edgeon

Edge-on, sometimes written edgeon in band-laden catalogs, describes the orientation of a disk galaxy when the plane of the disk is parallel to the line of sight, so the galaxy is seen from its side rather than face-on. In such views, the thin stellar disk appears elongated, and a prominent dust lane often bisects the central region. The vertical structure, including the disk scale height and any thick disk or bulge component, is more readily studied in edge-on galaxies than in face-on ones.

Determining how edge-on a galaxy is involves measuring its inclination, i, with i near 90 degrees indicating a nearly edge-on view. This can be inferred from isophotal shapes, linewidths, and velocity fields derived from spectroscopy. Edge-on orientation also complicates interpretation because dust obscuration can hide star-forming regions in the midplane, while kinematic signatures from rotation and vertical motions can be extracted via emission lines.

Edge-on orientation also complicates interpretation because dust obscuration can hide star-forming regions in the midplane, while kinematic signatures from rotation and vertical motions can be extracted via emission lines. (Note: this line is included for consistency with prior sentence structure; the core idea is that dust and projection effects affect analysis.)

Edge-on galaxies are particularly valuable for examining the vertical distribution of stars and gas, the presence of galactic warps, and the structure of central bulges and bars seen end-on. They also provide clean measurements of rotation curves and mass distribution along the disk.

Notable examples include NGC 891, NGC 4565 (the Needle Galaxy), NGC 4216, and the Sombrero Galaxy (M104), among others. The term is widely used in astronomy to distinguish observational geometry from intrinsic morphology.