Dorado Group


The Dorado Group is a loose concentration of galaxies containing both spirals and ellipticals. It is generally considered a 'galaxy group' but may approach the size of a 'galaxy cluster'. It lies primarily in the southern constellation Dorado and is one of the richest galaxy groups of the Southern Hemisphere. Gérard de Vaucouleurs was the first to identify it in 1975 as a large complex nebulae II in the Dorado region, designating it as G16.

Characteristics

A rough distance estimate from NGC 1549 put the cluster at 18.4 megaparsecs. The Cepheid distance estimate from Freedman et al. 2001 is 15.3 Mpc. Based upon the 2001 work of Tonry et al. the surface brightness fluctuation of six member galaxies was averaged and adjusted to estimate the group's distance at 19.1 ± 0.8 Mpc in 2007.
At the center of the cluster lie interacting galaxies NGC 1549 and NGC 1553. The dominant group members, ordered by luminosity, are: spiral NGC 1566, lenticular NGC 1553, and elliptical NGC 1549. The group spans an area of the sky 10° square, corresponding to an actual area of around 3 Mpc square. The group exhibits a relatively small harmonic mean radius due to the concentration at its core of more luminous galaxies. All together, the group has an overall luminosity of 7.8 ± 1.6 L.
The Dorado Group contains three dominant smaller groups within itself, NGC 1672 Group, NGC 1566 Group and the NGC 1433 Group, as evidenced by the H I distribution of the region. The Dorado Group is in the Fornax Wall that connects these three groups. Due to its location in the Fornax Wall, the group is at a similar distance as the Fornax Cluster. The Dorado Group is richer than the Local Group, while still being dominated by disk types of galaxies and its member galaxies have H I masses on par with non-interacting galaxies of the same morphological type. With the group's apparent crossing time being 12.6 ± 0.6 % of the universe's age, recent analyses deduce that the group is unvirialized, and thus this may explain the abundance of spirals and H I.

Members

The table below lists eighteen galaxies that were identified in 1982 as associated with the Dorado Group by John Peter Huchra and Margaret J. Geller with the ones later dropped stuck out. In 1989, the list of members was expanded to 46 by Maia, da Costa, & Latham. In 1990-1991, Henry C. Ferguson and Allan Sandage identified 34 other possible candidate members of the group with magnitudes greater than 19 and eliminated one member from Maia et al. putting the list at 79 galaxies. Kilborn et al. 2005 confirmed 26 members from the list. In 2006, the list was refined again by Firth et al. Using redshift data, they excluded eleven from the Ferguson et al. list, confirmed the membership of twenty on the list, and left 48 unconfirmed.
NameTypeR.A. Dec. Redshift Apparent Magnitude
NGC 2082SABc1184 ± 612.6
NGC 1947S0 pec1100 ± 2411.7
NGC 1796SBdm:1014 ± 912.9
NGC 1688SBdm1228 ± 612.6
NGC 1672SBbc Sy21331 ± 310.3
IC 2056SABb1133 ± 1012.5
NGC 1559SBcd1304 ± 411.0
NGC 1543SB001176 ± 711.5
NGC 1574SA01050 ± 2511.4
NGC 1533SB00790 ± 511.7
NGC 1546SA0+?1284 ± 1411.8
NGC 1553SA001080 ± 1110.3
NGC 1549E0−11220 ± 1510.7
NGC 1566SABbcSy11504 ± 210.3
NGC 1617SABa1063 ± 2111.4
NGC 1515SABbc1175 ± 712.1
NGC 1705SA0 pec633 ± 612.8
NGC 1596SA0: sp1510 ± 812.1

is located about 30 million light-years away in the constellation of Eridanus.
The Maia et al. 1989 thirty-four added members were: IC 2049, NGC 1536, IC 2058, IC 2032, NGC 1602, NGC 1581, IC 2085, NGC 1522, PGC 15149, NGC 1556, NGC 1527, NGC 1494, NGC 1493, PGC 14416, IC 2000, NGC 1483, NGC 1433, PGC 14078, NGC 1495, NGC 1510, NGC 1510, NGC 1512, IC 1959, IC 1986, NGC 1448, NGC 1487, IC 1933, NGC 1311, IC 1954, IC 1914, NGC 1411, IC 1970, PGC 13818, NGC 1249, and PGC 11139. And the six dropped from the 1982 list were: NGC 2082, NGC 1947, NGC 1796, NGC 1688, NGC 1672, and NGC 1559. In 2007, a study of ultracompact dwarfs identified one definite and two possible UCD members of the group. The thirty-four added by Ferguson et al. 1990 included IC 2038 and IC 2039.
The NGC 1566 Group of Dorado contains H I with MHI = 3.5 M of which 40% alone comes from the NGC 1566 galaxy. More than half of its members are outside its virial radius of 580 kpc which suggests this group is a young non-virialized group. The 2005 Kilborn et al. set of confirmed NGC 1566 Group members is:
NameTypeR.A. Dec. Redshift Apparent Magnitude
IC 2049SABd?1469 ± 714.5
NGC 1536SBc pec1217 ± 1313.2
NGC 1543SB001176 ± 711.5
LSBG F157-081Irregular1215 ± 716.7
NGC 1533SB00790 ± 511.7
IC 2038Sd pec712 ± 5215.5
APMBGC 157+016+068Irregular1350 ± 416.3
NGC 1546SA0+?1284 ± 1411.8
IC 2058Sc1379 ± 113.9
IC 2032IABm pec1068 ± 714.7
NGC 1566SABbcSy11504 ± 210.3
NGC 1596SA0: sp1510 ± 812.1
NGC 1602IBm pec1568 ± 813.3
NGC 1515SABbc1175 ± 712.1
NGC 1522S00: pec898 ± 713.9
ESO 118-019S00 pec123914.9
ESO 157-030E41471 ± 2814.7
ESO 157-047S0/a? pec sp1655 ± 1015.5
ESO 157-049S?1678 ± 514.3
IC 2085S00 pec sp982 ± 1013.9
NGC 1549E0−11220 ± 1510.7
NGC 1553SA001080 ± 1110.3
NGC 1574SA01050 ± 2511.4
NGC 1581S01600 ± 2713.6
NGC 1617SABa1063 ± 2111.4
Abell 3202Irregular1135 ± 4016.9