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KIAA Lectures Beijing, July 2010

Ken Freeman, RSAA

Lecture 5: the Galactic disk

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The Thin Disk

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The 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 the most important goal of galaxyformation theory. Disks like these are difficult to form in CDM.

Galactic disks

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Out of the hierarchical galaxy formation process come galactic disks with ahigh level of regularity in

their structure and scaling laws

We need to understand the reasons for this regularity. How does our Galaxy fit in ?

The Structure of Disks

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Disks have a roughly exponential light distribution in R and z:

I(R,z) = Io exp (-R/hR) exp (-z/hz)

out to R = (3 to 5) hR, then often truncated

M33 (Ferguson et al)

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Reason for the form of the exponential radial light distribution is not understood : extreme options are

1. a torqued gas cloud with the right internal angular momentum distribution M(j) collapses within dark halo, conserving its M(j) -> exponential gas disk

2. gas and stars in disk are radially redistributed into exponential distribution by viscous or spiral arm torques

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The vertical structure of disks is directly associated withtheir star formation history and dynamical history: scattering, accretion, heating, warping … these processes generate a vertical scale height hz for the old thin disk that is usually about 200-300 pc. More on this later.

z

K s

urfa

ce b

r igh

tess

de Grijs et al 1997

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Radial gradient of disk scaleheight

de Grijs & Peletier 1997

For late-type galaxies, the scaleheight hz isalmost independent of radius - constraint on

heating mechanism

ScSb

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Disk Truncation

M33 - outer disk truncated, very smooth structure

NGC 300 - exponential disk goes for at least 10 scale-lengths without truncation

Bland-Hawthorn et al 2005Ferguson et al 2003

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Disk Heating

(the secular increase in stellar velocity dispersion with time through

interaction of disk stars with spiral waves, giant molecular clouds … )

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Solar neighborhood kinematics:

Several possible mechanisms for heating disk stars: eg transient spiral arms, GMC scattering accretion of satellites

Expect heating by spiral arm/ GMC scattering to saturate after a few Gyr, as the stars spend more time away from the galactic plane

What do the observations show ?

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What is the observed form of the heating with time ?The facts are not yet clear ...

• One view is that stellar velocity dispersion ~ t 0.2-0.5

eg Wielen 1977, Dehnen & Binney 1998, Binney et al 2000.

stellar age

velocitydispersion

(km/s)total

W = 0.4total

Wielen 1977

W is in the vertical (z) direction

(McCormick dwarfs, CaII emission ages)

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Edvardsson et al (1993) measured accurate individual velocities and ages for ~ 200 subgiants near the sun.

Edvardsson et al data indicate heating for the first ~ 2 Gyr,with no significant subsequent heating. Disk heating inthe solar neighborhood appears to saturate when z ~ 20 km/s.

• Another view is that heating occurs for the first ~ 2 Gyr, then saturates.

14Freeman 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

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Soubiran et al (2008) agree, using distant clump giantsand isochrone ages

W

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In contrast, the Geneva-Copenhagen survey (Nordstrom et al 2004,Holmberg et al 2007) shows a steady increase in velocity dispersion with age : no saturation, like Wielen (1977)

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but there is some disagreement about stellar age estimates

Edvardsson et al (1993) ages Nordstrom et al (2004) ages

against isochrone ages from Valenti & Fischer (2005)

(Reid et al 2007)

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age (VF)

age (Holmberg 08)

The age-velocity dispersion relation is still not observationally secure. Measuring accurate stellar ages is difficult.

