The GEMS Project and Collapsed Groups
Duncan Forbes
Swinburne University
Motivation
• Groups poorly studied relative to 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
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 People
SwinburneForbes, Kilborn, Brough, 2 new PhD
students
Birmingham (Ponman)Nottingham (Pearce)UNSW (Couch)ATNF (Koribalski)
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)• Wide-field optical imaging (0.5
degrees)• Parkes HI mapping (5.5 degrees)• ATCA HI follow-up • 6dFGS spectra• XMM/Chandra imaging• Mock catalogues
X-ray Imaging
60 Groups
TX, LX, Z to R500
Osmond & Ponman 2004
LX ~TX4
Osmond & Ponman 2004
Revised
Halo Stripping ?
Helsdon etal. 2001
Isolated
BGGs
Optical Imaging
30 Groups
B,R,I filters
Luminosity Functions
Miles etal. 2004
Low LX
groups
Low sigma
Lots of spirals
Current merging
Pre-virialised
HI Imaging
• 16 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)
NGC 3783 Group
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 => use both σ and TX !
NGC 5044 group
NGC 3783 group
Isolated Galaxies
• Early-type galaxy, V < 9,000 km/s, B < 14No 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”)
HermitsSwinburneForbes, Reda, Proctor
Harvard (O’Sullivan)Durham (Hau)
Colour-magnitude Relation
Fundamental Plane
Deviant galaxies have young stellar populations and/or disturbed morphology
FP – Kappa style
Radial Kinematics
Collapsed Groups ?
• Typical isolated galaxy MB = -20.5, LX/LB = 30
• One potential collapsed group:NGC 1132MB = -22.0, log LX = 43.0 erg/s, LX/LB = 32
ΔM12 = 2.2, featureless morphology,
old stellar population
Future Work
• Stellar population analysis, eg NGC 821
• Focus on `transition’ galaxies, ie E + disk = spiral eg NGC 821
• HI in isolated ellipticals, ie 10% detected in HIPASS
• Measuring dark halo mass, eg via dwarfs or Xrays
NGC 821
Proctor et al. 2005
Future Work
• Stellar population analysis, eg NGC 821
• Focus on `transition’ galaxies, ie E + disk = spiral eg NGC 821
• HI in isolated ellipticals, ie 10% detected in HIPASS
• Measuring dark halo mass, eg via dwarfs or Xrays
NGC 4555
O’Sullivan & Ponman 2005
Web sites
• www.sr.bham.ac.uk/gems/
• astronomy.swin.edu.au/dforbes
• www.sc.eso.org/santiago/science/NGG/
(5-9 Dec. 2005, Chile)
Conclusions
• Xray halos of ellipticals not stripped
• Optical LF dip in young, low LX groups
• HI mapping probes dynamical processes
• Many group galaxies beyond Rvirial
• Most isolated ellipticals obey scaling relations, and are not collapsed groups