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

9. Halo Stripping ?
Helsdon etal. 2001

10. Optical Imaging 30 Groups B,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 group no IGM Rvirial (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

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

24. 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

25. 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

26. Web sites
astronomy.swin.edu.au/staff/dforbes