
Chemical studies of the old disk stars in the Galaxy can help to identify
disk stars that are the debris of common dispersed star-forming aggregates.
Chemical tagging will work if
• stars form in large aggregates - believed to be true
• aggregates are chemically homogenous
• aggregates have unique chemical signatures defined by
several elements which do not vary in lockstep from
one aggregate to another. Need sufficient spread in abundances
from aggregate to aggregate so that chemical signatures can be
distinguished with accuracy achievable (~ 0.05 dex differentially)
Testing the last two conditions were the goals of de Silva's work on open clusters :
they appear to be true.
See de Silva et al (2009) for more on chemical tagging
Chemical tagging is not just assigning stars chemically
to a particular population (thin disk, thick disk, halo)
Chemical tagging is intended to assign stars chemically
to substructure which is no longer detectable kinematically
We are planning a large chemical tagging survey of about a
million stars, using the new HERMES multi-object
spectrometer on the AAT.
The goal is to reconstruct the dispersed star-forming aggregates
that built up the disk, thick disk and halo within about 5 kpc of
the sun
HERMES is a new high resolution multiobject spectrometer on the AAT
spectral resolution 28,000
(high resolution option 50,000)
400 fibres over  square degrees
4 VPH gratings ~ 1000 Å
First light ~2012 on AAT
The four wavelength
bands are chosen to detect
lines of elements needed
for chemical tagging
Strong synergy with Gaia:
accurate parallaxes (~ 1% errors)
and proper motions
Galactic Archaeology with HERMES
400 fibers in  deg2 matches the stellar density at V ~ 14
at intermediate Galactic latitudes
4-m telescope, resolution 28,000, expect SNR = 100 per
resolution element at V = 14 in 60 minutes
We are planning a large survey reaching to V = 14
Old disk dwarfs are seen out to distances of about 1 kpc
Disk giants ……………………………………… 6
Halo giants ……………………………………… 15
Fractional contribution from galactic components
Thin disk
Thick disk
Halo
Dwarfs
Giants
0.58
0.10
0.02
0.20
0.07
0.03
About 9% of the thick disk stars and about
14% of the thin disk stars
pass through our 1 kpc dwarf horizon
Assume that all of their formation aggregates are azimuthally mixed
right around the Galaxy, so all of their formation sites are represented
within our horizon
Simulations (JBH & KCF 2004) show that
a complete random sample of 1.2 x 106 stars
with V < 14
would allow detection of about
• 20 thick disk dwarfs from each of ~ 4,500 star formation sites
• 10 thin disk dwarfs from each of ~ 35,000 star formation sites
* A smaller survey means less stars from a similar number of sites
HERMES and GAIA
GAIA is a major element of
a HERMES survey
GAIA will provide precision astrometry for HERMES sample
For V = 14,  = 10 as,  = 10 as yr -1 : this is GAIA at its best
(1% distance errors for dwarfs at 1 kpc, 5 km s -1
transverse velocity errors for giants at 6 kpc)
 accurate transverse velocities for all stars in the
HERMES sample, and
 accurate distances for most of the survey stars
 therefore accurate color-(absolute magnitude) diagram for most
of the survey stars: independent check that chemically tagged
groups have common age.
The main goal of the survey is unravelling the star formation
history of the thin and thick disk and halo via chemical tagging.
The data products include:
• [Fe/H], [/Fe] and [X/Fe] for vast samples of stars
from each Galactic component:
thin and thick disks from Rg = 2 to 15 kpc
halo out to Rg = 20 kpc
• HERMES + Gaia data will give the distribution of stars
in [position, velocity, chemical] space for a million stars,
and isochrone ages for about 200,000 stars with V < 14
What can the HERMES survey contribute to asteroseismology ?
Chemical tagging in the
inner Galactic disk
(expect ~ 200,000 survey giants
in inner region of Galaxy)
The old (> 1 Gyr) surviving
open clusters
are all in the outer Galaxy,
beyond a radius of 8 kpc.
Young clusters are seen in the inner Galaxy but do not survive
the tidal field and the GMCs.
Expect many broken open and globular clusters in the inner disk : good
for chemical tagging recovery using giants, and good for testing radial
mixing theory. The Na/O anomaly is unique to globular clusters, and
may help to identify the debris of disrupted globular clusters.
Simulation of disk formation from cooling gas in a dark halo potential (Roskar et al 2008)
Radial mixing: transient spiral arm interactions can move stars from one
near- circular orbit into another (Sellwood & Binney 2002; Minchev & Famaey 2010).
How important is this effect in real galaxies ?
• Can we detect ~ 35,000 different disk sites using
chemical tagging techniques ?
Yes: we would need ~ 7 independent chemical element groups,
each with 5 measurable abundance levels, to get enough
independent cells (57) in chemical abundance space.
• Are there 7 independent elements or element groups ?
Yes: light elements (Na, Al)
Mg
other alpha-elements (O, Si, Ca, Ti)
Fe and Fe-peak elements
light s-process elements (Y, Zr)
heavy s-process elements (Ba, La)
r-process (Eu)
A survey of 1.2 x 106 stars to V = 14
with HERMES would take 3000 pointings
Bright time program
~ 9 fields per night for ~ 330 clear nights:
5 year program.
LMC
Sgr
Fornax
Sculptor
Pompeia, Hill et al. 2008
Sbordone et al. 2007
Letarte PhD 2007
Hill et al. 2008 in prep
+ Geisler et al. 2005
Carina Koch et al. 2008
+ Shetrone et al. 2003
Milky-Way Venn et al. 2004
Abundance ratios reflect different
star formation histories
•
•
•
SNII
+SNIa
•
•
•
Each galaxy has had a different
evolutionary track
The position of the knee forms a
sequence following SFH-timescales (and
somewhat the galaxy total luminosity)
s- process (AGB product) very efficient in
galaxies with strong SFR at younger ages
(<5Gyrs): Fnx > LMC > Sgr > Scl
r/s-process elements can be used as
another clock (or even 2 clocks: r/s
transition knee, and start of rise in s )
AGB lifetimes + s-process yields are
metallicity-dependent (seeds)
Abundance pattern in the metal-poor stars
everywhere undistinguishable ? Seems to
be the case for stars in the exended lowmetallicity populations.
rise in s-process
Venn 2008
Hyades Cr 261 HR 1614
De Silva et al 2009
De Silva et al 2009
De Silva et al 2009
There is still much disagreement about stellar age estimates
Edvardsson et al (1993) ages
for subgiants
Nordstrom et al (2004) ages
Stars mostly near main sequence
[Fe/H] and Te from Strömgren photometry
against isochrone ages from Valenti & Fischer (2005)
Measuring accurate stellar ages is difficult
(Reid et al 2007)
HERMES
wavelength
bands
Data reduction and
analysis:
AAO provides basic
reduction: extraction,
wavelength calibration,
scattered light removal,
sky subtraction
Science team provides
abundance analysis
pipeline, based on
MOOG (C. Sneden),