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KIAA Lectures Beijing, July 2010 Ken Freeman, RSAA, ANU. Lecture 2: dynamical processes which lose information. Dynamical processes which lose information or generate potentially misleading information for Galactic archaeology. • Accretion of satellites • Resonances • Disk Heating

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KIAA Lectures

Beijing, July 2010

Ken Freeman, RSAA, ANU

Lecture 2: dynamical processes which lose information


Dynamical processes which lose information or generate

potentially misleading information for Galactic archaeology

• Accretion of satellites

• Resonances

• Disk Heating

• Orbit swapping


Accretion and destruction of satellites

This is an important part of CDM theory: merging of smaller objects of the hierarchy to form larger objects. Small galaxies are accreted and destroyed by larger galaxies: the debris of the small ones becomes part of the halo, bulge or disk of the larger one.

The orbital energy and angular momentum of the smaller galaxy is absorbed by the dark halo and disk of the larger one. The existence of thin disks constrains the merger history since the disk formed, because disks can be destroyed by major mergers.

Goal here is to describe some of the essential dynamics of merging,

accretion and disruption


Disk galaxies interact tidally

and merge.

Merging stimulates star formation and disrupts the galaxies. This is

NGC 4038/ 9 - note the long tidal arms . The end product of the merger

is often an elliptical galaxy.


B

A

v

r(t)


NGC 5907: debris of a small accreted galaxy

Our Galaxy has a similar structure from the disrupting Sgr dwarf

APOD


The “field of streams” seen in SDSS star counts in the halo of our Galaxy

north galactic pole

l = 180, b = 25

Accretion of small galaxies is more important than major mergers for the evolution

of the Milky Way: look now at the two main processes involved in accretion.


m halo of our Galaxy


m halo of our Galaxy


Accretion of small satellites halo of our Galaxy


1996 halo of our Galaxy

Decay of

a prograde

satellite

orbit

The satellite sinks

into the plane of the

galaxy in < 1 Gyr.

The disk provides

about 75% of the

torque on the satellite:

dynamical friction

against the dark halo

provides the rest


The computations show that the satellite typically sinks into the plane of the parent in less than a Gyr. The disk provides about 75% of the integrated torque on the satellite (resonances are important): dynamical friction against the halo provides the rest.

• Abadi et al 2003 analysed an SPH simulation of the formation of a disk galaxy assembled hierarchically in CDM.

They identified a large spheroid and two disk components: thin and thick. The spheroid stars are old (> 8 Gyr). The disk stars cover a wide range of age, but most of the older (> 10 Gyr) disk stars did not form in the disk but came from accreted satellites whose orbits were circularized before disruption.


The 3 components: into the plane of the parent in less than a Gyr. The disk provides about 75% of the integrated torque on the satellite (resonances are important): dynamical friction against the halo provides the rest.spheroid,thickand thin disks

Abadi et al 2003


In the simulation, into the plane of the parent in less than a Gyr. The disk provides about 75% of the integrated torque on the satellite (resonances are important): dynamical friction against the halo provides the rest.

the thick disk is mostly old; the thin disk is mostly younger

Only a small fraction of the old disk stars formed in the disk: most of it comes from circularized satellite debris

Abadi et al 2003


How could we test whether much of the old disk stars came from outside ? Kinematically this would be difficult, because their kinematics would be much like those of in-situ disk stars. Chemical techniques look promising …

The overall metal abundance of the satellites depends on their stellar luminosity. Disk stars have abundances [Fe/H] > -1 so the absolute magnitudes of the infalling satellites must have been brighter than -15.

That is consistent with the Abadi et al (2003) satellites which fell into the disk: they were typically more massive than 109 M.

That makes sense too from dynamical friction theory: dynamical friction affects only the more massive satellites.


Dwarf Galaxies: chemical signatures and abundance range from outside ?

• Large range of abundance

in individual dwarfs

• [Fe/H] - L relation 

internal chemical evolution.

Ultra-faint dSphs contain

individual stars with [Fe/H] < -3

satellites

globular clusters

Mean abundance - luminosity relation

Mateo 2008


Chemical studies of the old disk stars in the Galaxy might help to identify disk stars which came in from outside in satellites which then disrupted.

