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Galaxies in LowDensity Environments

Galaxies in LowDensity Environments. Duncan Forbes Swinburne University. 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 ?

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Galaxies in LowDensity Environments

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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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