Absorbing Column Density (nH) ○◂ Entries

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Absorbing Column Density (nH) ○◂｜Definition｜1st｜20251119205401-00-⌔

Area density - Wikipedia#Astronomy

Astronomy

In astronomy, the column density is generally used to indicate the number of atoms or molecules per square cm (cm${}^2$) along the line of sight in a particular direction, as derived from observations of e.g. the 21 cm hydrogen line or from observations of a certain molecular species. Also the interstellar extinction can be related to the column density of H or H${}_2$.¹

The concept of area density can be useful when analysing accretion disks. In the case of a disk seen face-on, area density for a given area of the disk is defined as column density: that is, either as the mass of substance per unit area integrated along the vertical path that goes through the disk (line-of-sight), from the bottom to the top of the medium:

$\sigma =\int \rho \mathrm{d}z,$

where $z$ denotes the vertical coordinate (e.g., height or depth), or as the number or count of a substance—rather than the mass—per unit area integrated along a path (column number density):

$N=\int n\mathrm{d}z.$

Printed 2026-06-28.

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Footnotes

1. “Column Density | COSMOS”. ↩

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