NGC 922 is one of the nearest known collisional galaxies.[10] This object was described by theHerschels as "considerably faint, pretty large, round, gradually pretty much brighter middle."[7] The general form is described by themorphological classification ofSB(s)cd pec,[2] which indicates a peculiar (pec)barred spiral galaxy (SB) with no inner ring system around the bar (s) and loosely-woundspiral arms (cd).
This object was originally described as a dust-obscuredgrand-design galaxy – a term used to indicate a type of spiral galaxy with prominent and well-definedspiral arms. However, observation of its features suggests it has undergone a merger with a gas-rich dwarf galaxy.[11] This has resulted in a distinctive C-shaped ring ofH-alpha emission.[12] About104 kpc away from NGC 922, there is a dwarf galaxy called S2 that was once thought to have been responsible for NGC 922's structure.[6] A stellar plume is seen extending from the galaxy in the direction of S2,[6] located about8′ away.[10] However, the stellar plume is not connected to S2, and S2 also has it own tidal structure, which would not be possible if it had already "punched" through NGC 922.[11]
The main galaxy has a stellar mass estimated 5.47 billion times the mass of the Sun (solar mass), while the dwarf companion has 28.2 million solar masses. The net mass of NGC 922 is estimated as 75 billion solar masses within a radius of13.4 kpc, with 72% being in the form ofdark matter and 20% as neutral hydrogen.[6] It is astarburst galaxy,[12] and the collision has resulted in star formation at the rate of7–8M☉·yr−1. New star clusters have been formed in the ring or bar with a mean age of 16 million years, whereas older clusters of 50 million years or more are predominantly found in the nuclear region. The tidal plume pointing toward the companion mainly consists of much older stars, indicating not much new star formation has taken place in that feature.[10]
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