Transcript Chemical Evolution - University of Groningen

Valencia, June 27, 2006
The Stellar History of
The Galaxy
Rosemary Wyse
Bernard’s 5th PhD student, from Cambridge period
The Fossil Record
Stars of mass like the Sun live for the age of
the Universe – studying low-mass old stars
allows us to do Cosmology locally.
 There are copious numbers of stars nearby
that formed at redshifts > 2 (ages/lookback
times of > 10 Gyr)
 Complementary approach to direct study at
high redshift.
 Stars retain memory of initial/early conditions
– age, chemical abundances, orbital angular
momentum (modulo resonances, torques)

Exciting times to be studying resolved stellar
populations:
 Large, high-resolution simulations of
structure formation are allowing predictions
of Galaxy formation in a cosmological
context
 Large observational surveys of stars in Local
Group galaxies are now possible using widefield imagers and multi-object spectroscopy
 High-redshift surveys are now quantifying
the stellar populations and morphologies of
galaxies at high look-back times

Clues from the Fossil Record
Star formation history
 Chemical evolution
 Merging history: for which

systems have we derived SFH?
Match models? CDM?
Stellar Initial mass function
 Is the Milky Way typical?
 Is the Local Group typical?

The Local Group
The motions, spatial distributions and
chemical elemental compositions can be
measured (with varying accuracies!) for
individual stars in galaxies throughout the
Local Group
 The Milky Way, M31, M33, gas-rich and
gas-poor satellites
 Analyse to test models e.g. CDM

Formation of a disk galaxy in CDM
Abadi et al
2003
Face-on
Edge-on
Stars are colour-coded by age: red = old, blue = young
Consequences of mergers:
 Orbital energy goes into internal degrees of
freedom of the merging systems
 Low density outer regions of smaller systems
tidally removed
 Thin disks are heated: gas cools, stars do not
Angular momentum is redistributed – outer parts
gain and inner parts lose
 Gas and stars driven to the center (bar helps)
 Disk formed subsequently has short scalelength : corollary, need angular momentum
conservation to form extended disks as
observed (Fall & Efstathiou 1980)
Predictions for disk galaxies:
 Extended disks form late, after most merging complete, or
redshift ~ unity (~8Gyr ago) (mass-dependent, 1012M)
 Hundreds of satellite dark haloes
 Stellar halo formed from disrupted satellites
 Minor mergers (< 20% mass of disk ) heat thin disk,
create thick disk and add gas to bulge
 More significant mergers transform disk galaxy to SO
or even elliptical
 (Re-) accrete gas to re-form disk
 Perhaps accrete stars too into disks
CDM simulation of the Local group Moore et al. 2001
6Mpc box
300kpc box
Left : z=10, small haloes dominate. Red indicates possible
site of star formation at this time (very dense regions).
Right: Present time, many of the small haloes have merged
into the model Milky Way halo; oldest stars found
throughout the Milky Way (most in bulge) and in satellites
Stellar Components of the Milky
Way Galaxy:
 Thin disk: large-scale structure is exponential with
scale-length of ~3kpc and scaleheight of older stars of
~300pc. Mass ~ 6 x 1010 M 
 Thick disk: exponential scale-length ~3kpc, scale-
height of ~ 1kpc, local normalisation ~5%
 Central bulge: exponential scale-length ~500pc, mildly
triaxial, scale-height ~300pc, mass ~1010 M
 Stellar halo: power-law density profile beyond solar
circle, total mass ~ 109 M 
The Thin Disk: SFH
Best studied at the solar neighborhood
 Star formation history locally is consistent with
early onset, with oldest stars ~2-3 Gyr younger
than metal-poor globulars (e.g. Hipparcos data
analyses of Binney et al 2000 & Sandage et al
2003; Nordstrom et al 2004), or ~11Gyr
 Evidence for ‘bursts’ of amplitude 2—3, perhaps
superposed on slow decline (e.g. Gilmore et al
2000; Rocha-Pinto et al 2000); spiral arm
passages?

Old stars in local thin disk
formed at redshift z > 1.5
 = 0.7, M =0.3
 = 0, M = 0.3
Ages of oldest stars from Binney et al 2000
The Thin Disk: old stars
Scale length of old stars is ~ 2 - 4 kpc (e.g. Siegel
et al 2002) thus if the old stars were formed in the
disk, star formation was initiated at ~ 3
scalelengths at z > 1.5
 Then the formation of extended disks was not
delayed until after a redshift of unity, as has
been proposed in CDM-models with feedback
(e.g. Weil et al 1998; Thacker & Couchman 2001)
 M31 also shows extended disk in older stars

(Ferguson & Johnson 2001; Guhathakurta 2004).

