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Which features distinguish Irr I galaxies from Irr II galaxies?
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Galaxy
The Whirlpool Galaxy (left), also known as M51, an Sc galaxy accompanied by a small irregular companion galaxy, NGC 5195 (right).
The classification of irregular galaxies into Irr I and Irr II types was originally proposed by Edwin Hubble, though the Irr II category is no longer considered a useful classification system.1
The primary features distinguishing these two types are as follows:
- Irr I Galaxies: These are the most common irregular systems. They appear blue in color, are highly resolved into individual stars and luminous areas, and possess little to no central nucleus. They are often viewed as an extension of the spiral galaxy sequence, representing systems that lack discernible spiral structure.1
- Irr II Galaxies: These are rare, red objects. Unlike the Irr I type, they exhibit a smooth but nonsymmetrical shape. These systems are generally chaotic, often resulting from galaxy-galaxy interactions, such as tidal distortions or galactic cannibalism.1