Galaxy Classification, AGN, and Galaxy Evolution
This description matches an elliptical galaxy (likely E0-E3 given the roughly spherical shape): the absence of cold gas and the uniformly old, red stellar population are exactly the properties associated with ellipticals in Lecture 11, since without gas there is no raw material for new (blue, young) star formation.
| Criterion | Points |
|---|---|
| Correct classification (elliptical) | 8 |
| Correct physical justification (gas content/age link) | 7 |
M_BH ∝ σ^4, so M_BH,A/M_BH,B = (σ_A/σ_B)^4 = 2^4 = 16.
| Criterion | Points |
|---|---|
| Correct exponent application | 8 |
| Correct final ratio | 7 |
A 1-day coherent variation implies an emitting region no larger than about 1 light-day ≈ 173 AU across — smaller than the orbit of Neptune (about 30 AU) is not required, but the region is still remarkably compact (order of the outer solar system's scale) for a source that can outshine an entire galaxy of hundreds of billions of stars.
| Criterion | Points |
|---|---|
| Correct light-travel-time reasoning | 8 |
| Correct AU conversion and scale comparison | 7 |
Full-credit answers should explain: the unified model attributes different observed AGN classes to the same basic structure (black hole, accretion disk, dusty torus, sometimes jets) viewed from different angles (a face-on view can show the luminous accretion disk directly, e.g. quasars/Seyfert 1, while an edge-on view is obscured by the torus, e.g. Seyfert 2) and at different accretion rates and jet strengths (radio galaxies/blazars for strong jets viewed at different angles).
| Criterion | Points |
|---|---|
| Correct viewing-angle explanation | 8 |
| Correct accretion-rate/jet explanation | 7 |
Award full marks for correct method, units, and a numeric result consistent with the arithmetic shown (allow reasonable rounding). Verify the student re-derives the number rather than quoting it from the lecture without adapting it to the problem's specific inputs.