Dorado Group

Lore & Background

The Dorado Group lies primarily in the constellation Dorado and is part of the Fornax Wall, which connects three smaller groups within it: the NGC 1672 Group, NGC 1566 Group, and NGC 1433 Group. The group is at a similar distance as the Fornax Cluster. At its center are the interacting galaxies NGC 1549 and NGC 1553. The dominant members, ordered by luminosity, are spiral NGC 1566, lenticular NGC 1553, and elliptical NGC 1549. The group spans an area of 10° square on the sky, corresponding to roughly 3 Mpc square, but its harmonic mean radius is relatively small (230 ± 40 kpc) due to the concentration of more luminous galaxies at its core.

The group is richer than the Local Group and is dominated by disk-type galaxies; its two brightest members are a spiral and a lenticular. Member galaxies have H I masses comparable to non-interacting galaxies of the same morphological type. The apparent crossing time is 12.6 ± 0.6% of the universe's age, leading to the conclusion that the group is unvirialized, which may explain the abundance of spirals and H I.

Membership has been refined over time. In 1982, John Peter Huchra and Margaret J. Geller identified eighteen associated galaxies. Maia, da Costa, and Latham expanded the list to 46 in 1989. Henry C. Ferguson and Allan Sandage added 34 possible candidates in 1990–1991, bringing the total to 79. Kilborn et al. confirmed 26 members in 2005. In 2006, Firth et al. excluded eleven as background galaxies or interlopers, confirmed twenty, and left 48 unconfirmed. A 2007 study identified one definite and two possible ultracompact dwarf members.

Reader's Guide

The Dorado Group is significant as one of the richest galaxy groups in the Southern Hemisphere, bridging the gap between small groups like the Local Group and larger clusters. Its unvirialized state, indicated by a crossing time only 12.6% of the universe's age, suggests it is still forming, which accounts for its high proportion of spiral galaxies and neutral hydrogen. The group's location in the Fornax Wall places it at a similar distance to the Fornax Cluster, offering a comparative laboratory for studying galaxy evolution in different density environments. The presence of three distinct subgroups within it—the NGC 1672, NGC 1566, and NGC 1433 groups—highlights its complex structure. The interacting central pair NGC 1549 and NGC 1553 provides a local example of galaxy interactions. The group's luminosity of 7.8 ± 1.6 L☉ and its relatively small harmonic mean radius underscore its dense core. The ongoing refinement of its member list, from 18 to 79 candidates and back to a confirmed core, reflects the challenges of defining group boundaries in a loose, unvirialized system. The identification of ultracompact dwarfs in 2007 adds to its interest as a site for studying compact stellar systems.

Did You Know?

• The Dorado Group was first identified by Gérard de Vaucouleurs in 1975 as G16.
• The group's crossing time is only 12.6 ± 0.6% of the universe's age, indicating it is unvirialized.