1. Formation of Galaxies Robert Feldmann, Rovinj 2003

2. 11.09.2003 Galaxy formation2 Outline 1. Introduction 2. ELS scenario 3. S-Z scenario 4. Massive elliptical galaxies 5. Summary 6. Literature

3. 11.09.2003 Galaxy formation3 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. 11.09.2003 Galaxy formation4 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. 11.09.2003 Galaxy formation5 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. 11.09.2003 Galaxy formation6 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

7. 11.09.2003 Galaxy formation7 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. 11.09.2003 Galaxy formation8 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

9. 11.09.2003 Galaxy formation9 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. 11.09.2003 Galaxy formation10 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. 11.09.2003 Galaxy formation11 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

12. 11.09.2003 Galaxy formation12 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. 11.09.2003 Galaxy formation13 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. 11.09.2003 Galaxy formation14 ELS scenario  Questions?

15. 11.09.2003 Galaxy formation15 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. 11.09.2003 Galaxy formation16 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. 11.09.2003 Galaxy formation17 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

18. 11.09.2003 Galaxy formation18 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

19. 11.09.2003 Galaxy formation19 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. 11.09.2003 Galaxy formation20 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. 11.09.2003 Galaxy formation21 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. 11.09.2003 Galaxy formation22 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<z f 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. 11.09.2003 Galaxy formation23 S-Z scenario  Second parameter  Diversity of colour distribution (for a fixed Fe/H ratio) could be explained by:  Age spread (10 9 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. 11.09.2003 Galaxy formation24 S-Z scenario  Collapse of central region rapidly (10 8 ) yrs  Collapse of outer fringes over longer period of time (>10 9 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

25. 11.09.2003 Galaxy formation25 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. 11.09.2003 Galaxy formation26 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. 11.09.2003 Galaxy formation27 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. 11.09.2003 Galaxy formation28 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

29. 11.09.2003 Galaxy formation29 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 (> 10 11 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. 11.09.2003 Galaxy formation30 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. 11.09.2003 Galaxy formation31 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

32. 11.09.2003 Galaxy formation32 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

33. 11.09.2003 Galaxy formation33 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. 11.09.2003 Galaxy formation34 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