STELLAR POPULATIONS

Erasmus Mundus Padova - Italy 2010-2011

Prof. Antonio Bianchini Dipartimento di Astronomia Università di Padova antonio.bianchini@unipd.it

STELLAR POPULATIONS - 1

Erasmus Mundus Padova - Italy 2010-2011 Prof. Antonio Bianchini

A basic unit of stellar population studies is the "SIMPLE STELLAR POPULATION" (SSP):

• Coeval • Chemically homogenous (at least at

birth) •  Similar orbits/kinematics

One example of an SSP is the typical star cluster (either open or globular).

“Basis Vectors” Some examples of composite, principal simple component populations in our

Milky Way might be:   halo   disk   bulge

Each of the above represent complex groupings of stars and matter, but with distinct global properties/distributions of chemistry/age/kinematics from one another.

  These differences presumably relate to differing mixtures of SSPs.

  The smaller the basis set (n, mn), the easier is the composite population (and, ultimately, the Galactic history) to unravel

  Identifying individual SSPs may be hard in complex galaxy, but, perhaps SSPs are strung together in somewhat simple patterns. . .

MORE SPECIFICALLY: WE SEEK: Correlations between attributes such as:

  SPATIAL DISTRIBUTIONS, e.g., stellar density laws

  KINEMATICS, velocities, velocity dispersions (i.e. observable dynamical features)

  CHEMISTRY, e.g. mean [Fe/H], chemical abundance patterns ([O/Fe], [Ca/Fe], [Zn/Fe], ...)

  AGES, reflected, e.g., in the types of stars seen (their evolutionary state)

TO IDENTIFY AND DEFINE : Principal component populations that will allow us

TO RECONSTRUCT: A complete, physical, chemodynamical evolutionary model of the Milky Way (or other galactic systems)

One of the primary reasons for studying stellar populations in galaxies is to improve our understanding of the formation of galaxies and their evolution with time.

The distance of the Sun from the Galactic center is between 7.6 (Eisenhauer et al. 2005) and 7.2 Kpc (Bica et al. 2006). From this , and the knowledge of the rotational velocity of the Sun (v=220 +/- 20 Km/s) we immediately obtain the rotation period of the Galaxy P=2πR/v=230 million years.

The mass of the Galaxy can be estimated from the third Kepler Law MG = R3/P2 (in standard units: MG = 4π2/G R3/P2) where R is given in AU and P in years. The result is 1011 M quite close to more accurate determinations based on the galactic rotation curve MG = 4π2/G R3/P2 (4x1011 M)

Metals in Stars Absorption lines almost exclusively from hydrogen: Population II

Many absorption lines also from heavier elements (metals): Population I At the time of

formation, the gases forming the Milky Way consisted exclusively of hydrogen and helium. Heavier elements (“metals”) were later only produced in stars.

=> Young stars contain more metals than older stars

During World War II American scientists served on the Manhattan Project. Walter Baade was however an exception. He was born in Germany and his citizenship documents were lost. He was therefore not allowed to work on war related projects. Baade was a staff astronomer at Mount Wilson Observatory, just north of Los Angeles. Without the usual competition for telescope time from other astronomers, such as Edwin Hubble, Baade had more access to the Hooker telescope, which at 100 inches was the world's largest at the time. Wartime blackouts reduced the light pollution from nearby Los Angeles and gave Baade better sky conditions.

With ideal conditions, Baade obtained excellent photographs that for the first time resolved individual stars in the Andromeda Galaxy. Baade and proved the existence in that galaxy of two distinct populations of stars: Population I and Population II.

Important historical discovery

HIGH VELOCITY STARS

A high velocity star is a star moving faster than 65 km/s to 100 km/s rehlative to the average motion of the stars in the Sun's neigbourhood. The velocity is also sometimes defined as supersonic relative to the surrounding interstellar medium.

The three types of high velocity stars are: runaway stars, halo stars and hypervelocity stars.

High-velocity halo stars are very old stars that do not share the motion of the Sun or most other stars in the solar neighbourhood which are in similar circular orbits around the centre of the Galaxy. Rather, they travel in elliptical orbits, which often take them well outside the plane of the Galaxy.

Although their orbital velocities in the Galaxy may be no faster than the Sun’s, their different paths result in the high relative velocities. Typical examples are the halo stars passing through the disk of the galaxy at steep angles.

Rotational velocity of nearby stars relative to the sun vs [m/H] (V = -232 km/s corresponds to zero angular momentum)

rapidly rotating disk &

thick disk

slowly rotating

halo

|Zmax| < 2 kpc

THE LARGER THE RADIAL VELOCITIES ..THE LARGER THE PROPER MOTIONS

For low metallicity stars the vertical velocities are comparable with the tangential ones

Stellar Populations Population I: Young stars: metal rich; located in spiral arms and

disk

Population II: Old stars: metal poor; located in the halo

(globular clusters) and nuclear bulge

Modern astronomers have expanded Baade's initial classification of Population I and II stars based on how strongly stars display the Population I or II properties. The five populations are: Extreme Population I, Older Population I, Disk Population II, Intermediate Population II, and Halo Population II. Some astronomers also think that there was a short lived initial generation of massive Population III stars that no longer exists.

Further complicating the classifications, there are Population I stars in the Magellanic that contain few metals. There are also Population II stars near the core of the Milky Way galaxy that contain metals.