Problem for CDM models…(?)
Or is the old thin disk stellar debris from
accreted satellites? cf. Abadi et al 2003
 Ongoing (e.g. RAVE, SDSS2/SEGUE)
spectroscopic surveys will detect substructure

in the thin disk,
and constrain
the merger history
M. Williams poster
Tides: Satellite Snacks
K.V. Johnston
Ongoing snacking…..
Sgr dSph as known
in 1997
Wyse, Gilmore & Franx 1997
2Mass revealed streams from Sagittarius dwarf
around the sky (Majewski et al 2003)
Field of Streams
Belokurov et al (2006)
disk accretion?
SDSS data, 19< r< 22, g-r < 0.4 colour-coded by
magnitude/distance, blue (~10kpc), green, red (~30kpc)
Belokurov et al 06
Thin disk IMF:
Salpeter
slope, or slightly steeper, for massive
stars
Slope flattens around 0.5 M, perhaps peaks
Only low-significance evidence for variations,
especially when take binarism, variable
extinction and mass-segregation in clusters into
account and observe wide area e.g. Kroupa 2002
-- for central Arches cluster dynamical evolution
can cause sufficient mass segregation to explain
observations (Stolte et al 2002; Kroupa 2004)
The Thick Disk
Defined
20 years ago (Gilmore & Reid 1983)
through star counts
 Local normalisation ~5%, scaleheight ~1kpc,
factor ~ 3 thicker than thin disk, same
scalelength ~3kpc; mass ~10--20% of thin disk,
i.e. ~1010M
Well-established now as a distinct component,
not tail of stellar halo or of thin disk, by
kinematics, metallicity and age distributions.
 Similar structures seen in external disk galaxies
Mould 2004, Yoachim & Dalcanton 2005
The Thick Disk: OLD
Gilmore, Wyse
& JB Jones 1995
Scatter plot of
Iron abundance
vs B-V for F/G
stars 1—2 kpc
above the
Galactic Plane
Few stars are bluer than the old turnoff at a given
metallicity, indicated by x or *. Consistent with old
age, ~ same as 47 Tuc, ~ 12 Gyr (open circle)
The Thick Disk:
Different pattern of elemental abundances than
in thin disk: different star formation histories
 Same ‘type II plateau’ value implying invariant
massive star IMF.
 Downturn implies >
~ 1Gyr age spread

Bensby et al 2004
Thick (filled)
and thin disk
(open) stars
show distinct
trends
Elemental Abundances
 Type II supernovae have progenitors > 8 M
and explode on timescales ~ 107 yr, less than
typical duration of star formation
 Main site of -elements, e.g. O, Mg, Ti, Ca, Si
 Low mass stars enriched by only Type II SNe
show enhanced ratio of -elements to iron,
with value dependent on mass distribution of
SNe progenitors – if well-mixed system, see
IMF-average
 Type Ia SNe produce very significant iron, on
longer timescales, few x 108 – 109 yr (binaries)
Type II
Supernova
yields
Salpeter IMF
gives
[/Fe] ~ 0.4
Gibson 1998
Progenitor mass
Schematic [O/Fe] vs [Fe/H]
Wyse & Gilmore 1993
IMF biased to most massive stars
Type II only
Plus Type Ia
Slow enrichment
SFR, winds..
Fast
Self-enriched star forming region.
Assume good mixing so IMF-average yields
LMC stars show sub-solar ratios of [/Fe], consistent
with expectations from extended star formation.
Hiatus then burst
gas
Continuous star formation
Smith et al 2003
Gilmore & Wyse 1991
Formation of Thick Disk
High stellar velocity dispersions (W ~ 40 km /s
and tot ~ 80 km/s) argue against normal disk
heating mechanisms e.g. GMC, spiral arms, as
they saturate at W ~ 20 km/s of old disk
Lack of vertical gradients difficult for slow
dissipational settling (e.g. Burkert et al 2002)
 Old age plus continual star formation in the
thin disk argues against exceptional heating
of thin disk (e.g. by massive halo black holes,
Lacey & Ostriker 1985) unless only very early
 Merger-induced heating of thin disk, by
accretion of fairly massive and dense satellite?

