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1 Introduction Astr 555 Class Notes { Fall 2016 1 1 Introduction • What are galaxies? What are they composed of? { Galaxies: gravitationally bound collections of stars, gas/dust, dark matter, black holes • Galaxies as crossroads of astronomy: stellar physics, gas physics, cosmology • Understanding galaxy formation one of the current largest problems in astron- omy; almost all fields can be regarded as important for understanding galaxy formation! • Galaxies are luminous tracers of large scale structure, act as cosmological probes Goals of course: • Understand general current picture of galaxies and galaxy evolution • Basic understanding of how galaxies are observed • Review statistical properties of the galaxy population, and specific properties of different types of galaxies • Understand physical tools by which we learn and characterize different compo- nents of galaxies • If time permits: what we can learn from the Milky Way Note content cannot be complete, and that ideas/data change with time! Note place of ASTR555 in curriculum: ASTR555 to provide basic overview of galaxies and their components, ASTR616 to focus on galaxy formation and evolution. Class organization and assignments: • web page accessible through Canvas • basic order of presentation : observing galaxies, galaxy properties and charac- teristics, galaxies toolkit: stars, gas, dust, central black holes/galactic nucleii, dark matter/masses • web material/notes • books/other resources • responsibilities of students { Students will think about/review material covered in each class, and ask questions either during presentation or at the beginning of the following class { Formal assignments Astr 555 Class Notes { Fall 2016 2 ∗ reading / questions ∗ problems / projects ∗ exams Note that the main content of the course is a survey of material. Basic compre- hension of physical properties of the galaxy population and how we learn about this is the primary goal. Don't only focus on equations! 1.1 History (see also Whittle notes) • By eye, only three galaxies known: M31, LMC, and SMC. • With telescopes, many more observed, but not distinguished from galactic neb- ulae: { Messier objects 1700's (32 of 103 in original catalog are galaxies; 39 of 110 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 (related question: size of the Milky Way); { resolved by Hubble's discoveries of Cepheids in M31. • 1920's-30's: measurements of galaxy redshifts by Slipher; Hubble's law • Most modern types of galaxies recognized by 1940-50, e.g. Hubble atlas and Hubble sequence. Speculation about Hubble sequence as evolutionary. Evolu- tion mostly considered in isolation. • 1950's-60's: galaxies put into cosmological context: discovery of microwave background and the Hot Big Bang model. Galaxy formation considerations: Eggen, Lynden-Bell, Sandage (\monolithic collapse") • 1970's-80's: More careful/quantitative consideration of galaxy evolution as a result of stellar evolution and populations (Tinsley et al., "population syn- thesis"). Galaxy formation considerations: Searle and Zinn (\formation from fragments"). • 1970's-80's Importance of dark matter recognized (predominantly from spiral rotation curves) (but note earlier suggestions by Zwicky!). Large scale struc- ture (among other things) suggests CDM (cold dark matter) {> more power at smaller scales {> merging is important (but now recognized that full idea of hierarchical clustering, i.e. small to large, is an oversimplification) Astr 555 Class Notes { Fall 2016 3 • 1980's: Importance of environment recognized: morphology-density relation; \nature vs. nurture". Inflation. First large scale surveys, both nearby (e.g., CfA) and at medium (z <= 1)redshift: large-scale structure in the Universe. • 1990's: Techniques for finding and confirming high redshift galaxies (z>2, i.e. at significant lookback time): Lyman break galaxies. HST imaging of distant galaxies (e.g. HDF). COBE and anisotropy in the microwave background. N- body (dark matter) simulations (e.g. Millenium simulation). • 2000's: Precision cosmology and ΛCDM (type Ia supernovae (e.g. here) and WMAP (image and power spectrum). Multiwavelength observations of galax- ies, in particular, IR (Spitzer) and submm (JCMT). Larger scale (e.g., SDSS) and deeper spectroscopic surveys: quantitative/statistical description of galaxy distribution (e.g., to z=0.15, to z=0.6, to z=5) and population (e.g. SDSS cor- relations) Very high redshift galaxies and epoch of reionization. Recognition of importance of central black holes, through measurement of galaxy masses and stellar masses. Simulations with gas physics (and star formation prescriptions), e.g. Illustris. • 2010's: