Galaxy Formation and EvolutionOpen Problems

Alessandro Spagna

Osservatorio Astronomico di Torino

Torino, 18 Febbraio 2002

Galaxy Structure Flat disk:

•1011 stars (Pop.I)

• ISM (gas, dust)

• 5% of the Galaxy mass, 90% of the visible light

• Active star formation since 10 Gyr.

Central bulge:

• moderately old stars with low specific angular momentum. • Wide range of metallicity• Triaxial shape (central bar)• Central supermassive BH

Stellar Halo• 109 old and metal poor stars (Pop.II)

• 150 globular clusters (13 Gyr)

• <0.2% Galaxy mass, 2% of the light•Dark Halo

Open Questions• Do galaxies, such as the Milky Way, form from accumulation of many smaller systems which have already initiated star formation?

• Does star formation begin in a gravitational potential well in which much of the gas is already accumulated?

• What is the nature and composition of matter in the galactic dark halo? What is its physical extent 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?

• ...

Galaxy formation: monolithic collapse

Fast dissipative collapse of a monolitic protogalactic cloud > ~108 yr and no chemical gradient in the halo

Galaxy formation: fragmented accretion

Prolonged aggregation of protogalactic fragments -> no radial gradient but age and metallicity spread.

CDM - Hierarchical scenario

Springel et al, 2001, MNRAS, 228, 726: high resolution N-body simulation of the evolution of clusters of galaxies

CDM - Hierarchical scenario

Helmi, White & Springel (2002, astro-ph/0201289) rescaled of a factor 10 the Springel’s simulation in order to study the evolution of CMD galactic halo and investigate the kinematics of CMD streams in the solar neighborhood.

* Note: baryonic - CDM interactions (e.g. central bar) have been neglected.

Merging History of the Galaxy

“Evidence of a continuous accretion process of satellites and fragments in the past 10-13 Gyr:

• a few tens star-forming dwarfs like Sagittarius or Carina galaxies

• 1000 metal poor fragments and dwarfs like Draco or Ursa Minor”

Buser, 2000, Science, 287, 69

The Milky Way is part of the Local Group: about 30 galaxies, half of them clustered in two subgroups (our Galaxy and Andromeda).

Note that there are 5 systems with 70<R<100 kpc and only 1 (Sagittarius) with R<70 kpc.

Halo streams

Simulated halo stream (105 particles, T=12 Gyr) for a spherical halo (q=0, left), and a flattened halo (h=0.75, right). (Ibata et al, 2001, ApJ 551, 294)

High Velocity Clouds

Dark Halo: Rotation curves of galactic disks

Stars and gas in the galactic disks follow circular orbits whose velocity depends on the inner mass only:

v2(r) = G M(<r) / r

A flat rotation curve means that the total M(<r) 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 1012 M (< 6 1012 M )

• Size: R ~ 200 kpc

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 2001, Science, 292, 698): 38 Halo WDs in 5000 deg² in the Southern Hemisphere towards the SGP.

• They estimate the lower limit of the space density to ~ 1% of the expected local halo density

3sun

4 pc M 103.1 -

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:

Rz hRRhz eezR /)(/0

0),(

hz =250 pc W = 20 km/s

Galactic disk(s)

Thick Disk:

• Pop.II Intermediate

• hz=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

Age-metallicity relation

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

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

Open Questions• Do galaxies, such as the Milky Way, form from accumulation of many smaller systems which have already initiated star formation?

• Does star formation begin in a gravitational potential well in which much of the gas is already accumulated?

• What is the nature and composition of matter in the galactic dark halo? What is its physical extent 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?

• ...