Transcript Bildungskonzepte von Galaxien - uni

Formation of Galaxies
Robert Feldmann, Rovinj 2003
Outline
1. Introduction
2. ELS scenario
3. S-Z scenario
4. Massive elliptical galaxies
5. Summary
6. Literature
11.09.2003
Galaxy formation
2
Introduction
 Investigation of the history of galaxies
 First approach:
 Chemical content
 Kinematics
 Spatial distribution
 Second approach:
 Snapshots, observe evolution directly
 Not really understood but many models
 Two paradigms
 Monolithic collapse
 Hierarchical merging
11.09.2003
Galaxy formation
3
Introduction
 Theoretical framework:
 structure formation by growth of mass fluctuations by
gravitational instability
 Fluctuation as initial conditions imposed on the early
universe
 Currently favoured : “hierarchical structure formation”
 Dark matter dominates overall mass density
 Dictates structure of visible matter
 Large density enhancements made by successive
merging
 Details set by cosmological model
11.09.2003
Galaxy formation
4
Introduction
 What should a modern theory yield?



Distribution of dark matter
number of halos as function of mass and time
Physics of normal baryonic matter




11.09.2003
Star formation
Energy dissipation
Metal enrichment
Main point: Relate underlying dark matter to
observed baryonic matter
Galaxy formation
5
Introduction
 Star formation


At redshifts z>1 conventional spectroscopic samples
become inefficient
 photometric methods
 Large Scale distribution


galaxies as tracer for dark matter
Clustering
 Morphologies
 Most challenging: Establishing links between
samples at different cosmic epochs
11.09.2003
Galaxy formation
6
ELS scenario

O.J.Eggen, D.Lynden-Bell, A.R.Sandage 1962
 Top-Down scenario
 Galaxy contains types of objects with large range in kinematical
properties
 Young main sequence stars (disk)
 Globular clusters
 Extreme subdwarfs
 time for energy, angular momentum exchange long
compared to age of galaxy
 Energy, momenta  initial dynamic conditions
 Stellar evolution  age of the subsystems
  Reconstruct galactic past
11.09.2003
Galaxy formation
7
ELS scenario
 Correlation exist between
 Chemical composition
 Eccentricity of their galactic orbit
 Angular momenta
 Maximal height above galactic plane
 Interpretation:
 Protogalaxy condensing out of pregalactic medium
 Collapsing toward galactic plane
 Shrinking in diameter until forces balance
 Fast collapse 100 Myr, rapid star formation
 Original size > 10 times present diameter
11.09.2003
Galaxy formation
8
ELS scenario
 Stellar dynamics:



General potentials
Nearly decoupling of motions in plane and
perpendicular
In contracting galaxy



11.09.2003
Assuming: axial symmetry
Masses with greatly differing angular momenta do
not exchange momenta
Thus, each matter element will conserve its
angular momentum
Galaxy formation
9
ELS scenario
 Stellar dynamics (2):
 Contracting galaxy: two extreme cases
 Potentials changing slowly
 Eccentricity is invariant
 Potentials changing rapidly
 Eccentricity increase with mass concentration
 Thus
 Angular momentum conserved
 Slow potential change: eccentricity is conserved,
height above galactic plane
 Fast changing potential: more eccentric orbits, height
spread
11.09.2003
Galaxy formation
10
ELS scenario
 Correlations
 between eccentricity and ultraviolet excess:
  eccentricity higher for older stars
 First idea: galaxy as hot sphere in equilibrium
supported by pressure, stars condensing out, falling
toward centre  to hot for stars to form
 From angular momenta observations: galaxy were not
in its present state of equilibrium at the time of first star
formation
 Rate of collapse: since there are highly eccentric orbits
 rapid collapse w.r.t. galactic rotation , i.e. 100 Myr
 Ratio of apogalactic distances of first and successive
order stars  10:1 collapse radially, 25:1 in z-direction
11.09.2003
Galaxy formation
11
ELS scenario
 Correlations (2)
 Between perpendicular velocity and excess:
  oldest objects were formed at almost any height,
youngest were formed near the plane
 Thus: collapse of galaxy into a disk after or during
formation of the oldest stars
 History of collapsing gas:
 Collide with other streams
 loosing kinetic energy by radiation
 Take up circular orbits
 First stars
 Not suffering collisions
 Continue on their eccentric orbits
11.09.2003
Galaxy formation
12
ELS scenario
 Summary:

10 Gyr ago: proto-galaxy started to fall together out of
intergalactic material (gravitational collapse)
Condensations formed, later becoming globular clusters
Collapse in radial direction stopped by rotation but continued
in z-direction  disk
Increased density  higher star formation
Gas, getting hot, cools by radiation

