Transcript Galaxy Formation and Evolution Open Problems

Galaxy Formation and Evolution
Open 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
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.
“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
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
4
  1.3 10 Msun 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 / hz
hz =250 pc  W = 20 km/s
e
 ( R R0 ) / hR
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
Age-metallicity distribution
of 5828 stars with /<0.5
and Mv<4.4
• 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 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?
• ...