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 (more later), we may be able

to detect or put observational limits on

the satellite accretion history of the galactic disk


LMC help to identify disk stars which came in from outside in satellites which then disrupted.Pompeia, Hill et al. 2008

Sgr Sbordone et al. 2007

FornaxLetarte PhD 2007

Sculptor Hill et al. 2008 in prep

+ Geisler et al. 2005

CarinaKoch et al. 2008

+ Shetrone et al. 2003

Milky-Way Venn et al. 2004

Abundance ratios reflect different

star formation histories

  • Each galaxy has had a different evolutionary track

  • The position of the knee forms a sequence following SFH-timescales (and somewhat the galaxy total luminosity)

  • s- process (AGB product) very efficient in galaxies with strong SFR at younger ages (<5Gyrs): Fnx > LMC > Sgr > Scl

  • r/s-process elements can be used as another clock (or even 2 clocks: r/s transition knee, and start of rise in s )

  • AGB lifetimes + s-process yields are metallicity-dependent (seeds)

  • Abundance pattern in the metal-poor stars everywhere undistinguishable ?Seems to be the case for stars in the exended low-metallicity populations.

SNII

+SNIa

rise in s-process

Venn 2008


• galactic stars help to identify disk stars which came in from outside in satellites which then disrupted.

other dSph stars

Hercules

Hercules dSph : M* = 4 x 104 M , large abundance spread

low Ca/Fe but …

very large [Mg/Ca]

and undetected Ba, Sr, Eu

The high Mg/Ca suggests

enrichment by just

1 or 2 high mass SNII

(M ~ 35 M)

Koch et al 2008


The metal-poor stellar halo help to identify disk stars which came in from outside in satellites which then disrupted.

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

mass of stellar halo ~ 1 x 109 M

(total stellar mass of the Galaxy is about 6 x 1010M)

Probably made up at least partly from debris of lower-mass accreted satellites


Tidal streams in the galactic halo simulation of accretion of 100 satellite galaxies
Tidal Streams in the Galactic Halo help to identify disk stars which came in from outside in satellites which then disrupted.(simulation of accretion of 100 satellite galaxies)

y (kpc)

RVGC (km s-1)

x (kpc)

RGC(kpc)

(Spaghetti: Harding)


Accretion in integral space (E,L help to identify disk stars which came in from outside in satellites which then disrupted.z)

Input - different colors

represent different

satellites

  • Output after 12 Gyr

  • stars within 6 kpc of

  • the sun - convolved with

  • GAIA errors

Helmi & de Zeeuw


Helmi & de Zeeuw adopted a time-independent help to identify disk stars which came in from outside in satellites which then disrupted.

gravitational field for their simulation

Cosmological simulations (eg Gill et al,

Gao & White) indicate that the dark halo

has doubled its mass since z =1

Gill et al showed that satellite debris retains

its identity in the (E, Lz) plane,

although its average (E, Lz) does change

Dynamically reconstructing at least some of the objects

that formed at high redshift and then became part of the

Galactic halo seems feasible: GAIA will contribute greatly.


Got to here help to identify disk stars which came in from outside in satellites which then disrupted.

Now look at some nasty stellar dynamical effects

First need briefly to discuss stellar orbits in galaxies


Stellar Orbits help to identify disk stars which came in from outside in satellites which then disrupted.

Lz is the z-component of the angular momentum


help to identify disk stars which came in from outside in satellites which then disrupted.

Side-on projection

Galactic plane projection

A typical near-circular rosette orbit in an axisymmetric potential, bounded by its energy and Lz values: see the effect of the third integral: this is also an isolating integral but cannot be written down in analytical form. Galactic disks are made up of such orbits.


help to identify disk stars which came in from outside in satellites which then disrupted.

In real galaxies


help to identify disk stars which came in from outside in satellites which then disrupted.

A rosette orbit is the vector superposition of these two components:

the epicycle + the circular guiding centre motion.


Now look at the motions of stars near the sun, in the light help to identify disk stars which came in from outside in satellites which then disrupted.

of the orbit theory.

This defines the UVW system

of stellar motions relative to the Local Standard of Rest (a coordinate System at the solar radius, going around the Galaxy at the circular velocity of 220 km/s. V is in the sense of Galactic rotation.

