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