Glossary

Arcsecond: Unit of an angle denoting how large something appears in the sky.
Bulge: Dense, spherical core of the galaxy.
Central mass density: The density of dark matter at the center of the galaxy. Changing this value changes the magnitude of the dark matter curve.
Centripetal force: A force that causes circular motion.
Concentration parameter: Also known as the bulge Sersic parameter. “Describes the curvature of the profile in a radius-magnitude plot” [Noordermeer2008].
Core radius: Indicates the distance from the galactic center where the density falls off by a factor of e (~2.7). Adjusting this factor changes where the “bump” of the curve is located. Synonyms: cutoff radius, scale radius.
Dark matter: A placeholder to account for the matter in spiral galaxies that is missing based on our observations of galactic motion. Related words: Dark Matter halo, Halo.
Dark matter halo: A spherical shaped distribution of Dark Matter that is most dense at some distance from the galactic center.
Density profile: The mass distribution of matter in the galaxy.
Disk: A large, relatively flat component of the galaxy.
Enclosed mass: The total mass of the area between the center of the galaxy or other body and the specified radius.
Edge-on spiral galaxy: A galaxy which lies in the same plane as our viewing direction which results in us only seeing the outer edge of the galaxy and part of the central bulge.
Face-on spiral galaxy: A face-on galaxy is perpendicular to our line of sight revealing its spiral structure.
Fitting: The process of adjusting unknown parameters in order to match a theoretical equation to measured data.
Flat Rotation: Rotation whose curve plateaus at large radii.
Gravitational constant (G): The constant of proportionality used to calculate effects of gravity. In SI units, G = 6.67e-11 m^3 kg^-1 s^-2
Inclination angle: The angle of the galaxy from the horizontal, as viewed from Earth. For example, a face-on galaxy has an inclination angle of 0, and an edge-on galaxy has an inclination angle of 90°.
Interstellar gas: Gas between stars and galaxies – mostly Hydrogen. Synonyms: atomic gas.
Isothermal: Having a constant temperature. In thermal physics, this affects the behavior of temperature-related properties, such as density.
Keplerian model: A model of the orbital motion of planets in solar systems such that the orbital velocity is inversely proportional to the square root of the radius. Related words: planet-like motion.
kpc: Unit of distance; kiloparsec. 1 kpc = 3262 light years = 3.086e+19 meters = 1.917e+16 miles.
Luminous matter: All matter that emits visible light in galaxies. Synonyms: light matter, visible matter, baryonic matter.
MACHO: MAssive Compact Halo Object. A type of object that emits little to no light, such as brown dwarfs, rogue planets, or black holes.
Mass-to-light ratio (M/L ratio): The ratio of how much mass there is compared to how much light the body emits; how much mass it is expected from the light source. Variation in the mass-to-light ratio means there is a wider range of masses that are plausible for the stellar component.
Measured datapoints: Measured velocities of stars and gas in a galaxy. Synonyms: measured velocities.
Mpc: Unit of distance; megaparsec: 1 Mpc = 1000 kpc.
NGC: New General Catalogue of galaxies.
Photometry: The measurement of light emitted from astronomical objects.
Planet-like rotation: Rotation whose curve resembles that of planets orbiting a star. Related words: Keplerian model.
Prefactor: A parameter that multiplies/directly scales the other information in an equation. Synonyms: scaling parameter. Related words: M/L ratio
Radial velocity: The speed stars and gas are moving at different distances from the center of the galaxy.
Reduced chi-squared value: A measure of how close the data and theory are to one another. In a strictly statistical sense, a reduced chi-squared of zero represents a perfect fit. However, a reduced chi-squared much less than 1 often indicates that more parameters were used than represented by the physical system, and we may be simply connecting the measured data points.
Rigid body rotation: Rotation whose curve resembles that of a rigid object. Synonyms: frisbee rotation
Rotation curve: A plot of a galaxy’s radial velocity versus the radius or distance from the center.
Solar mass: Mass of the Sun, equal to 1.989e30 kilograms.
Spiral arms: In a spiral galaxy, these are the long parts protruding from the disc’s otherwise elliptical shape. Related words: spiral galaxy.
Spiral galaxy: Galaxies that have spiral structures. Components are bulge, disk, gas, and Dark Matter halo. Spiral galaxies are less luminous than elliptical galaxies on average and have active star formation. Related words: spiral arms.
Surface brightness: The amount of flux from the galaxy per square arcsecond on the sky.
Theoretical rotation curve: A rotation curve that has been modeled from known parameters that is then used to match observed data.
UGC: Uppsala General Catalogue of galaxies.
Uncertainty: A value representing what range of values a data point might differ from what we measured. Synonyms: error.
Zero-point luminosity: A reference luminosity for a star that has a zero absolute bolometric magnitude [Mamajek2015].