Gas and first stars take separate orbits near perigalacticum






11.09.2003
gas settles down in circular orbits
first stars remain on their highly eccentric orbits
Galaxy formation
13
ELS scenario
 Questions?
11.09.2003
Galaxy formation
14
S-Z scenario
 L.Searle, R. Zinn 1978
 Bottom-Up scenario
 Precise abundance measurements
 Observing red giants, reddening-independent
characteristics
 Measuring correlations of



11.09.2003
Abundance with distance
Abundance with colour distribution
Abundance distribution in the outer halo
Galaxy formation
15
S-Z scenario
 Methods:




low-resolution spectral flux distribution
Obtaining intrinsic spectrum which is reddening independent
Dependent only on age, composition, absolute magnitude
One parameter abundance classification
S   wi Qi 
i



11.09.2003
w
i
i
 abundance ranking
Comparison with other spectroscopic measurements (Butler)
shows good agreement
Homogenous metal abundance within each cluster (Fig 7)
Galaxy formation
16
S-Z scenario
 Known main characteristics [Woltjer(75),Harris (76)]



Distributed with spherical symmetry
No disk component
Metal-rich clusters confined within 8kpc of galactic centre
(inner halo)
 But what about outer halo?
 Used a sample of 16 clusters with high precision distance
and abundance measurement
 and 13 clusters with rougher estimates
 All with distance > 8kpc
11.09.2003
Galaxy formation
17
S-Z scenario
 Is there a abundance gradient in the outer
halo?




11.09.2003
Metal abundance of inner halo higher than outer
halo, but do we find only very metal-poor clusters
at large distances?
No, great range of abundance at all galactic
distances (Fig 9)
Mean abundance not decreasing with distance for
d>15kpc
Contradiction with ELS measurements
Galaxy formation
18
S-Z scenario




11.09.2003
Probably included some metal-rich subdwarf
of the inner halo in their bins
 no statistical evidence that kinematics of
subdwarfs more metal-poor than 1/10 of the
sun is correlated with abundance.
Further abundance measurement in very
remote clusters by Cowley, Hartwick, Sargent
(78)  spread of abundance at all distances
Conclusion: abundance distribution in outer
halo independent of distance to galactic center
Galaxy formation
19
S-Z scenario
 Second parameter
 Colour distribution only loosely correlated with
abundance in clusters
 Second parameter (whatever it is) must be
closely correlated with abundance for the inner
halo and loosely correlated for the outer halo
 Inner halo: tightly bound clusters
 Outer halo: coexistence of tightly bound and
loosely bound clusters
 Fraction of loosely bound clusters increase
with distance
11.09.2003
Galaxy formation
20
S-Z scenario
 The abundance distribution in the outer halo

Using generalized histograms (i.e. fuzzy membership
using Gaussian distributions)
f z    z  
1
N
 K z  z 
i
i


11.09.2003
Probability density decreases roughly exponentially
with increasing distance
Thus: random sampling from exponential density
distribution
Galaxy formation
21
S-Z scenario
 Interpretation
 Lack of abundance gradient




11.09.2003
Slow contraction of pressure supported galaxy 
abundance gradient (for mean metal abundance as well
as range of abundance)  ruled out
Free falling collapsing gas  clusters with all
abundances 0<z<zf will occur, kinematics independent of
abundance.
ELS concluded that stars within this abundance range
were formed in this free falling case.
However, every theory were kinematical properties are
uncorrelated with abundance could be possible, e.g.
forming of small protogalaxies and subsequent
merging to form galactic halo
Galaxy formation
22
S-Z scenario

Second parameter

Diversity of colour distribution (for a fixed Fe/H
ratio) could be explained by:






Assuming same age leads to unknown
mechanism
 age spread as plausible explanation
Thus:


11.09.2003
Age spread (109 yrs)
Spread in helium abundance
Spread in C,N,O abundance
Loosely bound clusters  large age spread
Tightly bound clusters  small age spread
Galaxy formation
23
S-Z scenario





11.09.2003
Collapse of central region rapidly (108) yrs
Collapse of outer fringes over longer period of
time (>109 yrs)  remain in loosely bound outer
halo
Gas fallen from distances > 100kpc
Dissipation needed (before cluster formation)
since apogalactical distances of clusters are
today smaller than 100kpc
E.g. by collisions of the infalling gas flows
Galaxy formation
24
S-Z scenario

Abundance distribution

Simple model: homogeneous, closed system,
without stars at beginning, converting gas into
metals with a fixed yield