We will see that the distribution of

stars in the (U,V) plane is lumpy: are these lumps the remains of star forming events which would be interesting archaeologically ?


(Hipparcos data for nearby stars) The Hercules group is probably associated with local resonant kinematic disturbance by the inner bar (Dehnen 1999)

Sirius and Hyades

streams - mainly

earlier-type stars

  • Hercules disturb-

  • ance from OLR

  • mainly late-type

  • stars

V

U

Dehnen 1999


Hercules group probably associated with

o field stars

Chemically, the Hercules group looks just like a random sample of disk stars.

No hint that its stars may be related by birth.

The Hercules group is believed to be a resonance or dynamical group

Bensby et al 2007


probably associated with

p

Resonant Groups

Some stars are in resonance with a rotating

gravitational pattern: bar or spiral structure

Guiding center frequency  and epicyclic frequency  depend on R.

The bar frequency pis a constant.

Outer Lindblad Resonance occurs where p = (R)+ (R) /2 : stars with such values of  see same bar potential on each -oscillation. They are locked into the resonance instead of drifting off on their orbits. For the Galactic bar and disk, OLR lies near the Sun.

Stars in OLR have guiding center radius lying inside solar radius, so their angular momentum is a bit lower than the LSR’s and their V ~ -50 km/s, like the Hercules group


probably associated with

 + /2

Resonances can occur not only from the bar but also from any azimuthally propagating disturbance like a transient spiral structure that has a well-defined pattern speed.

The /6 resonances lie between the 4:1 resonances and corotation

Many (most ?) groups of stars observed to be moving together are probably resonant phenomena. There are p =   /2 resonances and also p =   /4, p =   /6 etc resonances, all of which can

generate bogus (ie dynamical) moving stellar groups

Real coeval chemically homogeneous moving groups do exist. They are good but rare. The dynamical groups are a nuisance for archaeology.

Fuchs 2007


Some potential metal-poor moving groups probably associated with

Histograms of Jz for Gratton's (2003) sample of nearby metal-poor stars with well-measured chemical abundances

The retrograde  Cenfeature isthought to be associated with the accretion event that brought  Ceninto the MW.

Its stars have the same chemical peculiarities as  Cen (Wylie et al 2010) - Na, Mg, s-process over-abundances.

The Arcturus feature was also thought to be the debris of an accretion event associated with the thick disk (Navarro et al 2004). Williams et al (2008) believe it is from a 6:1 resonance, because it shows no chemical identity.

Meza et al 2006


Omega Centauri probably associated with

Omega Cen is the most massive globular cluster in the Galaxy. It is the only cluster which is very inhomogeneous in its heavy element abundances, and it has some unusual element abundance ratios.

The chemical properties and very bound retrograde orbit of w Cen suggest that it is the nucleus of a satellite of mass ~ 108 M which was dragged in to the Galaxy by dynamical friction and then tidally disrupted

This may have been the event that thickened the thick disk

(Bekki & Freeman 2003)

Finding the debris of the w Cen satellite galaxy is an interesting archaeological goal : see Wylie et al 2010


Sirius probably associated with and Hyades

streams - mainly

earlier-type stars

For small epicyclic amplitudes, resonances occur at a particular guiding center radius, eg where p = (R)+ (R) /2

When the epicyclic amplitude A is larger than a few hundred pc, the linear theory breaks down. Orbit can still be represented as guiding center + epicycle, but  is no longer just a function of R: now  = (R,A).

The resonant condition is still p =   /2 and  is still (R) but the non-linearity in  broadens the resonance in R, so resonances become yet more widespread.

Identity of Sirius and Hyades moving groups is still undecided:

They could be real or dynamical (I think real)


Nonlinear epicycle probably associated with

Rosette orbit as seen from nonrotating frame

Orbit as seen from frame rotating at angular velocity 


Radial Mixing: orbit swapping probably associated with

An azimuthally propagating disturbance with pattern frequency p affects stars near corotation (where p = ). Depending on the relative phase of the star and the disturbance, the star can be flipped from a near-circular orbit at one radius to another near-circular orbit at a different radius (Sellwood & Binney 2002). The amplitude and sign of the radius change depends on the strength and phase of the disturbance

This adds to the difficulties of Galactic archaeology: we have always believed that stars in near-circular orbits are at radii close to where they were born. This need not be true.

eg recent simulations by Roskar et al (2008), aimed at understanding

the outer truncation that is seen in many galactic disks. See also Schoenich & Binney papers (2009) for application to evolution of the Galactic disk.