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Need to resolve this issue

We need to know whethercontinuing internal secular

dynamical heating is importantfor the evolution of the disk:

it provides a baseline for understanding the dynamical evolution of the disk, including theeffect on the disk of interactions with subhalos

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Chemical evolution of the Galactic disk

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The age-metallicity relation in the solar neighborhood is still uncertain

Rocha-Pintoet al 2006

Edvardsson et al 1993Nordstrom et al 2004Valenti & Fisher 2005

Estimating agesfor field stars is

difficult

(Reid et al 07)

The large scatter in [Fe/H] at all ages was part of the reason to invoke largescale radial mixing : bring stars from inner and outer Galaxy into the solar neighborhood

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KCF & Wylie de Boer are making a study of the age-metallicity relation and the age-velocity relation near the sun, using subgiants from the RAVE survey for which more reliable ages should be possible. We use high-resolution spectroscopic log g and Te to estimate ages.

With LAMOST and Gaia, it should be possible to make similar studies of the chemical evolution away from the solar neighborhood, out to at least 1.5 kpc from the sun.

log age

subgiants

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Our ages against the Nordstrom ages. This figure looks very similar to the figure by Reid et al (2007) comparing the Nordstrom ages against the Va lenti & Fisher isochrone ages. We appear to be on the same age scale as Va lenti & Fisher. This figure supports the view that the Nordstrom ages have substantial errors, particularly for the younger stars. The ages are in Gyr

Nordstrom ages vs Wylie de Boer

Nordstrom vs VF

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Our preliminary age-metallicity relation for about 400 GCS + Reddy/Bensby stars. The age is in Gyr. Will have about 1000 stars - still collecting data. Consistent with Pont & Eyer (2004) etc: gently declining A-M relation with rms scatter of only 0.15 dex in [M/H] (scatter includes the [M/H] error of ~ 0.10)

green stars at zero age are unevolved dwarfs

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The galactic disk shows an abundance gradient(eg galactic cepheids: Luck et al 2006) ....

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but it is not a simple axisymmetric gradient(Luck et al 2006: cepheids)

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Carney & Yong 2005

+ cepheids, other symbols are open clusters in the Galaxy. Clusters have ages 1-5 Gyr, cepheids are youngerThe abundance gradient and [/Fe]-gradient in the disk has flattened with time,tending towards solar values.

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Metallicity gradient in outer regions of M31disk also bottoms out, as in the Milky Way (Carney & Yong 2005)

Worthey et al 2004

M31

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The abundance gradient is seen also for older star clusters Yong & Carney 2005; Carney & Yong 2005 For the clusters (ages 1 to 5 Gyr) the abundance gradientbottoms out at RG = 12 kpc (RG = 15 kpc in M31), and at an abundance of [Fe/H] = -0.5 (as in M31).

Old stars in the outer disk are -enhanced, with [/Fe] = + 0.2 indicating fairly rapid star formation history in the outer disk(unlike the solar neighborhood).

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If the outer gas has evolved in situ, then the -enrichment of the 5 Gyr old clusters in the outer Galaxy suggests that their chemical evolution was quick: i.e. star formation in the outer disk has been going for only ~ 6 Gyr (5 Gyr + no more than 1 Gyr before SNIa become active)

This is an internal version of down-sizing, depending on surface density rather than total mass. Not only is the star formation timescale longer in the outer Galaxy (Chiappini et al 2001) but star formation and chemical evolution started later

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Is this reversal related to the orbit swapping

phenomenon ? Is it, and the lack of

truncation of the disk, due to strong radial mixing of stars from

the inner disk ?

Vlajic et al 2008

Roskar et al 2008

Reversal of abundance gradient in NGC 300

NGC 300

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Summary of outer disks

The disks of some spirals (eg NGC 300) extend out beyond 10 scale lengthswithout truncation, while other galaxies truncate at a few scalelengths.

The outer disks of M31, M33 and the Milky Way include a component that is at least several Gyr old.

The abundance gradients in the outer disks of M31 and the Galaxy bottomout at [Fe/H] = - 0.5

The older stars of the outer Galactic disk are -enhanced, indicatingthat they formed rapidly. This -enhancement is less for the younger starsof the outer disk. We still know little about the chemical and kinematical properties of the outer Galactic disk: great opportunity for LAMOST.