Merger-heating is re-expression of
out-of-equilibrium heating of Jones &
Wyse?
The Thick Disk: merger-heating

If merger origin through heated thin disk, last
significant (> 20% mass ratio to disk, robust dense
satellite) dissipationless merger happened a long
time ago,
(~12 Gyr or z~ 2)
And disk in place
then. Velazquez & White
1999
Thick disk will be
mix of satellite debris
plus heated disk – seen?
Gilmore, RW & Norris 02
CDM, 1000
realisations
of MW-mass
halo, now
1012M
Berlind, priv comm
Halo of the mass of the Milky Way will typically
have experienced 1—2 mergers with mass ratio of
> 0.2 satellite halo: total halo in the past 10Gyr.
Do not reach regime for thick disk : many more.
Shredded satellite will contribute
to ‘thick disk’
Huang & Carlberg 1997
The local thick disk
is quite metal-rich;
if accreted debris
dominates, need
large system to be
this enriched long
ago when thick-disk
stars formed.
The Thick Disk: OLD – but how old?
Reliable ages very important since dates last
significant merger to heat disk: typically in
CDM expect several 10-20% to TOTAL mass
mergers after z=2: need higher-resolution
simulations for the 20% to disk mass mergers
that can form thick disks
In situ sample selection also important since
can have contamination of local ‘thick disk’ by
local thin disk stars ejected by e.g. binary
supernova
The Central Bulge:


Age of the dominant population constrained by HST and
ISO Color-Magnitude Diagrams : for projected
Galactocentric distances of > 300pc, typical age is OLD,
≥10 Gyr; closer in, see younger stars (disk?) van Loon et al 03
Mean metallicity ~ –0.2 dex (e.g. McWilliam & Rich 1994; Ibata
& Gilmore 1995) :
 ~ solar metallicity, low gas fraction at z ~ 2, like red galaxies!
 Enhanced alpha elemental abundance ratios (Fulbright
McWilliam & Rich 06; Cunha et al 06) some decline as [Fe/H]

increases: fixed massive IMF
Low-mass IMF same as metal-poor globulars (Zoccali et al
2000) – same as in Ursa Minor dSph (Wyse et al 2002) and in
local disk
The Central Bulge: old
l,b=0,1
Van Loon
et al 2003
BW=0.9,-4
Age distributions determined from ISO
color-magnitude data.
Old age also from HST CMDs e.g. Zoccali et al 2003
Low-Mass MF in Bulge:
Zoccali et al
2000
UMi dSph I-band LF
Wyse et al 2002
M92

M15

NGC7078
Piotto et al 97;
Shifted and
renormalised
50% completeness
I=27.2
M814 =+8.1,
M  0.3M 
I-band luminosity functions are indistinguishable.
STIS/LP data and V-band data similar limits, agree.
The Central Bulge: Formation
During mergers, expect disk stars and gas to
be added to the bulge (cf. Kauffmann 1996)
 Also expect gas inflows driven by the bar

(Gerhardt 2001)
Bulge is dominated by old, metal-rich stars,
with high [/Fe], not favoring recent
mergers, or recent disk instability to form a
bar/pseudo-bulge
 All point to intense burst of star formation in
situ a long time ago, SFR ~ 10 M/yr
 Early merger – related to thick disk? – or
simply low angular momentum gas?

Bulge—Stellar halo connection?
Bulge, halo
Wyse & Gilmore 1992
Thick, thin disks
Bulge angular momentum distribution consistent with
dissipational collapse of gaseous ejecta from stellar
halo star-forming regions -- mass ratios also agree
with low metallicity of stellar halo cf Hartwick 1979
The Stellar Halo :
Stellar halo traced by high-velocity stars locally
-- ~ 30% of total mass of ~ 2 x 109 M-- is
rather uniform in properties: old and metal-poor,
enhanced elemental abundances indicating short
duration of star formation, in low-mass starforming regions, with ‘normal’ IMF.
 Unlike most stars in satellite galaxies now (cf.

Tolstoy et al 2003)

Accretion from stellar satellites not important
for last ~8Gyr for local halo (cf. Unavane et al 1996)
– no more than 10% from typical satellite since
then, biased to metal-rich stars.
Stellar halo is
OLD
Unavane, Wyse
& Gilmore 1996
Scatter plot of [Fe/H] vs B-V for local high-velocity
halo stars (Carney): again few stars bluer (younger)
than old turnoffs (5Gyr, 10Gyr, 15Gyr Yale)
Hernandez, Gilmore & Valls-Gabaud 2000
Carina dSph
Leo I dSph
Intermediate-age population dominates in typical
dSph satellite galaxies – Ursa Minor atypical, has
dominant old population, and narrow metallicity
spread (also normal IMF Wyse et al 2002)
Caveat: assume fixed metallicity, but intermediate-age secure
Field stellar (inner) halo cannot have formed
from dSph that were accreted after the
formation of the dSph dominant
intermediate-age population – this limits
accretion to have occurred > 8Gyr ago.
Perhaps more stringent limits come from the
different elemental abundances, since
timescale for Type Ia SNe only a few Gyr,
but need detailed chemical evolution models.
 Halo can be formed from any system that
formed stars early on, for only brief period ,
and did not self enrich significantly.
Tolstoy et al 2003
Large open colored symbols are stars in dwarf
Spheroidals, black symbols are Galactic stars:
the stars in typical satellite galaxies tend to have
lower values of [/Fe] at a given [Fe/H],
Consistent with fixed IMF and extended SFH.
How well-mixed was the stellar halo?
There
is a remarkable lack of scatter in the
elemental abundance ratios of [/Fe] for
metal-poor local halo stars, implying enrichment
by a well-sampled massive-star IMF and
good mixing – how was this achieved?
Few star-forming progenitors?
In CDM form halo only from the ~10 most
massive, earliest collapsing satellites
(Bullock & Johnston 05)

