increases linearly with

r,

while the total luminosity approaches a finite asymptotic limit as

r

increases. Clearly a large amount of

invisible

gravitating mass (more than 90% of the total mass in the case of the Milky Way and other examples) is needed to explain these flat rotation curves.

No evidence exists of disk DM in the solar neighborhood (from analysis of stellar velocity dispersions). Rotation curve of the spiral galaxy NGC 6503 as established from radio observations of hydrogen gas in the disk (K Begeman et al MNRAS 249 439 (1991)). The dashed curve shows the rotation curve expected from the disk material alone, the chain curve from the dark-matter halo alone.

Dark Halo: basic parameters

Physical extent

•Total mass ~ 2 10 12 M  • Size: R ~ 200 kpc (< 6 10 12 M  ) Values based on a Bayesan statistical analysis of the motions of a sample of halo tracers (globular clusters, dwarf galaxies) from

Wilkinson & Evans

(1999, MNRAS, 310, 645)

Composition

: •Mixture of

baryons

(stars, Macho’s) and

non-baryonic

particles (CMD candidates: neutralinos, axions) percentages still controversial

Dark Halo:

Microlensing results

~20% of the galactic halo is made of compact objects of ~ 0.5 M



MACHO

: 11.9 million stars toward the LMC observed for 5.7 yr  13-17 events  8%-50% (C.L. 95%) of halo made of 0.15-0.9 M  compact objects.

EROS-2

: 17.5 million stars toward LMC for 2 yr   2 events (+2 events from EROS-1) less that 40% (C.L. 95%) of standard halo made of objects < 1 M 

Candidate MACHOs

: • Late M stars, Brown Dwarfs, planets • Primordial Black Holes • Ancient Cool White Dwarfs Limits for 95% C.L. on the halo mass fraction in the form of compact objects of mass M, from all LMC and SMC EROS data 1990-98 (Lassarre et al 2000). The MACHO 95% C.L. accepted region is the hatched area, with the preferred value indicated by the cross (Alcock et al. 1997)

Dark Halo: search for Ancient cool WDs

• The most extensive survey to date (

Oppenheimer et al

Science, 292, 698):

38

2001, Halo WDs in 5000 deg² in the Southern Hemisphere towards the SGP. • They estimate the lower limit of the space density to

~ 1%

expected local halo density of the   1 .

3  10  4 M sun pc

-

3

Galactic disk

The

galactic disk

is the most conspicuous component of the Milky Way. This is a

thin

,

flat

structrure entirely supported by rotation.

The galactic disk is an “

evolving

” component since 10 Gyr, because of

dynamical processes

(e.g. gas accretion, mergers, disk instabilities, etc.) and continuous

star formation

.

The distributions of the stars over

position

and

velocity

are linked through the gravitational forces, and through the star formation rate as a function of position and time.

Galactic disk

The galactic disk is a complex system including stars, dust and gas clouds, active star forming regions, spiral arm structures, spurs, ring, ...

However, most of disk stars belong to an “axisymmetric” structure, the Thin disk, with an exponential density law:  (

R

,

z

)   0 

e



z

/

h z



e

 (

R



R

0 ) /

h R

h z =250 pc   W = 20 km/s

Thick Disk

: • Pop.II Intermediate • • h z =1000 pc  W = 60 km/s

Formation process

• Dynamical heating of the old disk because of an ancient major merger (

bottom-up

) • Halo-disk intermediate component (

top-down

)

Galactic disk(s)

Galactic disk Age-metallicity relation

Age-metallicity distribution of 5828 stars with   /  <0.5 and Mv<4.4

Feltzing et al

. (2001, A&A), who investigated the age metallicity in the solar neighbourhood, claimed that: • the age-metallicity diagram is well populated at all ages and especially old metal-rich stars do exist • the scatter in metallicity at any given age is larger than the observational errors

Open Questions

• Do

galaxies

, such as the Milky Way, form from

accumulation

smaller systems which have already initiated star formation?

of many • Does

star formation

begin in a gravitational potential well in which much of the gas is already accumulated?

• What is the nature and

composition

What is its physical

extent

of matter in the galactic

dark halo

? and

shape

? How much does it “weigh”? How does it

interact

with the visible component? • Does the

bulge

pre-date, post-date, or it is contemporaneous with, the halo and inner disk? Is it a

merger

remnant? Is it a remnant of a disk

instability

? • Is the

thick disk

a mix of the early disk and a later major

merger

?

• Is there a

radial

age and chemical gradient in the older stars?

• Is the history of

star formation

relatively smooth, or highly episodic?

• ...