General Outlook on Galaxies  Basic concepts  Important ideas about galactic structureBasic Concepts  Galaxies are the “ultimate synthesis” of astronomy • Look down to smaller structures: stars, gas, atoms, energetic particles, BHs • Look up to larger structures: clusters, superclusters, cosmology • Conclude: no subject for specialists!  Galaxies exhibit regularity of structures Scaling laws: T vs M, L vs M, R vs M, colors vs M, Z vs M • Stars wipe out all signs of their initial conditions – they obey rules given by their present state only  makes studies of stellar structure “easy”, but studying star formation is very difficult • Galaxies have preserved evidence of their initial conditions: scaling laws. Once you decode these laws, galaxy formation becomes “easy”…  Galaxies are “living structures”: they can change and they are still changing now • Moving, colliding, merging, harassment, ram-pressure stripping… • Must get used to the idea that we are looking at a process, not an end-resultGalaxy InteractionsGalaxy InteractionsGalaxy InteractionsGalaxy MergersGalaxy Interactions in the Distant UniverseImportant Ideas about Galactic Structure  Galaxies are two-part structures (visible portion) • Disks: flattened, near-circular rotation • Spheroids: ellipsoidal, slow rotation, eccentric orbits • Relative amount of two components varies – but internally the structure of each component shows remarkable regularity from galaxy to galaxyImportant Ideas about Galactic Structure  Galaxies consist of two kinds of matter • Baryonic (mostly luminous) & Non-baryonic (dark) • Iceberg effect: ~90% is hidden (dark) • Profound consequences for our picture of galaxy formation and evolution!  Two kinds of matter are separated radially • Condensed baryons, diffuse dark-matter halo • How did this separation occur?  Thermal radiation by baryons (dissipation)  radial collapse# DM is non-interacting, dissipationless material  can’t collapse  So, baryons once filled a greater volume than now • Collapse is accompanied by star formation and build-up of heavy elements • Positions and motions of stars contain some information about where and when they were born • Conclude: there are correlations between positions, motions, and composition of stars that contain a sort of code about galaxy formation and history • Our job is to analyze and decode these!Important Ideas about Galactic Structure  Supermassive (> 105 Msun ) black holes in the centers of galaxies • All galaxies with a spheroid contain a SMBH • Correlation between spheroid mass (velocity dispersion) and BH mass • How does the spheroid (>kpc) know about the BH (AUs) and vice-versa?Morphological Classification  Basic aims of a classification scheme • Complete: every galaxy is included under the scheme • Unambiguous: no conflicting criteria; no question as to where each galaxy belongs • Illuminate the evolutionary processes: hope that an underlying theory will emerge (e.g., stellar structure) • No classification system today really satisfies these requirements. There are particular problems in each category:  Peculiar galaxies: when are they physically important and when are they merely interesting side-shows?  Ambiguities: e.g. bulge-disk ratio vs appearance of arms and disk (e.g., NGC 4932 – smooth arms but no large central bulge)  Physical insight: as yet, relatively little… Hubble Classification (1936 – 1st version; “Tuning-fork diagram”) • Primary discriminant: small-scale light distribution (i.e. “lumpiness”) • En – S0: where n = 10 [1 – b/a] where b/a = apparent axial ratio • Spirals: Sa  Sc Along this sequence, arms become more prominent and galaxy looks lumpier due to HII regions • Hubble sequence is basically a sequence of star formation rate (per stellar mass) • Secondary discrimant: relative importance of bulge (spheroid) and disk • This is the difference between E’s and S0’s • Plays a role in S’s as well • Role of S0’s: a true transition class, not a parallel class • Irr. II and S0 pec.: a wastebasket for strange objects De Vaucouleurs’ Classification (1959) • What it does: • Improve resolution for late-type systems (Sd, Sdm, Sm, Im) and S0’s (S0-, S00, S0+) • Add interesting but not very fundamental detail about internal and external rings • Fundamental philosophy is the same as Hubble’s, otherwise • Morphological type T: -5 = E, -2 = S00, 0 = S0/a, 1 = Sa, 3 = Sb, 5 = Sc, 7 = Sd, 10 = Im Classification summary (Fig. 1.11 S&G) Using: L* ≅ 8 x 109 h-2 Lsun ≅ LMW (where h = H0/100 ≈ 0.75) CLASS LUMINOSITY “V/σ” cD ~10 L* < 1 E (0.05) – 10 L* < 1 Sa – Sc 1 – 5 L* > 1 Sd – Sm 0.1 – 2 L* > 1 dE 0.002 – (0.05) L* < 1 dSph < 0.002 L* < 1 (lower stellar density than dE) dIm < 0.01 L* ~ 1 (often forming new stars)Groups & Clusters  Effects of the environment on galaxy classification • Galaxy mergers in groups • Galaxy harassment in clusters • Morphological “segregation” • Ram-pressure stripping • Falling rotation curve? • Butcher-Oemler effect at 0.4 < z < 1 • …Local Group  Bound group of about 36+ galaxies to which the MW belongs  Size: ~ 1 Mpc in radius  Members: • 3 spirals: MW, M31, M33 (~90% of visible light of LG) • 1 elliptical: M32 (companion of M31) • 4 Irregulars: LMC, SMC (companions to MW), NGC 6822, IC 10 • 3 dE’s: NGC 205, NGC 147, NGC 185 (companions to M31)