A Supergroup in 6dFGS

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A Supergroup in 6dFGS
Piccy here

• Sarah Brough, Swinburne University
• Virginia Kilborn, Duncan Forbes, Swinburne
  University
• Warrick Couch, UNSW
• Heath Jones, Mount Stromlo
• Matthew Colless, AAO

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• A key prediction of hierarchical structure
  formation is that galaxy clusters assemble at
  late times from the merging and accretion of
  smaller structures (e.g. Peebles 1970).
• We lack clear early-stage examples of clusters
  being assembled from an ensemble of galaxy
  groups.
• Two of the groups in the GEMS sample (NGC 1332
  and NGC 1407) are associated with a much larger
  system the Eridanus cloud

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Eridanus - a Supergroup?

• Eridanus is a cloud of galaxies 23 Mpc away
  (Baker 1933,1936 de Vaucouleurs 1975)
• Forms a filamentary structure with the Fornax
  cluster (da Costa et al. 1988).
• There is intra-group X-ray emission around NGC
  1395 and NGC 1407, but not NGC 1332.

NGC 1407
NGC 1332
Eridanus
Fornax
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X-Ray Images
Osmond Ponman (2004)
Omar Dwarakanath (2005)
Osmond Ponman (2004)
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Advent of 6dFGS

• The 6dFGS database provides a unique source of
  positions, velocities, 2MASS near-IR magnitudes
  and spectra with which to analyse the properties
  of this region in detail.
• Galaxies with velocities lt 2500 km s-1
• 470 NED sources
• 114 new 6dFGS sources
• 249 total sources in Eridanus region (25 more
  than NED).

NGC 1407
NGC 1332
Eridanus
Fornax
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Previous Work

• Willmer et al (1989) studied 139 galaxies in the
  region, concluding that Eridanus is a cluster
  with 3-4 sub-clumps (around NGC 1332, NGC 1395
  and NGC 1407) each with different morphological
  mixes. They determined that these are all bound
  and will merge to form a 1013 M? cluster.
• Omar Dwarakanath (astro-ph/0503536) studied 181
  galaxies in the region and claim that the
  properties of Eridanus indicate that it is at an
  evolutionary stage intermediate to that of the
  field and a cluster of galaxies.

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FOF Determined Groups

• In order to study region need to define which
  galaxies are associated with which
  sub-clump/group/cluster.
• Use friends-of-friends technique (Huchra Geller
  1982)
• Following the prescriptions of 2dF (Eke et al.
  2004)
• ????? 150
• 2??r 1200

NGC 1407
NGC 1332
Eridanus
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Velocity Distribution
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Derived Properties
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Brightest Group Galaxies

• Galaxy at centre of structure gives some clue as
  to evolutionary status.
• No bright, centrally-dominant galaxy or X-ray
  emission at centre of Eridanus group
• Large elliptical at centre of NGC 1407 group and
  X-ray emission.
• Luminosity weighted centre and mean velocity of
  group correspond to those of NGC 1407.

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Neutral Hydrogen Content

• Omar Dwarakanath (2005) conclude that galaxies
  in the highest density regions are deficient in
  HI by a factor of 2-3 as a result of tidal
  interactions
• We observe that are no HI sources within NGC 1407
  but several within NGC 1332 groups.

NGC 1407
NGC 1332
Kilborn et al. in prep
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Colour Distributions

• Kodama et al. 2002 observe bimodality in colours
  of galaxy populations dependent on their
  environment.
• This region spans a dense cluster, groups,
  filaments and the field.
• Obtained B mags from Hyper-leda, resulting in
  225/536 galaxies with B-K colours.

Median
Fornax
NGC 1407
NGC 1332
NGC 1395
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High Density (43 galaxies) Low Density (182
galaxies)
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NGC 1407
NGC 1332
NGC 1395
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Future Directions

• Analyse whether groups are infalling and on what
  timescale.
• Compare observations with N-body simulations.
• Examine substructure in Eridanus group.
• Study morphological mix of region.
• Obtain star formation rates from 6dF spectra.

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Summary

• NGC 1407 is a massive group at a late stage in
  group evolution.
• Eridanus appears to be made up of 2 sub-groups
  each at an early stage of their evolution.
• These groups will coalesce to form a cluster of
  mass 6x1013 M?. The individual BGGs will merge
  to form a large central elliptical (Brough et al.
  2002 Lin Mohr 2004).
• The colours of the individual galaxies indicate
  that this structure straddles the density around
  which any galaxy transformation is occurring.