Although spiral galaxies seem to get all the attention, the irregular type galaxy is probably the most frequently encountered galactic type among the star forming galaxies and make up 1/3 to 1/2 of all galaxies! They are a diverse group and display a wide range of physical characteristics. Hubble originally defined irregular types as having a chaotic, nonsymmetrical light distribution without spiral structure although some irregulars do display rudimentary spiral arms. Irregulars are usually smaller, less massive and dimmer than the giant spirals and classical elliptical types and are subdivided into two major groups, the Magellanic type systems (Irr I) (similar to the Magellanic clouds) and Amorphous type systems (Irr II or IO). Most Irregular types are of low surface brightness which is why they are not commonly found in catalogs. The ones that attract attention and find their way into catalogs are the less frequent brighter specimens. They are the bluest of all galaxies, an indicator of young stellar populations and high star formation rates. The young stellar populations are often associated with Giant HII regions which are abundant in irregular galaxies.
NGC 4449 is a dwarf irregular galaxy and
a member of the Canis Venaticorum I cloud which contains a few
other prominent galaxies like NGC 4244, 4214 and IC 4182. It has
a high degree of star formation which has given rise to numerous
bright HII regions, superbubbles and giant shells of heated gas,
as well as super-luminous supernova remnants. Nearly the entire
galaxy is covered in HII regions. The two most luminous HII regions
known as CM 16 and CM 39 rival the prototypical Giant HII region
30 Doradus in size and luminosity. Several superbubbles and supergiant
shells have been identified in NGC 4449. These are immense bubbles
of super heated gas blown out by the fierce stellar winds of luminous
OB stars and their recent supernovae.
The distinction between the two is one of size. Superbubbles are smaller than 500 pc (1630 light years) while supergiant shells are greater than 500 pc. One particular supergiant shell, SGS2 has a diameter of 5800 light years and is the largest and most energetic supergiant shell known. It is twice the size of the 30 Doardus superbubble and releases ten times its energy in x-radiation.
What has caused all the chaos within NGC 4449? Its structure provides some forensic clues to its violent history. Radio wave observations have shown that NGC 4449 has an immense HI (neutral hydrogen gas) halo ten times larger than its optical diameter. A disk like feature and a lopsided arm structure have been found in its halo and are likely the fossil remains of a merger in its remote past. NGC 4449 most likely had an encounter with its irregular companion galaxy, DDO 125 some 500 million years ago and the disturbance brought on by that encounter has led to the perturbed state we see today in NGC 4449.