KIAA Lectures Beijing, July 2010

Ken Freeman, RSAA, ANU

Lecture 1: Introduction

What does our Galaxy look like ?

Near infrared imagefrom COBE/DIRBE -dust is transparent innear-IR

NGC 891: our Galaxy probablylooks much like this in visiblelight

We would like to understand how our Galaxy came to looklike this

The Milky Way is a disk galaxy: the disk is the primary stellar component

Some galaxies have much larger bulges

Also see a small central bulge

Overview of our Galaxy

dark halo

stellar halo

thin disk

thick disk

bulge

Each one of these components has something totell us about its formation history

Our task is to understand how the formation andevolution of the Milky Way took place: how does it compare with the predictions of CDM simulations ?

The thin disk is metal-rich and covers a wide age rangeThe other stellar components are all relatively old(note similarity of [Fe/H] range for thick disk and globular clusters)

Total mass ~ 2 x 1012 M : Wilkinson & Evans (1999), Sakamoto et al (2003)

Stellar mass in bulge 1 x 1010 M

disk 6 x 1010 M

stellar halo 1 x 109 M

Ages of components: globular clusters ~ 10-12 Gyrthick disk : > 10 Gyrthin disk : star formation started about 10 Gyr ago and star formation in the disk has continued at a more or less constant rate to the present time

How did the Galaxy come to be like this ?

To study the formation of galaxies observationally, we have a choice ...

we can observe distant galaxies at high redshift : we see the galaxies directly as they were long ago,at various stages of their formation and evolution

but not much detail can be measured about theirchemical properties and motions of their stars

so we cannot follow the evolution of any individual galaxy

or we can recognise that the main structures of our Galaxy formed long ago

at high redshift.

We can study the motions and chemical properties of stars in our Galaxy

at a level of detail that is impossible for other galaxies, and probe into the formation epoch of the Galaxy.

This is near-field cosmology

the halo formed at z > 4the disk formed at z ~ 2

The ages of the oldest stars in the Galaxy are similar to the lookback time

for the most distant galaxies

Both give clues to the sequence of events that led to the formation of galaxies

like the Milky Way

Now show a numerical simulation of galaxy formation.

The simulation summarizes our current view of how a disk galaxylike the Milky Way came together from dark matter and baryons

MOVIE

• much dynamical and chemical evolution• halo formation starts at high z• dissipative formation of the disk

Simulation ofgalaxy formation

• cool gas • warm gas • hot gas

QuickTime™ and aMicrosoft Video 1 decompressorare needed to see this picture.

• z ~ 13 : star formation begins - drives gas out of the protogalactic mini-halos. Surviving stars will become part of the stellar halo - the oldest stars in the Galaxy

• z ~ 3 : galaxy is partly assembled - surrounded by hot gas which is cooling out to form the disk

• z ~ 2 : large lumps are falling in - now have a well defined rotating galaxy.

Movie synopsis

The movie showed the formation and evolution ofa large spiral in a CDM simulation.

What does each component of the Milky Waycontribute to our understanding of the formation and evolution of disk galaxies in the CDM context ?

Living inside the Milky Way has advantages anddisadvantages. The Milky Way will be very good for assessing some CDM issues and not so good for others

What are the issues with galaxy formation in CDM in the context of what our Galaxy can contribute towards understanding these issues?

• Structure of the inner dark halo - core or cusp• Number of predicted satellites• Forming disks with small bulges in CDM• Active accretion history• Baryonic angular momentum

What are the issues with galaxy formation in CDM ?

• Structure of the inner dark halo - core or cusp Simulations predict cusp, observers claim core

NFWoptical rotation curves

give inner slope ofdensity distribution

NFW halos have = -1

Flat cores have = 0

eg de Blok et al 2002: sample of about 60 LSB galaxies

Distribution of inner slope of density ~ r

Very long argument !!

• Number of predicted satellites

What are the issues with galaxy formation in CDM ?

From simulations, we would expect a galaxy like the Milky Way to have ~ 500 satellites with bound masses > 108 M. These are not seen optically or in HI. New very faint satellites arebeing discovered but unlikely to find 500

B. Moore et al

Are some (or all) globular clusters the nuclei of accreted fragments ?

Are there large numbers of dark satellites ?

• Forming disks with small or no bulges in CDM

It is currently difficult for CDM to generate galaxies with small or no bulges. Understanding how the bulge of the Galaxy formed is important for this problem

What are the issues with galaxy formation in CDM ?

