Shapeless galaxies may be the closest living relatives of the universe's first ones
About a quarter of all galaxies fit neither the spiral nor the elliptical mould. These irregulars have no central bulge, no arms and no symmetry. The smallest of them, dwarf irregulars rich in gas and poor in heavy elements, are thought to resemble the very first galaxies that formed after the Big Bang.
Edwin Hubble's famous sequence sorts galaxies by form, from smooth ellipticals to elegant spirals. Irregulars slip through it. Their starlight, viewed in blue, shows no axis of symmetry, and many look chaotic. Some were once orderly spirals or ellipticals bent out of shape by a neighbour's uneven pull; others may have been assembled in collisions, or be young systems not yet settled into a type. A typical irregular weighs in at about a tenth of the Milky Way's mass, though outliers such as UGC 6697 are large.
Astronomers split them into groups. Irr-I galaxies show some structure, just not enough to fit Hubble's scheme; those with hints of spiral pattern are labelled Sm, those without Im. Irr-II galaxies show no usable structure at all and are often sculpted by mergers. Then there are dwarf irregulars, faint, gas-rich, low in metals and heavy with dark matter. They may be nearby, more recent versions of the faint blue galaxies seen in deep surveys of the distant universe, which makes them valuable for understanding how galaxies evolve.
The Magellanic Clouds, the Milky Way's bright companions, show how slippery the category is. Both were once called irregular. The Large Magellanic Cloud has been reclassified as a barred Magellanic spiral, while the Small Magellanic Cloud is still labelled Im despite containing a bar.
Their messiness makes them useful laboratories. Spirals are thought to trigger star birth partly through density waves sweeping around their discs; irregulars have little of that. Stars still form, sometimes as fast as in spirals, but driven more by gas moving at random, by massive stars stirring their surroundings, and by giant clouds of ionised hydrogen that are unusually common in them.
Source: Irregular galaxy
