surface density

The surface density of something (often signified by Σ) is the amount of it per unit area rather than unit volume: i.e., a variant of density that ignores one dimension. A typical use is to refer to such a mass density, but some other aspect might be specified, e.g., the count of something (other than units of mass). To illustrate with an example, given a flat rectangular box filled with marbles, then the number of marbles in the box divided by the box's volume is a density, whereas the number of marbles divided in the box divided by the area that the box occupies (e.g., its length times its width) is a surface density.

The term surface density tends to be used in a number of situations, one of which is in characterizing observation data: sometimes the existence of something can be reliably located within the celestial sphere, but not sufficiently exact along the line of sight to quote a spatial (3D) density, and surface density accurately describes our most reliable knowledge. It can also be useful regarding flat objects (irrespective of the direction we are viewing them from), e.g., disks, such as circumstellar disks, accretion disks, or galactic disks. It also has been used to refer to distributions of something over two dimensions, such as across the celestial sphere.

Distinct surface densities might be used in a model for the same object, e.g., one for a galaxy's stellar mass and another for its gas mass.

Surface density is equivalent to column density, but each has its own connotation regarding the shape of what is being measured. Column density is an amount (i.e., a mass or other count of something) per area, but also implies the third dimension of what is being counted is large. The term column density is used for a quantification of the material over a long distance, such as to quantify the interstellar medium between Earth and some astronomical object.