We need to understand the dynamics, growth processes, chemical evolutionand gas acquisition for outer disks. Many great LAMOST opportunities.

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The Thick Disk

34NGC 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

The Galactic Thick Disk

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Our Galaxy has a significant thick disk

• its scaleheight is about 1000 pc, compared to 300 pc for the thin disk

• its surface brightness is about 10% of the thin disk’s.

• it rotates almost as rapidly as the thin disk

• its stars are older than 12 Gyr, and are

• significantly more metal poor than the thin disk (-0.5 > [Fe/H] > -2.2) and

• alpha-enriched so its star formation was rapid

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The Galactic thick disk is detected in star counts. Its larger scale height means its velocity dispersion is higher than for the thin disk and therefore its rotation lags the LSR by more.

Near the sun, the galactic thick disk is defined mainly by stars with [Fe/H] in the range -0.5 to -1.0, though its MDF has a tail extending to very low [Fe/H] ~ -2.2.

From its kinematics and chemical properties, the thick disk appears to be a discrete component, distinct from the thin disk

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Kinematics and structure of the thick disk

rotational lag ~ 30 km/s near the sun (Chiba & Beers 2000) and increases by about 30 km s-1 kpc-1 with height above the plane (Girard et al 2006)

velocity dispersion in (U,V,W) = (46, 50, 35) km/s

radial scale length = 3.5 to 4.5 kpc : uncertain

scale height from star counts = 800 to 1200 pc (thin disk ~ 300 pc)density = 5 to 20% of the local thin disk

38Freeman 1991; Edvardsson et al 1993; Quillen & Garnett 2000

Velocity dispersionsof nearby F stars

old disk

Thick disk is discrete component

thickdisk

appears atage ~ 10 Gyr

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Ivezic et al 2008

see the thick disk up to z ~ 4 kpc: [Fe/H] between -0.5 and -1.0

-2.0 -1.5 -1.0 -0.5 0.0 [Fe/H]

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

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[/Fe] enhanced by about 0.25 rapid chemical evolution[Eu/Fe] (r-process) enhanced by ~ 0.4

Thick disk element ratios

thick diskthin disk

higher [/Fe] more rapid formation

41Fuhrmann 2008

-1.0 -0.5 0.0 0.5

Mg enrichment in the thick disk: thick disk appears distinct

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Veltz et al (2008) analysed the kinematics of stars near the Galactic poles in terms of components of different W. The figure shows the weights of the components: the kinematically distinct thin and thick disks and the halo are evident.

thin disk 225 pc

thick disk 1048 pc

halo

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The old thick disk is a very significant component for studying galaxy formation, because it presents a kinematically recognizable ‘snap-frozen’ relic of the early galaxy.

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

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Yoachim & Dalcanton 2006

Baryonic mass ratio: thick disk/thin disk

Most disk galaxies have thick disks:

The fraction of baryons in the thick disk is typically small (~ 10-15%) in large galaxies like the MW but rises to ~ 50% in smaller disk systems

Got to here

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How do thick disks form ?

• a normal part of early disk settling : energetic early star forming events (Samland et al 2003, Brook et al 2004)

• accretion debris (Abadi et al 2003, Walker et al 1996). The accreted galaxies that built up the thick disk of the Galaxy would need to be more massive than the SMC to get the right [Fe/H] abundance (~ - 0.7) The possible discovery of a counter-rotating thick disk (Yoachim & Dalcanton 2008) would favor this mechanism.

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• heating of the early thin disk by disruption of massive clusters (Kroupa). The internal energy of the clusters is enough to thicken the disk

• early thin disk, heated by accretion events - eg the Cen accretion event (Bekki & KF 2003): Thin disk formation begins early, at z = 2 to 3. Partly disrupted during active merger epoch which heats it into thick disk observed now, The rest of the gas then gradually settles to form the present thin disk

Clump cluster galaxy at z = 1.6(Bournand et al 2008)

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How to test between these possibilities for thick disk formation ?

