formation of galaxies
DESCRIPTION
Formation of Galaxies. Robert Feldmann, Rovinj 2003. Outline. Introduction ELS scenario S-Z scenario Massive elliptical galaxies Summary Literature. Introduction. Investigation of the history of galaxies First approach: Chemical content Kinematics Spatial distribution - PowerPoint PPT PresentationTRANSCRIPT
Formation of Galaxies
Robert Feldmann, Rovinj 2003
11.09.2003 Galaxy formation 2
Outline
1. Introduction
2. ELS scenario
3. S-Z scenario
4. Massive elliptical galaxies
5. Summary
6. Literature
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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
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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
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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
Star formation Energy dissipation Metal enrichment
Main point: Relate underlying dark matter to observed baryonic matter
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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
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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
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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
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ELS scenario
Stellar dynamics: General potentials Nearly decoupling of motions in plane and
perpendicular In contracting galaxy
Assuming: axial symmetry Masses with greatly differing angular momenta
do not exchange momenta Thus, each matter element will conserve its
angular momentum
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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
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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
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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
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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
gas settles down in circular orbits first stars remain on their highly eccentric orbits
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ELS scenario
Questions?
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S-Z scenario
L.Searle, R. Zinn 1978 Bottom-Up scenario Precise abundance measurements Observing red giants, reddening-independent
characteristics Measuring correlations of
Abundance with distance Abundance with colour distribution Abundance distribution in the outer halo
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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
abundance ranking Comparison with other spectroscopic measurements (Butler)
shows good agreement Homogenous metal abundance within each cluster (Fig 7)
i
ii
ii wQwS
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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
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S-Z scenario
Is there a abundance gradient in the outer halo? 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
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S-Z scenario
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
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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
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S-Z scenario
The abundance distribution in the outer halo Using generalized histograms (i.e. fuzzy membership
using Gaussian distributions)
Probability density decreases roughly exponentially with increasing distance
Thus: random sampling from exponential density distribution
i
iN zzKzzf 1
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S-Z scenario
Interpretation Lack of abundance gradient
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
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S-Z scenario
Second parameter Diversity of colour distribution (for a fixed Fe/H
ratio) could be explained by: Age spread (109 yrs) Spread in helium abundance Spread in C,N,O abundance
Assuming same age leads to unknown mechanism
age spread as plausible explanation Thus:
Loosely bound clusters large age spread Tightly bound clusters small age spread
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S-Z scenario
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
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S-Z scenario
Abundance distribution Simple model: homogeneous, closed system,
without stars at beginning, converting gas into metals with a fixed yield
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
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S-Z scenario
Hierarchical Model Halo formation = merging of number of subsystems Subsystems = similar to very small, irregular, gas-
rich 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)
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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
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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:
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
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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
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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
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Massive galaxies
Results Redshift distribution has a median redshift of
0.8 and a high-z tail beyond z=2. 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
Mild Evolution of Luminosity function (LF) Hierarchical models fails: different shape of the
LF , predict substantial evolution PLE models are in good agreement
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Massive galaxies
Observations of EROs (Extremely Red Objects) imply:
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
Anticorrelation: Most massive galaxies being old, low-mass
galaxies dominated by young stellar population Opposite than expected in hierarchical merging
models
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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.
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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 HaloesKauffmann, 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 galaxiesCimatti, A.
http://xxx.lanl.gov/abs/astro-ph/0303023