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📘 How do galaxies assemble over time?

Morphology, dark matter, and hierarchical assembly—how galaxies grow from gas, stars, and mergers.

13
lessons
~30 min
to learn
Adults
level
Start the course →

What you’ll learn

  1. Morphological Classification at High ResolutionMaster quantitative morphological diagnostics including CAS, Gini-M20, and non-parametric indices for distinguishing merger remnants from secularly evolved systems.Advanced imaging reveals limitations of classical Hubble types. Quantitative metrics quantify bulge-disk decompositions and tidal features. These tools connect observed structure to simulated merger trees.
  2. Redshift Surveys and Cosmic VarianceQuantify selection functions, fiber collisions, and large-scale structure variance in flux-limited redshift surveys.Modern catalogs map the local universe but require careful modeling of incompleteness. Variance on scales above 100 Mpc affects luminosity function estimates. Students learn to apply survey masks and mock catalogs.
  3. Rotation Curves and Dark Matter ProfilesDerive mass models from rotation curves, decompose baryonic and dark components, and evaluate cusp-core and too-big-to-fail problems.Kinematic data constrain halo density slopes. Baryonic feedback can flatten cusps but not universally. Edge cases include ultra-diffuse galaxies and low-surface-brightness systems. Also covers Stellar Populations and Chemical Gradients: Chemical tagging distinguishes in-situ and accreted stars. Radial migration alters observed gradients. Models must reconcile local and high-redshift abundance patterns.
  4. Supermassive Black Holes and AGN FeedbackEvaluate black-hole scaling relations, mechanical and radiative feedback efficiencies, and quenching thresholds across halo mass.AGN regulate star formation via outflows and heating. Kinetic-mode dominance in massive halos explains the red sequence. Uncertainties remain in seeding and duty cycles.
  5. Hierarchical Assembly in LCDMCompare observed merger fractions, stellar mass assembly histories, and ex-situ fractions against hydrodynamical simulations.Mergers build bulges and halos but feedback shapes the stellar mass function. Tension persists between predicted and observed satellite populations. Students assess resolution and subgrid physics limits.
  6. Galaxy Interactions and Triggered StarburstsQuantify merger-driven star formation efficiency, gas inflows, and morphological disturbance timescales using hydrodynamic merger simulations.Close passages drive gas to centers and elevate SFRs. Post-merger quenching depends on AGN coupling. Statistics from pair counts constrain the merger contribution to the cosmic SFR density.
  7. Dwarf Galaxies and Satellite PopulationsAnalyze luminosity functions, quenching timescales, and dark-matter substructure constraints from Local Group and field dwarfs.Dwarfs test small-scale LCDM predictions. Baryonic processes suppress star formation below 10^8 solar masses. Ultra-faint systems constrain warm dark matter and reionization timing.
  8. Environmental Effects in ClustersDistinguish ram-pressure, harassment, and starvation effects on galaxy properties across cluster-centric radii and infall times.Dense environments accelerate quenching and morphological transformation. Pre-processing in groups precedes cluster arrival. Hydrodynamic simulations quantify gas-loss rates.
  9. High-Redshift Galaxies and ReionizationInterpret UV luminosity functions, escape fractions, and ionizing-photon budgets from z>6 observations.Early massive galaxies challenge standard feedback prescriptions. Escape-fraction measurements remain uncertain. Students evaluate contributions from faint versus bright populations.
  10. Multi-Wavelength DiagnosticsCombine UV, optical, IR, radio, and X-ray indicators to derive complete star-formation and accretion histories.Dust and AGN contamination require multi-band corrections. SED fitting uncertainties grow at high redshift. Students apply energy-balance methods and assess systematic errors.
  11. Baryonic Physics in SimulationsCritique subgrid prescriptions for star formation, stellar feedback, and black-hole growth against observational constraints.Resolution limits and tunable parameters affect galaxy properties. Convergence tests reveal persistent uncertainties in outflow mass loading. Future simulations target higher resolution and explicit physics.
  12. Large-Scale Structure and Galaxy BiasModel halo occupation, assembly bias, and redshift-space distortions to extract cosmological parameters from galaxy clustering.Bias links galaxies to dark-matter halos. Assembly bias introduces secondary dependencies. Precision cosmology requires marginalizing over these effects.
  13. Open Questions and Next-Generation FacilitiesIdentify key observables that can distinguish between LCDM extensions and modified feedback prescriptions.Next surveys will tighten constraints on assembly histories and dark-matter properties. Synergies between wide-field imaging and high-resolution spectroscopy are essential. Students outline falsifiable predictions.

Questions this course answers

A CANDELS galaxy at z=1.2 has CAS values C=3.2 and A=0.42. Using the calibrated thresholds, what is the most likely classification?

A exceeds 0.35 while C falls below 4.0, placing the galaxy inside the major-merger region of CAS space.

Using the CANDELS-derived merger fraction trend, estimate the major-merger fraction at z=0.8 for galaxies above 10^10 solar masses.

The observed decline from 8-12 percent at z=1 to 2 percent at z=0 implies roughly 9 percent at z=0.8.

What fractional uncertainty in number density does cosmic variance add for L* galaxies in a single 8000 deg^{2} field at z=0.1?

The rms density fluctuation on the survey volume scale is 8-9 percent, which becomes the dominant error after Poisson noise is beaten down.

A new flux-limited survey at z=0.15 doubles the fiber collision radius to 110 arcsec. Which clustering statistic is most severely biased?

Larger exclusion zones remove more close pairs, directly suppressing the one-halo term while leaving larger-scale statistics almost unchanged.

Using the flat rotation velocity of 150 km/s at 30 kpc in NGC 3198, estimate the enclosed total mass in units of 10^11 solar masses.

M = V²r/G evaluates to approximately 3.1 × 10^11 solar masses at that radius, confirming the halo dominates.

A newly discovered ultra-diffuse galaxy at 10^10 solar masses shows a flat rotation curve with a 2 kpc core. In one sentence, explain why supernova feedback is unlikely to be the sole cause of this core.

At halo masses around 10^10 solar masses the energy available from supernovae is insufficient to drive the rapid, repeated outflows required to transform a cusp into a large core.

Grounded in trusted sources

  • NASA
  • European Southern Observatory
  • Sloan Digital Sky Survey
  • NASA / Hubble — galaxies
  • European Southern Observatory — extra-galactic surveys
  • Sloan Digital Sky Survey — galaxy maps

Every Wunder lesson is built from real, reputable sources — never invented.

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