Sales et al 2009 looked at the expected orbital eccentricity distribution for thick disk stars in different formation scenarios. Their four scenarios are:

• accretion (Abadi 2003) - thick disk stars come in from outside• heating (of the early thin disk by accretion of a massive satellite)• radial migration (stars on more energetic orbits migrate out from the inner galaxy to form a thick disk at larger radii where the potential gradient is weaker (Schoenrich & Binney 2009)• a gas-rich merger (Brook et al 2004, 2005). The thick disk stars are born in-situ

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Sales et al 2009

Distribution of orbital eccentricity of thick disk stars predicted by the different formation scenarios.

Ruchti, Wyse et al 2010:f(e) for thick disk stars from RAVE - may favor gas-rich merger picture ?

Abadiby massive satellite

(gas-rich)

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Fossil recovery by chemical tagging

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We seek signatures or fossils from the epoch of Galaxy

formation, to give us insight about the processes that took place as the Galaxy formed.

Aim to reconstruct the star-forming aggregates that

built up the disk, bulge and halo of the GalaxySome of these dispersed and phase-mixed aggregates can be still recognised kinematically

as stellar moving groups in velocity space or integral (E, Lz) space

For others, the dynamical information was lost through disk heating processes, but they are still recognizable

by their chemical signatures (chemical tagging).

The goals of galactic archaeology

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A major goal of Galactic archaeology is to identify how important mergers and accretion events were

in building up the Galactic disk and the bulge.

CDM predicts a high level of merger activity which conflictswith many observed properties of disk galaxies.

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• heating of the early stellar disk by accretion events or minor mergers

• stellar debris of ancient merger events

• star formation associated with early large gaseous accretion events

Thick disks are very common in other galaxies: theyappear to be old (> 6-10 Gyr) and moderately metal-poor.Their formation is not yet understood. Some possibleformation routes:

The thick disk is particularly interesting

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The galactic disk shows kinematical substructure in the solar neighborhood: groups of stars moving together, usually called moving stellar groups (Kapteyn, Eggen)

• Some are associated with dynamical resonances (eg Hercules group): don't expect chemical homogeneity or age homogeneity (eg Antoja et al 2008, Famaey et al 2008)

• Some are debris of star-forming aggregates in the disk (eg HR1614 group and Wolf 630 group). Might expect chemical homogeneity; these could be useful for reconstructing the history of the galactic disk.

• Others may be debris of infalling objects, as seen in CDM simulations: eg Abadi et al 2003

Stellar Moving Groups in the Disk

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Look at the HR1614 group (age ~ 2 Gyr, [Fe/H] = +0.2) which appears to be a relic of a dispersed star forming event.Its stars are scattered all around us.

This group has not lost its dynamical identity despite its age.

De Silva et al (2007) measured accurate differential chemical abundances for many elements in HR1614 stars, and finds a very small spread in abundances. This is very encouraging for chemical tagging

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HR1614 moving group stars: the (U,V) plane

The small tilt is expected because epicyclic theory

is not valid for theselarger V-values.

De Silva et al 2007

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• HR 1614o field stars

The HR 1614 stars (age 2 Gyr)

are chemically homogeneous.

They are probably the

dispersed relic of an old star forming event.

De Silva et al 2007

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Chemical studies of the old disk stars in the Galaxy can help to identify disk stars which came in from outside in disrupting satellites, and alsothose that are the debris of dispersed star-forming aggregates

The chemical properties of surviving satellites (the dwarf spheroidal galaxies) vary from satellite to satellite, and are different in detail from the more homogeneous overall properties of the disk stars.

We can think of a chemical space of abundances of elements O, Na, Mg, Al, Ca, Mn, Fe, Cu, Sr, Ba, Eu for example. The dimensionality of this space is between about 7 and 9. Most disk stars inhabit a sub-region of this space. Stars which came in from satellites may be different enough to stand out from the rest of the disk stars.

