The Dorado Group is a loose concentration of galaxies[2] containing both spirals and ellipticals.[3] It is generally considered a 'galaxy group' but may approach the size of a 'galaxy cluster'.[6] It lies primarily in the southern constellation Dorado and is one of the richest galaxy groups of the Southern Hemisphere.[7]Gérard de Vaucouleurs was the first to identify it in 1975 as a large complex nebulae II in the Dorado region,[2] designating it as G16.[3]
Characteristics
A rough distance estimate from NGC 1549 (using the Hubble constant as 70) put the cluster at 18.4 megaparsecs (Mpc).[2] The Cepheid distance estimate from Freedman et al. 2001 is 15.3 Mpc.[8] Based upon the 2001 work of Tonry et al.[9] the surface brightness fluctuation (SBF) of six member galaxies was averaged and adjusted to estimate the group's distance at 19.1±0.8 Mpc in 2007.[10]
At the center of the cluster lie interacting galaxies[11]NGC 1549 and NGC 1553. The dominant group members, ordered by luminosity, are: spiral NGC 1566, lenticular NGC 1553, and elliptical NGC 1549.[6] The group spans an area of the sky 10° square, corresponding to an actual area of around 3 Mpc square.[2] The group exhibits a relatively small harmonic mean radius (230 ± 40 kpc) due to the concentration at its core of more luminous galaxies.[6] All together, the group has an overall luminosity of 7.8 ± 1.6 ×1010L⊙.[6]
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.[5] The Dorado Group is in the Fornax Wall that connects these three groups.[5] Due to its location in the Fornax Wall, the group is at a similar distance as the Fornax Cluster.[10] The Dorado Group is richer than the Local Group, while still being dominated by disk types of galaxies (i.e. its two brightest members are spiral NGC 1566 and lenticular NGC 1553) and its member galaxies have H I masses on par with non-interacting galaxies of the same morphological type.[10] With the group's apparent crossing time being 12.6 ± 0.6 %[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.[10]
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 struck out.[12] In 1989, the list of members was expanded to 46 by Maia, da Costa, & Latham.[2] 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[6] galaxies.[2][3] Kilborn et al. 2005 confirmed 26 members from the list.[5] In 2006, the list was refined again by Firth et al.[6] Using redshift data, they excluded eleven (as being background galaxies or interloper) from the Ferguson et al. list, confirmed the membership of twenty on the list, and left 48 unconfirmed.[6]
Huchra et al. 1982 Members of the Dorado Group (with strikeouts for ones dropped by Maia et al. in 1989)
The NGC 1566 Group of Dorado contains H I with MHI = 3.5×1010M⊙ of which 40% alone comes from the NGC 1566 galaxy.[5] 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 (within the Dorado Group) is:[5]
^J. L. Tonry; A. Dressler; J. P. Blakeslee; E. A. Ajhar; A. B. Fletcher; G. A. Luppino; M. R. Metzger; C. B. Moore (2001), "The SBF Survey of Galaxy Distances. IV. SBF Magnitudes, Colors, and Distances", Astrophysical Journal, 546 (2): 681–693, arXiv:astro-ph/0011223, Bibcode:2001ApJ...546..681T, doi:10.1086/318301, S2CID17628238
^ abcdDeGraaff, Regina Barber; Blakeslee, John P.; Meurer, Gerhardt R.; Putman, Mary E. (December 2007), "A Galaxy in Transition: Structure, Globular Clusters, and Distance of the Star-Forming S0 Galaxy NGC 1533 in Dorado", The Astrophysical Journal, 671 (2): 1624–1639, arXiv:0710.0893, Bibcode:2007ApJ...671.1624D, doi:10.1086/523640, S2CID14312626
^Bridges, Terry J.; Hanes, David A. (April 1990), "Globular clusters in the interacting galaxies NGC 1549 and NGC 1553", Astronomical Journal, 99: 1100–1107, 1340, 1341, Bibcode:1990AJ.....99.1100B, doi:10.1086/115399, ISSN0004-6256
