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**Galaxies in Low-Density Environments**: Exploring How Group Environment Impacts Galaxy Evolution in Loose Galaxy Group

Explore the impact of low-density environments on galaxy evolution in loosely connected groups. Investigate physical mechanisms and timescales involved in galaxy evolution within these environments. Study processes like ram pressure stripping, interactions, mergers, and strangulation.

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**Galaxies in Low-Density Environments**: Exploring How Group Environment Impacts Galaxy Evolution in Loose Galaxy Group

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  1. Galaxies in LowDensity Environments Duncan Forbes Swinburne University

  2. Low Density Environments • Poorly studied relative to clusters • Star formation suppressed in clusters • Suppression occurs at group-like densities (2dF, SDSS) • What is the timescale ? • Are galaxies in groups pre-processed ? • What is the physical mechanism ?

  3. Physical Processes in Groups • Ram pressure stripping • Interactions & Harassment • Mergers • Group tidal field • Strangulation/Suffocation • Overlapping dark matter halos

  4. People…in Crete • Trevor Ponman, Trevor Miles, Habib Khosroshahi, Louise Nolan (Birmingham) • Frazer Pearce (Nottingham) • Warrick Couch (UNSW) • Ewan O’Sullivan (CfA) • Sarah Brough, Robert Proctor, Virginia Kilborn (Swinburne) ...a loose group of people studying loose groups of galaxies

  5. GEMS Group Evolution Multi-wavelength Study Aim: to understand how the group environment affects galaxy evolution and how groups themselves evolve. Method: multi-wavelength data and mock catalogues.

  6. GEMS Sample Selection • Catalogues of Galaxy Groups • 14 < D < 43 Mpc • ROSAT PSPC 10,000sec => 60 nearby groups with a range of X-ray properties, ie dynamically young groups with no IGM to virialised old groups with a hot, dense IGM. Includes both compact and loose groups.

  7. GEMS Dataset • ROSAT imaging (1.5 degrees) • INT/ESO2.2m Optical imaging (0.5 degrees) • Parkes HI mapping (5.5 degrees) • ATCA HI follow-up • UK Schmidt 6dF/AAT 2dF spectra • XMM/Chandra imaging

  8. X-ray Imaging60 GroupsTX, LX, Z to R500 Osmond & Ponman 2004

  9. Halo Stripping ? Helsdon etal. 2001

  10. Optical Imaging30 GroupsB,R,I filters

  11. Structural Properties R/Rvirial Poster 3.15 Khosroshahi etal. 2004

  12. Luminosity Functions Low LX groups Low sigma Lots of spirals Current merging Pre-virialised Miles etal. 2004 Poster 3.19

  13. HI Imaging • 17 Groups mapped with the Parkes multibeam instrument over 5.5 degrees • Mass limit of ~ 108 Msun • 2x deeper than HIPASS survey • 10x better velocity resolution than HIPASS • 15 arcmin beam (hence ATCA followup) • Further details next talk…

  14. NGC 3783 Group

  15. New Galaxies • In 16/17 groups searched to date: • 30 new group galaxies (~ 10% new) • 20 new redshifts • 10 not optically catalogued • a few potential HI clouds M ~ 108 Msun • IGM HI is rare, but some tidal bridges => small effect on LF faint end => `missing satellites’ are not HI clouds (CenA group: a few M ~ 106 Msun HI clouds)

  16. New Galaxy in NGC 5044 group MHI = 109 Msun V = 2750 km/s MHI/LB = 1.7 McKay etal 2004

  17. Virial Radius • Calculate Rvirial for groups using σ or TX ? • Statistical problems calculating σ for low N systems • Systematic bias in σ • Physical reasons for a low σ (dynamical friction, orientation effects) • Xrays only probe small fraction of virial radius => do both ! (including new HI galaxies)

  18. NGC 1566 groupno IGMRvirial (σ)

  19. NGC 5044 group

  20. NGC 3783 group

  21. Isolated Galaxies • Early-type galaxy, V < 9,000 km/s, B < 14 No neighbours within: • 700 km/s • 0.67 Mpc in plane of the sky • 2 B mags (factor of 6 in mass) Formation? • old, undisturbed • recent merger • collapsed group/fossil

  22. Scaling Relations Colour-magnitude Relation Fundamental Plane Reda, Forbes etal. 2004

  23. Fossil Groups ? • Typical isolated galaxy MB = -20.5 • One potential collapsed group: NGC 1132 MB = -22.0, log LX = 43.0 erg/s ΔM12 = 2.2, featureless morphology, old stellar population

  24. Conclusions • Xray halos of ellipticals not stripped • Optical LF dip in young, low LX groups • HI mapping finds new group galaxies • Many group galaxies beyond Rvirial (backsplash galaxies?) • Most isolated ellipticals obey scaling relations, and are not collapsed groups

  25. Web sites • www.sr.bham.ac.uk/gems/ • astronomy.swin.edu.au/staff/dforbes

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