With this chemical tagging approach, we may be able to detect or put observational limits on

the satellite accretion history of the galactic disk

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Use the detailed chemical abundances of e.g. thick disk stars to tag or associate them to common ancient star-forming aggregates with similar abundance patterns (eg Freeman & Bland-Hawthorn 2002)

The detailed abundance pattern reflects the chemical evolutionof the gas from which the aggregate formed.

Chemical Tagging

Different supernovae provide different yields (depending on

mass, metallicity, detonation details, ejected mass ...)leading to scatter in detailed abundances,

especially at lower metallicities (enrichment by only a few SN)

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Scatter in neutron-capture element ratios: large at low metallicity but still useful for disk stars

Wallerstein et al 1997

light s

heavy s

r process

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For chemical tagging to work, need a few conditions:

• stars form in large aggregates - believed to be true

• aggregates are chemically homogenous

• aggregates have unique chemical signatures defined by several elements which do not vary in lockstep from one aggregate to another. Need sufficient spread in abundances from aggregate to aggregate so that chemical signatures can be distinguished with accuracy achievable (~ 0.05 dex differentially)

Testing the last two conditions were the goals of Gayandhi de Silva's thesis on open clusters: they appear to be true.See G. de Silva et al 2008 for more on chemical tagging

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Clusters vs

nearby field stars

HyadesColl 261HR1614

De Silva 2007

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Chemical tagging is not just assigning stars chemicallyto a particular population (thin disk, thick disk, halo)

Chemical tagging is intended to assign stars chemicallyto substructure which is no longer detectable kinematically

We are planning a large chemical tagging survey, using the new HERMES multi-object spectrometer on the AAT.

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HERMES is a new high resolution multi-object spectrometer on the AAT

The four wavelength bands are chosen to detect lines of elements needed

for chemical tagging

spectral resolution 28,000(high resolution option 50,000)400 fibres over square degrees4 VPH gratings ~ 1000 ÅFirst light ~2012 on AAT

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Data reduction and analysis:

AAO provides basic reduction: extraction, wavelength calibration, scattered light removal,

sky subtraction

Science team provides abundance analysis pipeline, based on MOOG (Sneden)

HERMES wavelength

bands

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• Galactic Archaeology• Stellar physics Galactic bulge and disk Globular and open clusters Interstellar medium

LMC Dwarf spheroidal galaxies

Science Drivers

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A major goal is to identify observationally how important mergers and accretion events were

in building up the Galactic disk and the bulge.

CDM predicts a high level of merger activity which conflictswith many observed properties of disk galaxies.

Try to find groups of stars, now dispersed, that were associated at birth either • because they were born together in a single Galactic star-forming event, or • because they came from a common accreted galaxy.

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Galactic Archaeology with HERMES

Imagine a large complete stellar survey down to V = 14(matches the fiber density at intermediate latitudes)

Cover about half the southern sky (|b| > 30) :10,000 square degrees = 3000 pointings

gives 1.2 x 106 stars

At V = 14, R = 30,000, expect SNR = 100 per resolution element in 60 minutes

Do ~ 8 fields per night for ~ 400 clear nights(bright time program)

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Fractional contribution from galactic components

Dwarf GiantThin disk 0.58 0.20Thick disk 0.10 0.07Halo 0.02 0.03

Old disk dwarfs are seen out to distances of about 1 kpcDisk giants ______________________________ 5Halo giants ______________________________ 15

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Searching for progenitor formation sites

How many sites can we detect with a HERMES survey ?