Small bulges are thought to be generated by instability processes within disks, rather than by merger activity.

If that is correct, then an even larger fraction of disk galaxies were born without bulges, and the problem of forming pure disk systems becomes even more evident (more in lecture on the bulge)

• CDM predicts an active ongoing accretion history, leaving debris of accreted satellites in the stellar disk and halo. (The first stars probably came from small dense accreted systems which formed before the Milky Way itself). A very active accretion history may be inconsistent with the presence of a dominant thin disk. Epoch of last major merger is particularly important for disk survival.

We are uniquely located in the Milky Way to evaluate accretion history of a large spiral and measure the distribution of its first stars

What are the issues with galaxy formation in CDM ?

Chou APOD

Sgr

NGC 5907

• Baryonic angular momentum in context of angular momentum loss from baryons to dark halo via hydrodynamicaland gravitational effects. Old problem, that baryons have predicted to have less angular momentum than observed: seems to be less of a problem with higher resolution simulations (eg Kaufmann et al 2007, Governato et al 2007).

Observationally probably better studied in other galaxies

What are the issues with galaxy formation in CDM ?

• Baryon acquisition is needed to fuel ongoing star formation, which would exhaust the current gas supply on a timescale ~ 1 Gyr.

How is this happening ? Is it related to the accretion history, high velocity HI clouds, the galactic warp ? Is it gas that was previously ejected from the disk ?

Milky Way is potentially well suited to investigate baryon acquisition.

This seems too difficult. In the process of galaxy formation and evolution from the CDM hierarchy, a lot of information about the proto-galactic hierarchy is lost. Now discuss how information is lost during galaxy formation and evolution.

Reconstructing galaxy formation

We would like to reconstruct the whole process of galaxy formation, as the Galaxy comes together from the CDM hierarchy.

What do we mean by the reconstruction of Galaxy formation ? We want to understand the sequence of events that led to the Milky Way

as it is now. Ideally, we would like to tag or associate the visible

components of the Galaxy to parts of the proto-galactic hierarchy :

i.e. to the baryon reservoir which fueled the stars in the Galaxy.

Epochs when information about the proto-hierarchy is lost:

• As dark matter virialises• As baryons dissipate within the dark halo to form the disk and bulge• As the disk restructures to form the bulge (if that is the way it formed)• Subsequent accretion of objects from the environment : information is lost, though some traces remain.• During the evolution of the stellar disk, as orbits are scattered by dynamical processes - resonances, molecular clouds …

At each epoch, some information remains: what does theGalaxy remember ? What can we hope to discover with Galactic Archaeology ?

Signatures remembered from each epoch

• Zero order: since dark matter virialised: • First order: since main baryon dissipation epoch• Second order: subsequent evolution

(This is idealized: galaxy formation is an ongoing process)

In later lectures, we will look at • ways in which we can derive information about the early Galaxy, and • some of the processes that cause loss of information or provide bogus information for us to misinterpet.

Zero order signatures (since dark matter virialization)

The virialization phase is dominated by merging and violent relaxation. Early stars form in small elements of the hierarchy - some will become the stars of the metal-poor halo. The total binding energy E, mass M andangular momentum parameter whereJ is the angular momentum, are more or less established at this phase, although they continue to evolve slowly: E, M and J determine the gross nature of the galaxy.

The globular cluster system formed around this time: its underlying structure has probably not changed much, though a lot of the clusters were destroyed. Note that old GCs in Milky Way, LMC and nearby Fornax dwarf spheroidal galaxy have similar ages within 1 Gyr.

GCs in interacting systems like the Antennae indicate that GC formation is associated with interaction, as in this very early phase.

QuickTime™ and a decompressor

are needed to see this picture.

Some of the properties of the metal-poor stellar halo wereprobably established in this epoch, as small satellites which had already formed stars were accreted by thevirializing halo

The Tully-Fisher law which relates the rotational velocity and the baryon mass of galaxies, is also probably established at this phase.

Massive young clusters in NGC 4038/9

Massive cluster formation in NGC 4038/9

The Tully-Fisher law for disks

Gurovich et al 2010

stellar baryonic

slope 3.8 ± 0.1 slope 3.2 ± 0.1

Sakai

McGaugh

HIPASS

(baryonic = stars + neutral gas)

First order signatures

What information remains from the epoch when baryons dissipated to form disk (and maybe the bulge) ?