11.09.2003
Limiting case: small evolution (large amount of gas
left)  no fit
Limiting case: complete evolution (no gas left) 
good fit
However, picture could only explain elliptical galaxies
but no spirals, otherwise no star formation today
In spirals: need temporary removal of gas from star
formation process
 assumption of closed, homogenous model
inappropriate
Galaxy formation
25
S-Z scenario

Hierarchical Model



11.09.2003
Halo formation = merging of number of subsystems
Subsystems = similar to very small, irregular, gasrich galaxies
Stochastic model (Searl ’77):

Each fragment makes a few clusters

Suddenly looses gas: supernova explosion,
sweeping though galactic plane

Alternatively gradually loosing gas (better fit)
Galaxy formation
26
S-Z scenario
 Summary:
 No isolated, uniform, homogeneous, collapsing galaxy,
rather more “chaotic” origin
 Collapse of central region
 Some time later gas from outer regions fell into the
galaxy and dissipated much of its kinetic energy
 Transient high-density protogalactic regions, forming
outer halo stars and clusters
 These regions underwent chemical evolution and
reached dynamical equilibrium with galaxy
 Gas lost from this protogalactic regions swept into disk
11.09.2003
Galaxy formation
27
Massive galaxies
 Techniques:
 So far using kinematics and evolutionary properties of
individual stars
 Now, high redshift surveys
 Scenarios
 “Monolithic collapse”
 Violent burst of star formation
 Followed by passive evolution of luminosity (PLE)
 Predictions:




11.09.2003
Conserved comoving number density of massive spheroids
Massive galaxies evolve only in luminosity
Such systems should exist at least up to z>1.5
Progenitor systems (z>2-3) with high star formation, gas
Galaxy formation
28
Massive galaxies

Hierarchical merging
 Moderate star formation
 Reaching final masses in more recent epoches (z<1)
 Predictions:


massive systems very rare for z>1
Comoving number density of massive galaxies (> 1011
solar masses) decreases for higher z
 First possibility: search for starburst progenitors
 Second possibility: search for passively evolving
spheroids up to highes z possible
 Believed so far: most cluster ellipticals form at high
redshift, but less known about field spheroidals
11.09.2003
Galaxy formation
29
Massive galaxies
 Various surveys made suggest:
 Field ellipticals do not form a homogeneous population,
some consistent with PLE others not.
 K-band survey:
 select galaxies according to their masses (not to star
formation activity)
 Larger sample of galaxies
 Covering two independent fields
 Combining spectroscopic and photometric redshift
measurements
11.09.2003
Galaxy formation
30
Massive galaxies
 Results
 Redshift distribution has a median redshift of
0.8 and a high-z tail beyond z=2.



Mild Evolution of Luminosity function (LF)


11.09.2003
Current models of hierarchical merging do not
match median redshift (to low), underpredict
number of galaxies at z>1.5
Current PLE predictions are consistent with the
data
Hierarchical models fails: different shape of the
LF , predict substantial evolution
PLE models are in good agreement
Galaxy formation
31
Massive galaxies

Observations of EROs (Extremely Red
Objects) imply:



Anticorrelation:


11.09.2003
massive spheroid formed at z>2.4 and were fully
evolved at z=1, consistent with PLE predictions
Hierarchical models underpredict the number of
EROs
Most massive galaxies being old, low-mass
galaxies dominated by young stellar population
Opposite than expected in hierarchical merging
models
Galaxy formation
32
Summary
 Two paradigms:
 Cosmological model can favour one or the other
 “monolithic collapse”:
 smallest fluctuations are on galaxy scale
 probably not the way our own galaxy evolved
 Driven by gravitation instability
 Slow collapse vs. free falling
 Hierarchical merging:
 Strong Fluctuations on dwarf galaxy scales
 Subsequent merging of small protogalaxies
 New measurements from massive ellipticals may
revive the “old-fashioned” top-down model in a certain
parameter context.
11.09.2003
Galaxy formation
33
Literature


•
•
•
Observing the epoch of galaxy formation,
Charles C. Steidel,
http://www.pnas.org/cgi/content/full/96/8/4232#B4
Evidence from the motions of old stars that the galaxy collapsed, Eggen, O.J.,
Lynden-Bell, D., Sandage, A.R.,
Astrophysical Journal 136, 748 (1962)
Composition of Halo clusters and the formation of the galactic Halo
Searle, L., Zinn, R.
ApJ 225, 357, (1978)
The Formation and Evolution of Galaxies Within Merging Dark Matter Haloes
Kauffmann, G.; White, S. D. M.; Guiderdoni, B.
R.A.S. MONTHLY NOTICES V.264, NO. 1/SEP1, P. 201, 1993
The formation and evolution of field massive galaxies
Cimatti, A.
http://xxx.lanl.gov/abs/astro-ph/0303023
11.09.2003
Galaxy formation
34