Galaxy Formation and Evolution Galactic Archaeology

Chris Brook

Modulo 15 Room 509 email: cbabrook@gmail.com

2

Lecture 5: Galactic Archeology

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.

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)

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)

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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)

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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)

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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

Stellar Halo Formation

Models of the accretion of multiple satellites.Do they look like the real MW halo?

Johnston & Bullock 2005

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

A problem?

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

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

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

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?

The oldest stars

Primordial 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 thediscrepencies with CDM? (again!)

Adding baryons makes halos more spherical

Kazantzidis et al. 2004

The Galactic Centre

The Bulge

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

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°

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

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

Thick Disk Formation

Sales et al 2009

Clues to Thick Disk Formation

Looking for kinematic signatures of different thick disk formation scenarios

Chemical Tagging

Chemical Tagging

Abundance ratios reflect different evolutionary histories

Venn 2008

Chemical Tagging

Combine evidence from “near field cosmology” with evidence from high redshift observations

Hubble Ultra Deep Field galaxiesElmegreen & Elmegreen 2007

Thick Disk Formation

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 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.