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

Khosroshahi etal. 2004R/RvirialPoster 3.15

Luminosity Functions

Miles etal. 2004

Low LX

groups

Low sigma

Lots of spirals

Current merging

Pre-virialised

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 < 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

Scaling RelationsColour-magnitude Relation

Fundamental Plane

Reda, Forbes etal. 2004

Fossil Groups ?

• Typical isolated galaxy MB = -20.5

• One potential collapsed group:NGC 1132MB = -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