A Supergroup in 6dFGSA Supergroup in 6dFGS

Sarah Brough, Swinburne UniversitySarah Brough, Swinburne UniversityVirginia Kilborn, Duncan Forbes, Swinburne UniversityVirginia Kilborn, Duncan Forbes, Swinburne University

Warrick Couch, UNSWWarrick Couch, UNSWHeath Jones, Mount Stromlo Heath Jones, Mount Stromlo

Matthew Colless, AAOMatthew Colless, AAO

Piccy here

A key prediction of hierarchical structure formation A key prediction of hierarchical structure formation is that galaxy clusters assemble at late times from is that galaxy clusters assemble at late times from the merging and accretion of smaller structures the merging and accretion of smaller structures (e.g. Peebles 1970).(e.g. Peebles 1970).

We lack clear early-stage examples of clusters We lack clear early-stage examples of clusters being assembled from an ensemble of galaxy being assembled from an ensemble of galaxy groups.groups.

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

Eridanus - a ‘Supergroup’?Eridanus - a ‘Supergroup’? Eridanus is a cloud of Eridanus is a cloud of

galaxies ~23 Mpc away galaxies ~23 Mpc away (Baker 1933,1936; de (Baker 1933,1936; de Vaucouleurs 1975)Vaucouleurs 1975)

Forms a filamentary Forms a filamentary

structure with the Fornax structure with the Fornax cluster (da Costa et al. cluster (da Costa et al. 1988).1988).

There is intra-group X-ray There is intra-group X-ray emission around NGC 1395 emission around NGC 1395 and NGC 1407, but not NGC and NGC 1407, but not NGC 1332. 1332.

NGC 1407

Fornax

NGC 1332Eridanus

X-Ray ImagesX-Ray Images

Osmond & Ponman (2004) Omar & Dwarakanath (2005)

Osmond & Ponman (2004)

Advent of 6dFGSAdvent of 6dFGS The 6dFGS database The 6dFGS database

provides a unique source provides a unique source of positions, velocities, of positions, velocities, 2MASS near-IR 2MASS near-IR magnitudes and spectra magnitudes and spectra with which to analyse the with which to analyse the properties of this region properties of this region in detail.in detail.

Galaxies with velocities < Galaxies with velocities < 2500 km s2500 km s-1-1:: 470 NED sources470 NED sources 114 new 6dFGS 114 new 6dFGS

sourcessources 249 total sources in 249 total sources in

Eridanus region (25% Eridanus region (25% more than NED).more than NED).

NGC 1407

NGC 1332

Fornax

Eridanus

Previous WorkPrevious Work Willmer et al (1989) studied 139 galaxies in the region, Willmer et al (1989) studied 139 galaxies in the region,

concluding that Eridanus is a cluster with 3-4 sub-clumps concluding that Eridanus is a cluster with 3-4 sub-clumps (around NGC 1332, NGC 1395 and NGC 1407) each (around NGC 1332, NGC 1395 and NGC 1407) each with different morphological mixes. They determined with different morphological mixes. They determined that these are all bound and will merge to form a 10that these are all bound and will merge to form a 101313 M M cluster.cluster.

Omar & Dwarakanath (astro-ph/0503536) studied 181 Omar & Dwarakanath (astro-ph/0503536) studied 181

galaxies in the region and claim that the properties of galaxies in the region and claim that the properties of Eridanus indicate that it is at an evolutionary stage Eridanus indicate that it is at an evolutionary stage intermediate to that of the field and a cluster of galaxies.intermediate to that of the field and a cluster of galaxies.

FOF Determined GroupsFOF Determined Groups In order to study region In order to study region

need to define which need to define which galaxies are associated galaxies are associated with which with which sub-clump/group/cluster.sub-clump/group/cluster.

Use friends-of-friends Use friends-of-friends technique (Huchra & technique (Huchra & Geller 1982)Geller 1982)

Following the Following the prescriptions of 2dF (Eke prescriptions of 2dF (Eke et al. 2004): et al. 2004): = 150= 1502r = 1200

NGC 1407

Eridanus

NGC 1332

Velocity DistributionVelocity Distribution

Derived PropertiesDerived PropertiesEridanusEridanus NGC 1407NGC 1407

No. of membersNo. of members 4848 2323

Mean Velocity (km sMean Velocity (km s-1-1)) 159515952727 169616967474

Velocity Dispersion (km sVelocity Dispersion (km s-1-1)) 1671671515 3203203636

Skewness Skewness 0.30.3 0.40.4

Kurtosis Kurtosis 0.50.5 0.60.6

Crossing time (HCrossing time (H00-1-1)) 0.010.01 0.0010.001

Virial Mass (10Virial Mass (101313 M M)) 1.31.3 4.04.0

Virial Radius (Mpc)Virial Radius (Mpc) 0.160.16 0.310.31

MMVV/L/LKK 35.35. 200.200.

Brightest Group GalaxiesBrightest Group Galaxies

Galaxy at centre of structure gives some clue as to Galaxy at centre of structure gives some clue as to evolutionary status.evolutionary status.

No bright, centrally-dominant galaxy or X-ray No bright, centrally-dominant galaxy or X-ray emission at centre of Eridanus groupemission at centre of Eridanus group

Large elliptical at centre of NGC 1407 group and Large elliptical at centre of NGC 1407 group and X-ray emission.X-ray emission.

Luminosity weighted centre and mean velocity of Luminosity weighted centre and mean velocity of group correspond to those group correspond to those NGC 1407. NGC 1407.

Neutral Hydrogen ContentNeutral Hydrogen Content

Kilborn et al. in prep

Omar & Dwarakanath Omar & Dwarakanath (2005) conclude that (2005) conclude that galaxies in the highest galaxies in the highest density regions are density regions are deficient in HI by a deficient in HI by a factor of 2-3 as a factor of 2-3 as a result of tidal result of tidal interactionsinteractions

We observe that are We observe that are no HI sources within no HI sources within NGC 1407 but several NGC 1407 but several within NGC 1332 within NGC 1332 groups.groups.

NGC 1332

NGC 1407

Colour DistributionsColour Distributions Kodama et al. 2002 Kodama et al. 2002

observe bimodality in observe bimodality in colours of galaxy colours of galaxy populations dependent populations dependent on their environment.on their environment.

This region spans a This region spans a dense cluster, groups, dense cluster, groups, filaments and the field.filaments and the field.

Obtained B mags from Obtained B mags from Hyper-leda, resulting in Hyper-leda, resulting in 225/536 galaxies with 225/536 galaxies with B-K colours.B-K colours.

Median

Fornax

NGC 1407NGC 1332

NGC 1395

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

NGC 1395

NGC 1407

NGC 1332

Future DirectionsFuture Directions

Analyse whether groups are infalling and on Analyse whether groups are infalling and on what timescale.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.

SummarySummary NGC 1407 is a massive group at a late stage in group NGC 1407 is a massive group at a late stage in group

evolution. evolution.

Eridanus appears to be made up of 2 sub-groups each Eridanus appears to be made up of 2 sub-groups each at an early stage of their evolution.at an early stage of their evolution.

These groups will coalesce to form a cluster of mass These groups will coalesce to form a cluster of mass ~6x10~6x101313 M M. The individual BGGs will merge to form a . The individual BGGs will merge to form a large central elliptical (Brough et al. 2002; Lin & Mohr large central elliptical (Brough et al. 2002; Lin & Mohr 2004).2004).

The colours of the individual galaxies indicate that this The colours of the individual galaxies indicate that this structure straddles the density around which any galaxy structure straddles the density around which any galaxy transformation is occurring.transformation is occurring.