Dark matter, the invisible cosmic mystery, plays a crucial role in shaping galaxies and clusters. Evidence for its existence comes from unexpected rotation curves, gravitational lensing effects, and velocity dispersions that can't be explained by visible matter alone.
The discrepancy between visible and dynamical mass in cosmic structures is a key indicator of dark matter's presence. This unseen substance forms massive halos around galaxies and clusters, profoundly influencing their formation and evolution.
Evidence for Dark Matter in Galaxies
Rotation curves and dark matter
- Rotation curves map orbital velocities of stars and gas vs distance from galactic center
- Expect velocities to decrease with distance (Kepler's laws)
- Observe velocities remain constant or increase with distance
- Flat rotation curves imply extra, unseen mass beyond visible galaxy disk
- Extra mass attributed to dark matter halos around galaxies
- "Galaxy rotation problem" = discrepancy between expected and observed rotation curves
- Dark matter provides extra gravity to explain flat rotation curves
Gravitational lensing in galaxy clusters
- Gravitational lensing bends light path from distant objects by massive foreground objects (galaxy clusters)
- Strong lensing creates multiple images, arcs, Einstein rings of background source
- Weak lensing causes small distortions in shapes of background galaxies
- Gravitational lensing in galaxy clusters much higher than expected from visible matter alone
- Suggests extra, unseen mass in clusters
- Gravitational lensing mass maps show dark matter more widely distributed than visible matter in clusters
- Dark matter forms large, diffuse halos beyond visible boundaries of galaxies and clusters
Dark Matter in Galaxy Clusters
Dark matter and galaxy velocity dispersions
- Velocity dispersion = range of velocities of galaxies within a cluster
- Higher velocity dispersions indicate more massive cluster
- Observed velocity dispersions in clusters much higher than expected from visible mass of member galaxies
- Suggests extra, unseen mass holding cluster together gravitationally
- Dark matter halos around galaxy clusters provide extra gravitational potential to explain high velocity dispersions
- Without dark matter, clusters would not be gravitationally bound and would disperse
Visible vs dynamical mass discrepancy
- Visible mass estimated from observable stars, gas, dust
- Measured using stellar population synthesis, multi-wavelength observations
- Dynamical mass = total mass inferred from gravitational effects
- Estimated using rotation curves, gravitational lensing, velocity dispersions
- Dynamical mass of galaxies and clusters consistently much larger than visible mass
- Mass discrepancy typically factor of 5-10 for individual galaxies, even higher for clusters
- Significant visible vs dynamical mass discrepancy key evidence for dark matter
- Dark matter accounts for "missing mass" to reconcile observations with laws of gravity