The scale length of the disk may be roughly constant since then. The mass of the disk continues to grow as gas falls in and stars form. Chemical gradients in old components like the thick disk may be conserved but are probably affected by radial mixingof stars by disk heating, spiral arms.

The vertical scale height of the old disk evolves with disk heating, at least for a few Gyr, but is probably roughly constant after ~ 3 Gyr from birth, so we see old disk as it was about 7 Gyr ago.

The old thick disk may have been heated by accretion long ago, or formed long ago in a gas-rich merger (Brook et al 2007): it is probablymuch as it was after birth. It is one of the most important fossil remnants.

Brook et al 2007

Thick disk stars form rapidly in merger, thin disk stars form later

The bulge has probably not changed much since its formation. If it formed by disk instability, then it has probably not much changed over the last 7 Gyr.

The shape of the dark halo is probably remained more or less constant, except near the disk plane

Second order signatures

What information remains from subsequent evolution after the disk formed ?

Objects fall in to halo and break up. Their stars conserve some dynamical properties. Streams like Sgr will gradually mix away structurally but some dynamical information survives.

Star forming events in the disk mostly dissolve and phase-mix aroundthe Galaxy. Some maintain their kinematical identity as moving stellar groups. A few survive as open clusters : some are almost as old as the disk. All maintain their chemical identity, so we can use chemical techniques to find the debris of old dissolved star forming events.

Surviving star clusters and moving groups are chemically very homogeneous

Chemical abundances in two open clusters and one old moving group

Da Silva et al, 2008

• Hyades• Collinder 261• HR1614 moving group

Carney Laird Latham (1996) survey of high proper motion stars : orbital eccentricity vs [m/H] - modern version of ELS

diskstars:low-eorbits

highly eccentrichalo stars

The CLL survey shows the striking kinematical difference between disk and halo

The metal-poor stellar halo

Rotational velocity relative to the sun vs [m/H] (V = -220 km/s corresponds to zero angular momentum)

rapidlyrotatingdisk &

thick disk

slowlyrotating

halo

Carney Laird Latham survey : different metallicity distributions of halo stars and globular clusters : halo stars now known down to [Fe/H] < -5

GCs

halostars

Not all of the metal-poor stars ([Fe/H] < -1) are in the halo. A fraction belong to the thick disk.

More on the thick disk later: although most of the stars in the old thick disk have abundances of [Fe/H] = - 0.7 to -1.0, the thick disk has a long metal weak tail extending down to [Fe/H] ~ -2.

About 25% of the stars with [Fe/H] = -1.5 nearthe sun belong to thethick disk

40

Halo & thick disk are ancient structures, very important for GA

Halo Streams

Long orbital timescales survival of identifiable debris

eg Sgr tidal stream, discovered near the bulge, extends rightaround the Galaxy (probably a few times around)

Ibata et al 1995 Majewski et al 2003

2MASS M giants

These tidal streams from the currently disrupting Sgr dwarf are interesting (the halo is still under construction)

as are the ancient streams from small objects accreted long ago into the halo

The long orbital periods allow these ancient streams to survive,so the metal-poor halo is the best place to attempt reconstruction

of their accretion events.

They are too faint to see in configuration space - may see them inphase space, eg (RG , VG ), or in integral space

ie the space of integrals of the motion for stellar orbits,like energy and angular momentum (E , Lz )

Accretion is important for building the stellar halo, but not clear yet how much of the halo comes from discrete accreted

objects (debris of star formation at high z) versusstar formation during the baryonic collapse of the Galaxy

At one extreme, simulations of pure dissipative collapse (eg Samland et al 2003) suggest that the halo

may have formed mainly through a lumpy collapse, with only ~ 10% of its stars coming from

accreted satellites

In any case, we can hope to trace the debris of these lumps and accreted satellites from

theirphase space structure. But we can also use

chemical techniques to trace their debris

References

Binney & Tremaine: Galactic Dynamics (1987, 2008). This is the book on Galactic Dynamics: some of the material in these lectures comes from B&T

Binney & Merrifield: Galactic Astronomy (1998). A more descriptive book and well worth reading for background.

Sparke & Gallagher: Galaxies in the Universe (2007).Ditto : good book, with some theory

Turon & Primas: ESA-ESO Working Group Report #4: Galactic Populations, Chemistry and Dynamics (2008).A very useful compendium of Galactic knowledge, problems, techniques, surveys