8. GALAXY FORMATION

Note: The origin and early evolution of fluctuations belongs in the Cosmology course. Some discussion of hierarchical clustering and the formation of large-scale structure is inevitable here, because the study of galaxy formation cannot be removed from its cosmological context. Basically, the aim here is to discuss galaxy formation starting at a time when individual objects are well separated from the expansion of the universe.

8.1 Introduction

• Eggen, Lynden-Bell, & Sandage (1962)
  
• Hydrodynamic collapse simulations
  
  

8.2 Galaxy Mergers:Theory

• Dynamical friction and orbital decay
  
• Chandrasekhar’s formula
  
• Implications:
  
  • Toomre’s hypothesis that all bulges and ellipticals are merger remnants
    
  • Galaxy formation must be studied in the context of large-scale structure formation
    
• Conservation of phase space density
  
  

8.3 Simulations of Hierarchical Clustering and Galaxy Formation

• Cold dark matter simulations
  
  • Properties of the galaxies that form
    
  • Comparison with observations:successes and failures
    
• Dissipationless simulations of group and binary mergers
  
  • Properties of the galaxies that form
    
  • Comparison with observations: successes and failures
    
• The need for dissipation
  
• Dissipational simulations of group and binary mergers
  
  • Properties of the galaxies that form
    
  • Comparison with observations: successes and failures
    
    

8.4 Observational Constraints on Galaxy Formation

• Observations of mergers in progress
  
  • Merger sequence from close binary galaxies --> violence --> remnants
    
• Starbursts: IRAS galaxies
  
• Possible connection between ultraluminous IRAS galaxies and quasars
  
• Recognizing old mergers: tails, shells, and other fine structure
  
• Recognizing completed mergers: embedded stellar disks
  
  • Photometric signatures
    
  • Kinematic signatures
    
• Small accretion events
  
  • Not every elliptical with fine structure needs to be a major merger remnant
    
    

8.5 Tentative Verdict: Mergers versus Dissipative Collapse

• Strengths and weaknesses of the merger picture
  
• Evidence from high-z galaxies
  
• Bottom line: both mergers and dissipative collapse were important
  
  

8.6 Physical Processes During Galaxy Formation:Origin of the Fundamental Plane

• Dissipation: cooling times and galactic scales
  
• Complications: energy input during starbursts, etc.
  
• Formation of ellipticals
  
• Rotation of disk galaxies
  
• Late infall of cold gas
  
• Ongoing problems
  
• Connections with observations at high z