galactic disk
Definition: If nearly all mass were interior to the visible disk (as for the solar system, where nearly all mass is in the Sun), V(R) should decline as 1/√R beyond the visible mass, the way planetary orbital speeds decline with distance from the Sun.
Why it matters here: A flat rotation curve, combined with Newtonian gravity, implies a mass distribution that keeps growing well beyond the visible disk — the central piece of evidence for the Milky Way’s dark matter halo.
Relationship: Stars and gas clouds far outside the Sun’s orbital radius (measured via 21-cm radio observations of neutral hydrogen, unaffected by dust) are observed orbiting the Galactic center at roughly the same speed (about 220 km/s) as material near the Sun’s orbit, rather than slowing down as visible starlight and gas density drop off.
Boundary or common confusion: Assuming "dark matter" means "dark, ordinary matter we just haven’t seen yet" (like cold gas or dim stars). Multiple independent lines of evidence (rotation curves, gravitational lensing, the cosmic microwave background, large-scale structure) point to a non-baryonic form of matter that does not emit, absorb, or scatter light at all, distinguishing it from merely faint ordinary matter.
galactic bulge
Definition: Stars and gas clouds far outside the Sun’s orbital radius (measured via 21-cm radio observations of neutral hydrogen, unaffected by dust) are observed orbiting the Galactic center at roughly the same speed (about 220 km/s) as material near the Sun’s orbit, rather than slowing down as visible starlight and gas density drop off.
Why it matters here: A flat rotation curve, combined with Newtonian gravity, implies a mass distribution that keeps growing well beyond the visible disk — the central piece of evidence for the Milky Way’s dark matter halo.
Relationship: The Sun orbits the Galactic center at radius R₀ ≈ 8.2 kpc with orbital speed V₀ ≈ 220 km/s.
Boundary or common confusion: Assuming "dark matter" means "dark, ordinary matter we just haven’t seen yet" (like cold gas or dim stars). Multiple independent lines of evidence (rotation curves, gravitational lensing, the cosmic microwave background, large-scale structure) point to a non-baryonic form of matter that does not emit, absorb, or scatter light at all, distinguishing it from merely faint ordinary matter.
galactic halo
Definition: The Galaxy’s visible stellar and gas mass is estimated at only about 6.0e+10 M☉ — far less than the 3.07e+11 M☉ implied by the rotation curve at 25 kpc, requiring roughly 5.1 times more mass than is visible, attributed to a dark matter halo.
Why it matters here: A flat rotation curve, combined with Newtonian gravity, implies a mass distribution that keeps growing well beyond the visible disk — the central piece of evidence for the Milky Way’s dark matter halo.
Relationship: Stars and gas clouds far outside the Sun’s orbital radius (measured via 21-cm radio observations of neutral hydrogen, unaffected by dust) are observed orbiting the Galactic center at roughly the same speed (about 220 km/s) as material near the Sun’s orbit, rather than slowing down as visible starlight and gas density drop off.
Boundary or common confusion: Assuming "dark matter" means "dark, ordinary matter we just haven’t seen yet" (like cold gas or dim stars). Multiple independent lines of evidence (rotation curves, gravitational lensing, the cosmic microwave background, large-scale structure) point to a non-baryonic form of matter that does not emit, absorb, or scatter light at all, distinguishing it from merely faint ordinary matter.
rotation curve
Definition: Explain how a galaxy’s rotation curve is measured and what a "flat" rotation curve implies about its mass distribution.
Why it matters here: A flat rotation curve, combined with Newtonian gravity, implies a mass distribution that keeps growing well beyond the visible disk — the central piece of evidence for the Milky Way’s dark matter halo.
Relationship: The Galaxy’s visible stellar and gas mass is estimated at only about 6.0e+10 M☉ — far less than the 3.07e+11 M☉ implied by the rotation curve at 25 kpc, requiring roughly 5.1 times more mass than is visible, attributed to a dark matter halo.
Boundary or common confusion: Assuming "dark matter" means "dark, ordinary matter we just haven’t seen yet" (like cold gas or dim stars). Multiple independent lines of evidence (rotation curves, gravitational lensing, the cosmic microwave background, large-scale structure) point to a non-baryonic form of matter that does not emit, absorb, or scatter light at all, distinguishing it from merely faint ordinary matter.
dark matter halo
Definition: The Galaxy’s visible stellar and gas mass is estimated at only about 6.0e+10 M☉ — far less than the 3.07e+11 M☉ implied by the rotation curve at 25 kpc, requiring roughly 5.1 times more mass than is visible, attributed to a dark matter halo.
Why it matters here: A flat rotation curve, combined with Newtonian gravity, implies a mass distribution that keeps growing well beyond the visible disk — the central piece of evidence for the Milky Way’s dark matter halo.
Relationship: A flat rotation curve, combined with Newtonian gravity, implies a mass distribution that keeps growing well beyond the visible disk — the central piece of evidence for the Milky Way’s dark matter halo.
Boundary or common confusion: Assuming "dark matter" means "dark, ordinary matter we just haven’t seen yet" (like cold gas or dim stars). Multiple independent lines of evidence (rotation curves, gravitational lensing, the cosmic microwave background, large-scale structure) point to a non-baryonic form of matter that does not emit, absorb, or scatter light at all, distinguishing it from merely faint ordinary matter.
21-cm line
Definition: If nearly all mass were interior to the visible disk (as for the solar system, where nearly all mass is in the Sun), V(R) should decline as 1/√R beyond the visible mass, the way planetary orbital speeds decline with distance from the Sun.
Why it matters here: A flat rotation curve, combined with Newtonian gravity, implies a mass distribution that keeps growing well beyond the visible disk — the central piece of evidence for the Milky Way’s dark matter halo.
Relationship: Because V(R) instead stays roughly flat out to large R, M(R) must keep growing linearly with R, implying an extended, largely invisible ("dark") mass distribution well beyond the visible disk.
Boundary or common confusion: Assuming "dark matter" means "dark, ordinary matter we just haven’t seen yet" (like cold gas or dim stars). Multiple independent lines of evidence (rotation curves, gravitational lensing, the cosmic microwave background, large-scale structure) point to a non-baryonic form of matter that does not emit, absorb, or scatter light at all, distinguishing it from merely faint ordinary matter.