1. A Supergroup in 6dFGS Sarah Brough, Swinburne University Virginia Kilborn, Duncan Forbes, Swinburne University Warrick Couch, UNSW Heath Jones, Mount Stromlo Matthew Colless, AAO Piccy here

2. 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). 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. 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 Two of the groups in the GEMS sample (NGC 1332 and NGC 1407) are associated with a much larger system – the Eridanus cloud

3. Eridanus - a ‘Supergroup’? Eridanus is a cloud of galaxies ~23 Mpc away (Baker 1933,1936; de Vaucouleurs 1975) 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). 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. There is intra-group X-ray emission around NGC 1395 and NGC 1407, but not NGC 1332. NGC 1407 Fornax NGC 1332 Eridanus

4. X-Ray Images Osmond & Ponman (2004)Omar & Dwarakanath (2005) Osmond & Ponman (2004)

5. 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. 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 < 2500 km s -1 : Galaxies with velocities < 2500 km s -1 : 470 NED sources 470 NED sources 114 new 6dFGS sources 114 new 6dFGS sources 249 total sources in Eridanus region (25% more than NED). 249 total sources in Eridanus region (25% more than NED). NGC 1407 NGC 1332 Fornax Eridanus

6. 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 10 13 M  cluster. 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 10 13 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. 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.

7. FOF Determined Groups In order to study region need to define which galaxies are associated with which sub- clump/group/cluster. 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) Use friends-of-friends technique (Huchra & Geller 1982) Following the prescriptions of 2dF (Eke et al. 2004): Following the prescriptions of 2dF (Eke et al. 2004):  = 150 2  r = 1200 NGC 1407 Eridanus NGC 1332

8. Velocity Distribution

9. Derived Properties Eridanus NGC 1407 No. of members 4823 Mean Velocity (km s -1 ) 1595  27 1696  74 Velocity Dispersion (km s -1 ) 167  15 320  36 Skewness  0.30.4 Kurtosis  0.50.6 Crossing time (H 0 -1 ) 0.010.001 Virial Mass (10 13 M  ) 1.34.0 Virial Radius (Mpc) 0.160.31 M V /L K 35.200.

10. Brightest Group Galaxies Galaxy at centre of structure gives some clue as to evolutionary status. 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 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. Large elliptical at centre of NGC 1407 group and X-ray emission. Luminosity weighted centre and mean velocity of group correspond to those  NGC 1407. Luminosity weighted centre and mean velocity of group correspond to those  NGC 1407.

11. Neutral Hydrogen Content Kilborn et al. in prep 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 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. We observe that are no HI sources within NGC 1407 but several within NGC 1332 groups. NGC 1332 NGC 1407

12. Colour Distributions Kodama et al. 2002 observe bimodality in colours of galaxy populations dependent on their environment. 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. 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. Obtained B mags from Hyper-leda, resulting in 225/536 galaxies with B-K colours. Median Fornax NGC 1407 NGC 1332 NGC 1395

13. High Density (43 galaxies) Low Density (182 galaxies)

14. NGC 1395 NGC 1407 NGC 1332

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

16. Summary NGC 1407 is a massive group at a late stage in group evolution. 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. 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 ~6x10 13 M . The individual BGGs will merge to form a large central elliptical (Brough et al. 2002; Lin & Mohr 2004). These groups will coalesce to form a cluster of mass ~6x10 13 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. The colours of the individual galaxies indicate that this structure straddles the density around which any galaxy transformation is occurring.