Formation of Galaxies

1
Formation of Galaxies

• Robert Feldmann, Rovinj 2003

2
Outline

1. Introduction
2. ELS scenario
3. S-Z scenario
4. Massive elliptical galaxies
5. Summary
6. Literature

3
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

4
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

5
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

6
Introduction

• Star formation
• At redshifts zgt1 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

7
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

8
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 gt 10 times present diameter

9
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

10
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
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 ? 101 collapse radially,
  251 in z-direction

12
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

13
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

14
ELS scenario

• Questions?

15
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

16
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)

17
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 gt 8kpc

18
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
  dgt15kpc
• Contradiction with ELS measurements

19
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

20
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

21
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

22
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 0ltzltzf 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

23
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

24
S-Z scenario

• Collapse of central region rapidly (108) yrs
• Collapse of outer fringes over longer period of
  time (gt109 yrs) ? remain in loosely bound outer
  halo
• Gas fallen from distances gt 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

25
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

26
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)

27
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

28
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 zgt1.5
• Progenitor systems (zgt2-3) with high star
  formation, gas

29
Massive galaxies

• Hierarchical merging
• Moderate star formation
• Reaching final masses in more recent epoches
  (zlt1)
• Predictions
• massive systems very rare for zgt1
• Comoving number density of massive galaxies (gt
  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

30
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

31
Massive galaxies

• Results
• Redshift distribution has a median redshift of
  0.8 and a high-z tail beyond z2.
• Current models of hierarchical merging do not
  match median redshift (to low), underpredict
  number of galaxies at zgt1.5
• 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

32
Massive galaxies

• Observations of EROs (Extremely Red Objects)
  imply
• massive spheroid formed at zgt2.4 and were fully
  evolved at z1, 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

33
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.

34
Literature

• Observing the epoch of galaxy formation,
• Charles C. Steidel,
• http//www.pnas.org/cgi/content/full/96/8/4232B4
  
• 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 HaloSearle, 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