rgal

Rgal is a notation commonly used in astrophysics to denote the galactocentric distance—the distance from the center of a galaxy. In many texts it appears as r_gal, R_gal, or simply rgal. It is a radial coordinate that helps describe the structure and dynamics of galaxies, including the Milky Way.

Mathematically, rgal is defined in a galactocentric coordinate system. If the galaxy's center is the origin, and x, y, z are Cartesian coordinates, rgal = sqrt(x^2 + y^2 + z^2). In a cylindrical system (R, phi, z) aligned with the galactic disk, a projected radial distance in the plane is often denoted R, with rgal sometimes defined as sqrt(R^2 + z^2) for 3D distance, though many studies use R for radial distance in the disk.

Applications of rgal include modeling radial density profiles of stars and gas, studying metallicity gradients, and describing rotation curves and mass distributions within galaxies. It provides a single scalar measure of distance from the galactic center that can be used to compare different regions and to constrain structural models.

Other uses: Outside astrophysics, rgal may appear in programming or data sets as a variable name signifying a radial distance from a chosen origin; its precise meaning is context-dependent. As a result, the term is not a universal standard and should be interpreted in light of its specific scientific or computational context. See also galactocentric coordinates.