Resolved Stellar Populations in the Milky Way

Stellar Populations 2003

Ken FreemanResearch School of Astronomy & Astrophysics Mount Stromlo ObservatoryThe Australian National University

Overview of our Galaxy

dark halo

stellar halo

thin disk

thick disk

bulge

Total mass ~ 2 x 1012 M_sun : ( 5 x 1011 M_sun out to 50 kpc)Wilkinson & Evans (1999), Sakamoto et al (2003)

Stellar mass in bulge ~ 2 x 1010 M_sun disk 6 x 1010 M_sun halo 1 x 109 M_sun

Ages of components: globular clusters ~ 13 Gyr; some outer clusters 1-2 Gyr youngerthick disk : > 10 Gyrthin disk : ~ 10 Gyr from white dwarfs (Oswalt et al 1996, Legget et al 1998)

8 Gyr from old subgiants (Sandage et al 2003)

The thin disk is metal-rich and covers a wide age rangeThe other stellar components are all relatively old(note similarity of [Fe/H] range for thick disk and globular clusters)

Now show a numerical simulation of galaxy formation.

The simulation summarizes our current view of how a disk galaxylike the Milky Way came together from dark matter and baryons

MOVIE

• much dynamical and chemical evolution• halo formation starts at high z• dissipative formation of the disk

Simulation ofgalaxy formation

• cool gas • warm gas • hot gas

QuickTime™ and aMicrosoft Video 1 decompressorare needed to see this picture.

• z ~ 13 : star formation begins - drives gas out of the protogalactic mini-halos. Surviving stars will become part of the stellar halo - the oldest stars in the Galaxy

• z ~ 3 : galaxy is partly assembled - surrounded by hot gas which is cooling out to form the disk

• z ~ 2 : large lumps are falling in - now have a well defined rotating galaxy.

Movie synopsis

The metal-poor stellar halo

abundance range [Fe/H] = -1 to -5 overlaps with the metal-poor tail of the thin disk

Density distribution ~ r -3.5, extends out to ~100 kpc

Inner halo probably flattened, outer halo more nearly spherical

Kinematics of halo

For [Fe/H] < -1.7,slow rotation(30 - 50 km/s);pressure-supported = (140,105,95) km/s

For [Fe/H] > -1.7rotation increases,probably due tocontribution of themetal weak tail of the thick disk

Chiba & Beers 2000

Rotation of halo decreases with height above the galactic plane

Chiba & Beers 2000

Little correlation of orbital eccentricity e with [Fe/H]

(Chiba & Beers 2000) (Beers et al 2002)

See the thick disk with its metal-poor tail, and a clump of high-e stars at [Fe/H] ~ -1.7 (ie at the [Fe/H] of the break in the Vrot - [Fe/H] relation. Could come from infalling gas with low angular momentum

Fraction F of (metal-weak thick disk) of the (total metal-weak population) near the sun increases with [Fe/H]

What is this MWTD ?Did it form during collapse of disk ?Remnant of very early thin disk heated by early merger ?Accreted debris ?

Some thick disk stars in the solarneighborhood have [Fe/H] abundances as low as the most metal-poor globular clusters.

(Chiba & Beers 2000)

eg decay of prograde satellite orbit around disk galaxy(Walker et al 1996) - dragged down into the disk plane

by dynamical friction (against disk and halo) on timescale ~ 1 Gyr

Would expect some accreted debris to settle to the disk

Rotation of debris would increase with z, as observed

Halo Streams

Long orbital timescales survival of identifiable debris

eg Sgr tidal stream

Ibata et al 1995 Majewski et al 2003

2MASS M giants

A large fraction of the halo stars in the meridional planecould be associated with Sgr debris

Spaghetti collaboration : Morrison et al 2003

colored pointsare different wrapsof simulated orbitof Sgr (Helmi)

black points arespaghetti halo giants

These tidal streams from the disrupting Sgr dwarf are interesting, but the ancient streams from small objects

accreted long ago into the halo could be even more interesting.

They are too faint to see in configuration space - may see them inphase space, eg (RG , VG ), or in integral space

ie the space of integrals of the motion for stellar orbits,like energy and angular momentum (E , Lz )

Tidal Streams in the Galactic Halo (simulation of accretion of 100 satellite galaxies)

x (kpc)

y (

kp

c)

RGC (kpc)

RV

GC

(km

s-1)

(Spaghetti: Harding)

Input - different colorsrepresent different satellites

Output after 12 Gyr-stars within 6 kpc of-the sun - convolved withGAIA errors

Helmi & de Zeeuw

Accretion in integral space (E,Lz)

