Formation of Galaxies

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


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


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


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


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


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


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


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


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


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


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


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


ELS scenario

Questions?


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


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


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


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


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


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


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


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


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


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


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


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)


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


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


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


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


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


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


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.


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

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Formation of Galaxies