Galaxy Study Guide

📖 Core Concepts Galaxy – A gravitationally bound system of stars, remnants, gas, dust, and dark matter. Dark Matter – Makes up 85 % of a galaxy’s total mass; inferred from flat rotation curves and velocity dispersions. Hubble Sequence – Visual classification into Ellipticals (E0‑E7), Spirals (Sa‑Sc, SB a‑c), Irregulars, plus special types (lenticular S0, dwarf, etc.). Density‑Wave Theory – Spiral arms are quasi‑stationary overdensities that trigger star formation as gas passes through. Active Galactic Nucleus (AGN) – Central supermassive black hole accreting matter; manifests as Seyfert, quasar, blazar, or radio galaxy depending on luminosity and viewing angle. Starburst Galaxy – Forms stars at rates > 10 M☉ yr⁻¹ (LIRGs) or > 100 M☉ yr⁻¹ (ULIRGs); usually merger‑driven. Large‑Scale Structure – Galaxies group → clusters → superclusters → cosmic web of filaments and voids. --- 📌 Must Remember Mass composition: 85 % dark matter, a few percent visible stars/gas. Elliptical sub‑classes: E0 = spherical, E7 = most elongated. Spiral sub‑classes: Sa (tight arms, large bulge) → Sc (loose arms, small bulge). Flat rotation curves → dark‑matter halo (V ≈ constant at large radii). Baryonic Tully–Fisher: \(M{\rm baryon} \propto V{\rm flat}^4\). Effective radius \(R{\rm e}\): radius enclosing 50 % of total light. Petrosian radius captures 100 % of light for exponential disks, 80 % for de Vaucouleurs profiles. LIRG luminosity threshold: \(L{\rm IR} > 10^{11}\,L{\odot}\) → SFR ≥ 18 M☉ yr⁻¹. ULIRG threshold: \(L{\rm IR} > 10^{12}\,L{\odot}\) → SFR ≥ 180 M☉ yr⁻¹. Milky Way: Barred spiral (SB bc) with a central SMBH of a few × 10⁶ M☉. Typical galaxy size: 1 kpc – 100 kpc (≈ 3 kly – 300 kly). --- 🔄 Key Processes Galaxy Formation (ΛCDM) Small dark‑matter overdensities collapse → halos. Gas falls into halos, shocks, then cools → first stars (Pop III). Starburst Trigger Interaction → tidal compression → gas inflow → rapid star formation. Merger → Morphology Change Major merger: two comparable‑mass disks → elliptical remnant + possible AGN. Minor merger (cannibalism): large galaxy absorbs dwarf, little structural change. AGN Fueling Bar or interaction drives gas inward → accretion disc → high‑energy emission. Spiral‑Arm Star Formation (Density Wave) Gas enters overdense wave → shock → collapse → OB stars light up arm. --- 🔍 Key Comparisons Elliptical vs Spiral Shape: Elliptical = ellipsoidal, same from any angle; Spiral = thin disk + bulge + arms. Gas/Dust: Elliptical = little, low SFR; Spiral = abundant, ongoing star formation. Stellar Orbits: Random (velocity dispersion) vs ordered rotation. Barred vs Unbarred Spiral Barred (SB) have a linear stellar bar; can channel gas to the nucleus. Unbarred (S) lack a bar; gas inflow relies on other mechanisms. LIRG vs ULIRG LIRG: \(10^{11}–10^{12}\,L{\odot}\), SFR ≈ 18–180 M☉ yr⁻¹. ULIRG: \(>10^{12}\,L{\odot}\), SFR > 180 M☉ yr⁻¹, often merger‑driven. Seyfert vs Quasar Seyfert: nucleus bright but host galaxy visible. Quasar: nucleus outshines host; seen at high redshift. --- ⚠️ Common Misunderstandings “All galaxies are spirals.” – Only 70 % of luminous galaxies are spirals; ellipticals dominate dense clusters. “Dark matter is visible in rotation curves only for spirals.” – Ellipticals show dark halos via stellar velocity dispersion and weak lensing. “Barred spirals are rare.” – Roughly half of all spirals host a bar. “AGN always require a merger.” – Secular processes (e.g., bar‑driven inflow) can also fuel AGN. --- 🧠 Mental Models / Intuition “Galaxy as a city”: Dark matter = city’s invisible foundation. Stars = citizens (old vs young districts). Gas/dust = roads and utilities; when traffic jams (interactions) occur, a construction boom (starburst) happens. Flat rotation curve = “speed limit” that stays high far from the downtown (visible stars), implying a hidden mass belt (halo). --- 🚩 Exceptions & Edge Cases Dwarf Ellipticals vs Dwarf Spheroidals: Both low‑mass, but spheroidals are often satellite galaxies with very low surface brightness. Flocculent vs Grand‑Design Spirals: Density‑wave theory best describes grand‑design arms; flocculent arms arise from local instabilities. LINERs: Weak AGN‑like emission; can also be powered by old stellar populations or shocks. --- 📍 When to Use Which Classify morphology: Use visual inspection → Hubble type (E, S, SB, Irr). Measure size: Large, well‑resolved galaxies: use effective radius \(R{\rm e}\). Survey data (SDSS): adopt Petrosian radius for consistent photometry. Near‑IR studies: apply 2MASS isophotal method (Kₛ‑band, 20 mag arcsec⁻²). Estimate dark‑matter content: Spirals: fit rotation curve (V ≈ constant → halo mass). Ellipticals: use stellar velocity dispersion + Jeans modeling or weak lensing. Identify AGN: Optical spectra: broad vs narrow lines → Seyfert 1/2. Radio/X‑ray: strong jets → radio galaxy or blazar (orientation dependent). --- 👀 Patterns to Recognize Flat rotation curve → dark halo (look for constant V beyond optical radius). Tidal tails + disturbed morphology → recent merger / starburst. Strong 21 cm emission but weak optical light → gas‑rich dwarf or irregular. High infrared luminosity + compact radio core → ULIRG with embedded AGN. --- 🗂️ Exam Traps “All elliptical galaxies are featureless.” – Ellipticals can host faint shells or dust lanes from past minor mergers. “Barred spirals always have higher star‑formation rates than unbarred.” – Bars can both enhance central SFR and deplete gas in the disk; net effect varies. “A flat rotation curve proves dark matter alone.” – Modified gravity theories exist, but dark matter is overwhelmingly supported by multiple lines of evidence (lensing, dispersion). “Quasars are always more distant than Seyferts.” – Luminosity, not distance, defines quasars; some low‑z objects qualify if nuclear output dominates. “All LIRGs are merger remnants.” – While many are, some are isolated, gas‑rich disks with unusually high SFRs. ---