Accretion is important for building the stellar halo, but not clear yet how much of the halo comes from discrete accretedobjects (debris of star formation at high z, as in movie) versus

star formation during the baryonic collapse of the Galaxy

Recent simulations of pure dissipative collapse (eg Samland et al 2003) suggest that the halo

may have formed mainly through a lumpy collapse, with only ~ 10% of its stars coming from

accreted satellites

In any case, we may be able to trace the debris of these lumps and accreted satellites from their

phase space structure

Chemical Properties of the metal-poor Halo

enhancement associated with the short duration ofstar formation and enrichment

large scatter in heavier elements at low [Fe/H],associated with a small number of discreteenrichment events

insights into the nature of the earliest SN from detailedchemical abundances of very metal poor halo stars

Scatter in elementratios at lower [Fe/H]

Wallerstein et al 1997

elements have less scatter; Mg,Ti not rigidly coupled to Si,Ca

light s

heavy s

r process

The large observed scatter in [X/Fe] for metal-poor stars

suggests that the neutron capture elements

in metal-poor stars are products of

only a few nucleosynthesis events - confirmed by simulations

White Dwarfs in the Halo

Discovery of high proper motion WDs which could contribute some fraction of the dark halo density (Oppenheimer et al 2001)

Much discussion - emerging view that these WDs are probably thick disk objects - no real consensus yet

the number of true halo WDs appears consistent with the stellar halo (eg D. Carollo 2003, Salim et al 2003, Mendez 2002, Torres et al 2002)

Reid et al 2001

• Oppenheimer WDs

contour for halo

contour for disk

nearby M-dwarfs

Many of these WDsare probably

associated with the thick disk

The thin disk is the defining stellar component of disk galaxies.

It is the end product of the dissipation of most of the baryons, and contains almost all of the baryonic angular momentum

Understanding its formation is an important goal of galaxyformation theory.

The thin disk

star formation history in galactic thin disk : roughly uniform, with episodic star bursts for ages < 10 Gyr,

but lower for ages > 10 Gyr

Rocha-Pinto et al (2000)

Solar neighborhood kinematics:

Several mechanisms for heating disk stars: transient spiral arms, GMC scattering (eg Fuchs et al 2001), large-scale bending modes of anisotropic disk (*Sotnikova 2003), accretion events, star cluster dissolution (Kroupa 2001)

Expect heating mechanisms to saturate after a few Gyr: stochastic heating : heated stars spend less time near galactic plane bending modes : heating decreases as vertical heating reduces the anisotropy

What do observations show ?

Freeman 1991; Edvardsson et al 1993; Quillen & Garnett 2000

Velocity dispersionsof nearby F stars

old disk

thickdisk

Disk heating saturates at 2-3 Gyr

appears atage ~ 10 Gyr

exponential in R and z : scaleheight ~ 300 pc, scalelength 2-4 kpc (!) velocity dispersion decreases from ~ 100 km/s near the center

(similar to bulge) to ~ 15 km/s at 18 kpc

Lewis & KCF 1989

2

1.5

1

R (kpc)

log

(vel

ocity

dis

pers

ion)

Structure of the thin disk

Moving Stellar Groups

These are stars in the solar neighborhood with commonmotions and chemical properties : some are survivingfossils of star forming events in the disk.

HR 1614 group (Feltzing 2000). Thin disk group, age ~ 2 Gyr, [Fe/H] ~ 0.2

Arcturus group (Eggen 1971). Old thick disk group, velocity V = -116 km/s relative to LSR, [Fe/H] ~ -0.6,

These are nice examples of substructures surviving in the galactic disk. Gayandhi da Silva is working on the chemical homogeneity of these groups for her thesis at RSAA.

These moving groups in the disk will become very interesting with RAVE and GAIA

Some moving groups are probably associated with local resonantkinematic disturbances by the inner bar : OLR is near solar radius (Hipparcos data) : Dehnen (1999), Fux (2001), Feast (2002)

Sirius and Hyadesstreams - mainlyearlier-type stars

Hercules disturb-ance from OLR -mainly later-type stars

Dehnen 1999

Chemical properties of the nearby disk

The age-abundance relation

Edvardsson et al 1993

old disk

thick disk

young disk

Chemical properties of the nearby disk : [X/Fe]

Edvardsson et al 1993

thin diskthin + thick

(see also Prochaska et al 2000; Bensby et al 2003; Yong et al 2003)

s

Chemical properties of the nearby disk : [´/Fe] = [(Ca, Si)/H]

thin disk

thin + thick

Edvardsson et al 1993(Rm is mean orbital radius)

Abundance gradient in the old disk

Abundance gradient for the old open clusters(age > Hyades)

Friel 1995

NGC 4762 - a disk galaxy with a bright thick disk (Tsikoudi 1980)

Most spirals (including our Galaxy) have a second thicker disk component, believed to be the early thin disk heated by an accretion event. In some galaxies, it is easily seen

The thin disk The thick disk

More on the thick disk ...