Adopt 105 M_sun as the mass of the basic disk star-forming aggregate

About 9% of the thick disk stars and about 14% of the thin disk stars pass through our 1 kpc dwarf horizon

Assume that all of their formation aggregates are azimuthally mixed right around the Galaxy, so all of these formation sites are represented within our horizon

For the halo, the HERMES halo giants are visible out to 15 kpc, so we sample a large fraction of the galactic halo

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A complete random sample of 1.2 x 106 stars with V < 14 would include about

• 20 thick disk dwarfs from each of about 4,500 star formation sites• 10 thin disk dwarfs from each of about 35,000 star formation sites

* A smaller survey means less stars from a similar number of sites

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• Can we detect ~ 35,000 different disk sites usingchemical tagging techniques ?

Yes: we would need ~ 7 independent chemical element groups, each with 5 measurable abundance levels, to get enough independent cells (57) in chemical abundance space.

• Are there 7 independent elements or element groups ?

Yes: light elements (Na, Al) Mg other alpha-elements (O, Si, Ca, Ti) Fe and Fe-peak elements light s-process elements (Y, Zr) heavy s-process elements (Ba, La) r-process (Eu)

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HERMES and GAIA

GAIA (2015) will give precision astrometry for HERMES stars

For V = 14, = 10 as, = 10 as yr -1 : this is GAIA at its best

(1% distance errors for dwarfs at 1 kpc, 5 km s -1 transverse velocity errors for giants at 6 kpc)

accurate transverse velocities for all stars in the HERMES sample, and accurate distances for most of the survey stars therefore accurate color-(absolute magnitude) diagram for most of the survey stars: independent check that chemically tagged groups have common age.

GAIA is a major element ofa HERMES survey

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The data products include:

• [Fe/H], [/Fe] and [X/Fe] for vast samples of stars from each Galactic component: thin and thick disk stars from Rg = 2 to 15 kpc halo stars out to Rg = 20 kpc

• HERMES + Gaia data will give the distribution of stars in [position, velocity, chemical] space for a million stars, and isochrone ages for about 200,000 stars

The main goal of the survey is unravelling the star formation history of the thin and thick disk and halo via chemical tagging.

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Bertelli et al 1994

HERMES+ GAIA will give accurate abundances and 3D motions,and isochrone ages for subgiants in the thin disk, thick disk and halo

• subgiants are numerous (about 10% of the sample) and are observable out to about 1 kpc

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HERMES: other (PI) science Some needs longer exposures, down to V ~ 15 and fainter.

Some needs the high resolution option (R ~ 50,000) Much of this science involves GAIA data also

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Chemical tagging in the inner Galactic disk

(expect ~ 200,000 survey giantsin inner region of Galaxy)

The old (> 1 Gyr) surviving open clusters

are all in the outer Galaxy, beyond a radius of 8 kpc.

Young clusters are seen in the inner Galaxy but do not survive the tidal field and the GMCs.

Expect many broken open and globular clusters in the inner disk : good for chemical tagging recovery using giants, and good for testing radial mixing theory. The Na/O anomaly is unique to globular clusters, and may help to identify the debris of disrupted globular clusters.

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Star Formation History in nearby thin disk : roughly uniform, with episodic star bursts for ages < 10 Gyr,

but lower for ages > 10 Gyr

Rocha-Pinto et al (2000)

We need to know the SFH over the whole Galactic disk

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Vertical abundance gradient for thin disk from SDSS agrees well with independent estimate from thin disk clump giants (Soubiran et al 2008)

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Why is this interesting ?

Because we see thick disk stars in the solar neighborhoodwith [Fe/H] abundances as low as the most metal-poor globular clusters. Did these stars form as part of early disk formation, or were they acquired ?

Important to find observational

constraints on the merger history of our

Galaxy

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Many of the oldest stars in the disk are debris from accreted satellites which ends up in the thin and thick disk.

CDM simulations of formation of an early-type disk galaxy (Abadi et al 2003) show that not all disk stars form in the disk

Satellite orbit is dragged into disk plane by dynamical friction :acts like dissipation, although system is collisonless

Formation of disk stars outside the disk

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old disk

thickdisk

No significant chemical evolution in the nearby old disk for ages 2-10 Gyr