Lesson 6 of 6 · galaxy groups and the cosmic web
Map the neighborhoods between galaxies
10 minute lesson · 3 checks along the way
Your goal for this lessonDistinguish bound galaxy systems from projected crowds and evaluate a survey’s completeness before comparing densities.
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Words with a dotted underline open a short definition right here in the lesson.
Move outward one physical level at a time
The Milky Way belongs to the , which includes Andromeda, Triangulum, and many smaller galaxies. A galaxy group is not a star cluster: its major members are whole galaxies. Rich galaxy clusters contain many more galaxies, together with hot diffuse gas and substantial gravitating mass that is not visible starlight. Membership is a physical question involving positions and motions, not simply which objects fit in one picture.
On larger scales, galaxies trace a network of filaments, sheets, and denser knots separated by lower-density . Matter also exists between the visible galaxies. A void is relatively underdense, not necessarily completely empty. can describe extended associations that are not all gravitationally bound into one object. The cosmic web is therefore not one enormous solid structure, and its connecting material need not glow like the colored strands in a visualization.
Check 1 of 3
10 pointsUse the reading above. You can try again without losing points.
Add depth before calling a crowd a neighborhood
A telescope image puts foreground stars, nearby galaxies, and much more distant galaxies onto the same apparent sky. The background objects in the Tadpole field illustrate this overlap. Their proximity on the page does not establish that they share its orbit or belong to its group. Spectra and distance estimates help distinguish true associations from chance projection.
Euclid combines wide-area imaging with distance-related information to investigate galaxy distributions and gravitational lensing. Cosmological redshift can help estimate distance within a stated expansion model, while complicate nearby redshift-based positions. An apparent stretched structure along the line of sight may partly reflect those velocities or measurement error. A reconstructed map must explain how sky positions and redshifts were converted into spatial coordinates.
The Tadpole field also records many background galaxies. Their placement in the same optical and near-infrared composite illustrates sky projection: a small separation in this picture need not mean a small physical separation. Display color alone does not assign a distance or group membership.
Look closely
Choose two small sources that appear close together. Does that make them members of one gravitationally bound group?
See what to notice
Angular proximity is only one coordinate constraint. Distance estimates and motion information are needed to test physical association; background objects can overlap on the sky while lying far apart in depth.
Work through a decision
Worked survey: correct counts before comparing density
Consider two fictional regions, each with volume 100 cubic , using the same defined galaxy population. Region A contains 80 detections with independently calibrated 50 percent . Region B contains 120 detections with 75 percent completeness. Assume negligible false detections and uniform completeness for that population in each region. The estimated counts are 80/0.50 = 160 and 120/0.75 = 160 galaxies.
Both estimated densities are therefore 160/100 = 1.6 galaxies per cubic megaparsec. Comparing raw counts would have made B appear 50 percent denser. The correction does not establish exact equality in nature: counting uncertainty and imperfect completeness estimates remain. Nor can a single correction factor repair a survey where detection probability changes unpredictably with galaxy size, surface brightness, color, or local crowding.
Compare the same population after selection
A fictional survey comparison with independently calibrated completeness for the same population, equal volumes, and negligible false detections. Dashed outlines are inferred count totals, not newly observed galaxies. Counting and completeness uncertainties still affect the estimates.
Open full-size diagram in a new tab: Compare the same population after selectionCheck 2 of 3
10 pointsUse the reading above. You can try again without losing points.
Distinguish a measured map from a model picture
A map made from measured galaxy positions is a data product; a simulated predicts structure under chosen assumptions. Both may use assigned colors and simplified symbols. Neither is automatically a photograph of all the matter. Lensing, gas emission, and absorption against background sources add complementary information about material that galaxy counts do not directly measure.
To test whether two surveys disagree, compare their covered volumes, sample definitions, detection limits, distance methods, and uncertainties. Then ask whether a physical difference remains after those effects are addressed. The goal is a connected explanation from individual galaxies to their surroundings. A convincing cosmic map tells you what each mark represents, which regions were missed, and how the reconstruction could be challenged by another measurement.
Check 3 of 3
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Put it together
Your lesson progress
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Go to the source
Explore the evidence behind this lesson. These optional readings and videos open in a new tab.
- Read: NASA: groups, clusters and larger structuresKeep galaxy clusters distinct from star clusters and bound objects distinct from larger associations.
- Read: ESA: the cosmic-web questions Euclid addressesRead why large-area imaging and redshifts serve different parts of the reconstruction.
- Read: NASA: a measured early galaxy filamentCompare the observational arrangement with its proposed later evolution.