‘Type II plateau’
Cosmic scatter in
elemental
abundances of metal
poor halo stars
is extremely low,
0.05 dex – fully
sampled IMF of
massive stars?
Invariant IMF
[/Fe] from Type II
Supernovae depends
on progenitor mass
Cayrel et al. 2004
Outer Stellar Halo
Outer
halo may be younger: globular clusters
indicate perhaps half around 8–10 Gyr, including
Sgr dSph clusters, rather than 10-12Gyr
Accreted as dwarf galaxies plus globulars?
Structural and HB morphologies similar to those
in Fornax dSph, Sgr dSph, LMC (Mackay &
Gilmore 2004)
 Halo stars with low [/Fe] may be accreted, or
may just have formed in denser more-bound blobs.
Those known have high-energy, radial orbits.
Outer Stellar Halo:
 The outer halo, with dynamical timescales of >
1Gyr, is best place to find structure. Several
streams found, in both coordinate space and
kinematics
 Most due to the Sagittarius Dwarf e.g. Ibata et al
2001; Majewski et al 2003
 Very fast-moving field! Several (~ 5) candidate
new dSph and streams announced this year (spot
them in the Field of Streams…)
 mass function crucial for ‘satellite problem’
Concluding remarks
All stellar components of the Milky Way
contain very old stars (but where are first stars?)
 Little evidence for variations in stellar IMF,
over wide range of metallicity, age, local
density…
 Small-scale problems with CDM persist, but
things are evolving rapidly and the next few
years will really see model predictions and
observations able to confront one another

Complementary high-redshift data:
 Analysis of sizes of disks as a function of
luminosity and of stellar mass, based on restframe optical imaging for galaxies out to z ~ 3,
(Trujillo et al 2005) concluded little evolution in
sizes, significantly less than predicted by semianalytic CDM models e.g. Mo, Mao & White 1999
Consistent with simple gaseous infall and starformation in fixed potential, SFR highest in
central regions: ‘stellar disks form from earlyon in large halos’ (Trujillo et al 2005)
Field Inner Stellar Halo as
Dissolved Star-Forming Regions
 Most stars form in unbound systems, not
clusters; indeed present cluster population
(1% of halo) occupies ‘survivor’ phase
space (cf. Fall & Rees 1977)
 Short duration of star formation in
individual regions leads to enhanced [/Fe]
 Low angular momentum -- Gas loss leads
to reduced mean metallicity cf true yield,
flows to central regions…
Dwarf Spheroidals
 Low luminosity, low surface-brightness satellite
galaxies, L ~ 106L, V ~ 24 mag/•

Extremely gas-poor
Apparently dark-matter dominated
 ~ 10km/s, 10 < M/L < 300
~ metallicity <
 Metal-poor, mean stellar
~ –1.5 dex
Extended star-formation histories typical, from
earliest epochs
Important tests for CDM models: mass function,
ρ(r), luminosity function, tidal effects….
Cosmological context
 ()CDM predicts dwarf galaxies are the first to
form stars: building block of bigger systems?
How do survivors differ?
Need to hide most dwarf dark haloes (Klypin et al
1999; Moore et al 1999)
Radiative feedback perhaps as important as SNe
in truncating star formation particularly at lowest
potential wells (Efstathiou 1998; Bullock et al 2000)
Can we understand their star formation history?
Dark halo scaling properties define dSph (Dekel &
Silk 1986; Dekel & Woo 2003; Kormendy & Freeman 2004)
Merger History:
Constrain by characterizing the stellar
populations in components predicted to be
predominantly formed in mergers: stellar
halo, thick disk, bulge
 For the Milky Way, dominantly OLD, seems
to have been rather quiescent since z ~ 2;
atypical in CDM?
 M31 more violent history? (A. Ferguson et al
2002; Brown et al 03)
 M33 more quiet – is there a thick disk, bulge,
halo?

LMC as template thick disk progenitor?
Cole et al 2000
Hill et al 2000
Metallicity distribution of inner LMC disk agrees, and
total stellar mass about right, but stars are intermediateage -- took many more Gyr to self-enrich to this level.
Globular clusters also can give rise to streams;
streams not necessarily a signature of accretion
Odenkirchen et al 2003; Pal 5
Rgc=18.5kpc