Galaxy Formation and Evolution
Galactic Archaeology
Chris Brook
Modulo 15 Room 509
email: cbabrook@gmail.com
Lecture 5:
Galactic Archeology
2
The Structure of our Galaxy
Old components of the Milky Way
Formation of the Milky Way
Formation of the Milky Way
2 collapse scenarios were
postulated, based on
kinematics and abundances
The stellar halo, bulge, thick and thin disks have different
mean metallicities, as indicated
Age-Metallicity relation of the Components
The Milky Way’s history is reflected both in the abundances of key
chemical elements in stellar atmospheres, and in stellar motions
The motions of local stars can be decomposed into circular (V), radial
(U) and perpendicular to disk (W) components. Galaxy components
the thin disk, thick disk and halo have different motions.
√ U2 +W2 (km/sec)
Tangential orbital speed V (km/sec)
Thin-disc stars follow nearly circular orbits, with most of their motion
being tangential. Halo stars are equally likely to follow prograde or
retrograde orbits and cross the midplane with high speeds.
Ratio of Iron to hydrogen,
relative to that of the Sun
Tangential orbital speed V (km/sec)
These orbital distinctions are mirrored by differences in iron content, with
halo stars being the most metal-poor, as if they were formed from
relatively primordial material. Thin disk stars are the most metal rich.
Ratio of alpha elements to Iron,
relative to that of the Sun
Tangential orbital speed V (km/sec)
The Galaxy’s different populations also differ in their alpha-to-Iron
ratios, where alpha means elements such as oxygen and magnesium
that are synthesised in core-collapse supernovae.
Ratio of alpha elements to Iron,
relative to that of the Sun
Tangential orbital speed V (km/sec)
The Galaxy’s different populations also differ in their alpha-to-Iron
ratios, where alpha means elements such as oxygen and magnesium
that are synthesised in core-collapse supernovae.
Stellar Halo Formation
Ryan & Norris 1991
Halo stars have high velocities compared to the local standard of
rest (which rotates with the galaxy)- they also have low metallicity
Stellar Halo Formation
Models of the accretion of
multiple satellites.
Do they look like the real
MW halo?
Johnston & Bullock 2005
Looking for accretion events
Evidence of accretion from stellar kinematics. Stars may retain
coherence in phase space longer than they will remain spatially
associated
Helmi et al. 1999
A problem?
Stellar Halo Formation
Models of the accretion of multiple satellites.
Do they look like the real MW halo? More sophisticated models seem to
be able to account for this
Johnston et al 2008, see also e.g. Robertson et al. 2005
Dual Stellar halo?
See Carrollo, Beers et al. 2010
How have 2 halos formed?
In situ halo stars?
i.e. not all halo stars come from satellites
Is this the return of the original
ELS rapid collapse scenario?
That accretion plays a role in
halo formation is not in doubt,
and in particular the outer halo
is almost certainly accreted.
Zolotov et al. 2009
But the contribution of stars
born in the disk and later
knocked into the halo, is inner
halo remains under debate
Extremely Metal Poor Stars
We can use old stars found in the halo of the Milky Way to learn
about the earliest stages of galaxy formation. The particular
abundances found in the lowest metallicity stars can tell us about the
types of stars that first polluted the Universe.
Where are primordial stars found?
Brook et al. 2007
Where are primordial stars found?
Where are primordial stars found?
Primordial stars
The oldest stars
Probing Dark Matter
Probing the shape of the Dark Halo
Probing the shape of the Dark Halo
Yet CDM halos are triaxial/prolate (e.g. Jing & Suto 2002)
Probing the shape of the Dark Halo
Can the effect of
baryons explain the
discrepencies with
CDM?
(again!)
Adding baryons makes
halos more spherical
Kazantzidis et al. 2004
TheThe
Bulge
Galactic
Centre
The bulge Metallicity
Distribution Function
Bulge Formation: evidence from abundances
Along with other galaxies, the bulge of the MW has been thought to have similarities
to Elliptical galaxies: alpha-enhanced stellar populations, dominated by old stars,
and seem to have formed on short timescales, possibly in less than 1 Gyr (e.g.
Thomas et. al. 2005). Did it form in the same way as Ellipticals? Maybe through
starbursts that are driven by mergers at high redshift?
The Bulge
Recent Bulge Observations
Metallicity Gradient detected
along minor axis.
Recall that metallicity gradients
may be signatures of formation
mechanisms
Ness et al. 2012
Recent Bulge Observations
Metallicity distributions at different radii, all taken at lattitude -5°
Indications of a complex overlap of components in the central regions?
See Ness et al. 2012
The Thick Disk
Milky Way Thick Disk: properties
• large scale height~ 0.6-1 kpc (e.g. Phelps et al `99)
unclear scale-length compared to thin disk
(Juric 2008 cf Bensby et al. 2011)
• ~5-10% of the mass of the thin disk
• lags thin disk by~40 km/s
• dynamically hot
• old stars ~10 Gyrs (e.g. Gilmore & Wyse `95)
• -1<[Fe/H]<-0.2 (peak~-0.6)
• no vertical metallicity gradient
• distinct chemical abundance patterns
Kinematics, metal abundances and ages support the
hypothesis that it is a distinct component
The Thick Disk: ages and metallicities
Like halo stars,
thick disk are old
Clues to Thick Disk Formation
Thick Disk Formation
 A slow, pressure supported collapse (Larson 1976);
 Enhanced kinematic diffusion of the thin disk stellar orbits
(Norris 1987);
 A rapid dissipational violent dynamical heating of the early thin
disk (Quinn et al. 1993, Jones & Wyse 1983)
 stars accreted directly from satellites (Statler 1988; Abadi et al 2003)
 collapse triggered by high metallicity (Wyse & Gilmore 1988).
 Gas rich mergers at high redshift (disks born hot, Brook et al. 2004)
 Star cluster “popping” (Kroupa et al. 2003)
 Radial migration (Loebmann et al 2010, Schronich & Binney 2009)
-information of the metallicity, ages, and chemical abundances
of thick disk stars can be compared to the predictions that the
various scenarios make
Thick Disk Formation
Clues to Thick Disk Formation
Looking for
kinematic signatures
of different thick disk
formation scenarios
Sales et al 2009
Chemical Tagging
Chemical Tagging
Abundance ratios reflect
different evolutionary histories
Venn 2008
Chemical Tagging
Thick Disk Formation
Combine evidence from “near
field cosmology” with evidence
from high redshift observations
Hubble Ultra Deep Field galaxies
Elmegreen & Elmegreen 2007
The Thin Disk: what fuels ongoing star formation?
• The Milky Way is forming stars at ~1-5 solar masses/year,
essentially all of it in the thin disk. Where is the gas coming from?
• Stripped from satellites? Accreted through filaments?
The Thin Disk: what fuels ongoing star formation?
• A significant amount of current star formation may be fueled by
recycling of gas ejected from star formation cites in the galaxy.
The Milky Way and Environment
Galaxies in the Local Group
Galaxies in the Local Group
Probing Dark Matter
More sophisticated extensions of these methods attempt to probe dark
matter distributions in local dwarf galaxies, using dispersion as a measure
of mass, rather than using rotation curves which can only be used in discs.