Last updated: July-7-99

Magellanic Irregular and Amorphous Galaxies

Holmberg [51] first divided irregular galaxies into two types: Type I Irregulars are highly resolved systems similar to the Magellanic Clouds, while Type II Irregulars show a smoothed ``amorphous'' distribution of light, often broken by irregular patches of obscuration. Type I Irregulars have subsequently found a natural home as Im galaxies following the late spirals of class Sd and Sm [27 ]. Their star-formation history is summarized in [54]; while global radio, optical and spectral properties are reviewed in [42, 55]. On the other hand, Sandage and Brucato [159] have suggested that the original Irr II galaxies can be sub-divided, and that a single class should be abandoned. The properties of the Irr II class galaxies (also called I0 in [31]) have been summarized in [85]. Those few objects which show no spiral structure, but have unresolved disks, are now termed amorphous galaxies [159]. Examples are NGC 3077, NGC 5253, and M82.

**The Numerical Code t of the Morphological Types**
Morphological Type	(Hubble Revised)	Code t
Compact (high density) Ellipticals	cE	-6
Dwarf (low density) Ellipticals	dE	-5
Normal Elliptical Systems	E	-7
Giant Ellipticals with extended optical coronae (in particular the Morgan type cD)	E+	-4
Lenticular Systems	L- L L+	-3 -2 -1
Irregular Systems of Type II	I0
Lenticular-Spiral Systems	S0/a	0
Spiral Systems	Sa Sab Sb Sbc Sc Scd Sd Sdm	1 2 3 4 5 6 7 8
Spirals of Magellanic Type	Sm	9
Irregular of Magellanic Type (=Irr of Type I)	Im	10
Compact Blue Irregulars (Isolated Extragalactic H II Regions)	cI	11

Heidmann et al. [50] (above table) introduced a numerical code of the morphological types in the revised Hubble system. This code was extended and used in the Second Reference Catalogue of Bright Galaxies, RC2 ([8] in Secondary Galaxy Catalogs).

More detailed tables for coding of Hubble types, revised morphological types (de Vaucouleurs [27]), DDO (van den Bergh) types and Yerkes (Morgan) types etc. are given in the RC2.