The initial big bang made no metals other than trace amounts of lithium and beryllium. Elements other than hydrogen and helium were made in stars and blasted back into space by supernova explosions of massive stars. The first generation Population II stars therefore contain only hydrogen and helium. Later generation Population I stars contain 2% metals that were manufactured by the massive Population II stars that have short lifetimes and have died out in supernovae

In astrophysics, the metallicity of an object is the proportion of its chemical elements other than hydrogen and helium. Since stars are composed mostly of hydrogen and helium, astronomers use for convenience the blanket term "metal" to describe all other elements collectively.

Stellar populations I, II, and III, have decreasing metal content and increasing age. The populations were named in the order they were discovered, which is the reverse of the order they were created. Thus, the first stars in the universe (low metal content) were population III, and recent stars (high metallicity) are population I.

THE EGGEN, LYNDEN-BELL, SANGAGE

MODEL OF THE FORMATION OF

THE GALAXY

This suggests stellar formation in the central Bulge

Our Galaxy has a significant thick disk

• its scale height 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 10 Gyr, and are

• significantly more metal poor than the thin disk : mostly (-0.5 > [Fe/H] > -1.0)

• alpha-enriched so its star formation was rapid

[(α + Eu)/H] vs [Fe/H] for thin and thick disks near the sun

The thick disk is chemically distinct

The thick disk and halo of NGC 891 (Mouhcine et al 2010): thick disk has scale height ~ 1.4 kpc and scale length 4.8 kpc, much as in our Galaxy.

Most spirals (including our Galaxy) have a second thicker disk component

• Thick disks are very common.

• In our Galaxy, the thick disk is old, and kinematically and chemically distinct from the thin disk. What does it represent in the galaxy formation process ?

• The orbital eccentricity distribution will provide some guidance.

• Chemical tagging will show if the thick disk formed as a small number of very large aggregates, or if it has a significant contribution from accreted galaxies. This is one of the goals for the HERMES survey.

The Galactic bar/bulge

The boxy appearance of the bulge is typical of galactic bars seen edge-on. Where do these bar/bulges come from ? They are very common. About 2/3 of spiral galaxies show some kind of central bar structure in the infra-red.

The bars come naturally from instabilities of the disk. A rotating disk is often unstable to forming a flat bar structure at its center.

This flat bar in turn is often unstable to vertical buckling which generates the boxy appearance.

This kind of bar/bulge is not generated by mergers

• Is there a halo component that formed dissipationally early in the Galactic formation process ?

Hartwick (1987) : metal-poor RR Lyrae stars show a two-component halo: a flattened inner component and a spherical outer component.

Carollo et al (2010 ) identified a two-component halo plus thick disk in sample of 17,000 SDSS stars, mostly with [Fe/H] < -0.5

Describe kinematics well with these three components: <V> σ [Fe/H] Thick disk 182 51 -0.7 Inner halo 7 95 -1.6 Outer halo -80 180 -2.2 (retrograde)

From comparison with simulations, Zolotov et al (2009) argue that the inner halo has a partly dissipational origin while the outer halo is made up from debris of faint metal-poor accreted satellites.

NGC 5907: debris of a small accreted galaxy Our Galaxy has a similar structure from the disrupting Sgr dwarf

Summary for stellar halos: • most disk galaxies have a stellar halo

• stellar halo is made up mainly of debris of small accreted galaxies, although there may be an inner component which formed dissipatively

M33: RR Lyrae stars show an old disk + halo structure (Sarajedini et al 2006) M31: Spectra of red giants in the outer regions of M31 identify a non-rotating metal-poor halo (Kalirai et al 2006, Chapman et al 2006) with <[Fe/H]> ~ -1.4

Tanaka et al (2009): M31 halo shows at least 16 substructures, so at least 16 events contributed to building halo. Each event has mass 107 to 109 M and patchy metallicity distribution : ie not yet fully mixed.

•  The substructure of the outer Galaxy becomes clear:

The "field of streams" from the SDSS survey. From Belokurov et al. (2006, ApJ, 642, L137).

• Missing satellites problem.

Current CDM models predict that the Milky Way should have hundreds of satellites. Model from Ben Moore's Zurich group. From http://cfcpwork.uchicago.edu/seminars/talks/040206/slideshow/3.html

 Discovery and mapping of actual stellar streams around the Milky Way and Andromeda, and convergence of observation and theory of halo substructure.

(Left) Known Milky Way streams as mapped by Majewski & Law. (Right) Model of Milky Way from Bullock & Johnston model.

 In past few years (!) a substantial increase in the number of known Local Group dwarf galaxies (mostly satellites of Milky Way and Andromeda).

About a dozen new Milky Way dwarf galaxy satellites have been found in the SDSS data (doubling known number).

High velocity stars may have formed early in the Galaxy's history, or they may be remnants of smaller galaxies that have merged with ours.

NGC 5907: debris of a small accreted galaxy Our Galaxy has a similar structure from the disrupting Sgr dwarf

APOD

Introduction - The Basic Picture

59

Elliptical formation by merging spiral galaxies

The radio galaxy MRC 1138-262, also called the "Spiderweb Galaxy" is a large galaxy in the making. At 10.6 billion light years away, we see it in the process of forming only 3 billion years after the Big Bang. Note the small, thin "tadpole" and "chain" galaxies that are merging together to create a giant galaxy.

Basically, there are three types of galaxies:

SPIRALS, ELLIPTICALS and IRREGULARS

Spirals: >50% of most luminous within 100 Mpc eventually with bars Ellipticals: 10% of most luminous … Irregulars: several, rather faint.. Lenticulars: 20% of more luminous … , elongated ellipticals

These galaxies possess distinctive combinations of different stellar populations.

PAUSE