Class introduction

• Galaxies as crossroads of astronomy: stellar physics, gas physics, cosmology
• Galaxies are luminous tracers of large scale structure, act as cosmological probes
• Understanding galaxy formation one of the current largest problems in astronomy; almost all fields can be regarded as important for understanding galaxy formation!

Goals of course:

• Understand how galaxy properties are measured
• Review latest knowledge of observational properties of galaxies, including observations about both nearby galaxies and galaxies at higher redshift.
• Understand physical processes related to galaxy formation and/or evolution.
• Review basic concepts of modern theories of galaxy formation.
• (time permitting) Review of how galaxies are used as cosmological probes: classic cosmological tests as well as tests about structure formation in the Universe.

Class organization and assignments:

• basic order of presentation
• responsibilities of students
  • problems / projects
  • papers, papers, papers
  • exams
• web material/notes

Note content cannot be complete, and that ideas/data change with time! Comparison with ASTR615.

Observational history

• By eye, only three galaxies known: M31, LMC, and SMC.
• With telescopes, many more observed, but not distinguished from galactic nebulae:
  • Messier objects 1700's (32 of 103 in original catalog are galaxies; 39 of 106 in final catalog).
  • W. Herschel and his son John discovered thousands more in following decades - General Catalog in 1864, NGC in 1888.
• understanding of galaxies as extragalactic objects in 1920s
  • ``Great Debate'' between Curtis and Shapley about whether galaxies were located within or exterior to our own Galaxy;
  • resolved by Hubble's discoveries of Cepheids in M31.
• Most modern types of galaxies recognized by 1940-50, e.g. Hubble atlas and Hubble sequence. Speculation about Hubble sequence as evolutionary. Evolution mostly considered in isolation.
• Importance of dark matter recognized 1970-80's. Importance of environment recognized.

What are galaxies made of?

• Stars:
  • Observed properties of stars depend primarily on mass, age, composition
  • Galaxy have stars with multiple masses, ages, compositions, hence luminosities and colors
• ISM:
  • multiple phases (molecular, atomic, ionized)
• dark matter:
  • possibly, both baryonic and non-baryonic).
  • dominates mass of galaxies

How are galaxies formed?

General current picture of galaxy formation is that of gravitational instability in an expanding universe. In this picture, dark matter plays a crucial role as the dominant mass component. Gravitational instability picture:

• collapse: different scales collapse at different time.
  • collapse time depends on overdensity
  • overdensity on different size scales depends on relative size of initial perturbations and physics that affect their growth
  • hierarchical cluster vs. fragmentation vs. near-simultaneous collapse of all scales
• merging of dark matter halos,
• gas cools inside of dark matter halos: dissiipational collapse
• stars form from gas; black holes form?
• stars/AGN eject energy back into gas: feedback,
• interactions, both dissipationless (dark matter and stellar) and dissipational, continue

Galaxies evolve, both internally and in number.

• evolution from formation processes: merging, gas supply, etc.
• internal evolution: stellar evolution/formation, dynamics

Basic questions about galaxy formation - when do each of these steps happen and what are their relative importances? some issues:

• what sets the masses of galaxies? sizes? luminosities?
• what sets the distributions of number of galaxies as a function of mass/luminosity?
• does the ratio of baryonic mass/total mass change for different galaxies?
• what triggers star formation in galaxies?
• what is responsible for the range of galaxy morphology?
• how much of present structure is determined by initial conditions, e.g. initial overdensity, angular momentum (and what are initial conditions?),
• how much does present appearance depend on basic physics within galaxies, e.g. dynamics and chemical evolution,
• how much depends on environment, e.g. mergers and interactions, background radiation.
• what is the sequence of the different processes involved in galaxy formation?

Initial conditions/internal processes vs. environment (c.f. stars whose nature are determined by internal basic physics.) Possible/likely that relative importance of these effects is different for different galaxies.

Three approaches to study of galaxy formation/evolution:

• theories based on observations of present-day galaxies,
• theories based on observations of high redshift galaxies,
• theories based on understanding of cosmology and formation of structure.

At current time, these are being put together.

What limits the size of galaxies?

• Note that galaxies appear to have a maximum size of order $M\sim10^{11}-10^{12}$, even though there are larger agglomerations of matter, e.g., clusters
• Can be crudely explained by understanding of dissipation
• Self-gravitating cloud has two timescales:
  • dynamical or free-fall time, $t_{dyn}\sim (G\rho)^{-1/2}$
  • cooling time, $t_{cool}\sim kT_g/\Lambda(T)$, where $\Lambda$ is the cooling function.
• If $t_{cool}>t_{dyn}$, then a cloud can be in quasi-static equilibrium, i.e. cooling is unimportant
• If $t_{cool}<t_{dyn}$, cloud cools, kinetic energy is coverted to radiation, and the cloud collapses.
• Given a cooling curve for primordial composition, one can calculate the relevant timescales, and finds that collapse is unlikely to occur for $M>10^{12}Msun$ - giving our scale for galaxies. This implies that dissipation really is important, at least for objects that we observe as galaxies (i.e., luminous objects)!