A probably associated with

B

Corotation  = p

p

Star A gains angular momentum and is flipped to larger radius

Star B loses angular momentum and is flipped to smaller radius

Transient disturbances have different corotation radii so as time goes on, different parts of the disk are affected


Disk Truncation probably associated with

M33 - outer disk truncated,

very smooth structure

NGC 300 - exponential disk

goes for at least 10 scale-

lengths without truncation

Ferguson et al 2003

Bland-Hawthorn et al 2005


The truncations are not understood: may be associated with probably associated with

• the star formation threshold

• angular momentum redistribution by bars and spiral waves

• the hierarchical accretion process

• bombardment by dark matter subhalos (de Jong et al 2007)

Roskar et al (2008) - SPH simulation of disk formation from cooling

gas in an isolated dark halo : includes star formation and feedback.

The break is seeded by rapid radial decrease in surface density of

cool gas : break forms within 1 Gyr and gradually moves outwards

as the disk grows. The outer exponential is fed by secularly

redistributed stars from inner regions via spiral arm interactions

(Sellwood & Binney 2002) so its stars are relatively old.


stellar surface density probably associated with

gas surface density

star formation rate

mean stellar age

Roskar et al (2008)


break probably associated with

Secular radial distribution of stars via spiral arm interaction

into outer (break) region of truncated disk

Roskar et al (2008)


Very important now to assess how well radial mixing works for orbits with large epicyclic amplitudes. Does radial mixing significantly affect the old stars in the disk, or is it mainly a young disk effect ?

My guess is that it should work, but will be diluted by the z-motion: orbits with large z-amplitudes will spend much of their time away from the Galactic plane where the spiral disturbance is greatest.


Vlajic et al 2008 for orbits with large epicyclic amplitudes. Does radial mixing significantly affect the old stars in the disk, or is it mainly a young disk effect ?

NGC 300

Radial mixing may explain the reversal in the abundance gradient seen in NGC 300, and its lack of disk truncation

Is the outermost disk

of NGC 300 populated

by stars scattered out

from the more metal-rich

inner regions ?


The point of all this is that kinematics alone cannot reliably show whether a moving group is a resonance effect or the debris of an ancient accretion event or star formation event.

We cannot assume that stars are near their birth radius, not even for stars in near-circular orbits.

Chemical signatures are essential for Galactic archaeology


Meza et al (2006): evolution of disrupting satellite in V reliably show whether a moving group is a resonance effect or the debris of R - R plane

Curves show a few loci of constant (E, Lz)


Histogram of V reliably show whether a moving group is a resonance effect or the debris of rot

for

metal-poor stars

Arcturus peak is

a thick disk feature:

maybe debris of

an accreted galaxy

Meza et al (2006): the Chiba-Beers metal poor stars - histogram

of Vrot: see a few substructure peaks including possible

retrograde omega Cen galaxy debris


Histogram of J reliably show whether a moving group is a resonance effect or the debris of z

see the omega Cen

and Arcturus features

again

Gratton's (2003) sample of metal-poor stars with

well-measured chemical abundances

Meza et al 2006


Element abundances reliably show whether a moving group is a resonance effect or the debris of

[/Fe] vs [Fe/H]

(Meza et al 2006)

The red points are potential omega Cen debris candidates.

Less -enriched than other halo stars : implies a longer

history of chemical evolution, as observed in omega Cen itself


+ reliably show whether a moving group is a resonance effect or the debris of

S

simulation of Can Maj accretion event, seen from N pole.

Cold system,  ~ 11 km/s

Martin et al 2005


Also the Monoceros/Canis Majoris feature in the outer half of

the Galaxy, near plane : probably remains of an accreted

dwarf but may just be a feature in the galactic disk. Not easy

to visualize - look at current best-fit simulation of accretion

of dwarf (Martin et al 2005)


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