Extended gas halos in galaxies; the baryon cycle and connections with IGM/CGM. Integral field spectroscopy of galaxies (e.g., CALIFA, SDSS/MaNGA, MUSE). ultra diffuse galaxies (UDGs, e.g., in Coma). GAIA and big Milky Way spectroscopic surveys (SDSS/APOGEE, GAIA-ESO, GALAH). • beyond: Dark matter detection?? Dark energy?? Understand qualitatively, i.e., be able to describe in a few sentences, what all of these developments were. 1.2 Approaches Multifaceted approach to studying galaxy formation and evolution: • Galaxy "archaeology": study nearby galaxies in detail, attempt to understand processes that led to their current appearance. { Advantages: can resolve structure, individual stars in very nearest galaxies, high S/N observations { Disadvantages: some information may be erased by physical processes (e.g., merging), degeneracies in integrated light • Distant galaxies: look at galaxy samples at different lookback times, study distribution of properties (galaxy population) as a function of time { Advantages: direct probe of different stages. Lookback time-redshift rela- tion Astr 555 Class Notes { Fall 2016 4 { Disadvantages: brightness/selection effects, lack of detail, difficulty in as- sociating objects at one redshift to those at another • Physics of galaxy formation { Advantages: some physics (e.g., gravity) is well understood { Disadvantages: some physics (e.g., star formation, feedback) is not! Dy- namic range of the problem is huge ∗ Dynamic range in distances, from stellar scales to largest scale struc- ture ∗ Dynamic range in mass, from stellar scales to largest scale structure Know what the three approaches to studying galaxies are, and be able to describe advantages and challenges of each approach. 1.3 Overview of galaxies and galaxy formation Cosmological context of galaxy formation and evolution: • Baryonic (and some leptons!) matter, non-baryonic matter, dark energy • Current composition of the Universe • Past composition of the Universe Several main components of galaxies: • dark matter: { usually refers to non-baryonic matter, but note that some baryonic matter can be hard to see! { dominates mass of galaxies { currently detectable only by its gravitational effect on motions, or back- ground light (lensing) • Stars: { Observed properties of stars depend primarily on mass, age, composition { Galaxy have stars with multiple masses, ages, compositions, hence lumi- nosities and colors • gas/dust : ISM but also CGM, IGM { gas: multiple phases (molecular, atomic, ionized) { observed properties depends on density, temperature, composition: low density gas can be hard to detect! Astr 555 Class Notes { Fall 2016 5 { dust : observe via emission and absorption • central black holes { observed indirectly via gravitational effects, accretion Goal is to understand how galaxies come to appear as they do. Some of the impor- tant processes in galaxy formation (not necessarily a unique chronological sequence!): • Gravitational collapse (of dark matter, and later, baryons) in cosmological framework { How big are initial lumps at different size scales? { How much angular momentum? { How fast do lumps grow? • Condensation of gas and cooling { "Hot" vs "cold" accretion • Star formation { Under what conditions do stars form? { What is, and what drives, efficiency of star formation? { What types (masses) of stars form? { Drives chemical evolution, which in turn may impact cooling and star formation • Black hole formation { Primordial formation vs. formation from early stars { How common? • Feedback / mass loss { How much energy? Mechanical or thermal? Does mass escape or just delay accretion? { What objects generate it? Winds, supernovae, galactic nucleii? How? • Continued accretion from IGM { How much? { What mode? { What composition? • Merging Astr 555 Class Notes { Fall 2016 6 { halo merging; dynamical friction { galaxy merging: Gas-rich (\wet") vs gas-poor (\dry") merging • Other environmental processes { Cluster (group?) environment: ram pressure, tides • Dynamical evolution { Dynamical instabilities, e.g. bars and spiral arms { Migration { Internal vs. external triggers See a schematic flowchart and some of the complicated links from Mo et al. Each process has characteristic scales, and the relation between these scales may influence how galaxies form, evolve, and appear. • Hubble time −1 { t / H0 • cosmological densities { critical density today 3H2 ρ = 0 ∼ 10−29gm=cm3 ∼ 5:5−6atoms=cm3 crit 8πG { critical density scales (increases) with redshift { \collapsed" halo hasρ ¯ =∼ 200ρcrit • gravitational timescales: orbital time / free-fall time { orbital timescale 3π !1=2 t = orb Gρ¯ { freefall timescale:
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