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 ? What is the physical mechanism ?

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

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

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.

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.

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

X-ray Imaging 60 Groups TX, LX, Z to R500 Osmond & Ponman 2004

Halo Stripping ? Helsdon etal. 2001

Optical Imaging 30 Groups B,R,I filters

Structural Properties R/Rvirial Poster 3.15 Khosroshahi etal. 2004

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

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…

NGC 3783 Group

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)

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

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)

NGC 1566 group no IGM Rvirial (σ)

NGC 5044 group

NGC 3783 group

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

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

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

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

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