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.


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