galaxy mergers or non-interacting line-of-sight overlap, though Arp disputed that idea, claiming that apparent associations were examples of ejections. Notable Arp galaxies Catalog list Spiral galaxies Low surface brightness These are mostly dwarf galaxies or poorly defined spiral galaxies (with the designation Sm) that have low surface brightnesses (i.e. they emit little light per unit area). Low surface brightness galaxies are actually quite common. The exception is NGC 2857 (Arp 1), which is an Sc spiral galaxy (which means that it has a definite structure with loosely wound spiral arms and a faint but well-defined nucleus). Split arms This category contains spiral galaxies with arms that split into two separate parts. Detached segments This category contains spiral galaxies with arms that appear to be segmented. Some spiral arm segments may appear detached because dust lanes in the spiral arms obscure the arms' starlight. Other spiral arms may appear segmented because of the presence of bright star clusters (or discontinuous chains of bright star clusters) in the spiral arms. Three-armed Usually, most spiral galaxies contain two clearly defined spiral arms, or they contain only fuzzy filamentary spiral structures. Galaxies with three well-defined spiral arms are rare. One-armed One-armed spiral galaxies, including Magellanic spirals, are also rare. In this case, the single spiral arm may actually be formed by a gravitational interaction with another galaxy (as with the Large Magellanic Cloud itself, although it is not a member of the catalog). One heavy arm The spiral arms in these galaxies have an asymmetric appearance. One spiral arm may appear to be considerably brighter than the other. In the photographic plates produced by Arp, the bright arm would look dark or "heavy". While most of these galaxies (such as M101 and NGC 6946) are simply asymmetric spiral galaxies, NGC 6365 is an interacting pair of galaxies where one of the two galaxies is viewed edge-on and just happens to lie where the spiral arm for the other face-on galaxy would be visible. Integral sign These are galaxies that look like a stretched-out S shape (or like the integral sign used in calculus). Some objects, such as IC 167, are simply ordinary spiral galaxies viewed from an unusual angle. Other objects, such as UGC 10770, are interacting pairs of galaxies with tidal tails that look similar to spiral arms. Low surface brightness companions Many of these spiral galaxies are probably interacting with the low