galaxy formation and evolution galactic archaeology chris brook modulo 15 room 509 email:...
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Galaxy Formation and Evolution Galactic Archaeology
Chris Brook
Modulo 15 Room 509 email: [email protected]
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2
Lecture 5: Galactic Archeology
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The Structure of our Galaxy
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Old components of the Milky Way
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Formation of the Milky Way
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Formation of the Milky Way
2 collapse scenarios were postulated, based on kinematics and abundances
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The stellar halo, bulge, thick and thin disks have different mean metallicities, as indicated
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Age-Metallicity relation of the Components
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The Milky Way’s history is reflected both in the abundances of key chemical elements in stellar atmospheres, and in stellar motions
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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.
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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.
Tangential orbital speed V (km/sec)
√ U
2 +
W2 (
km/s
ec)
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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.
Tangential orbital speed V (km/sec)
Rat
io o
f Ir
on t
o hy
drog
en,
rela
tive
to t
hat
of t
he S
un
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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.
Tangential orbital speed V (km/sec)
Rat
io o
f al
pha
elem
ents
to
Iron
, re
lativ
e to
tha
t of
the
Sun
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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.
Tangential orbital speed V (km/sec)
Rat
io o
f al
pha
elem
ents
to
Iron
, re
lativ
e to
tha
t of
the
Sun
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Stellar Halo Formation
Halo stars have high velocities compared to the local standard of rest (which rotates with the galaxy)- they also have low metallicity
Ryan & Norris 1991
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Stellar Halo Formation
Models of the accretion of multiple satellites.Do they look like the real MW halo?
Johnston & Bullock 2005
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Helmi et al. 1999
Evidence of accretion from stellar kinematics. Stars may retain coherence in phase space longer than they will remain spatially associated
Looking for accretion events
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A problem?
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Stellar Halo FormationModels 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
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Dual Stellar halo?
See Carrollo, Beers et al. 2010
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How have 2 halos formed?
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In situ halo stars?i.e. not all halo stars come from satellites
Zolotov et al. 2009
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.
But the contribution of stars born in the disk and later knocked into the halo, is inner halo remains under debate
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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.
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Where are primordial stars found?
Brook et al. 2007
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Where are primordial stars found?
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Where are primordial stars found?
The oldest stars
Primordial stars
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Probing Dark Matter
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Probing the shape of the Dark Halo
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Probing the shape of the Dark Halo
Yet CDM halos are triaxial/prolate (e.g. Jing & Suto 2002)
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Probing the shape of the Dark Halo
Can the effect of baryons explain thediscrepencies with CDM? (again!)
Adding baryons makes halos more spherical
Kazantzidis et al. 2004
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The Galactic Centre
The Bulge
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The bulge Metallicity Distribution Function
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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?
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The Bulge
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Recent Bulge Observations
Metallicity Gradient detected along minor axis.
Recall that metallicity gradients may be signatures of formation mechanisms
Ness et al. 2012
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Recent Bulge Observations
Indications of a complex overlap of components in the central regions?
See Ness et al. 2012
Metallicity distributions at different radii, all taken at lattitude -5°
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The Thick Disk
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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
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The Thick Disk: ages and metallicities
Like halo stars, thick disk are old
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Clues to Thick Disk Formation
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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
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Thick Disk Formation
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Sales et al 2009
Clues to Thick Disk Formation
Looking for kinematic signatures of different thick disk formation scenarios
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Chemical Tagging
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Chemical Tagging
Abundance ratios reflect different evolutionary histories
Venn 2008
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Chemical Tagging
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Combine evidence from “near field cosmology” with evidence from high redshift observations
Hubble Ultra Deep Field galaxiesElmegreen & Elmegreen 2007
Thick Disk Formation
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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?
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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.
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The Milky Way and Environment
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Galaxies in the Local Group
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Galaxies in the Local GroupProbing 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.
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