Formation of Galaxies

Formation of Galaxies Robert Feldmann, Rovinj 2003

Outline • Introduction • ELS scenario • S-Z scenario • Massive elliptical galaxies • Summary • Literature Galaxy formation

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 Galaxy formation

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 Galaxy formation

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 Galaxy formation

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 Galaxy formation

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 Galaxy formation

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 Galaxy formation

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 Galaxy formation

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 Galaxy formation

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 Galaxy formation

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 Galaxy formation

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 Galaxy formation

ELS scenario • Questions? Galaxy formation

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 Galaxy formation

S-Z scenario • Methods: • low-resolution spectral flux distribution • Obtaining intrinsicspectrum 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) Galaxy formation

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 whatabout 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 Galaxy formation

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 Galaxy formation

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 Galaxy formation

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 Galaxy formation

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 Galaxy formation

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 Galaxy formation

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 Galaxy formation

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 Galaxy formation

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 Galaxy formation

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) Galaxy formation

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 Galaxy formation

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 Galaxy formation

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 Galaxy formation

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 Galaxy formation

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 Galaxy formation

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 Galaxy formation

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. Galaxy formation

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 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 Galaxy formation

Formation of Galaxies