Local standard of rest
Definition: A conventional velocity frame that removes an adopted Solar motion before Galactic interpretation.
Why it matters here: It is needed to answer At fixed longitude, how does a positive non-circular velocity perturbation propagate into inferred R in this model? Give the derivative sign. and to interpret The 21-cm line is observable through much of the Galactic disk, including dusty sightlines.
Relationships: Use it with Rotation curve and Tangent point; the lecture model is \(R=\frac{R_0}{1+v_{\rm LSR}/(\Theta_0\sin l)}\).
Boundary or common confusion: The flat, axisymmetric rotation curve is an idealization; streaming velocities of order tens of km/s can shift inferred radii and distances substantially.
Tangent point
Definition: The minimum Galactocentric radius sampled along an inner-Galaxy line of sight.
Why it matters here: It is needed to answer At fixed longitude, how does a positive non-circular velocity perturbation propagate into inferred R in this model? Give the derivative sign. and to interpret The 21-cm line is observable through much of the Galactic disk, including dusty sightlines.
Relationships: Use it with Local standard of rest and Terminal velocity; the lecture model is \(R=\frac{R_0}{1+v_{\rm LSR}/(\Theta_0\sin l)}\).
Boundary or common confusion: The flat, axisymmetric rotation curve is an idealization; streaming velocities of order tens of km/s can shift inferred radii and distances substantially.
Terminal velocity
Definition: The extremal line-of-sight velocity associated with gas near the tangent point under circular rotation.
Why it matters here: It is needed to answer At fixed longitude, how does a positive non-circular velocity perturbation propagate into inferred R in this model? Give the derivative sign. and to interpret The 21-cm line is observable through much of the Galactic disk, including dusty sightlines.
Relationships: Use it with Tangent point and Kinematic distance; the lecture model is \(R=\frac{R_0}{1+v_{\rm LSR}/(\Theta_0\sin l)}\).
Boundary or common confusion: The flat, axisymmetric rotation curve is an idealization; streaming velocities of order tens of km/s can shift inferred radii and distances substantially.
Kinematic distance
Definition: A distance inferred by matching line-of-sight velocity to an adopted Galactic rotation model.
Why it matters here: It is needed to answer At fixed longitude, how does a positive non-circular velocity perturbation propagate into inferred R in this model? Give the derivative sign. and to interpret The 21-cm line is observable through much of the Galactic disk, including dusty sightlines.
Relationships: Use it with Terminal velocity and Rotation curve; the lecture model is \(R=\frac{R_0}{1+v_{\rm LSR}/(\Theta_0\sin l)}\).
Boundary or common confusion: The flat, axisymmetric rotation curve is an idealization; streaming velocities of order tens of km/s can shift inferred radii and distances substantially.
Rotation curve
Definition: The circular orbital speed of Galactic material as a function of Galactocentric radius.
Why it matters here: It is needed to answer At fixed longitude, how does a positive non-circular velocity perturbation propagate into inferred R in this model? Give the derivative sign. and to interpret The 21-cm line is observable through much of the Galactic disk, including dusty sightlines.
Relationships: Use it with Kinematic distance and Local standard of rest; the lecture model is \(R=\frac{R_0}{1+v_{\rm LSR}/(\Theta_0\sin l)}\).
Boundary or common confusion: The flat, axisymmetric rotation curve is an idealization; streaming velocities of order tens of km/s can shift inferred radii and distances substantially.