Transcript The thin disk

Resolved Stellar Populations in the Milky Way
Ken Freeman
Research School of Astronomy & Astrophysics
Mount Stromlo Observatory
The Australian National University
Stellar Populations 2003
Overview of our Galaxy
dark halo
stellar halo
thick disk
thin 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 younger
thick disk : > 10 Gyr
thin 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 range
The other stellar components are all relatively old
(note similarity of [Fe/H] range for thick disk and globular clusters)
MOVIE
Now show a numerical simulation of galaxy formation.
The simulation summarizes our current view of how a disk galaxy
like the Milky Way came together from dark matter and baryons
• much dynamical and chemical evolution
• halo formation starts at high z
• dissipative formation of the disk
Simulation of
galaxy formation
• cool gas
• warm gas
• hot gas
QuickTime™ and a Microsoft Video 1 decompressor are needed to see this picture.
Movie synopsis
• 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.
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 ~ 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
s = (140,105,95) km/s
For [Fe/H] > -1.7
rotation increases,
probably due to
contribution of the
metal 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
Some thick disk stars in the solar
neighborhood have [Fe/H] abundances
as low as the most metal-poor
globular clusters.
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 ?
(Chiba & Beers 2000)
Would expect some accreted debris to
settle to the disk
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
Rotation of debris would increase with z, as observed
Halo Streams
Long orbital timescales  survival of identifiable debris
eg Sgr tidal stream
2MASS M giants
Ibata et al 1995
Majewski et al 2003
A large fraction of the halo stars in the meridional plane
could be associated with Sgr debris
colored points
are different wraps
of simulated orbit
of Sgr (Helmi)
black points are
spaghetti halo giants
Spaghetti collaboration : Morrison et al 2003
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 in
phase 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)
(Spaghetti: Harding)
RGC (kpc)
Accretion in integral space (E,Lz)
Input - different colors
represent different
satellites
Output after 12 Gyr
-stars within 6 kpc of
-the sun - convolved with
GAIA errors
Helmi & de Zeeuw
Accretion is important for building the stellar halo, but not
clear yet how much of the halo comes from discrete accreted
objects (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
a-enhancement associated with the short duration of
star formation and enrichment
large scatter in heavier elements at low [Fe/H],
associated with a small number of discrete
enrichment events
insights into the nature of the earliest SN from detailed
chemical abundances of very metal poor halo stars
light s
heavy s
r process
Scatter in element
ratios at lower
[Fe/H]
a elements have less
scatter; Mg,Ti not
rigidly coupled to Si,Ca
Wallerstein et al 1997
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)
• Oppenheimer WDs
2s contour for halo
2s contour for disk
nearby M-dwarfs
Many of these WDs
are probably
associated with
the thick disk
Reid et al 2001
The thin 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 galaxy
formation theory.
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 ?
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
Structure of the thin disk
log (velocity dispersion)
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
2
1.5
1
R (kpc)
Lewis & KCF 1989
Moving Stellar Groups
These are stars in the solar neighborhood with common
motions and chemical properties : some are surviving
fossils 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 resonant
kinematic disturbances by the inner bar : OLR is near solar radius
(Hipparcos data) : Dehnen (1999), Fux (2001), Feast (2002)
Sirius and Hyades
streams - mainly
earlier-type stars
Hercules disturbance from OLR
-mainly later-type
stars
Dehnen 1999
Chemical properties
of the nearby disk
The age-abundance
relation
thick disk
young disk
old disk
Edvardsson et al 1993
Chemical properties of the nearby disk : [X/Fe]
a
s
thin + thick
Edvardsson et al 1993
thin disk
(see also Prochaska et al 2000; Bensby et al 2003; Yong et al 2003)
Chemical properties of the nearby disk : [a´/Fe] = [(Ca, Si)/H]
thin disk
thin + thick
(Rm is mean orbital radius)
Edvardsson et al 1993
Abundance gradient in the old disk
Abundance gradient for the old open clusters
(age > Hyades)
Friel 1995
More on the thick disk ...
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
NGC 4762 - a disk galaxy with a bright thick disk (Tsikoudi 1980)
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 very
low [Fe/H] ~ -2.2.
The element abundance data indicate that the thick disk has
abundance patterns different from those for the thin disk,
consistent with time delay between formation of thick disk stars
and the onset of star formation in the current thin disk.
Thick disks are very common - but not ubiquitous
Formation 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)
If the heating by accretion picture is correct, the
thick 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/s
scale length = 3.5 to 4.5 kpc
scale height from star counts = 800 to 1200 pc
density = 4 to 10% of the local thin disk
current opinion is that the thick disk shows no vertical
abundance gradient (eg Gilmore et al 1995)
not much is known about the radial extent of the thick disk
- important, if the thick disk really is the heated early 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 which
heats it into thick disk observed now
The rest of the gas then gradually settles to form the
present thin disk
The Galactic
Bar- Bulge
small exponential
bulge - typical of
later-type galaxies.
M31
Unlike the large r1/4
bulge of M31
Launhardt 2002
Pritchet & van den Bergh 1994
Later type galaxies mostly have near-exponential bulges, rather
than 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 bars
eg Bureau & KF 1999 - believed to come from bar buckling
instability of disk.
Our bar-bulge is ~ 3.5 kpc long, axial ratio ~ 1:0.3:0.3
pointing 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 2000
K giants from several sources
and planetary nebulae (+)
Velocity dispersion of inner
disk and bulge are fairly similar
- not easy to separate inner disk
and 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
Abundance
gradient in
the bulge
( kpc )
Inhomogeneous collection of photometric (
) and
spectroscopic ( ) mean abundances - evidence for
abundance gradient along minor axis of the bulge
Zoccali et al (2002)
Minniti et al 1995
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 comes
from young main sequence stars.
The bulge globular clusters
3D kinematics of 7 globular
clusters in the bar/bulge
Their velocities show:
• all of them are confined to the
bulge region
• the metal-poor clusters (o) are
part of the inner halo
• the metal-rich clusters include
• a bar cluster
• clusters belonging to a
rotationally supported system
Dinescu et al 2002
thick disk
s = 42 km/s
old thin disk
s = 21 km/s
continuos
s ~ t 1/2
Cumulative ranked sum test: straight segments show age intervals over
which the velocity dispersion remains constant. Abrupt changes of slope
show appearance of discrete component
Freeman 1991
Decay of
a prograde
satellite
orbit
• spaghetti giants in
fields away from
the Sgr orbit
• globular clusters
there are halo stars
not associated with
Sgr debris
Spaghetti collaboration : Morrison et al 2003
Outer disk
Indication of truncation at about 15 kpc (eg Ruphy et al 1998), as
seen in most disk galaxies
Outer regions of some disks (M33, NGC 2403) show strong
intermediate age AGB population well beyond the region of
current star formation (Davidge 2003)
What is this ? Star formation going on relatively recently
at larger R than now ? Not expected in usual inside-out
picture for star formation in disks.
Does our Galaxy have such a population ?