Near the sun, the galactic thick disk is defined mainly by stars with[Fe/H] in the range -0.5 to -1.0, though it does extend to verylow [Fe/H] ~ -2.2.

Thick disks are very common - but not ubiquitousFormation pictures ...• a normal part of disk settling (Samland et al 2003)• accretion debris (Steinmetz et al 2003, Walker et al 1996)• early thin disk, heated by accretion events - eg the Cen accretion event (Bekki & KF 2003)

The element abundance data indicate that the thick disk hasabundance patterns different from those for the thin disk,consistent with time delay between formation of thick disk starsand the onset of star formation in the current thin disk.

If the heating by accretion picture is correct, thethick disk may be one of the most significant components for studying galaxy formation, because it presents a kinematically recognizable ‘snap-frozen’ relic of the (heated) early disk.

Secular heating thereafter is unlikely to affect its dynamics significantly, because its stars spend most of their time away from the galactic plane.

Kinematics and structure of the thick disk

rotational lag ~ 30 km/s (Chiba & Beers 2000)

velocity dispersion in (U,V,W) = (46,50,35) km/sscale length = 3.5 to 4.5 kpc

scale height from star counts = 800 to 1200 pcdensity = 4 to 10% of the local thin disk

not much is known about the radial extent of the thick disk - important, if the thick disk really is the heated early thin disk

current opinion is that the thick disk shows no verticalabundance gradient (eg Gilmore et al 1995)

The rest of the gas then gradually settles to form thepresent thin disk

My favored formation picture for the galactic disk

Thin disk formation begins early, at z = 2 to 3

Partly disrupted during merger epoch whichheats it into thick disk observed now

The Galactic Bar- Bulge

small exponentialbulge - typical of later-type galaxies.

Unlike the large r1/4 bulge of M31

Launhardt 2002 Pritchet & van den Bergh 1994

M31

Later type galaxies mostly have near-exponential bulges, ratherthan r1/4 bulges - hint that their bulges are not merger products - more likely generated by disk instability (eg Balcells et al 2002)

Boxy bulges, as in our Galaxy, are associated with barseg Bureau & KF 1999 - believed to come from bar buckling instability of disk.

Our bar-bulge is ~ 3.5 kpc long, axial ratio ~ 1:0.3:0.3pointing about 20o from sun-center line into first quadrant (eg Bissantz & Gerhard 2002)

The galactic bulge is rotating, like most other bulges: (Kuijken & Rich (2002) HST proper motions)

Beaulieu et al 2000K giants from several sourcesand planetary nebulae (+)

Velocity dispersion of innerdisk and bulge are fairly similar- not easy to separate inner diskand bulge kinematically

Bulge ends at |l| ~ 10o

Age and metallicity of the bulge

Zoccali et al 2002 : stellar photometry at (l, b) = ( 0º.3, -6º.2) :old population > 10 Gyr. No trace of younger population.

Extended metallicity distribution,from [Fe/H] = -1.8 to +0.2 (ie not very metal-rich at |b| = 6º )

Bulge MDF covers similar interval to (thin disk + thick disk) near sun

Inhomogeneous collection of photometric ( ) and spectroscopic ( ) mean abundances - evidence for abundance gradient along minor axis of the bulge

Minniti et al 1995

( kpc )

Abundance gradient inthe bulge

Zoccali et al (2002)

Near the center of the bar/bulge is a younger population,

on scale of about 100 pc : the nuclear stellar disk (M ~ 1.5 x 109 M_sun)

and nuclear stellar cluster (~ 2 x 107 M_sun )in central ~ 30 pc. (Launhardt et al 2002)

~ 70% of the luminosity comesfrom young main sequence stars.

The bulge globular clusters

Dinescu et al 2002

3D kinematics of 7 globularclusters in the bar/bulge

Their velocities show:• all of them are confined to thebulge region• the metal-poor clusters (o) arepart of the inner halo• the metal-rich clusters include • a bar cluster • clusters belonging to a rotationally supported system

Cumulative ranked sum test: straight segments show age intervals overwhich the velocity dispersion remains constant. Abrupt changes of slopeshow appearance of discrete component

Freeman 1991

thick disk= 42 km/s

old thin disk= 21 km/s

continuos~ t 1/2

Decay ofa prograde

satelliteorbit

• spaghetti giants in fields away from the Sgr orbit

• globular clusters

Spaghetti collaboration : Morrison et al 2003

there are halo starsnot associated withSgr debris

Outer disk

Indication of truncation at about 15 kpc (eg Ruphy et al 1998), asseen in most disk galaxies

Outer regions of some disks (M33, NGC 2403) show strongintermediate age AGB population well beyond the region ofcurrent star formation (Davidge 2003)

What is this ? Star formation going on relatively recentlyat larger R than now ? Not expected in usual inside-outpicture for star formation in disks.

Does our Galaxy have such a population ?